PALLET SENSOR SYSTEM Field The present disclosure relates to a pallet sensor system for monitoring loading of a pallet onto a pallet support structure. Background A common requirement in warehouses is to provide racking systems on which articles are stored. Often articles are supported on wooden pallets on which they are also transported, carried by forklift vehicles. It can be a requirement that pallets must not be sat directly on the ground, and hence require something to support them, thereby spacing them apart from the ground. This helps to keep the articles dry, as it keeps the articles away from any spillages or damp and promotes air circulation. Also, in an emergency, having an air gap under the article also means fire suppression measures (for example fire suppression fluids and powders) may circulate effectively. To keep articles off the ground, shelving can be provided as pallet support structures between racking members on which the pallets sit. Alternatively, or in addition, a mesh-structured pallet support member resting on one or more riser blocks may be provided as a pallet support structure. Shelving and/or pallet support members can also be provided at one or more higher levels of the racking system to provide additional pallet storage. While it is a common requirement to keep pallets off the ground, in some scenarios, the warehouse floor can be used as a pallet support structure for the ground level of a racking system. Loading of pallets onto pallet support structures can lead to hazards. For example, a forklift vehicle driver may drive the forks at the wrong height and damage the shelving or pallet support member. A second hazard can arise if during loading, a first pallet is pushed too far back on a pallet support structure and pushes a second loaded pallet, stored behind the first pallet in an adjacent warehouse isle, outwards towards the adjacent aisle which may result in injury to passing pedestrians or damage to goods. A third hazard can arise if a pallet is not loaded sufficiently far back on the pallet support structure and a front edge of the loaded pallet is left protruding at the front of the pallet support structure. This can result in damage to passing vehicles, injury to passing pedestrians or damage to the goods if the pallet topples from the support structure. The pallet sensor system of the present disclosure may address one or more of these hazards. Summary According to a first aspect of the present disclosure there is provided a pallet sensor system for monitoring loading of a pallet onto a pallet support structure, the pallet sensor system comprising: a back-stop structure for positioning at a rear-end of the pallet support structure; a pallet detection sensor configured to monitor a location of the pallet in relation to the back-stop structure; and a controller configured to: receive a sensing signal from the pallet detection sensor; and output a pallet loading signal, representative of a loading state of the pallet, based on the sensing signal. The pallet sensor system can advantageously monitor loading of the pallet onto the pallet support structure and output an alert when incorrect loading is detected thereby mitigating one or more of the hazards described above. The pallet detection sensor may be configured to monitor a location of the pallet in relation to the back-stop structure as the pallet moves closer to the back-stop structure. The back-stop structure may be configured to provide an abutment to a rear-side of the pallet. The pallet detection sensor may be configured to monitor a proximity of a rear-side of the pallet to the back-stop structure. The pallet detection sensor may be configured to monitor a proximity of a rear-side of the pallet to the back-stop structure as the pallet moves closer to the back-stop structure. The pallet detection sensor may be coupled to, or housed within, the back-stop structure. The pallet detection sensor may comprise a radiation transceiver. The radiation transceiver may comprise a radar, an optical transceiver or an ultra-sonic transceiver. The radiation transceiver may be coupled to the back-stop structure. The radiation transceiver may be configured to: emit a radiation signal towards a front-end of the pallet support structure; and detect at least a portion of the radiation signal reflected back towards the radiation transceiver. The pallet detection sensor may comprise a pressure sensor configured to detect an engagement of the pallet with the back-stop structure. The pallet detection sensor may comprise a plurality of sensor elements, each sensor element being configured to monitor a proximity of a respective portion of the rear- side of the pallet to the back-stop structure. The back-stop structure may comprise a plurality of back-stop members. The pallet detection sensor may comprise a plurality of sensor elements, each sensor element coupled to one of the plurality of back-stop members. The back-stop structure may comprise at least one back-stop member and a coupling for moveably coupling the at least one back-stop member to the pallet support structure such that the at least one back-stop member can move between a loaded position and an unloaded position. The pallet detection sensor may comprise at least one pressure sensor configured to provide a sensing signal indicative of whether the at least one back-stop member is in the loaded position or the unloaded position. The coupling may comprise a hinge coupling for rotatably coupling the at least one back-stop member to the pallet support structure. The pallet sensor system may comprise a biasing mechanism for urging the at least one back-stop member into the unloaded position. The controller may be configured to: output the pallet loading signal as a warning signal if the sensing signal indicates a proximity of a rear-side of the pallet to the back-stop structure does not satisfy a threshold condition; and output the pallet loading signal as a successful loading signal if the sensing signal indicates a proximity of a rear-side of the pallet to the back-stop structure does satisfy the threshold condition. The controller may be configured to not output the pallet loading signal if the sensing signal indicates the absence of a pallet. The controller may be configured to stop outputting the successful loading signal after a predetermined time period. The pallet detection sensor may comprise a radiation transceiver configured to output a sensing signal indicative of a distance between a rear-side of the pallet and the back- stop structure. The pallet loading signal may be representative of the distance between the rear-side of the pallet and the back-stop structure. The controller may be configured to output the pallet loading signal via an audible signal generator and/or visible output signal generator. The controller may be configured to output the pallet loading signal as a data signal to a server and/or a user device. The pallet detection sensor may further comprise a height sensor configured to detect a proximity of an underside of the pallet to a support surface of the pallet support structure. The pallet detection sensor may further comprise a load sensor configured to detect a mass of the pallet on the pallet support structure. The pallet sensor system may further comprise: a second back-stop member for positioning at a rear-end of a second pallet support structure; and a second pallet detection sensor configured to monitor a location of the pallet in relation to the back- stop structure. The controller may be configured to: receive a second sensing signal from the second pallet detection sensor; and output a second pallet loading signal, representative of a loading state of a second pallet on the second pallet support structure, based on the second sensing signal. The back-stop structure may be configured to engage with one or more hollow cells of a meshed pallet support member of the pallet support structure. According to a second aspect of the present disclosure there may be provided a pallet support structure comprising the pallet sensor system of any preceding claim. The pallet support structure may comprise a meshed pallet support member having a plurality of hollow cells, wherein the back-stop structure is configured to engage with one or more of the plurality of hollow cells of the meshed pallet support member. The pallet support structure may further comprise one or more riser blocks configured to engage with one or more of the plurality of hollow cells of the meshed pallet support member. According to a third aspect of the present disclosure there is provided a method of monitoring loading of a pallet onto a pallet support structure, the method comprising: receive a sensing signal from a pallet detection sensor monitoring a location of the pallet in relation to a back-stop structure positioned at a rear-end of the pallet support structure; and outputting a pallet loading signal, representative of a loading state of the pallet, based on the sensing signal. According to a fourth aspect of the present disclosure there is provided a pallet sensor system for monitoring loading of a pallet onto a pallet support structure, the pallet sensor system comprising: a back-stop structure for positioning at a rear-end of the pallet support structure; a pallet detection sensor comprising a radiation transceiver configured to: monitor a proximity of a rear-side of the pallet to the back-stop structure; and output a sensing signal indicative of a distance between the rear-side of the pallet and the back-stop structure; and a controller configured to: receive the sensing signal from the pallet detection sensor; and output a pallet loading signal, representative of a loading state of the pallet, based on the sensing signal. The pallet detection sensor may be coupled to, or housed within, the back-stop structure. The back-stop structure may comprise a structure for coupling to a back-stop abutment of the pallet support structure. The back-stop structure may comprise one or more of: a plate, a mount, a housing and a sleeve, for coupling to the back-stop abutment. The radiation transceiver may be coupled to the back-stop structure. The radiation transceiver may be configured to: emit a radiation signal towards a front-end of the pallet support structure; and detect at least a portion of the radiation signal reflected back towards the radiation transceiver. The pallet detection sensor may comprise a plurality of sensor elements. Each sensor element may be configured to monitor a proximity of a respective portion of the rear- side of the pallet to the back-stop structure. The pallet loading signal may be representative of the distance between the rear-side of the pallet and the back-stop structure. The controller may be configured to output the pallet loading signal as a warning signal if the sensing signal indicates a proximity of a rear-side of the pallet to the back-stop structure does not satisfy a threshold condition. The controller may be configured to output the pallet loading signal as a successful loading signal if the sensing signal indicates a proximity of a rear-side of the pallet to the back-stop structure does satisfy the threshold condition. The threshold condition may comprise the distance between the rear-side of the pallet and the back-stop structure being less than a range threshold. The pallet detection sensor may comprise a plurality of sensor elements. Each sensor element may be configured to: monitor a proximity of a respective portion of the rear- side of the pallet to the back-stop structure; and output a respective component of the sensing signal. The threshold condition may comprise a plurality of component threshold conditions corresponding to the component of the sensing signal from each sensing element. The threshold condition may be satisfied when each of the plurality of component threshold conditions are satisfied. The pallet detection sensor may comprise: a first radiation transceiver