GB2390771A

GB2390771A - Vehicle detection

Info

Publication number: GB2390771A
Application number: GB0215910A
Authority: GB
Inventors: Michael Alexander Graham Clark
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-07-10
Filing date: 2002-07-10
Publication date: 2004-01-14
Also published as: GB0215910D0

Abstract

A vehicle detector system has a Doppler detector for transmitting microwave radiation towards a vehicle, receiving the microwave radiation reflected by the vehicle and producing a Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector. The transmission frequency of the microwave radiation varies between a first frequency and a second frequency, the Doppler signals for the first and second frequencies are processed to determine a phase difference therebetween indicative of the distance between the vehicle and the detector, and an output signal is provided dependent upon the determined phase difference. Alternatively, a second Doppler detector is provided adjacent the first-mentioned Doppler detector for transmitting microwave radiation of a second, different frequency towards the vehicle, receiving the microwave radiation reflected by the vehicle and producing a second Doppler signal from the transmitted radiation of the second frequency. The corresponding received radiation, the first and second Doppler signals are processed to determine a phase difference therebetween indicative of the distance between the vehicle and the detectors, and an output signal is provided dependent upon the determined phase difference.

Description

239077 1

TITLE Vehicle Detection DESCRIPTION

This invention relates to vehicle detection.

More specifically, first and second aspects of the invention relate to a vehicle detector system comprising a Doppler detector for transmitting microwave radiation towards a vehicle, receiving the microwave radiation reflected by the vehicle and producing a Doppler signal, from 5 the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector.

Such detection systems are known and were first developed for commercial use by Philips in the early 1970s. The principle of operation relies on the Doppler effect where a transmitted electromagnetic wave T. reflected by a moving vehicle, results in a change in 10 frequency of the reflected wave R. An approaching vehicle gives an increase in frequency, a receding vehicle a corresponding decrease. The difference in frequency, or Doppler frequency, Fo is given by the equation FD=2VFr.cos8/c, where FT is the transmitted frequency, c is the speed of light, V is the velocity of the target and is the angle between the direction of travel of the vehicle and the direction between the transceiver and the vehicle. (For simplicity, for the 15 purposes of the remainder of this description, it is assumed that 8=0 so that FD=2VFT/c.) At

UK permitted frequencies (FT= 10.S77 to 10.597 GHz, X band), this frequency difference ED is 19 Hz per kph. In the K band (FT=24. 150 to 24. 250 GHz), it is 45 Hz per kph.

Such detector systems typically use Gunn diodes mounted in die-cast aluminium cavities to form an oscillator with two Schottky diodes acting as mixers mounted in a waveguide 20 connected to a horn antenna. A typical module is a Type MA86735 manufactured by MACOM.

The difference frequency signal appears at the outputs of the mixer diodes and can be used for detection purposes. The mixer diodes are located in the waveguide at critical positions, such that the phase difference between the signals gives an indication of the direction of travel. It does not give range information, only +90 phase dependent on the direction of travel. In the case of 25 vehicle detectors, to avoid pedestrians and other slow moving targets causing a detector unit to give an unwanted output signal, frequency discrimination is used such that only targets travelling above a detained speed (e.g. 8 kph) will give a detection signal.

- 2 The amplitude of the signal can be used as a crude range indicator, but clearly a good target (e.g. a bus) will give a much larger signal than a poor target (e.g. a cyclist). This gives a wide variation in the range at which a target is first detected. This imprecision limits the application of these devices to simple traffic control applications. They also require accurate s 'tilt' setting up for best results.

Several techniques have been tried by others in an attempt to provide more accurate range information. These include frequency modulated carrier wave (FMCW) techniques, but they have not been applied primarily because of the lack of available bandwidth at permitted frequencies and the complex processing required to cope with a multiplicity of targets.

10 The present invention employs a different technique to provide accurate range information in connection with vehicle detection.

If radiation of a first microwave frequency FT] is transmitted from a detector to a vehicle that is at a distance R from the detector and is approaching the detector and if the radiation is reflected back to the detector with a frequency FT,+FD,, where FD] IS the Doppler i 15 frequency, the angular progression ó' of the microwave radiation will be 21r(2FT! +FDl)R/C (i.e. 2'r times the distance travelled divided by the wavelength). Similarly, if radiation of a second, different microwave frequency FT: is transmitted from the detector to the vehicle and if the radiation is reflected back to the detector with a frequency Fr:+FD:, the angular progression 42 of that microwave radiation will be 21r(2FT2+FD2) R/C. The phase difference ó. between the 20 resulting Doppler signals at the two Doppler frequencies will be 024-2n(2Fz-2FT'+FD: FD')R/c. If AF-T (=FT2-FTI) is much greater than FDz-FD, (in other words, if the velocity V of the vehicle is much less than the speed of light), then the expression for the phase difference can - be simplified to 041R/C. In other words, the distance R of the vehicle is related to the difference AFT between the microwave transmission frequencies, the phase difference between 25 the Doppler signals and the speed c of light by R0C/(4AF1).

