GB2210227A

GB2210227A - Radar systems

Info

Publication number: GB2210227A
Application number: GB8602526A
Authority: GB
Inventors: Peter J Sinnock
Original assignee: Racal Radar & Displays Ltd
Current assignee: Racal Radar & Displays Ltd
Priority date: 1986-02-01
Filing date: 1986-02-01
Publication date: 1989-06-01
Also published as: GB8602526D0; GB2210227B

Abstract

A CW radar system is capable of operating in ranging and non-ranging modes. In non-ranging mode an unmodulated signal identifies a target, whereupon the ranging mode starts and the transmitted radiation is frequency modulated so that the frequency of the output varies linearly in time. This causes a frequency shift to be detected between the received radiation reflected from a target and the radiation being transmitted at the time of reception. A proportion of this frequency shift will be due to the Doppler effect and the remainder will be due to the modulation. That component of the frequency shift due to the modulation is proportional to the time taken for the return journey from the radar to the target and can be used to derive the range of the target. The frequency of the signal transmitted by a slotted array antenna 2 is modulated by range modulator 10 which controls an oscillator 4 having a PROM store of frequency control data. Received signals from slotted array antenna 20A-D are mixed 24 and filtered 30 and then processed at 32 to give frequency difference data and hence target identification data. <IMAGE>

Description

RADAR SYSTEMS The present invention relates to radar systems and, more particularly, to continuous wave (CW) radar systems.

CW radar systems are used in various applications to identify moving targets which reflect the radiated energy back to the radar. Unlike pulsed radar systems, which can readily provide an indication of range from the time delay between transmission and reception of the reflected pulse, CW radars cannot readily indicate the range of the target.

The present invention is concerned with the technical problem of providing an indication of the range of a detected target.

The present invention accordingly provides a continuous wave radar system comprising means for transmitting a continuous wave signal into a target area, means for selectively frequency modulating the transmitted radiation, means for receiving radiation reflected from a target and processing the received radiation to produce an output signal having a frequency corresponding to the frequency shift between the frequency of the received radiation and the frequency of the radiation being transmitted at the time of reception, and means for identifying from the received signal the presence of a target and subsequently modulating the signal transmitted towards said target.

The system may be used in conjunction with a computer for analysing the frequency or frequencies of the output signal when the transmitted radiation is frequency modulated in order to derive the range of the target.

Such a radar system is preferably provided with an antenna system which, once a target has been identified, can take a "second-look" at the target within a short time. Such a system can thus be operated first in a non-ranging mode using an unmodulated OW transmission to identify targets.

In the non-ranging mode any frequency shift between the received and transmitted frequencies is due to the Doppler shift caused by the velocity component of the target in the direction between it and the radar. The second-look is taken in a ranging mode using the modulated ChT transmission. The frequency of the output signal during this second-look then represents a combination of the Doppler frequency shift and the frequency shift due to the frequency modulation. For a target where the Doppler frequency shift will not have changed significantly between looks the frequency of the output signals in the different modes can be processed to produce the time taken for the radiation to reach the target and return, from which the range can be computed.

In a preferred embodiment the frequency modulating means comprises means for frequency modulating the transmitted signal to produce one cf a plurality of predetermined frequency variations in time in dependence on the signal strength of the unmodulated radiation reflected from the target.

For a relatively low signal strength a frequency variation in time is selected where the frequency increases in a linear sweep during the dwell time of the antenna system on the target. For a target which reflects only a low signal strength the range is likely to be relatively great and for this reason the frequency shift due to the Doppler effect will only vary by a small amount, if at all, during the two distinct 'looks' at the target. Thus the measured Doppler shift in the non-ranging mode can be used in deriving the range during the ranging mode.

For a relatively high signal strength of the reflected signal a frequency variation in time is selected where the frequency increases in a linear sweep and decreases in a linear sweep of the same slope during the dwell time of the antenna system on the target. With such a frequency variation the output signal has peaks at two distinct frequencies during the ranging mode. The Doppler frequency shift is represented by the average of the two measured frequencies so that when using this type of frequency modulation the Doppler shift is remeasured during the ranging mode so that an accurate estimate of the range is still possible even where the velocity of the target in the direction between it and the radar has varied significantly between 'looks'. The use of this type of modulation results in a lower signal strength of the output signal during the ranging mode.However for close targets such a loss can generally be accepted and the varation in Doppler shift is usually only significant for targets which are relatively close to the radar.

