TECHNICAL FIELD

The present invention relates to a method and a system for surveying the functionality of a video link from an aircraft to an object. The present invention further relates to a computer program product and to a computer-readable storage medium.

BACKGROUND ART

Unmanned aerial vehicles, UAV, have during the past years become increasingly popular. When using UAV in populated areas and in controlled airspace, a very important aspect is safety.

A way to operate aerial vehicles is to equip the aerial vehicle with a camera arrangement or any other image sensor(s). The images from the aerial vehicle are then transferred to an object at a remote place which is not aboard the vehicle via a video link. This allows operating the aerial vehicle from the remote place, or any other remote place, based on the images from the aircraft received via the video link. As an example, a person on the ground can act as a remote pilot for the aerial vehicle and control the vehicle based on the received images.

There is especially a need for improving the safety for aerial vehicles which provide a video link.

SUMMARY OF THE INVENTION

The fact that the data bandwidth available in air to ground data links is limited drives a need to apply video compression instead of transferring the raw uncompressed video. This however means that some aspects of the video encoding and decoding need to be ensured, in order to ensure the integrity of the complete link. Approaches like inserting test patterns are considered not being enough, since it doesn't address that the image that was captured by a sensor or a camera arrangement is the one that is displayed on the ground or at another object.

It is an object of the present invention to improve the safety when operating aerial vehicles. It is a further object of the present invention to improve the safety when operating aerial vehicles which are not operated by a person aboard the aerial vehicle.

It is a further object of the present invention to present an alternative way to provide a comparably safe operation of aerial vehicles.

At least some of the objects of the present disclosure are achieved by a method for surveying the functionality of a video link from an aircraft to an object. In that method the video link comprises a video compression and transmission system aboard the aircraft. The video compression and transmission system is arranged to provide and/or receive video images, to compress the provided and/or received video images, and to transmit the compressed video images to the object via the video link. The object is equipped with a video decoder system which is arranged to decode the compressed video images which are transmitted from the aircraft. The method comprises the following step of transmitting an uncompressed region of a video image from the aircraft to the object. The method further comprises the step of comparing the transmitted uncompressed region of the video image with a corresponding decoded region of the video image, wherein the decoded region has been compressed and transmitted via the compression and transmission system aboard the aircraft. The method even further comprises the step of deciding whether the video link functions properly based on the result of said comparing.

This has the advantage that it is possible to determine whether the video link as a whole functions properly. In other words, it is not only checked that each component of the video link functions properly. As will be explained in the reminder of the disclosure there are effects which can occur in between the components of a video link and which thus cannot be detected by surveying each component for its functionality individually. By determining whether the video link as a whole functions properly it is thus possible to detect more errors and thus increasing the safety when operating the aircraft. This is especially useful when operating the aircraft beyond visual line of sight, BVLOS, and/or when providing a first person view, FPV, video link. In one example the video link is a FPV video link.

In one example of the method, the step of comparing comprises the step of determining whether the uncompressed region and the corresponding decoded region show the same picture. This further increases the safety as a wrong picture might cause the operator to base decisions on wrong facts without realising it.

In one example of the method, the step of comparing the transmitted uncompressed region of the video image with a corresponding decoded region of the video image comprises the step of determining a difference map between the uncompressed region and the corresponding decoded region. The step of comparing the transmitted uncompressed region of the video image with a corresponding decoded region of the video image further comprises the step of determining whether the sum of all differences of the difference map is below a predetermined threshold. This allows for a relatively easy implementation of the comparison step. Further such an implementation can be performed comparably fast. In one example of the method, the step of comparing the transmitted uncompressed region of the video image with a corresponding decoded region of the video image further comprises the step of determining whether the number of pixels that differ more than a pre-determined noise level from the original image is less than a pre-defined threshold. This also allows for a relatively easy implementation of the comparison step. Further such an implementation can also be performed comparably fast.

In one example of the method, the uncompressed region is transmitted via the video link. This also allows for an easy implementation as no additional link is needed.

In one example of the method, the steps of the method are repeated after each n:th video image, wherein n is a natural number larger than one, preferably larger than five, even more preferably larger than ten. This allows for saving bandwidth, while at the same time not negatively affecting safety too much.

