GB2421384A

GB2421384A - Bistatic radar having a variable carrier difference frequency

Info

Publication number: GB2421384A
Application number: GB0525289A
Authority: GB
Inventors: Roland Klinnert; Thomas Walter; Tore Toennesen
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2004-12-14
Filing date: 2005-12-09
Publication date: 2006-06-21
Anticipated expiration: 2025-12-09
Also published as: FR2879303B1; GB0525289D0; DE102004060087A1; FR2879303A1; GB2421384B

Abstract

A bistatic radar is disclosed whereby regulation and control of the image frequency is carried out by adjustment of the carrier frequency via at least one of the carrier frequency oscillators 21,22 of the transmitter and receiver sensors 11,12. The oscillators of the transmitters and receivers therefore do not need to be phase synchronised and may have very different frequencies. Direct echoes and cross echoes may both be evaluated, and a high angle resolution and classification of object contours is possible.

Description

Prior Art Device for radar applications, in particular bistatic radar applications A device for bistatic radar applications according to the precharacterising clause of Claim 1 is known from DE 102 139 87 Al. There, by utilising an image effect in the frequency range, cross-echo detection and distance measurement are possible in a bistatic pulse radar system, without carrier frequency oscillators of each transmitter/ receiver pair having to be phase-synchronised by elaborate measures, as is the case with other known arrangements. Advantages of the Invention The measures according to Claim 1, i.e. means for controlling or regulating the at least one image frequency by changing the carrier frequency of at least one of the carrier frequency oscillators of transmission and reception sensors assigned to each other, provide a very low-outlay embodiment of a bistatic radar system. In order to set an intended image frequency between its minimum 0 Hz (modulo pulse repetition frequency PRF) and its maximum - half the pulse repetition rate (modulo pulse repetition rate), for example with a pulse repetition rate PRF kept constant, it is sufficient to vary (pull) one of the two oscillating frequencies at most by the magnitude of the PRF, which can be done with very little outlay. In contrast to conventional direct-echo evaluations in monostatic operation with a large aperture angle, a high angle resolution is achieved by the invention. The cross-echo evaluation increases the spatial sampling of the motor vehicle environment, makes it possible to classify object contours and increases the redundancy of the sensor information. If synchronous frequency modulation in the transmission and reception sensors is provided according to the measures of Claim 2, and/or if the measures of the further claims are implemented, the following advantages (objects) are achieved: Encoding of the transmission signal so that, on the one hand, the interference sensitivity with respect to external signals in the reception frequency range can be reduced and, on the other hand, unique transmitter identification can be achieved in the sensor array, - Increasing the distance resolution, or transmission signal bandwidth, without any otherwise necessary shortening of the pulse duration, - Complying with spectral power distribution requirements, for example for frequency authorisation, - Suppressing possible misallocations of reception pulses to transmission pulses in the event of overshooting, - Allowing the transmission signal to be modulated with data for communication purposes. Time-synchronous pulse modulation of transmitters and receivers may advantageously be used. An image signal occurring in the mixed signal of the receiver by an aliasing effect is likewise utilised for the detection and distance measurement. The frequency fa of the resulting image signal (the image frequency) depends both on the pulse repetition frequency PRF and on the carrier difference frequency df of a transmitter and a receiver. The following applies with an integer factor n: df=n PRF fa The image frequency is not adjusted by varying the pulse repetition rate PRF as in DE 102 139 87 Al, but by varying one or both carrier frequencies of the transmitter/receiver sensor pair, and therefore the carrier difference frequency df. Furthermore, the oscillators of transmitters and receivers need not then be phase-synchronised and may even have very different frequencies (i.e. quite feasibly a difference greater than the bandwidth of the I/Q (in-phase/ quadrature phase) signal processing). The combination of the following measures is particularly advantageous: Synchronous pulse driving of transmitters and receivers in an array, i.e. at least pairwise, Utilising an image signal in I or Q signals, or signals derived from them, for each transmitter/ receiver pair, Controlling/regulating the mid-frequency of the image signal fa by changing one carrier frequency or both carrier frequencies of the transmitter/receiver sensor pair, Optionally, synchronous carrier frequency modulation for transmitters and receivers. This combination has the following advantages: A continuous LF signal for the cross-echo detection and distance measurement by power measurement or the like, for example