Preferably the transmitting means comprises an oscillator connected to a power supply and a transmitter antenna, the oscillator having associated therewith a memory containing data relating to the output frequency/supply voltage(s) characteristic of the oscillator, the modulating means comprising means for reading said data and varying the output(s) of the power supply in order to produce the required frequency variation in time.

With such a system it is not necessary to recalibrate the system when an oscillator is changed.

In an alternative embodiment the radar system comprises calibrating means comprising means for passing directly to the processing means a proportion of the modulated radiation to be transmitted, means for passing a further said proportion to the processing means via a predetermined time delay representative of a fixed range, and means for controlling the modulating means so that the output signal has a predetermined frequency.

Such a calibrating means can readily be incorporated into a radar system which already has built-in test equipment, which is capable of injecting a simulated received signal locally generated into the processing means.

Two embodiments of a radar system incorporating a ranging system in accordance with the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings: Figure 1 is a block diagram of a first radar system; Figure 2 is a diagram illustrating the alternative frequency variations in time of the modulated transmitted and received radiation during ranging mode operation of the system of Figure 1; and Figure 3 is a block diagram of calibrating means for use in a second embodiment.

A radar system as illustrated diagrammatically in Figure 1 comprises a transmitter antenna 2 which is coupled to an extended interaction oscillator (EIO)4. Tne antenna is, for example a slotted array antenna having an input port at one end and a dump load 3 connected to the other end via a cross-coupler 22. The EIO 4 is a special type of klystron suitable for continuous wave operation which produces radiation of a frequency dependent on the input voltages supplied. A programmable read only memory (PROM.) 6 is associated with the oscillator 4. In this PROM 6 there is stored data measured for that particular oscillator which relates to the volatage inputs/frequency output characteristic of that oscillator. The oscillator 4 is connected to a high voltage power supply unit (PSU) 8 which provides the required voltage inputs to the oscillator. The voltage outputs of the PSU are selectively variable by a modulator 10. A radar central processing unit (RCPU) 12 is coupled to the PROM 6, PSU 8 and modulator 10 via a system bus 13. The RCPU12 controls the operation of the PSU and modulator in accordance with the data read from the PROM and a stored control programme in a manner to be described more fully later.

Radiation reflected from a target is received by one of four receiver antennas 20A - 20D. The receiver antennas are four slotted array antennas arranged one above the other so that each defines a respective one of four continguous receive beams, each having a smaller angular height, say 30, within the angular height of the transmitted beam. Normally such a slotted array antenna for use as a receiver antenna would have a matching termination at the cpposite end of the slot to the output port. In the present case this termination is replaced by an attenuator so that test signals can be injected from a built-in test equipment microwave integrated circuit (BITE MIC) 16. The transmitter antenna and the receiver antennas are housed in a radome (not shown) which scans a target area in a repetitive manner.The antennas and associating scanning drive will be referred to in the following as the antenna system.

The received signals are each fed via a respective chain of a mixer 24, amplifier 28 and filter 30 to a signal processor 31. Only one such chain is shown for reasons of clarity but it will be appreciated that the signals from each antenna are processed similarly. The received signal is mixed in mixer 24 with a signal fed from the EIO 4 via an up-converter 26 which also receives an input from an IF oscillator 27. The signal processor 32 processes a portion of each of the received signals from the antennas.

These portions overlap in time so that the processor always analyses any part of the received signal from at least two antennas. Such a method results in more rapid processing of the received signal. The signal processor 32 also receives input data over a system bus 13 from the RCPU12.

The output of the signal processor is an output signal having a frequency representing the difference frequency between any received signal and the frequency being transmitted at the time of reception. The output signal is fed to the RCPU12. For some applications the output of the system is the output from the processor 32. In other applications this output is further procesed by the RCPU12 to enable the RCPU to produce an output to a display (not shown) giving data on targets identified.

The BITE MIC 16 receives an input from the transmitter antenna 2 via the cross-coupler 22 and a further input from an oscillator 18, the frequency of which is controlled by the RCPU12 via the system bus. This arrangement allows the BITE MIC 16 to generate an artificial "received" signal which can be injected into the receiver antennas 20A - 20D to simulate a received signal with a Doppler offset determined by the frequency of the oscillator 18. Such signals can be used to test and calibrate the system.