In one example of the method, the position of the region of the video image differs when the steps of the method are repeated. This further increases the safety. In one example of the method the deciding whether the video link functions properly is also based on whether the sum of all differences of the difference map is below a pre-determined threshold for a pre-determined number of repetitions of the steps of the method, and potentially whether the number of pixels that differ more than a pre-determined noise level from the original image is less than a pre-defined threshold for a pre-determined number of repetitions of the steps of the method. This provides avoiding or at least reducing false alarms.

In one example of the method, the region of the video image is smaller than a quarter of the video image, preferably smaller than a tenth of the video image. This allows for saving bandwidth. In one example, the method further comprises the step of indicating to an operator the result of deciding whether the video link functions properly. This further increases the safety.

In one example, the uncompressed region of the video image corresponds to an integer number of codeblock sizes of the compressed video image. This allows for an easy

implementation. This also provides better results. At least some of the objects are also achieved by a system for surveying the functionality of a video link from an aircraft to an object. The video link comprises a video compression and transmission system aboard the aircraft. The video compression and transmission system is arranged to provide and/or receive video images, to compress the provided and/or received video images, and to transmit the compressed video images to the object via the video link. The object is equipped with a video decoder system which is arranged to decode the compressed video images which are transmitted from the aircraft. The system for surveying comprises a transmission circuit adapted to transmit an uncompressed region of a video image from the aircraft to the object. The system further comprises a comparator circuit adapted to compare the transmitted uncompressed region of the video image with a corresponding decoded region of the video image, wherein the decoded region has been compressed and transmitted via the compression and transmission system aboard the aircraft. The system even further comprises a decision circuit adapted to decide whether the video link functions properly based on the result of the comparing. In one embodiment, the system further comprises an indicator unit adapted to indicate to an operator the result of deciding whether the video link functions properly.

At least some of the objects are also achieved by a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to the present disclosure.

At least some of the objects are also achieved by a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of the present disclosure.

The system, the computer program product, and the computer-readable storage medium show the advantages which are described in relation to the method of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts schematically a situation in which the present disclosure can be used.

Fig. 2 depicts schematically an embodiment of a system according to the present disclosure. Fig. 3a depicts schematically an image which might be provided from an imaging system aboard an aerial vehicle.

Fig. 3b, 4a, and 4b each schematically explains an aspect of an example of the present disclosure.

Fig. 5a, b schematically explains how the safety can be improved by the present disclosure. Fig. 6 shows schematically a flowchart of a method according to the present disclosure.

DETAILED DESCRIPTION

The present disclosure uses the expressions "aerial vehicle" and "aircraft" interchangeably. No different meaning is intended. It should also be understood that the expression "video" or "image" can refer to the representation of the video or image in a data format. Thus the video or image does not need to be visible but can as well refer to data which can be converted into a video or an image.

Throughout the figures, same reference numerals refer to same parts, concepts, and/or elements. Consequently, what will be said regarding a reference numeral in one figure applies equally well to the same reference numeral in other figures unless not explicitly stated otherwise.

Fig. 1 depicts schematically a situation in which the present disclosure can be used. An aerial vehicle 10 is depicted. The aerial vehicle can be any kind of aerial vehicles, such as, for example, an aeroplane, a helicopter, an unmanned aerial vehicle, UAV, a missile, a rocket, a glider, or the like. It should especially be noted that the form of the aircraft is only an example and that the aircraft in principle can have any form or shape. The aircraft 10 can be manned or unmanned. As the advantages of the present disclosure regarding safety for unmanned aerial vehicles might be easy to understand from the present disclosure, the same advantages will apply for manned aircrafts. This can be seen in cases where, for example, a pilot of the manned aerial vehicle becomes unconscious, dies, or for any other reason unable to control the aircraft. In case no second pilot is aboard the aircraft or able to control the aircraft, by establishing a video link from the aircraft 10 to the object 20 it might be possible for a person outside the aircraft 10 to gain reliable video information from the aircraft 10. Thus safety is improved when operating aerial vehicles even for manned aerial vehicles. That person outside the aircraft 10 can then, for example, control the aircraft 10 based on the gained video information.