arranged to monitor a proximity of a first end of the rear-side of the pallet to the back-stop structure and output a first component of the sensing signal; and a second radiation transceiver arranged to monitor a proximity of a second end of the rear-side of the pallet to the back-stop structure and output a second component of the sensing signal. The first and second radiation transceivers may be coupled to, or housed within the back-stop structure. The first radiation transceiver may be spaced apart from the second radiation transceiver along a length of the back-stop structure. The first end of the pallet may be spaced apart from the second end of the pallet in the lateral dimension. The first and second radiation transceivers may be optical time of flight radiation transceivers. The controller may be configured to determine, based on the sensing signal, one or more pallet parameters comprising one or more of: the distance between the rear-side of the pallet and the back-stop structure; and a skew of the pallet. The pallet loading signal may be representative of one or more of: the distance between the rear-side of the pallet and the back-stop structure; and a skew of the pallet. The controller may be configured to output the pallet loading signal as a warning signal if the sensing signal does not satisfy a threshold condition. The controller may be configured to output the pallet loading signal as a successful loading signal if the sensing signal does satisfy the threshold condition. The threshold condition may comprise one or more of: the distance between the rear-side of the pallet and the back-stop structure being less than a range threshold; and a skew of the pallet being less than a skew threshold. The pallet detection sensor may comprise a plurality of first sensor elements arranged as a first radiation transceiver array. The first radiation transceiver array may be configured to output a first ranging matrix as a first component of the sensing signal. Each matrix element of the first ranging matrix may be indicative of a ranging distance detected by a respective first sensor element. The first radiation transceiver array may comprise a time of flight infrared array sensor. The pallet sensor system may further comprise a plurality of second sensor elements arranged as a second radiation transceiver array. The second radiation transceiver array may be configured to output a second ranging matrix as a second component of the sensing signal. Each matrix element of the second ranging matrix may be indicative of a ranging distance detected by a respective second sensor element. The first radiation transceiver array may be arranged to monitor a proximity of a first end of the rear-side of the pallet to the back-stop structure. The second radiation transceiver array may be arranged to monitor a proximity of a second end of the rear- side of the pallet to the back-stop structure. The first and second radiation transceiver arrays may be coupled to, or housed within the back-stop structure. The first radiation transceiver array may be spaced apart from the second transceiver array along a length of the back-stop structure. The first end of the pallet may be spaced apart from the second end of the pallet in the lateral dimension. The second radiation transceiver array may comprise a time of flight infrared array sensor. The controller may be configured to determine, based on the sensing signal, one or more pallet parameters comprising one or more of: the distance between the rear-side of the pallet and the back-stop structure; a skew of the pallet; a height of the pallet above the pallet support structure; and a position of stock loaded on the pallet. The controller may be configured to identify matrix elements of the first ranging matrix and/or the second ranging matrix corresponding to the pallet. The controller may be configured to identify matrix elements of the first ranging matrix and/or the second ranging matrix corresponding to the pallet using edge detection. The controller may be configured to identify matrix elements of the first ranging matrix and/or the second ranging matrix corresponding to the pallet as one or more matrix elements indicative of the lowest ranging distance(s). The controller may be configured to identify matrix elements of the first ranging matrix and/or the second ranging matrix corresponding to stock. The controller may be configured to identify matrix elements of first ranging matrix and/or the second ranging matrix corresponding to stock as matrix elements: positioned above the matrix elements corresponding to the pallet; and indicating a distance greater than, and within a threshold difference to, the distance indicated by the matrix elements corresponding to the pallet. The controller may determine groups of adjacent matrix elements, in one or both of the first and second ranging matrices, that indicate distances within a threshold grouping range of each other. The threshold grouping range may be of the order of 1 to 10 cm. The controller 108 may determine the matrix elements corresponding to the pallet 2 as the group of matrix elements indicating the lowest distance. The controller 108 may determine matrix elements corresponding to the stock 40 as all other groups of matrix elements indicating a finite distance less than the detection range of the pallet detection sensor. The controller may be configured to determine the distance between the rear-side of the pallet and the back-stop structure based on matrix elements of the first ranging matrix and/or the second ranging matrix corresponding to the pallet. The controller may be configured to determine the distance between the rear-side of the pallet and the back-stop structure based on a lowest distance value or an average distance value indicated by the matrix elements of the first ranging matrix and/or the second ranging matrix corresponding to the pallet. The controller may be configured to determine the skew of the pallet based on matrix elements of the first ranging matrix corresponding to the pallet. The controller may be configured to determine the skew of the pallet based on a difference between a lowest distance value and a highest distance value indicated by the matrix elements of the first ranging matrix corresponding to the pallet. The controller may be configured to determine the skew of the pallet based on a difference between a first distance indicated by matrix elements of the first ranging matrix corresponding to the pallet and a second distance indicated by matrix elements of the second ranging matrix corresponding to the pallet. The controller may be configured to determine the height of the pallet above a loading surface of the pallet support structure based on a position, within the first ranging matrix, of matrix elements corresponding to the pallet and a distance indicated by the matrix elements. The controller may be configured to determine the position of stock loaded on the pallet based on a position within the first ranging matrix of matrix elements of the first ranging matrix corresponding to the stock. The pallet loading signal may be representative of one or more of: the distance between the rear-side of the pallet and the back-stop structure; a skew of the pallet; a height of the pallet; and a position of edges of the stock. The controller may be configured to output the pallet loading signal as a warning signal if the sensing signal does not satisfy a threshold condition. The controller may be configured to output the pallet loading signal as a successful loading signal if the sensing signal does satisfy the threshold condition. The threshold condition may comprise one or more of: the distance between the rear-side of the pallet and the back-stop structure being less than a range threshold; a skew of the pallet being less than a skew threshold; a height of the pallet being within a threshold height range; and a position of edges of the stock being within a threshold stock position range. The controller may be configured to stop outputting the successful loading signal after a predetermined time period. The controller may be configured to not output the pallet loading signal if the sensing signal indicates the absence of a pallet. The controller may be configured to output the pallet loading signal via an audible signal generator and/or visible output signal generator. The controller may be configured to output the pallet loading signal as a data signal to a server and/or a user device. The pallet detection sensor may further comprise a load sensor configured to detect a mass of the pallet on the pallet support structure. The pallet sensor system may comprise: a second back-stop member for positioning at a rear-end of a second pallet support structure; and a second pallet detection sensor comprising a second radiation transceiver configured to: monitor a proximity of a rear-side of a second pallet to the second back- stop structure; output a second sensing signal indicative of a distance between the rear- side of the second pallet and the second back-stop structure;, wherein the controller is configured to: receive the second sensing signal from the second pallet detection sensor; and output a second pallet loading signal, representative of a loading state of the second pallet on the second pallet support structure, based on the second sensing signal. According to a fifth aspect of the present disclosure, there is provided a method of monitoring loading of a pallet onto a pallet support structure, the method comprising: receiving a sensing signal from a pallet detection sensor comprising a radiation transceiver, wherein the sensing signal is indicative of a a distance between a rear- side of the pallet and a back-stop structure positioned at a rear-end of the pallet support structure; and outputting a pallet loading signal, representative of a loading state of the pallet, based on the sensing signal. There may be provided a computer program, which when run on a computer, causes the computer to configure any apparatus, including a circuit, controller, converter, or device disclosed herein or perform any method disclosed herein. The computer program may be a software implementation, and the computer may be considered as any appropriate hardware, including a digital signal processor, a microcontroller, and an implementation in read only memory (ROM), erasable programmable read only memory (EPROM) or electronically erasable programmable read only memory (EEPROM), as non-limiting examples. The software may be an assembly program. The computer program may be provided on a computer readable medium, which may be a physical computer readable medium such as a disc or a memory device, or may be embodied as a transient signal. Such a transient signal may be a network download, including an internet download. There may be provided one or more non-transitory computer-readable storage media storing computer-executable instructions that, when executed by a computing system, causes the computing system to perform any method disclosed herein. Brief Description of the Drawings One or more embodiments will now be described by way of example only with reference to the accompanying drawings in which: Figure 1A shows an example pallet sensor system installed on a pallet support structure installed in conjunction with a racking system; Figure 1B shows the pallet support structure of Figure 1A without the racking system or pallets; Figures 1C and 1D illustrate a back-stop structure of the example pallet sensor system in more detail; Figure 1E illustrates a back-stop member of the back-stop structure in isolation; Figure 1F illustrates a locking plate for affixing the back-stop member to the pallet support structure; Figure 1G illustrates a locking pin for affixing the back-stop member to the pallet support structure; Figure 2A illustrates a first stage of loading a pallet onto a first pallet support structure of a racking system, with an example pallet