The radar system 1 is capable of operating in two distinct modes under the control of the RCPU 12: a non-ranging mode in which an unmodulated continuous wave is transmitted by the transmitter 2; and a ranging mode in which the continuous wave is frequency modulated by the modulator 10.

In the non-ranging mode, any output signal produced by the signal processor 32 represents the presence of a target and its frequency represents the velocity component of the target in the direction between it and the radar due to the Doppler effect. In the ranging mode, the power supply unit 8 of the transmitter is controlled by the modulator 10 so as to frequency-modulate the continuous wave transmitted by the antenna 2 such that the frequency of the continuous wave varies in time as shown in the continuous line plot in Figure 2A or Figure 2B. In this embodiment, the modulator 10 is a digital-to-analogue converter which receives a series of digital inputs from the RCPU 12 and feeds its analogue output to a control input of the power supply unit 8 in order to vary the power supply to the oscillator 4 and therefore its frequency.The digital inputs supplied by the RCPU 12 are selected in accordance with the data read from the PROM 6 in order to reproduce the required frequency variation in time. In the ranging mode, the output from the signal processor 32 is a signal with a frequency which is the difference between the frequency of the received signal and the frequency of the signal being transmitted at the time of reception. The frequency of the output signal therefore represents a combination of the frequency shift due to the Doppler effect and the frequency shift due to the frequency modulation which is dependent on the range of the target.

The modulator 10 is adapted to impose two distinct types of frequency modulation as illustrated in Figures 2A and 2B of the drawings. The selection of the form of modulation to be used is made by the RCPU in dependence of the signal strength of the output signal during the nonranging mode. If the signal strength is below a predetermined threshold the modulation illustrated in Fig.

2A is used and if the signal strength is above that threshold the modulation of Figure 2B is used. Each cycle of both forms of frequency variation is selected to correspond to or be shorter than the dwell time of the antenna system on the target which is typically 4 ms.

When the frequency variation illustrated in Figure 2A is used the frequency of the transmitted signal decreases in a linear sweep with a relatively short fly back period between sweeps. Accordingly the signal reflected from the target is shifted by a constant amount which is proportional to the time for the return trip to the target and therefore the range. When the frequency variation illustrated in Figure 2B is used the frequency of the transmitted signal increases and then decreases in a linear sweep during its dwell on the target. This results in the output signal being split into two separate signals resulting in an approximately 3dB loss in the signal strength of the output signals in the ranging mode relative to the strength of the undivided signal in the non-ranging mode. Accordingly this form of modulation is not appropriate to targets which are sufficiently distant to produce low strength reflections. The frequencies of the two output signals represent the Doppler shift frequency plus the positive frequency shift during the downward sweep of the frequency variation and the Doppler shift frequency plus the negative frequency shift during the upward sweep of the frequency variation. Since the slope of the upward and downward sweeps is the same the Doppler shift frequency is the average of the frequencies of the two output signals in the ranging mode.

Accordingly use of the modulation of Figure 2B allows both velocity and range of the target to be measured in the ranging mode.

The described circuit operates as will now be described: In the non-ranging mode, the transmitter antenna 2 transmits OW radiation at constant frequency. The RCPU holds the modulator 10 off. During operation, the antenna system will normally be scanning a target area. If a target is detected then the receiver antennas 20A - D will receive the reflected radiation which is passed to the signal processor 32 of the radar. The frequency shift of the received radiation relative to the frequency of the transmitted radiation is due to the Doppler effect and depends on the velocity component of the target along a line between the radar and the target. The signal processor outputs â signal having a frequency representing the Doppler frequency shift.This signal can be fed to a computer (not shown) which produces an indication of the presence of a target and its velocity in the normal manner. The Doppler frequency shift is also stored in the computer for possible use later in the ranging mode.

The computer may be part of the radar system or a separate system. For example if the radar system is part of a defence system the computer can also control the launch of a missile in response to the data concerning the target produced by the radar system. However if the radar system is being used for surveillance the RCPU may perform the function of the computer and output target data to a display and tracking sensor.