The detector system of the first aspect of the invention enables this relationship to be -

exploited and is characterized by: means for modulating the transmission frequency of the microwave radiation between a first frequency and a second frequency; and means for a processing the Doppler signals for the first and second frequencies to determine a phase 30 difference therebetween indicative of the distance between the vehicle and the detector, and providing an output signal dependent upon the determined phase difference.

- 3 The detector system of the second aspect of the invention also enables this relationship between distance and Doppler phase difference to be exploited and is characterized by: a second Doppler detector adjacent the first-mentioned Doppler detector for transmitting microwave radiation of a second, different frequency towards the vehicle, receiving the microwave 5 radiation reflected by the vehicle and producing a second Doppler signal from the transmitted radiation of the second frequency and the corresponding received radiation; and means for processing the first and second Doppler signals to determine a phase difference therebetween indicative of the distance between the vehicle and the detectors, and providing an output signal dependent upon the determined phase difference.

10 In many vehicle detection applications, it is unnecessary to output a value for the distance or speed of the vehicle or other measured parameters, but merely to produce an output such as a binary "detected" or "not detected" signal. Accordingly, in both aspects of the invention, the processing means is preferably operable such that the value of the output signal is dependent upon one or more of the following: (a) whether or not the indicated distance is above 15 a first predetermined value; (b) whether or not the indicated distance is below a second predetermined value; (c) whether or not the speed indicated by one or both of the Doppler signals is above a third predetermined value; (d) whether or not the speed indicated by one or both of the Doppler signals is below a fourth predetermined value; and (e) whether or not the amplitude of one or both of the Doppler signals is above a fifth predetermined value.

20 As mentioned above, a known detector unit employs two mixer diodes to produce two Doppler signals having the same Doppler frequency. The mixer diodes are spaced apart so that, at the transmission frequency of the detector unit, one Doppler signal leads or lags the other Doppler signal by 90 depending on the direction of travel of the vehicle. With the present invention, it is unnecessary to employ two mixer diodes in this way because the direction of 25 travel can be determined from the phase difference between the two Doppler signals for the two different transmission frequencies. Accordingly, the processing means is preferably also operable to process the phase difference to provide an indication of the direction of travel of the vehicle. The processing means can then operate in such a way that the value of the output signal is dependent upon the indicated direction.

30 As will be described later in more detail, there can be a limit to the range of unambiguous detection of the system, in that, for example, the Doppler signals for a highly reflective receding vehicle at long range may be interpreted as the Doppler signals for an approaching vehicle at close range. The system may be given a degree of intelligence, for

- 4 example so that it does not change from a "not detected" state to a "detected state" in the above example, unless an approaching vehicle has earlier been detected in the medium range.

Accordingly, the processing means may operable such that a change of value of the output signal is dependent upon the indicated direction and upon one or both of the following: (a) 5 whether or not the indicated distance is above a sixth predetermined value; and (b) whether or not the indicated distance is below a seventh predetermined value.

Other refinements may be made to the system, for example to employ temporal averaging of the measured values and/or derived values, and different weighting may be placed on different values. Accordingly, the system may further comprise means for storing the 10 indicated distance, speed, direction and/or a value determined therefrom, and the processing means may be operable such that the value of the output signal and/or a change of value of the output signal is dependent upon the current distance, speed and/or direction and a previously stored distance, speed, direction and/or stored value.

The invention also provides corresponding methods of vehicle detection. In particular, a 15 third aspect of the invention provides a method of detecting a vehicle, comprising the steps of: transmitting microwave radiation towards the vehicle; receiving the microwave radiation reflected by the vehicle; and producing a Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector; characterized by the steps of: modulating the transmission frequency of the microwave 20 radiation between a first frequency and a second frequency; processing the Doppler signals for the first and second frequencies to determine a phase difference therebetween indicative of the distance between the vehicle and the detector; and providing an output signal dependent upon the determined phase difference.