sensor system monitoring the loading; Figure 2B illustrates a second stage of the loading of the pallet and the response of the example pallet sensor system; Figure 2C illustrates completion of correct loading of the pallet and the response of the example pallet sensor system; Figure 2D illustrates completion of correct loading of a pallet onto a second pallet support structure of the racking system and the response of the example pallet sensor system; Figure 3 illustrates a further example pallet sensor system according to an embodiment of the present disclosure; and Figures 4A to 4D schematically illustrate first and second ranging matrices output as respective first and second components of a sensing signal by respective first and second radiation transceiver arrays, as a pallet is loaded onto a pallet support structure. Detailed Description Figures 1A to 1G illustrate a pallet sensor system for monitoring loading of a pallet onto a pallet support structure, according to an embodiment of the present disclosure. Figure 1A shows the pallet sensor system installed on a pallet support structure 10. The pallet support structure 10 is installed in conjunction with a racking system 1. Pallets 2 are located on top of pallet support members 100 which form part of the pallet support structure 10. Figure 1B shows the pallet support structure 10 without the racking system 1 or pallets 2. Figures 1C and 1D illustrate a back-stop structure 102 of the pallet sensor system in more detail. Figure 1E illustrates a back-stop member of the back-stop structure 102. Figures 1F and 1G illustrate fixing means for coupling the back-stop member to the pallet support member 100. The pallet sensor system comprises a back-stop structure 102 for positioning at a rear- end 104 of the pallet support structure 10. The pallet sensor system also includes a pallet detection sensor 106 configured to monitor a location of the pallet 2 in relation to the back-stop structure 102. The pallet sensor system further comprises a controller 108. The controller 108 receives a sensing signal 110 from the pallet detection sensor 106 and outputs a pallet loading signal 112, representative of a loading state of the pallet 2, based on the sensing signal 110. In this example, the controller 108 outputs the pallet loading signal 112 via an optical signal generator 114 in the form of a strip of LED lights along a side edge of the pallet support structure 10 and via an audible signal generator 115. The pallet sensor system can advantageously monitor loading of the pallet 2 onto the pallet support structure 102 and output an alert when incorrect loading is detected thereby mitigating one or more of the hazards described above. The loading state may correspond to a simple binary indication of whether the pallet is correctly loaded or not. The loading state may also comprise: a distance from the back-stop structure 102 to the pallet 2; a skew of the pallet 2; a height of the pallet 2 above a loading surface of the pallet support structure 10; and the position of the edges of stock loaded on the pallet. In some examples, the back-stop structure 102 may comprise a back-stop abutment arranged to provide an abutment to a rear-side of the pallet 2 and prevent the pallet from protruding across the rear-end 104 of the pallet support structure 10. In this way the back-stop structure 102 provides a mechanical stop that can prevent the pallet 2 pushing through to an adjacent pallet support structure or an adjacent aisle of a warehouse. In the illustrated example, the back-stop structure 102 comprises a plurality of back-stop abutments 102-1, 102-3, 102-3. (As described herein the terms back-stop member and back-stop abutment may be used interchangeably.) In some examples, the back-stop structure 102 may be removably couplable to the pallet support member 100. In this example, each back-stop abutment 102-1, 102-2, 102-3 of the back-stop structure 102 is complementary in shape to, and locates within, hollow cells of a meshed pallet support member 100, as described in more detail below. In some examples, a back-stop abutment may comprise one or more mechanical fixings or couplings, such as clamps, clips, fastenings, pins, plates, moveable couplings etc, for (removably) coupling to the pallet support structure 10. In this way, the back- stop structure may be used in conjunction with shelving or any type of raised pallet support structure 10. As mentioned above, the warehouse floor may also serve as a pallet support structure 10. In such examples, the back-stop structure 102 may be provided with fixings to attach to the floor and/or a sufficiently weighted baseplate to maintain the back-stop structure 102 in position. In yet further examples, in which a pallet support structure 10 already has an integral back-stop abutment (such as a lip on shelving), the back-stop structure 102 may comprise a structure for attaching to the integral back-stop abutment, such as a plate, a mount, or a more robust abutment etc. In some examples, the back-stop structure 102 may comprise a unitary structure that may provide an abutment along a length of the rear-end 104 of the pallet support structure 10 and across a width of the pallet 2 (i.e. along a lateral dimension with respect to a pallet loading direction from the front-end to the rear-end of the pallet support structure 10). In other examples, the back-stop structure 102 may comprise a plurality of individual back-stop members. The plurality of back-stop members may be spaced apart along the length of the rear-end 104 of the pallet support structure 10 to provide an abutment to the rear-side of the pallet 2 across its width. In this example, the back-stop structure 102 comprises three back-stop members each comprising a respective back-stop abutment 102-1, 102-2, 102-3. The back-stop abutments 102-1, 102-2, 102-3 are spaced apart along the length of the rear-end of the pallet support structure 10. In this way, each back-stop abutment 102-1, 102-2, 102-3 can provide a mechanical stop to a different portion of the rear-side of the pallet 2. This can advantageously prevent unwanted rotation of the pallet 2 when pushed against the abutment during loading. Providing the back-stop structure 102 as a plurality of back-stop members can provide other advantageous including: (i) compatibility with a meshed pallet support structure; and (ii) providing a sensor system with multiple sensor types, as discussed further below. In some examples, the pallet detection sensor 106 may be coupled to the back-stop structure 102. For example, the pallet detection sensor 106 may be housed within the back-stop structure 102 and/or coupled to a surface of the back-stop structure 102. In some examples, the pallet detection sensor 106 may comprise a plurality of sensor elements 106-1, 106-2, 106-3. The plurality of sensor elements 106-1, 106-2, 106- 3 may be coupled along a length of the back-stop structure 102 with each sensor element 106-1, 106-2, 106-3 monitoring a respective portion of the rear-side of the pallet 2 to the back-stop structure. Each sensor element 106-1, 106-2, 106-3 may be coupled to a respective one of a plurality of back-stop members 106-1, 106-2, 106- 3 of the back-stop structure 102. Each sensor element may output a respective component of the sensing signal 108. In this example, the pallet detection sensor 106 comprises: a first sensor element 106-1 coupled to a surface of a first back-stop abutment 102-2; a second sensor element 106-2 housed within a second back-stop abutment; and a third sensor element 106-3 coupled to a surface of a third back-stop abutment 102-3. Providing a plurality of sensor elements 106-1, 106-2, 106-3 to monitor a respective portion of the rear-side of the pallet 2 can ensure that the pallet 2 is not rotated and is abutted to the back-stop structure across the whole of the rear- side of the pallet 2. In this way, the sensor system can provide improved monitoring of whether the pallet 2 is correctly loaded. It will be appreciated that in other examples (not illustrated), the pallet detection sensor 106 may be positioned separately to the back-stop structure 102 and arranged to monitor the position of a rear-side of the pallet 2 in relation to the back-stop structure 102. For example, an optical sensor, such as a camera, may be positioned above the pallet support structure 10 and directed downward towards the pallet support structure 10. In some examples, the pallet detection sensor 106 may comprise a radiation transceiver. The radiation transceiver may comprise a radar, an optical transceiver, an ultrasonic transceiver, a lidar transceiver, or any other known radiation transceiver. The radiation transceiver may emit a radiation signal towards a target and receive a reflected radiation signal in return. The reflected signal may change depending on the presence of the pallet 2 in a path of the radiation signal. In some examples, the radiation transceiver may be coupled to the back-stop structure 102 and arranged to: emit the radiation signal in a first direction from the rear-end of the pallet support structure 10 towards a front-end of the pallet support structure 10; and detect at least a portion of the radiation signal reflected back in a second direction towards the rear- end 104 of the pallet support structure 10 and the radiation transceiver. In this way, the radiation transceiver can monitor a proximity of the rear-side of the pallet 2 to the back-stop structure 102 and output the sensing signal 110 as indicative of the proximity. The controller 108 may determine the proximity using a time of flight measurement or other known distance determining techniques. In the illustrated example, the second sensor element 106-2 comprises an ultrasonic transceiver housed in the second back-stop abutment 102-2. An aperture in a front surface of the second back-stop abutment 102-2 provides a path for the radiation signal and reflected radiation signal. In some examples, the pallet detection sensor 106 may comprise a pressure sensor coupled to the back-stop structure 102. The pressure sensor may detect an engagement of the pallet 2 with the back-stop structure 102, for example when the pallet 2 abuts to the back-stop structure 102. The pressure sensor may be positioned on a front surface of the back-stop structure 102 such that the pallet 2 directly impinges on the pressure sensor. In other examples, the pressure sensor may be positioned on an internal or rear surface of the back-stop structure that is subject to compression when the pallet 2 abuts the back-stop structure 102. The pressure sensor may output a binary sensing signal based on whether the pallet 2 abuts the back-stop structure 102. In this way, the pallet detection sensor monitors a proximity of the pallet 2 to the back-stop structure 102. In some examples, the back-stop structure 102 may be coupled to the pallet support structure 10 and configured to move between an unloaded position and a loaded position. For example, the back-stop structure 102 may comprise a hinge coupling or a translational coupling, such as one or more guide rails, to move between the unloaded and loaded positions. The back-stop structure 102 may comprise a biasing mechanism, such as a spring, to urge the back-stop structure towards the unloaded position in the absence of a pallet 2. As the pallet 2 is loaded, the pallet 2 may be pushed towards the rear-end 104 of the pallet support structure 10 by a forklift vehicle. The motion of the pallet 2 can overcome the biasing mechanism and force the back- stop structure 102 into the loaded position as the pallet 2 is pushed back against and abuts the back-stop structure 102. The pressure sensor may be positioned on a surface of the back-stop structure 102 and output a binary sensing signal based on whether the back-stop structure is in the unloaded position or the loaded position. In this way, the pallet detection sensor monitors a proximity of the pallet 2 to the back-stop structure 102. In