The aerial vehicle 10 is arranged to send data 30 via a video link to an object 20. The object 20 is at a remote place from the aircraft 10. The object 20 can be land-borne, sea-based, or air- based. The object 20 can be any object being arranged to receive data 30 from the video link. Examples of the object 20 are a second aircraft, different from aircraft 10, a ground control station, an airport tower, a vessel, a land-based vehicle, a wearable or movable operator device, or the like. The data 30 comprises video data, i.e. data representing video images. The video data can comprise plain video images. The video data can comprise any kind of data which can be converted into video images. Especially it should be noted that the video data can comprise any kind of encoded and/or compressed video data. The data 30 can also comprise additional data, such as control data or any other kind of data.

In the shown example the video link is depicted as a one-way video link from the aircraft 10 to the object 20. This is, however, not a prerequisite. The video link can equally well be, for example, two-ways, broadcast, or the like.

In one example, the aircraft 10 is replaced by a remote object, such as a remote tower or the like. Thus, everything described in this disclosure in relation to the aircraft can be equally well applied to any remote object. In one example, the remote object is placed at a distance of more than 100 m, more than 1 km, or more than 10 km from the object 20.

Fig. 2 depicts schematically embodiments of a system 100 according to the present disclosure. It should be noted that not all depicted components are necessary for all embodiments of the system. The system 100 is a system for surveying the functionality of a video link from an aircraft to an object.

The system 100 can comprise an imaging system 110. The imaging system 110 is preferably aboard the aircraft. The imaging system 110 can be arranged to provide images from the surrounding of the aircraft. The imaging system 110 can comprise a camera arrangement. The camera arrangement can comprise one or several cameras. The imaging system 110 can comprise at least one sensor. The at least one sensor can be arranged to provide images. The imaging system 110 can operate at any wavelength, such as visible light, infrared, ultraviolet, or the like. Preferably the imaging system 110 is arranged to provide the images in a data format or to convert the images in a data format. The imaging system 110 is arranged to provide said images repeatedly. The rate at which the images are provided repeatedly can vary. In one example the rate is several times per second. In one example the rate is around once per second. In one example the rate is every few seconds. The system 100 comprises a video compression and transmission system 120. The video and transmission system 120 is arranged aboard the aircraft 10. The video compression and transmission system 120 is arranged to provide and/or receive video images. In one example the video images are received from the imaging system 110. The video compression and transmission system 120 is arranged to compress the provided and/or received video images. Compressing video images is well known in the art and therefore not discussed any further here. It should, however, be stressed that the present disclosure in principle works with any kind of compression for images or video images. An example of a possible compression is a video compression algorithm based on H.264. An example of a possible compression is a video compression algorithm based on intra frames, l-frames, such as H.264 l-frames. In one example the compression is a lossy compression. This is especially advantageous if the data bandwidth for video links from the aircraft to the ground typically does not allow transmitting video images at a desired rate with a desired resolution at lossless compression. However, under certain circumstances or with further technical developments regarding data transmission in the future the compression can be lossless in one example. The video compression and transmission system 120 can comprise a processing unit (not shown in the figure). In one example, the processing unit performs the compression.

The video compression and transmission system 120 is part of a video link 190 between the aircraft and the object. The video compression and transmission system 120 is arranged to transmit the compressed video images to the object via the video link 190. In one example, the video compression and transmission system 120 comprises a transmitter (not shown in the figure). The transmitter can be a data transmitter. In one example said transmitter is arranged to transmit the compressed video image via the video link 190.

The system 100 further comprises a video decoder system 130. The video decoder system 130 is arranged at the object 20. The video decoder system 130 is arranged to decode the compressed video images which are transmitted from the aircraft. The video decoder system 130 can comprise a processing unit (not shown). In one example the processing performs the decoding. The video link 190 can comprise a receiving unit (not shown). The receiving unit can be a separate unit or being comprised in the video decoder system 130. The receiving unit and/or the video decoder system 130 can be arranged to receive the compressed video images from the video compression and transmission system 120. The system 100 further comprises a transmission circuit adapted to transmit an uncompressed region of a video image from the aircraft to the object. The uncompressed region of a video image is an uncompressed region of one of the video images which are transmitted from the aircraft to the object via the video link 190. How the region can be chosen and how the video image can be chosen will be explained in further detail in relation to Fig. 3b and Fig. 6. The transmission circuit can be located outside the video link 190, such as in the form of an additional transmitter (not shown in the figure). The transmission circuit adapted to transmit an uncompressed region can be the video compression and transmission system 120. In one example the transmission circuit adapted to transmit an uncompressed region is a transmitter of the video compression and transmission system 120. In Fig. 2, the transmission of the uncompressed region is depicted by an extra line outside the video link 190, as will be further explained in relation to Fig. 5a-b. It is, however, also possible that the uncompressed region is transmitted via the video link (not shown in the figure). Further details are explained in relation to Fig. 6. The system 100 further comprises a comparator circuit 140 adapted to compare the transmitted uncompressed region of the video image with a corresponding decoded region of the video image, wherein the decoded region has been compressed and transmitted via the compression and transmission system 120 aboard the aircraft 10. The comparator circuit 140 can, for example, comprise a processing unit. The comparator circuit 140 is arranged to receive the transmitted uncompressed region of the video image. The comparator circuit 140 is arranged to receive the corresponding decoded region of the video image. The corresponding decoded region is preferably received from the decoder system 130. More details regarding the comparing are explained in relation to the following figures.