Also, a fourth aspect of the invention provides a method of detecting a vehicle, 2s comprising the steps of: transmitting microwave radiation of a first frequency towards the vehicle; receiving the microwave radiation reflected by the vehicle; and producing a first Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector; characterized by the steps: transmitting microwave radiation of a second, different frequency towards the vehicle; receiving 30 the microwave radiation reflected by the vehicle; producing a second Doppler signal from the transmitted radiation of the second frequency and the corresponding received radiation; processing the first and second Doppler signals to determine a phase difference therebetween

- s - indicative of the distance between the vehicle and the detectors; and providing an output signal dependent upon the determined phase difference.

Specific embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: 5 Figure I is a schematic view of one embodiment of detector system being used to detect a vehicle; Figures 2A-2H are timing diagrams illustrating various waveforms arising in the system of Figure 1; Figures 3A-3D are further timing diagrams, on a greater timescale, illustrating various further lO waveforms arising in the system of Figure 1; Figure 4 is a flow diagram illustrating one process carried out by a processor in the system of Figure 1; Figure 5 is a flow diagram illustrating another process carried out by a processor in the system of Figure 1; 15 Figures 6 & 7 show modifications to the process of Figure 5; and Figure 8 is a schematic view of another embodiment of detector system being used to detect a vehicle.

Referring to Figure 1, a microwave detector module 10 is statically mounted on, for example, a traffic light post 12 so as to have a field of view 14 along a road on which vehicles

20 16 are to be detected.

The microwave module 10 is known per se and may be implemented by a model MA87849 varactor-controlled oscillator transceiver produced by M/A-COM, Inc. of Lowell, Massachusetts, USA. This is a mechanically and electrically tunable Gunn oscillator/transceiver designed to operate in the 24.000 to 24.250 GHz range (K band). A Gunn diode and GaAs 2S varactor diode are coupled in a single high Q cavity to provide low am/fm noise and reasonable frequency/power output stability with respect to change in operating temperature. A Schottky diode mixer is assembled in an attached waveguide section and performs as the receiver portion of the module. The transmission frequency of the microwave module 10 can be tuned

( - 6 electrically over a range of 0.150 GHz in dependence upon the voltage of a signal M applied to a modulation input 18 of the module 10.

The module 10 transmits radiation T at the microwave frequency FT, and any radiation R that is reflected back at the module 10 by an object such as the vehicle 16 is received by the 5 module 10 and mixed to produce an IF signal I that is output from the module 10. As is known, due to the Doppler effect, the frequency ED of the IF signal I is related to the component V of the velocity of the vehicle 16 in the direction between the vehicle 16 and the module 10 by the relationship F,,=2VF,/c, where c is the speed of light.

The detector system also includes a modulator 20 that has a modulation output 22 and 0 two gate outputs 24,26. The modulation output 22 provides a square wave modulation signal M at a frequency FM of, for example, 200 kHz, as shown in Figure 2A, that is supplied to the modulation input 18 of the microwave module 10 so as to switch the frequency of the transmitted microwave radiation T between FT and FT+, as shown in Figure 2B, where, for example, FT=24.150GHz and A,=1 MHz. The frequency of the microwave radiation R 15 reflected by the vehicle 16 therefore switches between FT+FD' and FT+r+FD, as shown in Figure 2C, where FD=2VFT/c and FD:=2V(FT+r)/c. Accordingly, the IF signal I switches between a first Doppler signal D, having a frequency of FD,=2VFT/c and a second Doppler signal D: having a frequency of FD:=2V(FT+AFr)/c, as shown in Figure 2D. It will be appreciated that even at a fast vehicle speed V of, for example, 200 kph (56 m.s-'), the Doppler 20 signals D,,D: will have frequencies FD,,FDz of about 9 kHz, and so Figure 2D shows only a very small portion of a cycle of each Doppler signal D',D:.

The two gate outputs 24,26 of the modulator 20 provide rectangular-wave gate signals G,,G:, respectively, at the same frequency FM as the modulation signal M, as shown in Figures 2E,2F, respectively. Each gate signal G,,G: has a mark/space ratio of less than unity. Each 25 mark of the gate signal Gil lies within a space of the modulation signal M, whereas each mark of the gate signal G: lies within a mark of the modulation signal M. Each gate signal GAG: is supplied to a control input of a respective controllable switch 32,34, and the IF signal I is supplied to the signal inputs of both of the controllable switches 32,34. The outputs of the controllable switches 32,34 are connected to respective grounded capacitors 44,46, and 30 therefore the signals DAD: at the outputs of the controllable switches 32,34 are substantially equivalent to complete forms of the respective components of the Doppler signals in the IF signal 1, as shown in Figures 2G,2H, respectively.