the illustrated example, the first and third back-stop abutments 102-1, 102-3 are rotatably coupled to the pallet support member 100. Figures 1C and 1D illustrate the back-stop abutments 102-1, 102-3 in the unloaded position. First and third sensor elements 106-1, 106-3 each comprise a pressure sensor coupled to a first internal surface 116 of the respective back-stop abutment 102-1, 102-3. When the pallet is loaded into the correct position, it pushes and rotates the first and third back-stop abutments 102-1, 102-3 in the direction indicated by arrow A into the loaded position. As a result, the pressure sensors 106-1, 106-3 are compressed against a second internal surface 118 of the back-stop structure 102 (or alternatively against a surface of the pallet storage structure 10) and output a sensing signal 110 indicative of the increased pressure and the proximity of the pallet 2 to the back-stop structure 102. Some examples, such as the illustrated example, can provide a pallet detection sensor 106 comprising both a radiation transceiver 106-2 and a pressure sensor 106-1, 106- 3. Such a multi-sensor system can advantageously ensure a pallet 2 is correctly loaded by monitoring the abutment of the pallet 2 along the length of the back-stop structure 102. Furthermore, the radiation transceiver 106-2 can output a component of the sensing signal 110 that is indicative of a distance from the back-stop structure 102 to the rear-side of the pallet 2. As described below, the controller 108 may output the pallet loading signal as representative of this distance. The pressure sensors 106-1, 106-3 can provide lower cost pallet detection sensor elements to provide the monitoring of the pallet loading across the full width of the pallet support structure 100. The pallet detection sensor 106 may monitor a proximity of the rear-side of the pallet 2 to the back-stop structure 102. The pallet detection sensor 106 may output the sensing signal 110 indicative of the detected proximity of the rear-side of the pallet 2 to the back-stop structure 102. In some examples, such as those utilising pressure sensors, the detected proximity may be binary (unloaded/loaded position). In some examples, such as those utilising radiation transceivers, the detected proximity may be indicative of a relative distance from the back-stop structure 102 to the rear-side of the pallet 2. In systems comprising a plurality of sensor elements 106-1, 106-2, 106-3, such as that illustrated, the sensing signal 110 may comprise a signal component from each sensor element 106-1, 106-2, 106-3. The components of the sensing signal 110 may be indicative of a proximity of the corresponding portion of the rear side of the pallet 2 to the back-stop structure 102. As described above, each component of the sensing signal 110 may indicate the proximity in a binary or quantitative manner. The controller 108 may receive the sensing signal 110 from the pallet detection sensor 106 and determine whether the proximity of the rear-side of the pallet 2 to the back- stop structure 102 satisfies a threshold condition. The threshold condition may comprise one or more component threshold conditions corresponding to the component of the sensing signal from each sensing element 106-1, 106-2, 106-3. The threshold condition may be satisfied if each of the one or more component threshold conditions are satisfied. For binary components of the sensing signal 110, the component threshold condition may relate to the component of the sensing signal 110 indicating that the respective portion of the rear-side of the pallet 2 is abutted to the back-stop structure 102. For quantitative components of the sensing signal, the component threshold condition may relate to the component of the sensing signal 110 indicating that the respective portion of the rear-side of the pallet 2 is within a threshold range of the back-stop structure 102. The controller 108 may be configured to output the pallet loading signal 112 based on whether the sensing signal 110 satisfies the threshold condition. For example, the controller may be configured to: (i) output the pallet loading signal 112 as a warning signal if the sensing signal 110 indicates a proximity of a rear-side of the pallet 2 to the back-stop structure 102 that does not satisfy the threshold condition; and (ii) output the pallet loading signal 112 as a successful loading signal if the sensing signal 110 indicates a proximity of a rear-side of the pallet 2 to the back-stop structure 102 that does satisfy the threshold condition. In some examples, the controller 108 may stop outputting the successful loading signal after a predetermined time period (for example, from 1 to 5 seconds). This can advantageously conserve power, particularly advantageous in battery powered sensor systems. In some examples, the controller 108 may further determine whether a pallet 2 is present on the pallet support structure 10. For example, the controller 108 may receive a sensing signal 110 from a radiation transceiver 106-2 that indicates that no pallet 2 is present within the range of the depth (from the front-end to the rear-end 104) of the pallet support structure 10. In some examples, the pallet detection sensor 106 may further comprise a load sensor or further pressure sensor (not illustrated) configured to detect a mass of a pallet 2 on the pallet support structure 10 and output a corresponding component of the sensing signal 110. For example, a load sensor may be positioned between the pallet support member 100 and riser blocks supporting the pallet support member 100. In such examples, the controller 108 may not output the pallet loading signal 112 if the sensing signal 110 indicates no pallet is present on the pallet support structure 10. In some examples, the pallet sensor system may reside in a sleep mode or idle mode other than when a pallet 2 is in the process of being loaded or unloaded. In the sleep mode, all components of the pallet sensor system may be inactive with the exception of the controller and a sensor to detect a pallet presence (such as a load sensor or a radiation transceiver). When a pallet is first placed on the pallet support structure 10 and detected by the load sensor or radiation transceiver, the controller 108 may wake the other components, such as the remaining sensors and output signal generators and monitor the loading / unloading of the pallet 2 as described above. The sensor system may also reside in the sleep mode after successful loading of the pallet 2 with only the controller and one sensor active (such as the load sensor or one of the pressure sensors). When a driver of a forklift vehicle begins unloading the pallet, the sensor may detect the change in load or pressure and output an appropriate sensing signal 110. The controller 108 may wake the remaining components and output an appropriate pallet loading signal 112. In some examples, the pallet detection sensor 106 may comprise a height sensor configured to detect a proximity of an underside of the pallet 2 to a support surface 122 of the pallet support structure 10. The height sensor may be a radiation transceiver operating in a similar manner to the radiation transceivers described above. The height sensor may output a corresponding component of the sensing signal 110 indicative of a height of the underside of the pallet 2 above the support surface 122 of the pallet support structure 10. The controller 108 may output the pallet loading signal 112 as representative of the height. This can advantageously assist the driver and avoid collisions of the forks, pallet 2 and/or articles on the pallet 2 with the pallet support structure 10, which can be particularly advantageous when the pallet support structure 10 is elevated with respect to the driver, for example upper shelving. In some examples, the pallet detection sensor 106 may comprise a damage sensor configured to detect damage to the pallet support structure 10, such as the pallet support member 100 or pallet shelving. The damage sensor may comprise a collision sensor configured to detect impacts between the forks of a forklift vehicle and the pallet support structure 10. In some examples, the damage sensor may comprise a camera configured to capture an image of the pallet support structure 10 which may be intermittently or in response to a signal from the collision sensor. The controller 108 may output the pallet loading signal 112 via an audible signal generator 115 and/or an optical signal generator 114. Alternatively, or in addition, the controller 108 may output the pallet loading signal 112 as a data signal to a server and/or a user device, such as a cab display or mobile device of the driver of the forklift vehicle performing the pallet loading/unloading. Outputting a data signal can advantageously assist the driver to ensure correct loading of the pallet 2 and avoid damage. Outputting a data signal to a server, central control system and/or secure cloud can advantageously enable logging of data and provide a warehouse operator with an overview of pallet support structures via a user interface such as a dashboard. The data signal can be used to provide an overview of any of: (i) a duration of pallet loading / unloading; (ii) an occupancy (how often and current state) of a pallet bay; and (iii) alerts and warnings associated with loading/unloading at a pallet supports structure 10; and (iv) damage to the pallet support structure 10, for one or more pallet support structures 10 in the warehouse. In examples in which the sensing signal 110 comprises a component indicative of a relative distance between the rear-side of the pallet 2 and the back-stop structure 102, the controller 108 may output the pallet loading signal 112 as representative of the relative distance. For example, the optical signal generator 114 (LEDs) may flash at a frequency representative of the relative distance. Alternatively, or in addition, the audible signal generator may produce a series of pips with a frequency of pips indicative of the relative distance. A data signal may also include a relative distance for display on the user device of the forklift driver. In examples with a binary component of the sensing signal 110 indicative of whether the rear-side of the pallet 2 abuts the back-stop structure 102 (or not), the controller 108 may output the pallet loading signal 112 as representative of whether the pallet is in the correct loading position (or not). For example, the optical signal generator 114 may change a colour of emission, the audible signal generator may output a continuous tone (for successful loading) and / or the data signal may indicate successful loading. In the illustrated example, the controller 108 can determine the relative distance between the second back-stop abutment 102-2 and the rear-side of the pallet based on the component of the sensing signal 110 from the ultrasonics sensor 106-2. The controller 108 outputs the pallet loading signal via the audible signal generator 115 as a series of audible pips with a spacing between pips (frequency) dependent on the relative distance. As the pallet 2 approaches the second back-stop abutment 102-2, the pips get closer together in time. The controller 108 also outputs the pallet loading signal 112 via the LEDs 114 as a first colour (e.g. red) based on the binary components of the sensing signal 110 from the pressure sensors 106-1, 106-3 indicating the first and third back-stop abutments 102-1, 102-3 are in the unloaded position. The system may comprise first and second strips of LEDs 115 on respective sides of the pallet support structure 10, each indicative of the respective component of the sensing signal from the respective first and third pressure sensors. Following successful loading of the pallet 2, the controller 108 may output the pallet loading signal 112 as a successful loading signal via the LEDs 115 as a second colour (e.g. green). The controller 108 may also change the audible signal 114 to a continuous