The system 100 further comprises a decision circuit adapted to decide whether the video link functions properly based on the result of said comparing (not shown in the figure). How the deciding is performed is explained in more detail in relation to Fig. 6. The decision circuit adapted to decide whether the video link functions properly based on the result of said comparing can comprise a processing unit. The decision circuit adapted to decide whether the video link functions properly and the comparator circuit 140 can be comprised in a common unit, such as a common processing unit. The system 100 can comprise an indicator unit 150 adapted to indicate to an operator the result of deciding whether the video link functions properly. In one embodiment the indicator unit 150 indicates the result visually. In one embodiment the indicator unit 150 indicates the result acoustically. In one embodiment the indicator unit 150 is a tactile circuit for indicating the result. The indicator unit 150 can comprise any of a display, a lamp, a speaker, a vibrating element, a screen, or the like.

Fig. 3a depicts schematically an image 200 which might be provided from an imaging system aboard an aerial vehicle. In practice, the image 200 will usually be coloured. However, any kind of images is possible, such as black/white images, grey-scale images, or images with any other colour coding. The shown example indicates the lights of a runway as they might be seen from a pilot when trying to approach the runway for landing. When the aircraft 10 is equipped with an imaging system 110, the image 200 is an example of what kind of images might be provided from the aircraft and transmitted via the video link. In the present example the lights of a runway are depicted for explaining the principle of the invention in a manner easily understandable even in a black/white representation. It should, however, be emphasised that the present disclosure easily well can be applied when flying the aircraft without being involved in a start and/or landing process.

Fig. 3b depicts the same image 200 as in Fig. 3a. Further, a region 210 of the image is depicted. This is an example of an uncompressed region of the image which is transmitted from the aircraft to the object in connection with the present disclosure. In one example the uncompressed region is a different section of the image in consecutive runs of a method according to the present disclosure. This is indicated by the arrow 220, indicating that the region 210 is swept over the image 200 on consecutive runs of the method according to the present disclosure. It should, however, be emphasised that any other sweeping path than the indicated arrow 220 is possible as well. It is also possible to make the uncompressed region "jump" through the image 200. Fig. 4a depicts the same image 200 as in Fig. 3a and 3b. An example of an uncompressed region 240 is depicted in the figure. As can be seen, the example of an uncompressed region shows two lights in connection to the runway, one approximately in the middle of the uncompressed region and one close to the right border of the uncompressed region, however placed in the middle of the region in the vertical direction.

Fig. 4b depicts a second image 201 which differs from the image 200. The second image 201 is an image as might be provided from the imaging system aboard an aerial vehicle at a later moment of time compared to the image 200. This might, for example, be the case when the aircraft has approached the runway a little bit further compared to the situation in the Fig. 3a- 4a. An example of a second uncompressed region 241 is depicted in the figure. As can be seen, the example of an uncompressed region shows one light in connection to the runway, placed approximately in the lower left corner of the uncompressed region. The uncompressed regions 240 and 241 show the same section of the images 200 and 201, respectively. However, due to the fact that the images scenery of the images 200 and 201 differs, also the scenery in the uncompressed regions 240 and 241 differs in the shown example.