( - 7 The Doppler signals D,,D: are supplied as inputs to respective amplifiers and band-pass filters 48,50, that filter out frequencies below 450 Hz and above 9 kHz (equivalent to velocities V below 10 kph (2.8 m.s') and above 200 kph (56 m.s') and amplify the signals to a suitable level to produce output signals A,,A2, as shown in Figures 3A,3B, respectively. (It should be s noted that the timescale of Figures 3A-3D is substantially greater than the timescale of Figures IA-IH.) The signals A, ,A: are supplied to respective Schmidt trigger or limiter circuits 52,54, which, in effect, binarise the signals A',A: to produce respective square- wave signals L,,L:, as shown in Figures 3C,3D. The signals A,,A: are also supplied to respective signal strength detector circuits 56,58 that produce digital signal-strength signals S,,S2 according to the 10 amplitudes of the signals A,,A: respectively. The signals L',L:,S,,S: are all supplied to a digital processing circuit 60 that also has two manually-adjustable inputs 62,64 for setting the threshold values and the mode of operation of the detector system, and a detection output 66. The digital processing circuit 60 may be implemented by a general purpose microprocessor that is programmed by software to operate in the manner described below, but is more preferably 15 implemented by a dedicated microprocessor.

As illustrated in Figure 4, the processing circuit 60 analyses the input signals L,,L:, S,,S:. In a preliminary step 68, the process waits for a rising edge of the signal L,. Once detected, in step 70, a counter X is reset and commences counting a clock signal at a fixed frequency Fc of, for example, 1.250 MHz. In step 72, the process waits for a rising edge of the 20 signal L:. Once detected, in step 74, the counter X is stopped, and, in step 76, a counter Y is reset and commences counting the fixed frequency clock signal. In step 78, the process waits for a rising edge of the signal L,. Once detected, in step 80, the counter Y is stopped. In steps 80,82, the signal strength signals S,,S: are compared with a predetermined threshold 0,, and if either is less than the threshold en the process loops back to step 70. However, if not, in step 25 86, the values X,Y of the two counters are stored, and then the process loops back to step 70.

Referring back to Figures 3C and 3D, it will therefore be appreciated that the period between a rising edge of L, and the next rising edge L: is equal to X/Fc, that the period between that rising edge of L2 and the next rising edge L, is equal to Y/Fc, and that the period of the signal L, is equal to (X+Y)/Fc.

30 From the values X,Y, it is possible to calculate the speed V, range R and direction of travel r of the vehicle 16. Recalling that the period of the signal L is also equal to l/FD, and that F=2VFT/c, it will be appreciated that V=cFc/(2FT(X+Y)). It will be seen from Figures 3C and 3D that the phase difference p(where 0<p<) between L: and L, is given by

- 8 p=2X/(X+Y) if X<Y, and by p=2Y/(X+Y) if Y<X. It will be recalled that p=4RAFr/c. Therefore, R=cX/(2AFT(X+Y)) if X<Y, and R=cY/(2QT(X+Y)) if Y<X.

The direction of travel T of the vehicle 16 is given by T =sgn(X-Y), where T = I refers to one of approaching and receding depending on how the system is set up, and t=-l refers to the other s of approaching and receding, and where sgn(X-Y) refers to the sign of X-Y. For the remainder of this description it will be assumed that T=1 refers to approaching and T--1 refers to

receding. In cases where the detector system is merely required to compare speed, range and direction with predetermined criteria and produce a binary output, rather than to output values 10 indicative of the speed and range, the relationships set out above can be greatly simplified.

In order to determine whether the speed V of the vehicle 16 is greater than a threshold speed 02, i.e. to perform the test "cFc/(2FT(X+Y))>02?", it is merely necessary to perform the test "X+Y<?", where 0-cFc812F,. Conversely, in order to determine whether the speed V of the vehicle 16 is less than a threshold speed 02, it is merely necessary to perform the test 15 "X+Y>03?".

As mentioned above, the direction of travel of the vehicle 16 is simply given by T = sgn(X-Y).