tone to provide the successful loading signal. After a predetermined time period, for example 5 seconds, the controller 108 may stop outputting the successful load signal. In some examples, the controller 108 may be housed in the back-stop structure 102. In other examples, the controller 108 may be housed in a separate housing, for example in combination with the pallet detection sensor 106 and / or the output signal generator. The controller 108 may be communicatively coupled to the pallet detection sensor 106 and the output signal generator by any known wired or wireless means. The pallet sensor system may comprise a power supply unit (not shown). The power supply unit may be housed in the back-stop structure 102. The power supply unit may provide power to the controller 108, the pallet detection sensor 106 and / or the output signal generator 114, 115. The power supply unit may be mains powered or battery powered. In some examples, the output signal generator(s) 114, 115 may have separate power supplies, for example separate battery packs. As illustrated, in some examples the pallet sensor system may be used in conjunction with a pallet support structure 10 comprising a meshed pallet member 100. In particular, the back-stop structure 102 may be shaped to conform and/or engage with one or more of a plurality of hollow cells 124 of the meshed pallet member 100. The meshed pallet support member 100 may have a base side 120 and a support side 122. The base side 102 may face the substrate (i.e. floor) on which the pallet support structure 10 rests, and the support side 104 may support the pallet 2. That is to say, in use, the support side 104 is the side on which the pallet 2 will sit. The pallet support member 100 (best illustrated in Figure 1B) may include a plurality of hollow cells 124 which extend from an opening on the base side 120 to an opening on the support side 122. The cells may have a substantially uniform cross-sectional shape between the openings on either side of the pallet support member 100. That is to say, each cell 124 can define a passage which extends between the base side 120 to the support side 122, the passage being open on both the base side 120 and the support side 122. The passage may have a uniform cross-sectional shape along its length. In another example the passage may have a uniform cross-sectional shape along at least some of its height between the base side 120 and the support side 122. That is to say, each cell 124 may have the same width and length along at least part of its height between the base side 120 and the support side 122. The distance between opposing surfaces of the cell walls may be constant along the cell height. The cells walls may be polygonal, for example square or rectangular. The pallet support member 100 comprises a boundary wall which extends from the base side 120 to the support side 122 to define a first edge, a second edge, a third edge, and a fourth edge of the pallet support member 100. The first edge is provided on the opposite side of the pallet member 100 to the second edge. The third edge is provided on the opposite side of the pallet member 100 to the fourth edge. The cells 124 are arranged in rows that extend between opposite edges of the pallet support member 100. The cells 124 may be arranged in parallel rows that extend between opposite edges. The rows 108 may be consecutive, insofar as the rows are immediately adjacent one another across the pallet support member 100. Hence the cells 124 may be arranged in rows that extend between the third edge and the fourth edge. Alternatively the cells 124 may be arranged in rows extend between the first edge and the second edge. The cells of adjacent rows may be staggered. That is to say, the cells 124 of adjacent rows may be misaligned. Put another way, cells 124 in a first row of cells may be defined by walls such that the cells 124 of the row are offset relative to the cells 124 of an adjacent row. Cells 124 may be defined by primary dividing walls and secondary dividing walls, the primary dividing walls being spaced apart and extending between two different (e.g. opposite) edges of the boundary wall, and the secondary dividing walls extending between the primary dividing walls to define opposite ends of the cells 124 along the row. Hence the secondary dividing walls of one row may be offset from the secondary dividing walls of an adjacent row such that the cells 124 of the adjacent rows are staggered, and cell 124 of alternate rows are aligned, as shown in Figure 1B. The boundary wall, primary dividing wall and secondary dividing wall may be integrally formed. The boundary wall, primary dividing wall and secondary dividing wall may be formed from a material comprising a plastic. Alternatively, the boundary wall, primary dividing wall and secondary dividing wall may be formed from a plastic material, for example polyurethane. As shown in the Figure 1B, the first edge of the boundary wall (at the front end of the pallet support member 100) may comprise a first extension, a second extension and a third extension spaced apart along the first edge to define a first recessed region between the first extension and the second extension, and a second recessed region between the second extension and third extension. Hence the first edge may have a sculpted serpentine shape, with walls which extend towards and then away from the second edge. The first extension, second extension and third extension, and the first recessed region and second recessed region, extend from a row of cells 124 that extend between the third edge and the fourth edge. The first extension, second extension and third extension may each be configured as a bumper (i.e. buffer) to absorb shock locks in the plane in which the pallet member 100 extends. Thus the first extension, second extension and third extension can provide protection for the main body of the pallet support member 100. For example, these extensions provide buffers/bumpers which may be sacrificial such that although they may be damaged when struck (for example by the back or front of a forklift vehicle), the integrity of the remainder of the pallet support member 100 (e.g. the structure which defines the cells 124) may be protected. Figure 1E illustrates in isolation the central back-stop member 102-2 of the back-stop structure 102. The back-stop member 102-2 is shaped to conform with (or be complementary to) the shape of the cells 124 of the meshed pallet support member 100. In this way, the back-stop member 102-2 may engage with one or more of the cells 124. In this example, the back-stop member 102-2 comprises a body (i.e. base) 130 and a first engagement member 132 which extends from the body (i.e. base) 130 to a free end 136. The back-stop member 102-2 may comprise a second engagement member 138, comprising the same features as the first engagement member 132 and spaced apart from the first engagement member 132 along the length of the back-stop member 102-2. The second engagement member 138 may be configured to operate and integrate with the cells 124 in the same way as the first engagement member 132. The first engagement member 132 and second engagement member 138 may be complementary in shape to the cells 106. That is to say, the engagement members 132, 138 and each cell 124 may have the same cross-sectional shape along their length/height, with the engagement members 132, 138 having external dimensions which are slightly less than the internal dimensions of the cells 124 such that the engagement members 132, 138 may be entered into each of the cells 124. The engagement members 132, 138 and cells 124 may be configured, that is to say sized, such that there is a snug fit or interference fit between the engagement members 132, 138 and each cell 124. Put another way, each of the engagement members 132, 138 and cells 124 may be configured such that each of the engagement members 132, 138 are operable to be entered into any of the plurality of the cells 124 from the base side 120 of the meshed pallet support member 100, such that each of the engagement members 132, 138 are locatable in each of the cells 124. In this way the or each engagement member 132, 138 may fix to the pallet support member 100. Because of the configuration (for example relative sizes of the engagement members 132, 138 and each cell 124), and because the back-stop member 102-2 may be formed as a discrete member separate from the pallet support member 100, each of the engagement members 132, 138 may be operable to be removeable from each of the cells 124. That is to say each engagement member 132, 138 may be operable to be positioned in a first location on the pallet support member 100, removed from the pallet support member 100 and positioned at the same or a different location on the pallet support member 100. Hence each engagement member 132, 138 is operable to be removeable from the cell 124 in which it is engaged such that the position of the back-stop member 102-2 on the pallet support member 100 may be changed. The first engagement member 132 and cells 124 may also be configured (that is to say sized and shaped) such that when located in the cell 124, the first engagement member 132 extends part of the way, but not all of the way, from the base side 120 towards the support side 122. The body 130 of the back-stop member 102-2 forms an abutment for engagement with the pallet support member 100. The first engagement member 132 extends from the body 130 (and hence the abutment for engagement with the pallet support member 100) to a distance which is less than the height of the cell 124 so that the free end 136 of the first engagement member 132 is recessed beneath the support side 122. That is to say, the free end 136 is recessed into the opening on the support side 122 such that the free end 136 of the first engagement member 132 does not extend out of the opening on the support side 122 of the pallet support member 100. As shown in the figures, and in particular Figure 1B, the pallet support structure 10 may further comprise a locking pin 300 and a locking plate 400. An example locking plate 400 is shown in detail in Figure 1G, and an example locking pin 300 is shown in detail in Figure 1F. The engagement members 132, 138 of each element type may comprise a cavity 140 with a first engagement feature 142. The locking plate 400 may include an aperture 410. The locking pin 300 may comprise a shaft portion 302 and a head portion 304 at one end of the shaft portion 302. The head portion 304 may have a diameter larger than the shaft portion 302 such that it overhangs the shaft portion 302. The diameter of the head portion 304 may also be larger than the diameter of the locking plate aperture 410. The locking pin 300 may further comprise a second engagement feature 312 for lockable engagement with the first engagement feature 212 of the engagement member 132, 138. The second engagement feature 312 may be provided on the shaft portion 302. The locking plate 400 may comprise a cut out 412 in the wall of the aperture 410 to allow for the passage of the second engagement feature 312 of the locking pin 300 to pass through the locking plate 400. The shaft portion 302 may be configured to extend through the locking plate aperture 410 and a cell 124 to engage the first engagement feature 142 of the engagement member 132, 138 with the second engagement feature 312 of the locking pin 300. The first engagement feature 142 of the engagement member 132, 138 and the engagement feature 312 of the locking pin 300, may be configured such that as they are engaged, with the shaft portion 302 extending through, in series, the aperture 410 of the locking plate 400, a cell 124 of the pallet support member 100 and into the cavity 140 of the engagement member 132, 138, the head portion 304 is drawn towards the free end 136 of the engagement member 132, 138. The act of the head portion 304 being drawn towards the engagement member 132, 138 of the back-stop member 