GB2460691A

GB2460691A - FMCW radar for traffic monitoring

Info

Publication number: GB2460691A
Application number: GB0810325A
Authority: GB
Inventors: Peter Robert Williams
Original assignee: AGD Systems Ltd
Current assignee: AGD Systems Ltd
Priority date: 2008-06-06
Filing date: 2008-06-06
Publication date: 2009-12-09
Also published as: GB0810325D0; WO2009147406A1; GB2472559A; GB2472559B; GB201021284D0

Abstract

A FMCW radar is disclosed for detecting a moving target. A frequency modulated radio signal is transmitting towards the target at a given frequency modulation repetition rate. The return signal is received and digitized at a rate which is the same as, or an integral multiple of, the frequency modulation repetition rate. In this the higher frequency components are folded back into a single range bin, and therefore processing and memory requirements are considerably reduced. The velocity of the moving target can be determined in a conventional manner. The range of a target is determined by splitting the return signal and feeding it into two channels having different frequency responses so that the target signal amplitude is different in the two channels. The amplitude difference is detected and used to determine the range of the target. In another embodiment, the signal is split into two channels which are digitized at the same rate but with a preset time delay between the two channels. The relative phase difference between the relevant two target signals from the channels is determined thereby to determine the range of the target.

Description

I

Radar Methods and Apparatus This invention relates to methods and apparatus for detecting a moving target and in particular, but not exclusively, to such apparatus and methods for traffic monitoring.

For many years low cost radars have been used for traffic monitoring and speed enforcement devices.. These have commonly used continuous wave (CW) radars that are only capable of measuring the speed of the target but not its range. Frequency modulated continuous wave (FMCW) radar have been used in many high bandwidth applications and offer good range resolution together with the ability to determine the range and velocity of a target. In many applications, the signal processing needs can be reduced by filtering the signal and digitising the required part of the returned signal bandwidth. However, even in these radars, the amount of signal processing required can still be significant and this generally means that a low cost radar is difficult to realise. Existing such FMCW devices still have relatively high processing and memory requirements which increase their cost and also mean that they draw significant amounts of power and so their use is usually limited to applications where there is a mains supply available.

We have therefore designed methods and apparatus for detecting a moving target which make use of FMCW techniques but which have considerably reduced memory and power requirements.

Accordingly, in one aspect, this invention provides a method of detecting a moving target, which comprises: transmitting towards said target a frequency modulated radio signal at a given frequency modulation repetition rate; receiving the return signal and digitising it at a rate equal to said frequency modulation repetition rate or an integral multiple thereof; processing said transmitted and return signals to obtain data representative of the velocity of said target.

In specific implementations of the above method, the matching of the digitisation rate of the return signal with the repetition rate of the frequency modulation means that, in the frequency domain, the ground return signals all fold back onto the zero frequency. This means that a previous requirement of several tens of fast fourier transforms (FFT) operations is now considerably reduced thereby reducing the memory and processing requirements.

Although it would be possible to make use of such methods simply to determine the velocity of a target, in preferred embodiments the transmitted and return signals are processed to also obtain data representative of the range of said target. It will be appreciated that because the frequencies have effectively all been folded back into the first range bin, it is necessary to disambiguate the range bin signals. We have developed t*o preferred schemes for doing this namely an amplitude-based scheme and a phase-based scheme.

In the amplitude-based scheme, the return signal is split and fed into two channels having different frequency responses, and the data from each channel is then processed to obtain respective target signal amplitudes, with the respective target signal amplitudes then being compared to determine an estimate of the range of said target. Although various differential frequency responses are possible, in one airangement, both channels have a high pass frequency response (similar to the range compensation filter in existing FMCW arrangements), with one channel additionally having a low pass frequency response. Thus, knowing the signal amplitudes of the folded frequency of the target detected in each channel, together with the differential frequency responses of each channel, the pre-folded frequency of the target signal can be determined, and from that the range of the target deduced.

In the phase-based scheme, the return signal is split into two channels which are digitised at the same digitisation rate but with a preset time delay between two channels, the data from the channels then being processed to determine the relative phase difference between the relevant two target signals from the channels, thereby to determine the pre-folded frequency of the target signal and thus an estimate of the range of said target.

It will be appreciated that this phase-base technique also deduces an estimate of the relevant pre-folded frequency of the target signal to determine its range. In this instance by having a slight delay between the digitisation of both channels, the phase difference can be measured and, knowing this together with the time delay, the frequency can be deduced.