In order to determine whether the range R of the vehicle 16 is less than a threshold range 0, i.e. to perform the test "cX/(2AFr(X+Y))C8?- if X<Y, or 20 "cY/(2AFr(X+Y))<?" if Y<X, it is merely necessary to perform the test "(X/Y)r>Ds?", where 05=(2/c)-l and where T= I depending on the direction of travel. Conversely, in order to determine whether the range R of the vehicle 16 is greater than a threshold range D., it is merely necessary to perform the test "(X/Y)t < Ds?".

A simple detection algorithm is shown in Figure 5, to detect whether a vehicle is 25 approaching with a speed greater than a predetermined threshold 02 and is within a predetermined range 8. The process of Figure S runs in parallel with the process of Figure 4.

The process of Figure 5 is triggered in step 88 by values of X and Y having been stored in step 86 of the Figure 4 process. Once triggered, in step 90, the direction T=sgn(X-Y) is calculated, and in step 92 it is determined if the direction is "approaching", i.e. T= 1. If not, then the 30 process loops back to step 88, but if so then in step 96 it is determined if the speed is greater than the threshold, i.e. X+Y<. If not, then the process loops back to step 88, but if so then in step 98 it is determined if the range is less than the threshold, i.e. (X/Y)r::> 05. If not, then the

- 9 - process loops back to step 88, but if so then in step 100, the detection output 66 of the processing circuit 60 IS set to "1" for a predetermined period of time, and the process loops back to step 88 to await retriggering.

The type of algorithm that is performed by the processing circuit 60, Figure 5 showing 5 one example, is manually set by the set mode input 64, and the thresholds that are used by a particular algorithm are manually set by the set values input 62. The various modes that are possible involve: detecting approaching vehicles only, receding vehicles only, or vehicles travelling in either direction; detecting only vehicles closer than a set maximum range, vehicles farther than a set minimum range, or vehicles between a set minimum range and a set maximum 10 range; and/or detecting only vehicles travelling faster than a set minimum speed, vehicles travelling slower than a set maximum speed, or vehicles travelling between a set minimum speed and a set maximum speed.

The simple detection algorithm of Figure 5 may be improved. For example Figure 6 illustrates a modification to the Figure 5 algorithm that uses a counter F that is incremented 15 towards an upper limit of 10 with each positive detection and decremented towards a lower limit of O with each negative detection, and the output is set only if the counter value F is greater than 5. The Figure 6 algorithm is similar to Figure 5, except that there is a preliminary step 102 of resetting the counter F to 0. Also, a negative determination in any of steps 92,96,98 leads to step 104 in which the counter F is decremented if it is greater than 0. Furthermore, positive 20 determinations in all of steps 92,96,98 lead to step 106 in which the counter F is incremented if it is less than 10. After steps IW, 106, the detection output 66 IS set for the predetermined time only if in step 108 it is determined that the counter value F is greater than 5.

A possible problem that may arise with the system described above is that there is a maximum RMAX to the unambiguous range of detection, given by p=2=4RMAxAFT/c, or 25 RMAX=C/26-. For a change in modulation frequency of Ar=1 MHZ, RMAX=I5O m. It is therefore possible that a receding large reflective object at a range slightly less than ISO m may be detected as an approaching object at close range. The algorithm of Figure S may be further modified to take account of this, as shown in Figure 7. In addition to the threshold 95 corresponding to the maximum range that is detected for an approaching vehicle, a further 30 threshold 0 is also set corresponding to the minimum range at which a vehicle will initially be detected if it has not already been detected. Accordingly, after a positive determination in step 98, in step 110 it is determined if the range is less than the minimum range, i.e. (X/Y)r>. If not, then the process proceeds to step 100, but if so then in step 112, it is determined if the

( - 10 detection output 66 is already set. If not, then the process loops back to step 88 without setting the output, but if so, then in step 100 the detection output 66 is set for a further preset time, and the process then loops back to step 88 to await retriggering.

With the algorithm of Figure 8, the current state of the detection output 66 is used in s determining the next state of the detection output 66. Various further developments may be made to the detection algorithms, such as using previously detected speed(s), range(s) and/or direction(s) in addition to the currently detected speed, range and/or direction, and using different weightings for different values.