102-2 is such that the locking plate 400 is trapped between the head portion 304 and walls of the pallet support member 100 to thereby clamp the pallet support member 100 to the member 202-2. The cells 124 may comprise a shoulder on the support side 122 of the pallet support member 100. That is to say, the shoulder may define a recess feature in the support side 122 of one or more cells 124. The shoulder/recess may be sized so as to allow the locking plate 400 to enter the cell 124, such that when the locking plate 400 is in position in the cell 124, it is flush with, or beneath the surface of, the support side 122 of the pallet support member 100. It will be appreciated that the first and third back-stop members 102-1, 102-3 may comprise different engagement members, to those described for the central back-stop member 102-2, that enable the movable coupling of the back-stop abutment 150 to the meshed pallet support member 100. For example, the first and third back-stop members 102-1, 102-3 may not be affixed with the locking pin 300 and/or the locking plate 400 as shown in Figures 1C and 1D. Further the engagement members of the first and third back-stop members 102-1, 102-3 may comprise a similar outer profile to the engagement members 132, 138 of the second back-stop member 102-2, that is complementary to the shape of the cells 124. However, the engagement members of the first and third back-stop members 102-1, 102-3 may additionally comprise a moveable coupling, internal to the engagement member, that moveably couples an abutment member 150 (back-stop abutment) of the first and third back-stop members 102-1, 102-3 to the engagement member and hence to the pallet support member 100. In this way, the back-stop members 102-1, 102-3 can be said to be moveably coupled to the pallet support structure 10 as the abutment member of the first and third back-stop members 102-1, 102-3 can move between the unloaded position and the loaded position. Returning to Figure 1E, the back-stop member 102-1 comprises an abutment member 150 (back-stop abutment) which extends from the body 130. The abutment member 150 may be provided in series between the first engagement member 132 and the second engagement member 138. That is to say, the first engagement member 132 and the second engagement member 138 are spaced apart by the abutment member 150. The abutment member 150 may comprise a first portion 152 which extends from the body 130 of the back-stop member 102-2 and forms (for example terminates at) a support land 154. A second portion 156 extends from the support land 154 to a free end 158. Hence when the (or each) engagement member 132, 138 of the back-stop member 102-2 is located in one of the cells 124 of the pallet support member 100, the first portion 152 is located in a different one of the cells 124 with the support land 154 flush with, or recessed beneath, the surface of the support side 122, and the second portion 156 extends above the support side 122 to provide the back-stop abutment 150. The second portion 156 may defines a shoulder 160 which extends away from the support land 154 to overhang a back side of the first portion 152, such that when the first engagement member 132 of the back-stop member 102-2 is located in one of the cells 124 of the pallet support member 100, the shoulder 160 rests on the support side 122 of the pallet member 100 providing robustness to impacts from an impinging pallet 2. The illustrated pallet support structure 10 may further comprise one or more riser blocks 500 configured to support the pallet support member above the floor. The riser blocks 500 may comprise a supporting base and one or more engagement members with the same structure as the engagement members of the second back-stop member 102-2 described above. The riser blocks may also be secured with the same locking pin 300 and locking plate 400 as the back-stop member 202-2. In this way, the riser blocks 500 can be removable attached to any of the cells 124 of the mesh pallet structure and positioned at a desired location. The illustrated pallet support structure 10 may further comprise a kerb member 600. The kerb member 600 may define a barrier which is no greater in height than the distance between the base of the riser 500 and the base surface 124 of the pallet support member 100 such that when the pallet support member 100 is supported on the riser 500, the kerb member 600 is located under the pallet support member 100. There may be provided a small clearance between the top of the kerb member 600 and the base side 120 of the pallet support member 100. Alternatively, the kerb member 600 may have a height such that the pallet support member 100 rests upon the top side of the kerb member 600. The kerb member 600 may be elongate and a rectangular prism. The kerb member 600 may be provided adjacent to the pallet support member 100, or under the first edge of the boundary wall 110 to thereby prevent objects, for example parts of forklift vehicle and/or users’ body parts from extending underneath the pallet support member 100. This reduces the likelihood of damage to the pallet support member 100 and reduces the likelihood of harm to users of the system. Figures 2A to 2D illustrate the operation of a pallet sensor system according to an embodiment of the present disclosure. The Figures illustrate a racking system 1 with first and second pallet support structures 10, 20 respectively comprising a meshed pallet support member 100 and upper shelving 20. In this example, the pallet sensor system comprises a first back-stop structure 102 coupled to the rear-end of the first pallet support structure 10 and a second back-stop structure 202 coupled to the rear- end of the upper shelving 20. The first and second back-stop structures 102, 202 each comprise three back-stop members (not illustrated) with a radiation transceiver coupled to the central back-stop member and a pressure sensor coupled to each of two other rotatably coupled back-stop members, in the same way as described above in relation to Figures 1A to 1G. At a first time, illustrated in Figure 2A, the pallet sensor systems is in an idle or sleep mode as there is no pallet 2 within range of the sensor system. A forklift vehicle 3 approaches with a loaded pallet 2. At a later time, illustrated in Figure 2B, the driver of the forklift vehicle 3 brings the pallet 2 within range of the radiation transceiver of the first back-stop structure 102. The driver places the pallet onto the first pallet support structure 10. The radiation transceiver detects the presence of the pallet 2 and provides a corresponding sensing signal to the controller. The controller activates the remaining components of the first sensor system. The controller receives components of the sensing signal from the radiation transceiver and the two pressure sensors indicative of a proximity of the pallet 2 to the back-end structure 102. The controller determines that the proximity does not satisfy a threshold condition and outputs a pallet loading signal as a warning signal. The controller outputs the warning signal by: (i) illuminating LED lights 114 at the edge of the pallet support member 100 in a first colour; and (ii) outputting audible pips from an audible signal generator 115. As the driver of the forklift vehicle 3 pushes the pallet 2 closer to the first back-stop structure 102, the radiation transceiver detects the closer proximity of the pallet 2 to the first back-stop structure 102 and outputs the sensing signal indicative thereof. The controller determines the closer proximity (and the sensing signal components from the pressure sensors) still does not satisfy the threshold condition and increases the frequency of the pips of the audible warning signal, representative of the closer proximity. At a later time, illustrated in Figure 2C, the driver of the forklift vehicle 3 loads the pallet 2 correctly onto the first pallet support structure 10 such that the rear-side of the pallet abuts the first back-stop structure 102 along its length. The radiation transceiver detects the proximity of the pallet 2 to the first back-stop structure 102 and outputs a sensing signal indicative thereof. In addition, the two pressure sensors detect rotation of the left and right back-stop members into the loaded position by sensing a change in pressure indicating an abutment of the pallet 2 to the back-stop members. The pressure sensors output sensing signal components indicative of the abutment of the pallet 2. The controller receives the sensing signal components and determines that the proximity of the pallet 2 to the back-stop structure satisfies the threshold condition (all component threshold conditions satisfied). In response, the controller outputs the pallet loading signal as a successful loading signal. The controller outputs the successful loading signal by (i) illuminating LED lights 114 at the edge of the pallet support member 100 in a second colour; and (ii) outputting a continuous audible tone from the audible signal generator 115. After a predetermined period, the controller stops outputting the successful loading signal to conserve power. Figure 2D illustrates the operation of the pallet sensor system when loading the pallet 2 onto the upper shelving 20. The operation of the pallet sensor system is principally the same as described in relation to Figures 2A to 2C. However, the controller, housed in the first back-stop structure 102, receives a second sensing signal 210 from the second pallet detection sensor associated with the second back-stop structure. The controller may receive the second sensing signal by wired or wireless means (and associated transceivers at the second back-stop structure 202 and the controller). The controller outputs the pallet loading signal via the LED lights 114 on the first pallet support structure 10 because these are visible to the driver of the forklift vehicle 3. The controller also outputs the pallet loading signal 112 via the audible signal generator 115. Figure 2D illustrates that a pallet sensor system may comprise a second back- stop structure 202 for positioning at a rear-end of a second pallet support structure 20. The second pallet support structure 20 may be located directly above the first pallet support structure 10. Each back-stop structure 102, 202 may make use of one or more common output signal generators 114, 115. It will be appreciated that the illustrated system may be configured in other ways. For example, the system may comprise a second controller in the second back-stop structure which may communicate the sensing signal to the controller in the first back-stop structure or may determine a second pallet loading signal and communicate directly with the output signal generators 114, 115. Figure 3 illustrates a further example pallet sensor system according to an embodiment of the present disclosure. In this example, the pallet sensor system is arranged with respect to a racking system 1 with pallet support structure 10 comprising shelving of the racking system 1. The shelving 10 comprises a first pallet bay 10-1 and a second pallet bay 10-2. The shelving 10 includes an integral back-stop abutment 30. In this example, the pallet sensor system comprises a back-stop structure 102 for positioning at a rear-end of the pallet support structure 10. The pallet sensor system further comprises a pallet detection sensor 106, comprising a radiation transceiver, configured to: monitor a proximity of a rear-side of the pallet to the back-stop structure 102; and output a sensing signal 110 indicative of a distance between the rear-side of the pallet and the back-stop structure 102. The pallet detection sensor 106 may output the sensing signal 110 to a controller 108. The controller 108 outputs a pallet loading signal 112 representative of a loading state based on the sensing signal 112, as described above in relation to the earlier examples. In this example, the controller 108 outputs the pallet loading signal 