amplitude, quasi-peak, etc., of an image signal, Economical sampling of the LF power signal with small sampling rates, substantially determined by the scan rate and the intended resolution, is possible for the digital post-processing, Cross echoes can be evaluated in parallel with direct echoes, since the image signal is placed in a separate frequency range in the I signal and/or Q signal (frequency multiplex operation), Elaborate phase synchronisation of the carriers is unnecessary, although a minimum short-term frequency stability (during the pulse integration time) is a prerequisite, No great requirements on the bandwidths of mixers and LF amplifiers (at least above the selectable image frequency),Active suppression of otherwise sporadically occurring direct-echo crosstalk, which occurs in sensor arrays with unsynchronised operation having a fixed PRF when the image frequency randomly enters the frequency range of the direct echo (0 ... Doppler frequency), for example because of temperature drift of the carrier frequencies, Monitoring of the carrier frequencies (diagnostic function: detecting abnormal drifts or failure) by observing the behaviour of the image frequency regulation, especially when direct crosstalk from a transmitter to a receiver constantly occurs, Standard pulse compression methods can additionally be used, Very economical hardware embodiments are possible, for example alternative embodiments with a constant PRF and minor detuning of an oscillator by at most the PRF, Achieving some or all of the aforementioned objects by synchronous carrier frequency modulation. Drawings Exemplary embodiments of the invention will be explained in more detail with reference to the drawings, in which: Figure 1 shows a block diagram of the radar system according to the invention having a pulse radar transmitter and receiver pair driven time-synchronously, Figure 2 shows a power density spectrum of the mixed, unpulsed carriers of neighbouring sensors, Figure 3 shows a power density spectrum of the mixed, pulsed carriers of neighbouring sensors with a negligible pulse duration, Figure 4 shows a power density spectrum of the mixed, pulsed carriers of neighbouring sensors with a nonnegligible pulse duration, Figure 5 shows the power density spectrum of a real I(Q) signal in cross-echo reception, Figure 6 shows the image frequency regulation by carrier frequency detuning of at least one of the carrier frequency oscillators,Figure 7 shows the dependency of the detection signal y on the carrier frequency df with a delay T adapted to the crosstalk and illustration of maximum value regulation. Description of the Exemplary Embodiments Figure 1 shows details of two simple conventional pulse radar sensors 11, 12, the upper sensor 11 of which operators as a transmitter (Tx) and the lower sensor 12 as a receiver (Rx). Using their respective carrier frequency oscillators 21, 22, the sensors generate carrier signals X1 and X2 with individual carrier frequencies fLO1 and fLO2. These carrier signals are preferably modulated by the same pulse source 3 with the 0-1 pulse sequence p, i.e. pulses are impressed on the output signals of the carrier frequency oscillators by means of the modulators 51, 52. Naturally, each of the sensors 11, 12 may also be assigned a separate pulse signal source 3. Then, however, it is necessary to synchronise these pulse signal sources with one another.This may be done either by a connecting line or, alternatively, by recovering the transmission PRF from the reception signal and compensating for the phase shift. It is possible to find the phase shift by utilising redundancy, since normally two cross-echo measurements of an object and possibly also direct echo measurements are always available owing to the reversibility of the signal propagation paths (for example, let: 6 = phase lead of pulse signal source 1 over pulse signal source 2; tofK: cross-echo time-of-flight from Sll via object K to S12, or return direction; tofKl2: cross-echo measurement from Sll to S12 relative to pulse signal source 2; tofK21: crossecho measurement from S12 to Sll, pulse signal source 1; the following than apply: tofK=tofK-A and tofK=tofK21+ =(tofK12-tofK21)/2 tofK=(tofK12+tofK21)/2.Following reflection by an object, the signal radiated from the transmitter is picked up by the receiver after the time-offlight (tof). Using a delay circuit/delay line 6, the receiver delays the pulse sequence p by the delay time . If the set delay corresponds to the time-of-flight tof, then the signal m=p x1 x2 is obtained at the output of the mixer 7, which can be supplied with a transmission signal on the one hand and on the other hand with a reception signal, owing to the time-synchronous pulse modulation if =tof. This (ideal) mixed signal is lowpass filtered, for example, in an evaluation circuit 4 by the subsequent real amplifier 8 and by the real mixer 7 itself. The I signal is then available for the further LF signal processing at the output of the amplifier or impedance converter and, in the case of a second mixer which operates with the 90[deg] phaseshifted carrier, a Q signal is also available. The spectrum which is obtained for the I(Q) signal will be described below. 