A further embodiment of the invention will now be briefly described with reference to 10 Figure 8. The detector system of Figure 8 is similar to that described with reference to Figures 1 to 7, except in the following respects. Instead of employing a single microwave module 10 whose transmission frequency can be modulated, the system of Figure 8 employs two microwave modules lOA,lOB mounted adjacent each other on the support 12 and having overlapping fields of view 14A,14B. Each microwave module lOA,IOB has a Gunn oscillator

15 transceiver operating in the 10.577 to 10.597 GHz range (X band). The first module IDA uses a transmission frequency FT in the X band, and the second module IOB has a slightly different transmission frequency, FAT, where, for example, AFr=l MHz. The IF outputs of the microwave modules lOA,lOB are directly connected, as the signals D,,D:, to the inputs of the amplifiers and band-pass filters 48,50, and the modulator 20, controllable switches 32,34 and 20 capacitors 44,46 of the system of Figure 1 are omitted.

It should be noted that the embodiments of the invention have been described above purely by way of example and that many modifications and developments may be made thereto within the scope of the present invention.

Claims

1. A vehicle detector system comprising a Doppler detector for transmitting microwave radiation towards a vehicle, receiving the microwave radiation reflected by the vehicle and producing a Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector; 5 characterized by: means for modulating the transmission frequency of the microwave radiation between a first frequency and a second frequency; and means for processing the Doppler signals for the first and second frequencies to determine a phase difference therebetween indicative of the distance between the vehicle and the detector, 10 and providing an output signal dependent upon the determined phase difference.

2. A vehicle detector system comprising a Doppler detector for transmitting microwave radiation of a first frequency towards a vehicle, receiving the microwave radiation reflected by the vehicle and producing a first Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector; 15 characterized by: a second Doppler detector adjacent the first-mentioned Doppler detector for transmitting microwave radiation of a second, different frequency towards the vehicle, receiving the microwave radiation reflected by the vehicle and producing a second Doppler signal from the transmitted radiation of the second frequency and the corresponding received radiation; and 20 means for processing the first and second Doppler signals to determine a phase difference therebetween indicative of the distance between the vehicle and the detectors, and providing an output signal dependent upon the determined phase difference.

3. A system as claimed in claim I or 2, wherein the processing means is operable such that the value of the output signal is dependent upon one or more of the following: 25 (a) whether or not the indicated distance is above a first predetermined value; (b) whether or not the indicated distance is below a second predetermined value; (c) whether or not the speed indicated by one or both of the Doppler signals is above a third predetermined value;

- 12 (d) whether or not the speed indicated by one or both of the Doppler signals is below a fourth predetermined value; and (e) whether or not the amplitude of one or both of the Doppler signals is above a fifth predetermined value.

5 4. A system as claimed in any preceding claim, wherein the processing means is also operable to process the phase difference to provide an indication of the direction of travel of the vehicle. S. A system as claimed in claim 4, wherein the processing means is operable such that the value of the output signal is dependent upon the indicated direction.

10 6. A system as claimed in claim S. wherein the processing means is operable such that a change of value of the output signal is dependent upon the indicated direction and upon one or both of the following: (a) whether or not the indicated distance is above a sixth predetermined value; and (b) whether or not the indicated distance is below a seventh predetermined value.

IS 7. A system as claimed in any preceding claim, further comprising means for storing the indicated distance, speed, direction and/or a value determined therefrom, and wherein the processing means is operable such that the value of the output signal and/or a change of value of the output signal is dependent upon the current distance, speed and/or direction and a previously stored distance, speed, direction and/or value.

20 8. A vehicle detector system, substantially as described with reference to the drawings.

9. A method of detecting a vehicle, comprising the steps of: transmitting microwave radiation towards the vehicle; receiving the microwave radiation reflected by the vehicle; and

- 13 producing a Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector; characterized by the steps of: modulating the transmission frequency of the microwave radiation between a first frequency and s a second frequency; processing the Doppler signals for the first and second frequencies to determine a phase difference therebetween indicative of the distance between the vehicle and the detector; and providing an output signal dependent upon the determined phase difference.

10. A method of detecting a vehicle, comprising the steps of: 10 transmitting microwave radiation of a first frequency towards the vehicle; receiving the microwave radiation reflected by the vehicle; and producing a first Doppler signal, from the transmitted and received radiation, having a frequency indicative of the speed of the vehicle towards or away from the detector; characterized by the steps: 15 transmitting microwave radiation of a second, different frequency towards the vehicle, receiving the microwave radiation reflected by the vehicle; producing a second Doppler signal from the transmitted radiation of the second frequency and the corresponding received radiation; processing the first and second Doppler signals to determine a phase difference therebetween 20 indicative of the distance between the vehicle and the detectors; and providing an output signal dependent upon the determined phase difference.

11. A method of detecting a vehicle, substantially as described with reference to the drawings.