112 to an optical signal generator 114 in the form of a video display screen, to indicate the loading state to the driver of the forklift. The video display screen may indicate the loading state as the distance from the back-stop to the rear-side of the pallet, a skew of the pallet, a height of the pallet and/or a stock overhang state of the pallet. In this example, the back-stop structure 102 comprises a sleeve for coupling to the back-stop abutment 30. The back-stop structure 102 may be coupled to the back-stop abutment 30 with additional fixings to ensure that the back-stop structure remains in position. The back-stop structure 102 may comprise an impact resistant and/or impact absorbing material such as rubber. The pallet detection sensor 106 may be coupled to, or housed within, the back-stop structure 102 such that a height of the pallet detection sensor 106 coincides with a height of the pallet 2 when the pallet is resting on the pallet support structure 10. The pallet detection sensor 106 comprises a radiation transceiver for determining a distance to a rear-side of the pallet, as the pallet moves closer to (or further from) the back-stop structure 102 during the loading process (or unloading process). In the same way as some of the earlier examples, the radiation transceiver in this example is housed within the back stop structure 102 and arranged to emit a radiation signal towards a front end of the shelving 10 and receive and detect at least a portion of the radiation signal reflected back towards the radiation transceiver. The radiation transceiver may comprise a time of flight sensor or other known ranging sensor capable of measuring and outputting a sensing signal indicative of the distance. It will be appreciated that any time of flight or ranging calculation may be performed at the pallet detection sensor 106, the controller 108 or a combination of the two. In this example, the pallet detection sensor comprises: (i) a first radiation transceiver 106-1 arranged to monitor a proximity of a first end of the rear-side of the pallet to the back-stop structure 102 and output a first component of the sensing signal 110; and (ii) a second radiation transceiver 106-2 arranged to monitor a proximity of a second end of the rear-side of the pallet to the back-stop structure 102 and output a second component of the sensing signal 110. The first component of the sensing signal 110 is indicative of a first distance from the back-stop structure 102 to the first end of the rear-side of the pallet and the second component is indicative of a second distance from the back-stop structure 102 to the second end of the rear-side of the pallet. In this example, the first and second radiation transceivers 106-1, 106-2 are coupled to the back-stop structure 102 and the first radiation transceiver 106-1 is spaced apart from the second transceiver along the length of the back-stop structure 102. In some examples, the spacing between the first and second radiation transceivers 106-1, 106- 2 may be from 100 to 1000 mm. In some examples the spacing may be based on a standard pallet width (e.g. a fixed proportion of the standard pallet width, such as 80%, 90%, 95% or similar). As a result, the first radiation transceiver 106-1 can monitor a first end (e.g. a left end) of the rear-side of the pallet and the second radiation transceiver 106-2 can monitor a second end (e.g. a right end) of the rear- side of the pallet opposite the first end. The first and second radiation transceivers 106-1, 106-2 may be optical time of flight radiation transceivers, for example infrared time of flight sensors, as described above. The controller 108 may determine one or more pallet parameters based on the first and second components of the sensing signal 110. The one or more pallet parameters may include the distance between the rear-side of the pallet and the back-stop structure and/or a skew of the pallet. The controller 108 may determine the distance as the lowest value or average value of the first distance and the second distance indicated by the respective first and second components of the sensing signal 110. The controller 108 may determine the skew based on a difference between the first distance and the second distance. The controller 108 may output the pallet loading signal 112 as representative of the distance between the rear-side of the pallet and the back-stop structure and/or the skew of the pallet. The controller 108 may output the pallet loading signal 112 as a warning signal if the first distance, the second distance, the distance between the rear-side of the pallet and the back-stop structure, and/or the skew of the pallet, satisfy a threshold condition. The controller 108 may output the pallet loading signal 112 as a successful loading signal if the first distance, the second distance, the distance between the rear-side of the pallet and the back-stop structure, and/or the skew of the pallet, do not satisfy the threshold condition. The threshold condition may comprise one or more of: (i) the first distance, the second distance and/or the distance between the rear-side of the pallet and the back-stop structure, being less than a range threshold; and/or (ii) the skew of the pallet being less than a skew threshold. In this example, the first radiation transceiver 106-1 comprises a plurality of first sensor elements forming a first radiation transceiver array 106-1. The plurality of first sensor elements may be arranged in a grid to provide a first ranging matrix (or first ranging image), with each first sensor element indicating a range or time of flight distance for a corresponding matrix element of the first ranging matrix (or corresponding pixel of the first ranging image). In this example, the second radiation transceiver 106-2 comprises a plurality of second sensor elements forming a second radiation transceiver array 106-2. The second sensor elements may be arranged in the same way as the first sensor elements. The first and second radiation transceiver arrays 106-1, 106-2 are coupled to the back-stop structure 106-2. The second radiation transceiver array 106-2 is be spaced apart from the first radiation transceiver array 106-1 along a length of the back-stop structure 102, that is in a lateral direction parallel to the plane of the shelving when in use (i.e. parallel to a loading surface of the pallet support structure 10). As a result, the first radiation transceiver array 106- 1 can monitor a first end (e.g. a left end) of the rear-side of the pallet and the second radiation transceiver array 106-2 can monitor a second end (e.g. a right end) of the rear-side of the pallet opposite the first end. As explained below, the controller 108 may process sensing signals 110 from one or both of the first radiation transceiver array 106-1 and the second radiation transceiver array 106-2 to determine one or more pallet parameters including: (i) a distance between a rear-side of the pallet and the back-stop structure 102; (ii) a skew of the pallet; (iii) a height of the pallet above a loading surface of the pallet support structure 10; and/or a position of stock loaded on the pallet. The loading state may comprise one or more of these pallet parameters. Each first and second sensor element may comprise an infrared sensor element. Each of the first and second radiation transceiver arrays 106-1, 106-2 may comprise a time of flight infrared array sensor, such as the VL53L5CX Time-of-Flight 8x8 multizone ranging sensor from STMicroelectronics. Figures 4A to 4D schematically illustrate the first ranging matrix 110-1 and the second ranging matrix 110-2 output as respective first and second components of the sensing signal 110-1, 110-2, by the respective first and second radiation transceiver arrays 106-1, 106-2, as a pallet 2 is loaded onto the pallet support structure 10. The figures will be described with continuing reference to Figure 3. It will be appreciated that the figures are for illustrative purposes and, in some examples, the first and second ranging matrices 110-1, 110-2 may only exist as information bits forming components of the sensing signal 110 and processed by the controller 108. However, in some examples, the pallet sensor system 110 may output ranging images similar to the illustrations of Figure 4. For example, the pallet loading signal may comprise ranging images for outputting to a video display screen of the optical signal generator 114. The first and second ranging matrices 110-1, 110-2 each comprise 8 x 8 arrays of matrix elements 170 corresponding to 8 x 8 arrays of respective first and second sensor elements of the first and second radiation transceiver arrays 106-1, 106-2. Figure 4A illustrates the first and second ranging matrices 110-1, 110-2 when a pallet 2 is first brought within a detection range of the pallet detection sensor 106. For infrared array sensors, the detection range may be on the order of 1 m from the pallet detection sensor 106. As the pallet 2 is on the order of 1 m from the pallet detection sensor 106, the entire length of the rear surface of the pallet 2 (shown as black matrix elements 170) and the profile of the stock 40 (shown as partially shaded matrix elements 170) loaded on the pallet 2 can be detected by both the first and second radiation transceiver arrays 106-1, 106-2 and embedded in the first and second ranging matrices 110-1, 110-2. The total field of view of each of the first and second radiation transceiver arrays 106-1, 106-2 may be on the order of 60 degrees in both the lateral and vertical directions. In general, the pallet 2 should be the closest object to the pallet detection sensor 106. Therefore, the controller 108 may determine the matrix elements 170 corresponding to the pallet 2 as the matrix elements 170 indicating the lowest ranging distance. For example, the matrix elements 170 corresponding to the pallet 2 may indicate a ranging distance of 80 cm, the matrix elements corresponding to the stock 40 may indicate a ranging distance of 100 cm and the remaining matrix elements 170 may indicate an infinite ranging distance (beyond detection range). In some examples, the controller 108 may determine groups of adjacent matrix elements 170 (in one or both of the first and second ranging matrices 110-1, 110-2) that indicate substantially the same distance, that is within a threshold grouping range. The threshold grouping range may be of the order of 1 to 10 cm. The controller 108 may determine the matrix elements 170 corresponding to the pallet 2 as the group of matrix elements 170 indicating the lowest distance. The controller 108 may determine matrix elements 170 corresponding to the stock 40 as all other groups of matrix elements 170 indicating a finite distance less than the detection range of the pallet detection sensor 106. The controller 108 may determine groups of matrix elements 170 corresponding to stock based on the position of the group in the ranging matrix being above the matrix elements 170 corresponding to the pallet 2. Alternatively, or in addition, the controller 108 may use edge detection to determine the matrix elements 170 corresponding to the pallet 2 and/or the stock 40. At distances of ~ 1 m, the controller 108 may determine the distance to the pallet 2, the skew of the pallet 2, the height of the pallet 2 and the position of the stock 40 from a single ranging matrix because the entire profile of the pallet is within the field of view. As the pallet approaches the pallet detection sensor 106 (and the correct loading position), the use of the ranging matrices 110-1, 110-2 from both radiation transceiver arrays 106-1, 106-2 enables a more accurate determination of the distance and a determination of skew. Determination of the different pallet parameters is described below in relation to Figure 4A when the pallet is at a distance of ~ 1m. Determination of some of the pallet parameters at closer distances is described further below in relation to Figures 4B to 4D. Distance to the Pallet The