Technical Field of the Invention

The present invention relates to the field of radar signal processing and in particular to a method and apparatus for determining movement of an object from a reflected radar signal.

Background to the Invention

A common application of radar systems is in the measurement of the speed of movement of known and unknown targets. This is often achieved by exploiting the Doppler Effect, which results in a predictable change in the frequency of a reflected radar wave based on the relative motion between the target and the radar receiver.

There are a number of different types of radar systems which are able to take advantage of the Doppler Effect to determine the speed of a moving target. Such measurements of the speed of radar targets have a wide number of applications and are often used for safety or law enforcement systems including, but not limited to, air traffic control and the monitoring of road vehicle speeds. It is therefore particularly important to ensure that such measurements are accurate. However, radar systems which rely on measuring a change in frequency of a reflected wave to determine target speed may be subject to measurement errors and may be difficult to verify. It is therefore an aim of the invention to provide a more robust method of determining the movement of a radar target.

Summary of the Invention

According to an embodiment of the invention, there is provided a method for

determining the movement of an object comprising: transmitting a first radar signal having a first frequency and a first chip rate towards an object; receiving a second radar signal having a second frequency and a second chip rate, the second radar signal being a reflection of the first radar signal from the object; measuring the second frequency; measuring the second chip rate; determining that the object is moving if a Doppler shift of the second frequency from the first frequency matches a Doppler shift of the second chip rate from the first chip rate.

According to another embodiment of the invention, there is provided a radar system comprising: a transmitter configured to transmit a first radar signal having a first frequency and a first chip rate; a receiver for receiving a second radar signal having a second frequency and a second chip rate, the second radar signal being a reflection of. the first radar signal from an object; a processor configured to measure the second frequency, measure the second chip rate and to determine that the object is moving if a Doppler shift of the second frequency from the first frequency matches a Doppler shift of the second chip rate from the first chip rate.

The inventor has identified that both the frequency and the chip rate of the reflected radar signal change as a result of a reflection from a moving target. In particular, regarding the effects of Doppler Shift on a reflected radar signal as a manipulation of time, rather than frequency, it is clear that time-related aspects of the radar signal, including the chip rate will be measurably affected. Therefore, when the second radar signal is reflected from an object which is moving, with respect to a radar receiver, both the second chip rate and the second frequency of the second radar signal will be modified from those of the first chip rate and the first frequency of the first radar signal.

Accordingly, the method measures both the Doppler shift of the frequency and of the chip rate in order to provide a more robust system that is less prone to error and provides a higher degree of confidence. The extent to which the frequency is modified by the Doppler effect will typically match or substantially match the extent to which the chip rate is modified. Similarly, a parameter derived from the measured change in chip rate, for example the speed of an object, may match or substantially match the parameter derived from the measured change in frequency.

The method is particularly advantageous for safety critical and law enforcement applications, but would be equally applicable to any application where a verification of the measured speed of the target was desired, for example, when measuring the movement of competitors in sporting events.

Clearly the degree to which the Doppler shift of the second frequency from the first frequency matches the Doppler shift of the second chip rate from the first chip rate can be set for each application and they need hot be identical to achieve that aim of the invention. Advantageously, the degree of matching can be modified to the measurement integrity requirements of the particular application. For example, law enforcement applications may require a higher degree of certainty that an object is moving than some other applications and therefore require a lower tolerance for determining a match.

Clearly, the object may be any object which reflects a radar signal. Additionally, the object may already be of known interest, and to which the first radar signal is

intentionally directed. Alternatively, the object may be initially unknown and fall within a region defined by the range of the first radar signal.

It will be clear to those skilled in the art that the transmitter and the receiver of the radar system may be separate elements within the radar system or equally may be combined together within a transceiver.

Advantageously, a first object speed estimate may be determined from the Doppler shift of the second frequency from the first frequency and a second object speed estimate may be determined from the Doppler shift of the second chip rate from the first chip rate, for example using the processor of the radar system.

Generating both a first object speed estimate and a second object speed estimate from different aspects of the reflected second radar signal, when compared with the first radar signal, provides advantages over systems which singly determine an objects speed from analysis of the Doppler shifted frequency component of the second radar signal. Such an advantage includes the provision of a second check on the accuracy of the object speed estimate generated from the Doppler shifted frequency component of the reflected radar signal. An additional advantage is the provision of a second check to determine if the target object is a moving or stationary object.

Advantageously, an average object speed may be determined from the average of the first object speed estimate and the second object speed estimate, for example using the processor of the radar system.