Fig. 5a, b schematically explains how the safety can be improved by the present disclosure. The second image 201 as depicted in relation to Fig. 4b is provided aboard the aircraft and intended to be transmitted via the video link to an object. Turning to Fig. 5a, the second image 201 is compressed at the video compression and transmission system 120. The second image 201 is further decoded at the video decoder system 130. Also, an uncompressed region of the second image 201 is transmitted. The uncompressed region is shown in Fig. 5a. The uncompressed region is the second uncompressed region 241 as shown in Fig. 4b. The region of the image which has been decoded by the video decoder system 130 and which corresponds to the uncompressed region is also extracted. The comparator circuit 140 do then compare the transmitted uncompressed region of the video image with a corresponding decoded region of the video image. As sketched in Fig. 5a, the transmitted uncompressed region of the video image and the corresponding decoded region of the video image are basically the same. It can then be decided that the video link functions properly since the comparison reveals that the transmitted uncompressed region of the video image and the corresponding decoded region of the video image are basically the same.

Turning now to Fig. 5b, the second image 201 is provided aboard the aircraft as in the previous figure. As in the previous picture, the second uncompressed region 241 of that second image is transmitted. However, in the situation of Fig. 5b the video compression and transmission system 120 does not work properly. As an example, the part of the compression and transmission system 120 responsible for the compression has been "frozen", i.e. no longer the actual image is compressed, but instead a previous image. In the shown example, the previous image 200 as shown in Fig. 3a-4a is compressed and transmitted instead of the actual second image 201. Consequently, the previous image 200 will also be decoded at the video decoder system 130. The comparator circuit 140 does then compare the transmitted uncompressed region of the video image with a corresponding decoded region of the video image. However, since the decoded image is a different image than the image from which the uncompressed region of the video image 201 has been transmitted, the corresponding decoded region differs from the uncompressed region 241 of the video image. In the shown example, since the image which actually has been compressed is the image 200, the corresponding decoded region will be the region 240 which has been described in relation to Fig. 4a. The comparator circuit 140 does then compare the transmitted uncompressed region of the video image with the corresponding decoded region of the video image. As sketched in Fig. 5b, the transmitted uncompressed region of the video image 241 and the corresponding decoded region 240 of the video image are not the same. It can then be decided that the video link does not function properly since the comparison reveals that the transmitted uncompressed region of the video image and the corresponding decoded region of the video image are not the same.

One advantage of the present disclosure is that even a non-functioning of the video compression and transmission system 120 can be detected, such as the described freezing. Such a freezing might, for example, be hard to detect in solutions based on test images or test patterns only. As an example, in case the video compression and transmission system 120 of a prior art solution freezes on the test image, a test on the side of the object whether the test image is correctly transmitted will not indicate any errors in the video link. Fig. 6 shows schematically a flowchart of a method 600 according to the present disclosure. The method 600 is a method for surveying the functionality of a video link from an aircraft to an object. In the method 600, the video link comprises a video compression and transmission system aboard the aircraft. The video compression and transmission system is arranged to provide and/or receive video images, to compress the provided and/or received video images, and to transmit the compressed video images to the object via the video link. This is described in further detail in relation to Fig. 2. Especially the compression used by the video compression and transmission system can be any kind of compression method, for example a compression method based on code-blocks. The object is equipped with a video decoder system which is arranged to decode the compressed video images which are transmitted from the aircraft. This is also described in further detail in relation to Fig. 2. The method starts with step 610.

In step 610 an uncompressed region of a video image is transmitted from the aircraft to the object. This transmission is in one example via the video link. In one example the transmission is performed via a different link. In one example, the region of the video image is smaller than a quarter of the video image, preferably smaller than a tenth of the video image. Such an example is depicted in Fig. 3b-4b. It might often happen that the bandwidth of the video link is the limiting factor, which often is the reason why video compression is used. An

uncompressed region of a video image is thus preferably not too big in data size in relation to the compressed video for not counteracting the savings achieved due to compression. When transmitting the uncompressed region via the video link, uncompressed region can be transmitted as a separate data stream and/or be integrated in the stream of the compressed video images, for example in the headers, or the like.

In one example, the uncompressed region of the video image corresponds to an integer number of codeblock sizes of the compressed video image. This is especially advantageous in case the video images are compressed based on codeblock schemes. Since the decoding in this case usually is performed in codeblocks as well, having the uncompressed region of the video image corresponding to an integer number of codeblock sizes of the compressed video image allows later on comparing the uncompressed region with full codeblock(s), which facilitates comparison. The method 600 can be repeated. When repeating the method, it is not necessary to perform step 610 for every video image which is transmitted via the video link. Instead, it might be advantageous to perform step 610 and the following steps only every n:th time a video image is transmitted. This allows saving bandwidth. Further, especially if the repetition rate of the video images is high, such as several times per second, it is in many practical cases not decisive whether a non-functioning of the video link is detected in a fraction of a second or only after the a second or a few seconds.