1. Mixing (multiplying) the unpulsed carriers, see X1 and X2 in Figure 1, of two neighbouring sensors with the central difference frequency df=fLO1-fLO2 would lead to a spectrum with band-limited components around df=fLO1-fLO2 and fLO1+fLO2 (Figure 2). The sum component can subsequently be neglected owing to the lowpass behaviour of the mixer 7 and the amplifier 8. The width of the remaining spectral component around df is determined by the short-term frequency stability of the carrier frequency oscillators during the pulse integration time. What is essential is that such a band-limited spectrum occurs even with frequency or phase unsynchronised oscillators. 2. Pulse modulation of the product X1 X2, which finally leads to the ideal mixed signal m, corresponds to sampling, the sampling frequency being given by the pulse repetition rate PRF set for the pulse generator. In the spectrum, however, ideal sampling (5 sampling) leads to periodic continuation of the spectrum of the sampled signal. The spectrum distributed around df would then respectively be imaged twice in the frequency interval [z PRF, (z+l)-PRF], where z is an integer (Figure 3). It should be noted that a band-limited signal always occurs in the frequency range [0, PRF/2], i.e. even with the difference frequency df which is much more than the pulse repetition rate PRF (i.e. with undersampling).The mid-frequency fa of the "image signal" in [0, PRF/2] and the difference frequency df are therefore interdependent according to df=n-PRF±fa, where n N0 (integer divisor between df and PRF). Ideal sampling is approximately obtained when the pulse duration is very short compared to the smallest period of the sampled signal, i.e. Tp l/df. If this is not the case, then the amplitudes of the repeated spectral components fall off with an envelope which is defined by the pulse shape and the non-negligible pulse duration (Figure 4). With a square-wave pulse of length Tp, for example, the envelope is a sinx/x profile with the first zero at 1/Tp. 3. The spectrum of the real IQ signal falls off significantly above the limit frequencies of the mixer and the amplifier/impedance converter, which as a rule are much less than the difference frequency df, and generally approximates a profile as shown in Figure 5. This limited signal component with its essential frequency components below PRF/2, which occurs because of a cross echo in the I(Q) signal, will be referred to below as cross-echo Doppler. A direct echo of an extremely fast-moving object with an associated Doppler frequency around fD=df would lead to a similar signal. 4. It should be noted that the image frequency fa of the cross-echo Doppler having the predeterminable pulse repetition frequency PRF (with a slowly varying time frequency df) according to (1) can always be set to the intended value. In particular, by controlled adjustment of PRF, it is possible to ensure on the one hand that the image frequency fa always lies below the limit frequency of the mixer and the amplifier. With parallel reception of direct echoes of the sensor, on the other hand, it will be ensured that the image frequency fa always lies above the maximum Doppler frequency fDmax. This can be regarded as "frequency multiplex" utilisation of the I(Q) signal, in which direct echoes and cross echoes lie in mutually separated frequency ranges. An essential prerequisite for unique separation, of course, is that the local oscillators are so frequency-stable in the short term that the bandwidth of X1 X2 is always less than PRF/2-fDmax5. The divisor n and the image frequency fa characterise the instantaneous difference frequency of a sensor pair, for which there is cross-echo reception. In sensor arrays having more the 2 sensors, in which the difference frequencies of all relevant sensor pairs differ significantly from one another, transmitter identification is therefore possible even with parallel reception of a plurality of cross echoes. The device according to the invention is characterised, in particular, by the following features: synchronous pulse driving (connecting line or by recovering the transmitter PRF from the reception signal and compensating for the phase shift) utilising the cross-echo Doppler in I, Q signals, or signals derived therefrom below PRF/2 control/regulation of the image frequency (midfrequency fa of the cross-echo Doppler) by changing (detuning) the carrier frequency of at least one of the carrier frequency oscillators 21 and 22, in which case the difference between the carrier frequencies of a transmitter and receiver can vary at most by the pulse repetition rate PRF and can be adjusted so finely that the image frequency can be kept inside the bandwidth of a signal detector. As shown by Figure 1, one carrier frequency oscillator or both carrier frequency oscillators has or have a control input for frequency detuning (inputs denoted by fLO1and fL02). The essential disturbance variable of the image frequency control loop is usually the frequency drift of the carrier frequency oscillators with the temperature. In the case of direct crosstalk from a transmitter to a receiver, for example by reflection/conduction of the transmission signal on the bumper or other generated measures, there is constantly an image signal for at least one set delay T irrespective of the existence