The invention also extends to apparatus for detecting a moving target, which apparatus comprises: means for transmitting towards said target of frequency modulated radio signal at a given frequency modulation repetition rate; means for receiving the return signal; means for digitising said return signal at a rate equal to said frequency modulation repetition rate or an integral multiple thereof, and means for processing said transmitted and return signals to obtain data representative of the velocity of said target.

The frequency modulation may be of any convenient form for example sawtooth or triangular waveform with a sawtooth waveform being preferred.

Whilst the invention has beeh described above, it extends to any inventive combination or sub-combination of the features set out above, in the following or in the following description of claims.

The invention may be performed in various ways, and two embodiments thereof will now be described by way of example only, reference being made to the accompanying drawings in which Figure 1 is a schematic view of a typical prior art FMCW radar; Figure 2 is view of a typical FMCW radar mixer output in the frequency domain; Figure 3 is a first embodiment of a radar of this invention in which range is determined by amplitude comparison; Figure 4 is a view of a typical FMCW radar mixer output in.the frequency domain from the embodiment of Figure 3, and Figure 5 is a second embodiment of FMCW radar in accordance with this invention in which range is determined by phase comparison.

Referring initially to Figure 1, in a typical known FMCW radar, an oscillator 10 is frequency modulated by a sawtooth waveform 12 to provide a "chirp" signal which is coupled to an antenna 14 for transmission. The received signal is supplied to a mixer 16 to be mixed with the chirp signal to provide an intermediate signal whose real and imaginary parts 1(t) and Q(t) are processed through inphase and quadrature paths 18, 20 respectively. In each path the signal is amplified by an amplifier 22 to then pass through a high pass filter 24 which acts a range compensation filter, with the filtered output then passing to an analogue to digital converter 26 and the digitised signals then being supplied to a signal processor 28. The range compensation filter is designed so that signals from all targets have a similar level, Objects further away give rise to higher frequencies but their smaller size due to range means that the amplitude of the return signal decreases with distance and so the high pass filter compensate for this.

Figure 2 shows a typical frequency spectrum of a typical signal from the mixer 16. Each peak in the frequency spectrum represents a potential target.

From the spectrum it can be seen that there is a signal that repeats every 25kHz. These are signals due to reflections from the ground (zero velocity). In many instances these signals are not of interest as in many radio applications it is only moving targets that are of interest. Thus it is the signal peaks between these ground returns that are the wanted target signals. To process this signal the analogue to digital converters 26 would typically be required to digitise at a rate of 400 kilo samples per second (KSPS). The signal processor 28 typically applies a series of range FFTs followed by a series of Doppler FFTs to extract the target information. The following FFTs would have to be carried out in this example which would have 64 Doppler bins: * 64* 12 point range FFTs * 16* 64 point Doppler FFTs This amounts to a huge number of data samples which means that a large amount of memory is required and also significant processing power.

Turning now to the embodiments described below, in each of these the signals are digitised at a much lower rate than is usual in an FMCW radar. In the examples given, the signals are digitised at 25KSPS instead of 400 KSPS.

These signals are therefore being decimated by a factor of 16. This means that the samples are heavily undersampled and when digitised signals are FFT'd only a 25Khz bandwidth of signal can be seen. All the higher frequency components from the original bandwidth of 400Khz will have been folded back into the 25Khz bandwidth. The digitisation frequency has been chosen so that all the ground return signals fold back to 0Hz. This is achieved by choosing the frequency modulation repetition rate to be the same as the digitisation rate. In these embodiments, which we refer to as decimating FMCW radars, the moving target can be clearly seen in the frequency domain but its range can no longer be determined by the conventional technique. We describe two different embodiments in which the range can be determined. The first embodiment relies on an amplitude comparison step, and the second embodiment relies on a phase comparison step. Both embodiments essentially determine what the frequency of the target signal would have been in the unfolded spectrum. We refer to "pie-folded" and "folded" frequencies to differentiate between the signals before and after the processing which causes the folding.

Turning firstly to the amplitude comparison decimating FMCW radar shown in Figure 3, components similar to those used in the conventional arrangement of Figure 1 are given like reference numerals. As previously, a frequency modulation derived at 12 is applied to an oscillator 10 to provide a transmitted signal which is passed to the antenna 14. The return signal is mixed with the transmitted signal to obtain an intermediate signal which is processed along inphase and quadrature paths 18, 20. However, in each path, the signals are split and fed into two sets of filters having different frequency responses.