Once a target has been detected, the RCPU 12 of the radar system controls the antenna system to take a second look at the target within a short interval of the target's detection. For the second-look, the RCPU 12 switches on the modulator 10 so that a frequency-modulated CW signal is transmitted by the antenna 2. The transmitted signal has either of the frequency variations illustrated in Figure 2 in dependence on the signal strength of the output signal in the previous cycle. The signal reflected from the target and received by the receiver antenna 12 will have the frequency variation in time indicated by the dotted line plots of Figure 2. As will be appreciated the frequency of the output signal will now include an additional t f due to frequency modulation as well as the previous Doppler shift frequency.The magnitude of theLif can readily be obtained by use of the measurement of the Doppler shift in the previous cycle for the modulation of Figure 2A or as half the frequency difference of the two output signals for the modulation of Figure 2B. Since of is directly proportional to the time taken for the signal to make the return journey to the target with the constant of proportionality depending on the slope of the frequency sweep, the range can readily be computed.

A suitable slope for the linear parts of the frequency variation shown in Fig. 2 is one which provides a 10kHz frequency shift 6f for a range of 10 km. For a given slope the maximum frequency deviation of the frequency variation depends on the maximum range which is to be detected. Thus if the maximum frequency deviation is selected to be 187 kHz, a maximum range of 187 km can be detected. This gives a period T of the variation of 2.5 ms. For ranges between 187 km and 375 km there will be ambiguity with the waveform illustrated as the same frequency shift will be measured at two distinct ranges.

However this problem is generally avoided by selecting the maximum frequency deviation to represent the maximum detection range of the system.

In order to test the operation of the radar system provision may be made to inject signals generated by the radar system itself into the processing circuits. Such signals are derived from transmitter 2 via the crosscoupler 22 and MIC 16. The oscillator 18 is controlled by the RCPU and may produce suitably modulated signals to simultate various types of received signals. The MIC 16 and oscillator 18 under the control of the RCPU provide built-in test equipment for the system.

In the embodiment described the accurate reproduction of the frequency variation in time during the ranging mode is ensured by the data stored in the PROM 6 associated with the oscillator 4. If it is required to change the oscillator a new oscillator and PROM. unit is provided so that once the RCPU has read the new data from the PROM accurate operation can be resumed. In a second embodiment where the PROM is not provided, calibration is provided by a calibration circuit 40 which is connected between the cross-coupler 22 and an input 34 to the MIC 16 in the embodiment of Figure 1.

The calibration circuit 40 is brought into operation with the radar in the ranging mode. The calibration circuit artificially injects a delayed portion of the transmitted signal into the antennas 20A-2D via the MIC 16 to simulate a target at a fixed range. For example for the slope illustrated in Figure 2h a 66.667 ms delay representing a range of 10 km produces a frequency shift of 10 kHz. If the signal processor measures a frequency shift which varies from the expected value, the digital values output by the RCPU 12 to the modulator 10 are adjusted so as to bring the measured frequency shift, that is the frequency of the output signal, to the expected value for the given delay.

The calibration circuit 40 receives an input from the transmitter antenna 2 via the cross coupler 22 which feeds a -30dB proportion of the transmitter signal to one input of a further cross-coupler 42 while the remainder is transmitted from the antenna. The coupler 42 passes -10dB of the signal received from the coupler 22 to a mixer 44.

When the calibration circuit is not in use the signal from the cross coupler 42 is fed directly to a cross coupler 44 to input 34 to the MIC 16. The mixer 44 receives an input from an oscillator 48 which is preferably a Gunn or Impatt diode. The frequency of the oscillator 48 is set so that the output from the mixer 44 is at a suitable intermediate frequency (IF) for a delay line 50 which is connected to the output of the mixer 44 via an amplifier 52. The delay line 50 is preferably a surface acoustic wave (SAW) delay line and in this case it will have an intermediate frequency of 50 to 100 Mhz. The output of the delay line 50 is fed via a further amplifier 52 to a mixer 54 connected to the same oscillator 48 in order to mix the delayed signal back to its original frequency. The delayed signal is connected to cross coupler 46 which directs it to input 34 of the MIC 16.