and position of objects in the detection field being monitored. Particularly simple, robust and constantly engaged image frequency regulation is therefore possible. Uninterrupted monitoring of the carrier frequencies can furthermore be carried out. Such regulation will be described in more detail below. It will be assumed that a constant pulse repetition rate PRF is used, and that the power, amplitude or the like of the image signal y is detected by analogue or digital means in a particular fixed frequency band. According to Figure 6, frequency-selective evaluation of the I and/or Q signal is carried out in the unit 9 in order to obtain the image signal y. The inputs fLO1 and/or fLO2 of the carrier frequency oscillator are controlled via the control unit 10. A prerequisite in order to be able to measure direct and cross echoes in parallel is that the detection frequency band does not include 0 Hz and lies below PRF/2. With a delay T set suitably for the time-of-flight of the crosstalk, the detection signal y then in principle exhibits the dependency on the carrier difference frequency df as indicated in Figure 7.This profile is periodic with the PRF, only one period being represented here. Under the aforementioned conditions, there are always two local maxima within such a period. The object of the image frequency regulation is then to detune one or both carrier frequency oscillators so that the detection signal y assumes a maximum. A general method of maximum value regulation consists in varying the carrier difference frequency constantly so that the first derivative of the function y (dy) is estimated, for example by time differencing. From the sign of the first derivative, it is then possible to determine the average change of the carrier difference frequency according to the following rules (see also the block arrows in Figure 7). dy/d(df) >0: df T dy/d(df)<0: df Variants with synchronous frequency modulation in transmitters and receivers Synchronous changes (common-mode changes) of the instantaneous absolute carrier frequencies fLO1, fLO2 are irrelevant for the image frequency fa, since it depends only on the difference df of the carrier frequencies. Any synchronous frequency modulation of a transmitter and a receiver therefore has no effect on the described image frequency regulation and, in this sense, is orthogonal to the cross-echo evaluation. Frequency modulation allows a flexible design of the transmission spectrum. Frequency modulations (FM) with different time constants will be discussed in more detail below with their primary objectives. 1. Intra-pulse frequency modulation Intra-pulse FM, i.e. FM carried out over the duration of a pulse, is known and widespread in radar technology (linear FM or chirp, with one or more ramps, non-linear FM, etc.) and its primary objective is usually some degree of decoupling of distance resolution and pulse radar in the design, but also partly signal encoding or the controlled shaping of a transmission spectrum. Intra-pulse FM belongs to the pulse compression method, which is combined here with the cross-echo measurement with the image frequency regulation. The literature mostly describes a matched filter for the frequency demodulation in the receiver.The impulse response of the matched filter, then referred to as a pulse compression filter (PCF), is moreover known to be a copy of the time-inverted and perturbed frequency-modulated transmission signal, which means that the output signal of the PCF corresponds to the autocorrelation function of the transmission signal. The aforementioned implementation of frequency demodulation by mixing the reception signal with a copy of the transmission signal, generated in the receiver by a carrier frequency oscillator operated frequency-synchronously with the transmitter, and subsequent lowpass filtering, likewise corresponds to the formation of an autocorrelation function. This form of frequency demodulation may therefore be regarded as a practical implementation of a matched filter, or PCF. 2. Inter-pulse FM Continuous frequency changes with time constants in the range of the pulse intervals or abrupt frequency changes (frequency hopping) between the pulses, performed synchronously in a transmitter oscillator and a receiver oscillator, allow unique allocation of reception pulses to transmission pulses over a plurality of pulses. It is therefore also possible to achieve signal encoding or to deliberately influence the transmission spectrum. 3. Slow FM This is intended to mean continuous frequency changes with time constants in the range of the pulse integration time or abrupt frequency changes (frequency hopping) with time frames of constant frequencies, which comprise a plurality of pulses, likewise performed synchronously in a transmitter oscillator and a receiver oscillator. The object of slow FM is primarily the reduction of interference sensitivity, or signal encoding, for example by frequency avoidance methods in case of extraneous signal detection in the instantaneous reception frequency band of a receiver. To a limited extent, however, the transmission spectrum may again be influenced in the controlled way. Another potential application of slow FM is to modulate the transmission signal with data (FSK).