One filter 30 has the traditional FMCW high pass frequency response for range compensation while the other filter 32 has the same high frequency response but an additional low pass response superimposed on it, and below we refer to normal and low pass channels. It should be noted that it is not necessary to have a high pass filter response, indeed in some instances it may be preferred not to provide the range compensation function. The low pass filter in the example given discriminates in favour of targets that are closer. The filter responses should be selected so that they allow the pre-folded frequency of the target to be determined.

The output of the mixer in the frequency domain is shown in Figure 4 from which it will be seen that the presence of a target is evident from the signal peak at 36. The frequency at which this peak appears in Figure 4 indicates the velocity of the target. In order to determine the range of the target, the differentially filtered channels are FFTed and similar spectrums are observed.

However, because the low pass channel has an additional low frequency response, the target signal amplitude is different, from that in the normal channel, and the extent of the difference is indicative of the pre-folded frequency of the target return and thus its range.

In the above arrangement, the number of FFTs required has been reduced to just two followed by an amplitude comparison step to determine the targets range. This arrangement therefore considerably reduces the amount and speed of processing required and therefore allows a relatively inexpensive process to be used.

Referring now to the phase comparison embodiment, in this arrangement the FMCW radar is modified as shown in Figure 5. Similar components are given similar references. For this type of radar there is no additional filtering required for the extra channel. Instead, in this arrangement, the signal received from the high pass filter 24 is digitised at a low rate but, split into channels that are clocked by a clock generator 34 at the same frequency, but with one slightly delayed. When the two channels are digitised, the amplitude response should be the same if they are clocked synchronously. However, because of the slight time difference introduced between digitisation of the channels there will be a phase difference between the slightly delayed samples of the target return. The phase difference measured is dependant on the pre.-folded frequency of the target return that caused the FF1 output. The higher the original pie-folded frequency, the greater the phase difference measured will be, i.e.: Channell(t) = sin(wt) Channel2(t) = sin(w(t+t)) Phase difference = wt Therefore knowing the phase difference and the time delay the pre-folded frequency can be determined which in turn identifies the range of the target. This phase difference method has the advantage that it is not necessary to calibrate match the filters used in the amplitude-based method. Two options are available; firstly the delay between the samples may be small in terms of the period of the pre-folded frequency of the target return i.e. equivalent to a phase difference of a few degrees, or it may be much larger, approaching half the period of the sample frequency. A benefit of this latter arrangement is that it means that the analogue to digital converter has longer to carry out digitisation because the samples are more evenly distributed timewise.

Claims

CLAIMS1. A method of detecting a moving target, which comprises: transmitting towards said target a frequency modulated radio signal at a given frequency modulation repetition rate; receiving the return signal and digitising it at a rate equal to said frequency modulation repetition rate or an integral multiple thereof; processing said transmitted and return signals to obtain data representative of the velocity of said target.
2. A method according to Claim 1, wherein the transmitted and return signals are processed to also obtain data representative of the range of said target.
3. A method according to Claim 1 or Claim 2, wherein the return signal is split and fed into two channels having different frequency responses, and the data from each channel is then processed to obtain respective target signal amplitudes, with the respective target signal amplitudes then being compared to determine an estimate of the range of said target.
4. A method according to Claim 3, wherein both channels have a high pass frequency response with one channel additionally having a low pass frequency response.
5. A method according to Claim I or Claim 2, wherein the return signal is split into two channels which are digitised at the same digitisation rate but with a preset time delay between two channels, the data from the channels then being processed to determine the relative phase difference between the relevant two target signals from the channels, thereby to determine the pie-folded frequency of the target signal and thus an estimate of the range of said target.
6. Apparatus for detecting a moving target, which apparatus comprises: means for transmitting towards said target of frequency modulated radio signal at a given frequency modulation repetition rate; means for receiving the return signal; means for digitising said return signal at a rate equal to said frequency modulation repetition rate or an integral multiple thereof, and means for processing said transmitted and return signals to obtain data representative of the velocity of said target.
7. A method substantially as hereinbefore described with reference to any of Figures 3 to 5.
8. Apparatus substantially as hereinbefore described and illustrated with reference to any of Figures 3 to 5.