It may be preferable to compare the first object speed estimate to the second object speed estimate and to indicate that an error has occurred if the second object speed estimate is significantly different to the first object speed estimate, for example using the processor to signal that an error has occurred if the second object speed estimate is greater that a threshold value from the first object speed estimate and that the output target speed cannot be relied upon. Clearly this threshold value may be modified to the measurement integrity requirements of the particular application, for example, law enforcement applications may require a higher degree of certainty of measurement, and therefore require a lower threshold value, than other types of application.

Preferably the first radar signal may be a modulated continuous wave signal.

Advantageously modulating a continuous wave signal comprises modulating with a Pseudo random noise code sequence.

Advantageously, the radar system processor may further comprise a pseudo random code generator.

Pseudo random noise code sequences are well known in the art and can be applied in a broad number of applications. In particular, they are a binary sequence of known repetition period and would be applicable to the modulation of a continuous wave signal to generate a first radar signal of a known first chip rate. A pseudo random code generator may be used to generate the Pseudo random noise code sequence.

Brief description of the Drawings

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows a flow diagram of a method according to a first embodiment of the invention for determining the movement of an object, in which a camera is used to determine whether a racing car has performed a false start;

Figure 2 shows a schematic diagram according to a second embodiment of the invention for determining the speed of ah object, in which a road traffic camera is used to determine the speed of a motor vehicle;

Figure 3a, shows a diagram of a continuous wave radar signal used in the transmitter of the road traffic camera of the second embodiment; Figure 3b, shows a diagram of the continuous wave radar of Figure 3a when modulated with a Pseudo random noise code sequences in the road traffic camera of the second embodiment to produce a modulated continuous waveform radar signal;

Figure 3c, shows a diagram of a reflection of the modulated continuous waveform radar signal of Figure 3b, which has been affected by Doppler shift;

Figure 4, shows a schematic diagram of a waveform modulator forming part of the road traffic camera of the second embodiment; and

Figure 5, shows a schematic diagram of a radar system forming part of the road traffic ' camera of the second embodiment.

Detailed Description

A first embodiment of the invention will now be described with reference to the flow diagram of Figure! Firstly, a radar signal is transmitted 101 from a radar transceiver with a known frequency and a known chip rate. The known frequency is referred to herein as a first frequency and the known chip rate is referred to herein as a first chip rate.

Upon contact of the transmitted radar signal with an object, for example a racing car, a proportion of the transmitted radar signal is reflected. The reflected radar signal is received 102 at the radar transceiver, and the frequency of the reflected radar signal, which is referred to herein as a second frequency, is measured in a step 103. The chip rate of the reflected radar signal, which is referred to herein as a second chip rate, is also measured in a step 104.

The second frequency and the second chip rate of the reflected radar signal will differ from the first frequency and the first chip rate of the transmitted radar signal according to the Doppler shift resulting from relative movement between the object and the transceiver which received the reflected radar signal 102. The Doppler shift between the first frequency and the second frequency is measured 105, and additionally the Doppler shift between the first chip rate and second chip rate is measured 106. It is determined that an object is moving 107 if the shift of the second frequency from the first frequency matches a Doppler shift of the second chip rate from the first chip rate. For example, in the case where the object is a racing car, the method could be used to check that the racing car has not performed a "false start" and started moving prior to a starting signal given at the start of a race in which the racing car is taking part. If a shift in both the first and second frequencies and the first and second chips rates occurs prior to the starting signal being given, then it may be determined that the racing car has performed a false start.

A second embodiment of the invention will now be described with reference to Figures 2 - 5. The schematic diagram of Figure 2 shows a motor vehicle 201 that is moving along a road 202 in a direction indicated by the arrow 203. A road traffic camera 204 is positioned adjacent to the road 202.

The road traffic camera 204 transmits a first radar signal 205 towards the motor vehicle 201. The first radar signal 205 has a first frequency and a first chip rate. Upon contact with the motor vehicle 201 , a proportion of the first radar signal 205 is reflected towards the road traffic camera 204, forming a second radar signal 206. The second radar signal 206 has a second frequency and a second chip rate. The second frequency will differ from the first frequency due to the associated Doppler shift that results from the relative movement between the motor vehicle 201 and the road traffic camera 204. Similarly the second chip rate will experience a Doppler shift from first chip rate.

The use of a measured Doppler shift to calculate the speed of a motor vehicle is well known to those skilled in the art. Such techniques may be applied to the Doppler shift measured between the first and second frequencies and the Doppler shift measured between the first and second chip rates to determine a first object speed estimate and a second object speed estimate respectively. An average object speed estimate is then produced from the average of the first object speed estimate and the second object speed estimate.