In order to calibrate the system using the circuit 40, a proportion of the transmitter output is coupled by means of the couplers 22 and 42 to the delay line 50 which introduces a 66.667 ms delay. As explained, this time represents a range of 10 km. When this delayed signal is compared by the processing circuits 20A - 20D with the direct input from the transmitter a 10 kHz shift should be output, If the frequency shift varies from 10 kHz then a signal representing the deviation is fed back to the RCPU 12 in order that it can suitably adjust the operation of the modulator 10.

Since the radar may be capable of operating at different frequencies the frequency of the oscillator is controlled in order always to produce a value which when mixed with the radiated frequency produces an output at the IF of the delay line. To achieve this a proportion of the input to the delay line 50 is fed via a further amplifier 56 to a frequency discriminator 58. In dependence on the - deviation of the frequency of the input signal to the frequency discriminator 58 from the required IF of the delay line, a control signal is fed to the oscillator 48 in order to bring the frequency of that oscillator to a value such that when mixed with the transmitted signal the output is at the desired IF.

Suitable control software will be implemented by the RCPU12 in order to switch the system between the ranging and non-ranging modes, to convert the frequency of the output signals produced by the signal processor into the ranges of the detected targets taking due account of the velocity (unless a separate computer is provided for this purpose), and to control the operation of the-modulator in response to calibration errors detected during operation of the calibration circuit 40 (in the case of the second embodiment).

Claims

1. A continuous wave radar system comprising means for

transmitting a continuous wave signal into a target area, means for selectively frequency modulating the transmitted radiation, means for receiving radiation reflected from a target and processing the received radiation to produce an output signal having a frequency corresponding to the frequency shift between the frequency of the received radiation and the frequency of the radiation being transmitted at the time of reception, and means for identifying from the received signal the presence of a target and subsequently modulating the signal transmitted towards said target.

2. A radar system as claimed in claim 1, further comprising means for analysing the frequency or frequencies of the output signal when the transmitted radiation is frequency modulated in order to derive the range of the target.

3. A radar system according to claim 1 or 2, wherein the frequency modulating means comprises means for frequency modulating the transmitted signal to produce one of a plurality of predetermined frequency variations in time in dependence cn the signal strength of the unmodulated radiation reflected from the target.

4. A radar system as claimed in claim 3, wherein the frequency modulating means is adapted to produce a frequency variation in time in which the frequency increases in a linear sweep, and then drops substantially instantaneously or in which the frequency increases and subsequently decreases in linear sweeps of the same slope in dependence upon whether the signal strength of the refelected unmodulated radiation was below or above a predetermined threshhold respectively.

5. A radar system as claimed in any one of the preceding claims, wherein the transmitting means comprises comprises an oscillator connected to a power supply and a transmitter antenna, the oscillator having associated therewith a memory containing data relating to the output frequency/supply voltage(s) characteristic of the oscillator, the modulating means comprising means for reading said data and varying the output(s) of the power supply in order to produce the required frequency modulation.

6. A radar system as claimed in any one of claims 1 to 4, further comprising calibrating means for passing directly to the processing means a proportion of the modulated radiation to be transmitted, means for passing a further said proportion to the processing means via a predetermined time delay representative of a fixed range, and means for controlling the modulating means so that the output signal has a predetermined frequency.

7. A radar system substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A continuous wave radar system comprising means for transmitting a continuous wave signal into a target area, means for selectively switching between at least two modes, in a first mode the transmitted sigal being unmodulated and in the second and any further modes the transmitted signal being frequency modulated, a single receiving means for use in all modes for receiving radiation reflected from a target and processing the received radiation by mixing it with a signal derived from an oscillator controlling the transmitting means, to produce an output signal having a frequency corresponding to the frequency shift between the frequency of the received radiation and the frequency of the radiation being transmitted at the time of reception, and means for identifying from the received signal the presence of a target and subsequent to identification of a target, modulating the signal transmitted towards said target.

3. A radar system according to claim l or 2, wherein the frequency modulating means comprises means for frequency modulating the transmitted signal to produce one of a plurality of predetermined frequency variations in time in dependence on the signal strength of the unmodulated radiation reflected from the target.

5. A radar system as claimed in any one of the preceding claims, wherein the transmitting means comprises an oscillator connected to a power supply and a transmitter antenna, the oscillator having associated therewith a memory containing data relating to the output frequency/supply voltage(s) characteristic of the oscillator, the modulating means comprising means for reading said data and varying the output(s) of the power supply in order to produce the required frequency modulation.