When repeating the method 600, the position of the uncompressed region of the video image preferably differs when step 610 is repeated. In one example the position of the uncompressed region is swept over the video image, for example such as shown in Fig. 3b. In one example, the next uncompressed region overlaps partly with the previous region. In one example there is no overlap between the regions in consecutive repetitions of step 610. This is for example indicated by a consecutive uncompressed region 211 after an uncompressed region 210 in Fig. 3b. The sweeping direction can be any kind of sweeping direction. In one example the position of the uncompressed region "jumps" over the video image.

Differing the uncompressed region has at least two advantages. One advantage is that even failures of the video compression and transmission system 120 which only apply to a specific region of the video image can be detected. By moving the region this or these affected code- blocks will be detected. In another aspect, parts of video images might look the same between consecutive video images for quite a long time. As an example, region 210 in Fig. 3a-4b, when kept at that place, might look the same for quite some time. In the shown example that region will likely be completely black for quite some time, as indicated by the fact that this region still would be black when taken in Fig. 4b which is a video image later than Fig. 4a. A further advantage of moving the uncompressed region is thus that defects like freezing can be detected earlier, as they might be undetected in the example of Fig. 3a-4b where the region 210 would look the same for quite some time. After step 610 the method continues with step 620.

In step 620 the transmitted uncompressed region of the video image is compared with a corresponding decoded region of the video image. In this respect the decoded region has been compressed and transmitted via the compression and transmission system aboard the aircraft. The comparison can be performed by the comparator circuit 140 which is described in relation to Fig. 2. In one example, step 620 comprises step 621. Step 622 comprises determining a difference map between the uncompressed region and the corresponding decoded region. The concept of determining a difference map between two images is well known in the art and thus not described any further here. In general, the decoded region will nearly always look at least slightly different than the uncompressed region. This is due to the fact that some colour and/or detail information will usually get lost during lossy compression. Thus, a difference map can indicate how much the uncompressed region and the decoded compressed region differ from each other. If the uncompressed region and the decoded compressed region represent the same video image the differences which appear in the difference map should be rather small.

In one example, step 620 comprises step 622. In step 622 it is determined whether the sum of all differences of the difference map is below a first pre-determined threshold. The first predetermined threshold can be set depending on the compression method, compression settings, resolution, bandwidth of the video link, accuracy of compression and decoding, or the like.

In one example, step 620 comprises step 623. In step 623 it is determined whether the number of pixels that differ more than a pre-determined noise level from the original image are less than a second pre-defined threshold. The second pre-determined threshold and/or the pre-determined noise level can be set depending on the compression method, compression settings, resolution, bandwidth of the video link, accuracy of compression and decoding, or the like.

In one example, step 620 comprises step 624. In step 624 it is determined whether the uncompressed region and the corresponding decoded region show the same picture. In one example it is concluded that the same picture is shown when the sum of all differences of the difference map is below the first pre-determined threshold. In case the same picture is shown the differences between the corresponding regions should be rather small and thus the sum of all differences in the difference map. In one example it is concluded that the regions show the same picture in case the sum of all differences is below the first threshold. In one example it is concluded that the regions do not show the same picture in case the sum of all differences is above the first threshold.

In one example it is concluded that the same picture is shown based on whether the number of pixels that differ more than a pre-determined noise level from the original image are less than a second pre-defined threshold. If the same picture is shown, the number of pixels differing more than the pre-determined noise level from the original image should be rather limited. In one example it is concluded that the regions show the same picture in case the number of pixels that differ more than a pre-determined noise level from the original image is less than the second pre-defined threshold. In one example it is concluded that the regions do not show the same picture in case the number of pixels that differ more than a predetermined noise level from the original image is more than the second pre-defined threshold.

Difference maps are only examples of comparing images in step 620. Other methods are known in the art and might be applied additionally to any of steps 621-624 or instead of any of steps 621-624. After step 620, the method continues with step 630. In step 630 it is decided whether the video link functions properly based on the result of step 620. In one example it is concluded that the video link is not functioning properly if it has been concluded that the uncompressed region and the corresponding decoded region do not show the same picture. In one example it is concluded that the video link is not functioning properly if any of the compressed video image or the uncompressed region could not be received at comparison in step 620. In one example it is concluded that the video link is not functioning properly if any other error occurred during video compression, transmission, and/or decoding.