controller 108 may determine the distance to the pallet 2 (from the pallet detection sensor 106) based on the matrix elements 170 of the first and/or second ranging matrices 110-1, 110-2 corresponding to the pallet 2. In some examples, the controller 108 may determine the distance using an average distance indicated by the matrix elements 170 corresponding to the pallet 2. In other examples, the controller 108 may determine the distance as the minimum distance indicated by any one of the matrix elements 170 corresponding to the pallet 2. This distance determination can be implemented for all distances of the pallet 2 from the back-stop structure 102, i.e. for all stages of a loading/unloading process as illustrated by Figures 4A to 4D. As the pallet 2 approaches the pallet detection sensor 106, the entire width of the rear-side of the pallet 2 no longer fits within the field of view of each radiation transceiver array 106-1, 106-2 (see Figures 4C and 4D). Each radiation transceiver array 106-1, 106-2 will only sample a portion of the rear-side of the pallet 2 towards the respective left and right ends. Therefore, for closer spacings of the pallet 2 to the back-stop structure 102, determining the distance using both ranging matrixes 110-1, 110-2 can provide more accurate results. Skew of the Pallet When the pallet 2 is sufficiently far from the pallet detection sensor 106 such that the entire width of the rear-side of the pallet 2 is captured in the field of view of the first (or second) radiation transceiver array 106-1, the controller 108 may determine the skew of the pallet 2 based on the matrix elements 170 of the first (or second) ranging matrix corresponding to the pallet 2. In some examples, the controller 108 may determine the skew of the pallet 2 based on a difference between a minimum distance value and a maximum distance value indicated by the matrix elements 170 of the first (or second) ranging matrix corresponding to the pallet 2. In some examples, the controller 108 may determine the skew based on a difference between: (i) a matrix element 170 at a right end of the group of matrix elements 170 corresponding to the pallet 2; and (ii) a matrix element 170 at a left end of the group of matrix elements 170 corresponding to the pallet 2. As the pallet 2 approaches the pallet detection sensor 106, the entire width of the pallet 2 no longer fits within the field of view each radiation transceiver array 106-1, 106-2 (see Figures 4C and 4D). Therefore, the controller 108 may determine the skew of the pallet based on a difference between: (i) a first distance indicated by the matrix elements 170 of the first ranging matrix 110-1 corresponding to the pallet 2; and (ii) a second distance indicated by the matrix elements 170 of the second ranging matrix 110-2 corresponding to the pallet 2. The first and second distances may be determined using any of the distance determination approaches described above. In some examples, the controller 108 may determine the skew based on a difference between: (i) the maximum distance indicated by the matrix elements 170 of the first ranging matrix 110-1 corresponding to the pallet 2; and (ii) a minimum distance indicated by the matrix elements 170 of the second ranging matrix 110-2 corresponding to the pallet 2, or vice versa. In some examples, the controller 108 may determine the skew based on a difference between: (i) a matrix element 170 at a left end of the group of matrix elements 170 of the first ranging matrix 110-1 corresponding to the pallet 2; and (ii) a matrix element 170 at a right end of the group of matrix elements 170 of the second ranging matrix 110-2 corresponding to the pallet 2. It will be appreciated that the controller 108 may also determine the skew of the pallet 2 using both ranging matrices when the entire pallet width is in the field of view of each transceiver array 106-1, 106-2. In some examples, the controller 108 may determine a skew as the distance difference described above. In some examples, the controller 108 may determine the skew as a skew angle approximated as the distance difference divided by a width of a standard pallet 2. Height of Pallet The controller 108 may determine the height of the pallet 2 above a loading surface of the pallet support structure based on a position, within the first ranging matrix 110-1 and/or the second ranging matrix 110-2, of matrix elements 170 corresponding to the pallet 2 and the distance indicated by the matrix elements 170. For example, the controller 108 may determine the distance to the pallet 2 and determine a height of each row of matrix elements 170 based on the determined distance and the angular field of view, θ, of each matrix element 170 (e.g. height = θ/distance). The controller 108 may then determine the height of the pallet 2 above the loading surface based on a position of the matrix elements 170 corresponding to the pallet 2, within the first and/or second ranging matrices 110-1, 110-2. In some examples, the height of the pallet 2 above the loading surface may be determined when the pallet 2 is at a predetermined distance from the radiation transceiver arrays, for example at 1 m. The pallet sensor system may be pre-calibrated such that each row of matrix elements 170 corresponds to a predetermined height above the loading surface of the pallet support structure 10, when the pallet is at the predetermined distance. In this way, the height of the pallet 2 above the loading surface may be determined based on a lowest position of the matrix elements corresponding to the pallet 2. It will be appreciated that the height of the pallet 2 may only be determined when the bottom edge of the pallet 2 is distinguishable in the first and/or second ranging matrices 110-1, 110-2 (i.e. in Figures 4A to 4C but not 4D). Stock Position The controller 108 may determine the position of the stock 40 loaded on the pallet 2 based on a position within the first ranging matrix of the matrix elements of the first and/or second ranging matrices 110-1, 110-2 corresponding to the stock 40. It will be appreciated that the position of the stock 40 may only be determined when the extremities of the stock 40 are distinguishable in the first and/or second ranging matrices 110-1, 110-2 (Figure 4A). Figures 4B to 4D illustrate the first and second ranging matrices 110-1, 110-2 as the pallet is brought closer to the pallet detection sensor 106 and into the correct loading position. As the pallet 2 is brought closer to the pallet detection sensor 106, a portion of the left end of the rear-side of the pallet 2 begins to fill more of the field of view of the first ranging matrix 110-1 and a portion of a right end of the rear-side of the pallet 2 begins to fill more of the field of view the second ranging matrix 110-2. As a result, use of the first and second radiations transceiver arrays 106-1, 106-2 and respective ranging matrices 110-1, 110-2 can provide more accurate determination of distance and skew. The controller 108 may output the pallet loading signal 112 as representative of the loading state, in the same way as described above for earlier examples. The loading state may correspond to a simple binary indication of whether the pallet is correctly loaded or not. The loading state may also comprise one or more of: the distance from the back-stop structure 102 to the pallet 2; the skew of the pallet 2; the height of the pallet 2 above the loading surface; and the position of the edges of the stock 40. The controller 108 may output the pallet loading signal 112 as any combination of audible and visible indicators including a video display, as described above. The controller 108 may output the pallet loading signal 112 as a warning signal if the sensing signal 110 does not satisfy a threshold signal and output the pallet loading signal 112 as a successful loading signal if the sensing signal 110 does satisfy the threshold condition. The threshold condition may comprise the distance between the rear-side of the pallet 2 and the back-stop structure being less than a range threshold. The threshold condition may comprise: (i) a first distance between the rear-side of the pallet 2 and the back-stop structure 102 as indicated by the first radiation transceiver array 106-1 and the associated first ranging matrix 110-1; and (ii) a second distance between the rear-side of the pallet 2 and the back-stop structure 102 as indicated by the second radiation transceiver array 106-2 and the associated second ranging matrix 110-2, both being less than the range threshold. In this way, the pallet sensor system can signal when the pallet 2 is correctly loaded with the rear-side of the pallet 2 positioned close to the back-strop structure 102 across the width of the pallet 2, with minimal or no skew. The threshold condition may comprise the skew of the pallet 2 being less than a skew threshold. In this way, the pallet sensor system can signal that the pallet is correctly loaded with no skew. The threshold condition may comprise a height of the pallet above the loading surface of the pallet support structure 10 being within a threshold height range. A range is advantageous because if the height is too low, there is a risk that the forks of the forklift truck will collide with the pallet support structure 10 and if the height is too high, the stock 40 may collide with a second pallet support structure 20 above the pallet support structure 10. In some examples, the threshold height range may be specified as an absolute height and compared to the height determined above. In some examples, the threshold height range may correspond to a range of rows of matrix elements in which matrix elements corresponding to the pallet 2 should reside, when the pallet 2 is a predetermined distance from the pallet detection sensor 106 / back-stop structure 102. The threshold condition may comprise a position of edges of the stock 40 being within a threshold boundary. Left and right threshold boundaries may be defined with respect to the respective left and right edges of the group of matrix elements 170 corresponding to the pallet 2. For example, the left and right threshold boundaries may be defined as the column of matrix elements 170 corresponding to the left end and right end of the group of matrix elements 170 corresponding to the pallet 2. An upper threshold boundary may be defined as a particular row of matrix elements 170 when the pallet 2 is a predetermined distance from the pallet detection sensor 106 / back-stop structure 102. Outputting a warning signal if the position of the edges of stock 40 are not within the threshold boundary can advantageously reduce the risk of stock colliding with the racking system 1, other pallet support structures and/or other pallets and reduce the risk of stock topping off the pallet 2. It will be appreciated that while examples have been discussed predominantly in relation to loading, the disclosed pallet sensor systems can equally monitor unloading of pallets from pallet support structures and output pallet loading signals indicative of the load state (of the unloading process). In particular, the controller may output data signals indicative of the unloading process to provide an overview of pallet loading/unloading statistics for a plurality of pallet support structures in a warehouse. Throughout the present specification, the descriptors relating to relative orientation and position, such as “horizontal”, “vertical”, “top”, “bottom” and “side”, are used in the sense of the orientation of the structures and systems as presented in the drawings. However, such descriptors are not intended to be in any way limiting to an intended use of the described or claimed invention. It will be appreciated that any reference to “close to”, “before”, “shortly before”, “after” “shortly after”, “higher than”, or “lower than”, etc, can refer to the parameter in question being less than or greater than a threshold value, or between two threshold values, depending upon the context.