The first and second object speeds are compared to determine whether the Doppler shift measured between the first and second frequencies and the Doppler shift measured between the first and second chip rates match one another, and therefore whether the object which reflected the first radar signal back to the road traffic camera 204 was moving. Accordingly, reflected radar signals received from stationary objects such as roads signs and/or trees can be discounted and the road traffic camera 204 configured to react only to objects that are determined to be moving, such as the motor vehicle 201.

Furthermore, in this embodiment, an alert could be generated indicating that an error has occurred if the second object speed estimate is greater that a threshold value from the first object speed estimate. This alert could be tailored to the application, for example were the road traffic camera held by an operator the alert may be an audible cue, a visual cue a vibration cue or a combination of these or other cues. In the case of a remote operated road traffic camera an alert may be sorted with the record of the traffic incident. Similarly the threshold value could be tailored to comply with appropriate road traffic laws.

The first and second radar signals 205 and 206 will now be described in further detail with reference to Figures 3a, 3b and 3c. To form the first radar signal 205, a continuous wave radar signal 301 shown in Figure 3a is modulated with a Pseudo random noise code sequence 302 to produce a modulated continuous waveform shown in Figure 3b, the modulated continuous waveform being the first radar signal 205. The first radar signal 205 has a first frequency and first chip rate. The first chip rate being determined by the bit rate of the Pseudo random noise code sequence 302. The first radar signal 205 differs from the continuous wave radar signal 301 by the waveform 304.

As shown in Figure 3c, due to the Doppler Effect that results from the movement of the motor vehicle 201 towards the road traffic camera 204, the second radar signal 206, has an increased frequency compared with that of the frequency of the first radar signal 205. Furthermore, due to this Doppler Effect, the second chip rate of the Pseudo random noise code sequence 306 corresponding to the second radar signal 206 is increased with respect to the first chip rate of the first radar signal 205, Accordingly, the second radar signal 206 in Figure 3c is shown more compressed in time than the first radar signal 205 in Figure 3b to illustrate the increase in frequency.

In Figures 3a, 3b and 3c, the nature and length of the radar waveforms and the Pseudo random noise code sequence 302 are for illustrative purposes only.

The modulation of the continuous wave radar signal 301 by the Pseudo random noise code sequence 302 in this embodiment is done by a waveform modulator 402, as shown in Figure 4.The waveform modulator 402 comprises a mixer 403 and a pseudo random code generator 404 configured to output the Pseudo random noise code sequence 302 to the mixer 403.

In use, the continuous wave radar signal 301 is received by the waveform modulator 402. The pseudo random code generator 402, outputs the Pseudo random noise code sequence 302 to the mixer 403. The mixer 403 modulates the continuous wave radar signal 301 using the Pseudo random noise code sequence 302. The mixer 403 outputs the modulated continuous waveform as the first radar signal 205.

Further details of the road traffic camera 204 will now be described with reference to the schematic diagram of Figure 5, which shows the road traffic camera 204 comprising a transceiver 505, a processor 507, and a user view screen 510. The transceiver 505 comprises a transmitter 502 and a receiver 504. The transmitter 502 includes the waveform generator 402 described above in relation to Figure 4.

In use, the first radar signal 205 is transmitted by the transmitter 502. The second radar signal 206 is a reflection of the first radar signal 205 from the motor vehicle 201 . The second radar signal 206 is received by the receiver 504. The transceiver 505 outputs data 506 to a processor 507. The data 506 includes the first frequency and the first chip rate of the first radar signal 205 and the second frequency and the second chip rate of the second radar signal 206. The processor 507 determines a first object speed estimate from the Doppler shift of the second frequency from the first frequency and a second object speed estimate from the Doppler shift of the second chip rate from the first chip rate. The processor 507 determines the average object speed from the average of the first object speed estimate and the second object speed estimate. The processor outputs average object speed data 508 to the user view screen 510.

Optionally, the processor 507 may also compare the first object speed estimate to the second object speed estimate and output an error data message 509 to the user view screen 510 if the second object speed estimate is greater that a threshold value from the first object speed estimate.

In an alternate embodiment the user view screen 510 could be remote from the road traffic camera 204, rather than an integral part, for example if the road traffic camera 204 was a permanent installation rather than a mobile device. Further embodiments falling within the scope of the appended claims will also be apparent to the skilled person.