When performing any of the steps 610-630, checksum(s) and/or sequence numbers can be applied to the transmitted video image, and/or the uncompressed region of the video image, and/or the compressed and/or decoded region of the video image. This assures that the correct images and/or regions thereof are compared and/or can be used to detect and/or identify additional errors in the functionality of the video link. The advantage of the present disclosure is that not only the functioning of every component itself is monitored as in prior art, but also the interaction of all components. As an example, in case of "freezing", the compression itself, the transmission itself, and the decoding itself could still work properly. Thus monitoring the compression itself, the transmission itself, and the decoding itself, would not reveal an error. However, the method according to the present disclosure would reveal such a "freezing", thereby increasing safety. An operator usually immediately realises that a video link is not functioning properly in case no video is transmitted. However, in the present disclosure it can be realised that the video link does not functions properly even if images are still transmitted, for example as would be the case with "freezing".

After step 630, the method continues with the optional step 640. Step 640 comprises indicating to an operator the result of deciding whether the video link functions properly. That indication can be acoustically, visually, via a tactile circuit, or via any combination thereof. In one example the indicating is performed via a warning sound. In one example the indicating is performed via a warning lamp, or a warning message, or any other warning indication on a screen or a display, which, for example, can be lighted permanently or in a blinking manner. In one example the video image from the aircraft can be shut off at a screen and/or display for an observer outside the aircraft when it is decided that the video link is not working properly. This has the advantage that an observer is not trying to rely on outdated images anyhow. In one example the latest available image for which it was concluded that the video link functions properly is displayed continuously, possible with an indication of time how old that image is. This has the advantage that an observer can try to perform security actions based on the latest available information which was concluded to be transmitted on a properly functioning video link. The method ends after the optional step 640. As described earlier, the method 600 can be repeated and is preferably performed repeatedly. In one example, the steps of the method 600 are repeated after each n:th video image which is transmitted via the video link. The number n is a natural number larger than one, preferably larger than five, even more preferably larger than ten. As discussed before, bandwidth constraints might limit the performance of the method to every n:th image. However, as also discussed before, depending on the circumstance it might not give any advantage to perform the method too often, such as several times per second. In practice, a person skilled in the art will find reasonable parameters for how often video images are transmitted and how often method 600 is performed on these video images. These reasonable parameters might depend on the resolution of the video images, the degree and kind of compression, the bandwidth, the fact how critical the aircraft mission is, the speed of the hardware components involved, and so on. In one example the number n is allowed to vary.

In one example, the deciding whether the video link functions properly in step 640 is also based on whether the sum of all differences of the difference map is below a pre-determined threshold for a pre-determined number of repetitions of the steps of the method. In one example, the deciding whether the video link functions properly in step 640 is whether the number of pixels that differ more than a pre-determined noise level from the original image is less than a pre-defined threshold for a pre-determined number of repetitions of the steps of the method.

Thus, a single deviation, such as a single occurrence of a value or a number being on the side of a threshold which indicates that the video link is not functioning properly, will not lead to the conclusion that the video link is not functioning properly. This might be advantageous in case such single occurrences, or a low number of occurrences, can be caused occasionally by artefacts, or the like. It is then advantageous not to conclude that the video link does not function properly in case the next run of the method indicates that the video link indeed functions properly. Thus false alarms can be reduced. In general, the pre-determined number of repetitions can be determined based on the parameters of the system in use, especially on how often the parameters are expected to cause a false indication or the like.

The present disclosure also relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method 600. Said execution on a computer can be distributed among several physical units, such as the elements described in relation to Fig. 2. Especially, the instructions of the computer program product might be executed by any of the processing units described in relation to Fig. 2. The present disclosure also relates to a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method 600. The computer-readable storage medium can be a non-volatile medium.

What has been described before in relation to the method 600 can be equally well applied to the system 100. Especially, the corresponding elements of the system 100 can be arranged to perform any of the steps described in relation to the method 600. The method 600 can also comprise any of the actions described in relation to the system 100.

What is stated above in relation to the method 600 and the system 100 applies equally well to the computer program product and the computer-readable storage medium.