Claims

Patent Claims

1. Device for radar applications, in particular bistatic radar applications, consisting of at least two spatially separated radar sensors (11, 12) for transmission and/or reception operation, each radar sensor (11, 12) being assigned an independent, in particular free-running carrier frequency oscillator (21, 22) and a modulator (51, 52) for applying pulses from a pulse signal source (3) to the output signal delivered by the respective carrier frequency oscillator (21, 22), a time-synchronous control of the pulses being provided for at least two radar transmission/reception sensors (11, 12) assigned to each other, and an evaluation device (4) being provided for at least one image frequency, i.e. in particular a cross-echo Doppler signal using a mixing device (7) for transmission and reception signals, characterised in that means are provided for controlling or regulating the at least one image frequency by changing the carrier frequency of at least one of the carrier frequency oscillators (21, 22) of transmission and reception sensors (11, 12) assigned to each other.

2. Device according to Claim 1, characterised in that synchronous, in particular intra-pulse frequency modulation, synchronous inter-pulse frequency modulation or synchronous slow frequency modulation is provided in the transmission and reception sensors (11, 12).

3. Device according to one of Claims 1 and 2, characterised in that the transmission and reception sensors (11, 12) are configured so that there is always direct crosstalk from the transmitter to the receiver of a sensor, irrespective of the existence and position of objects in the detection field to be evaluated.

4. Device according to Claim 3, characterised in that an image signal or cross-echo Doppler signal constantly present owing to the direct crosstalk is used to control or regulate the image frequency.

5. Device according to Claim 3 or 4, characterised in that an image signal constantly present owing to the direct crosstalk is used to monitor the carrier frequency regulation in transmitters and receivers as a diagnostic function.

6. Device according to one of Claims 1 to 5, characterised in that a constant pulse repetition rate is provided for the transmitted radar pulses.

7. Device according to one of Claims 1 to 6, characterised in that a predeterminable phase shift can be set in the pulse repetition rate of the transmitted radar pulses for transmission and reception sensors (11, 12) respectively assigned to each other.

8. Device according to one of Claims 1 to 7, characterised in that the image frequency regulation is carried out as maximum value regulation of the output signal of an image signal detector.

9. Device according to one of Claims 1 to 8, characterised in that image frequency regulation can be carried out with the aid of a power and/or frequency estimate of the cross-echo Doppler.

10. Device according to one of Claims 1 to 9, characterised in that besides the continuous image frequency regulation, a search or capture mode is provided for initial or repeated finding of the image frequency.

11. Device according to one of Claims 1 to 10, characterised in that the image frequency regulation is designed so that simultaneous evaluation of direct and cross echoes is possible.

12. Device according to one of Claims 1 to 11, characterised in that means are provided for carrying out the image frequency regulation so that crosstalk of cross echoes is suppressed in the (Doppler) frequency range of direct echoes.

13. Device according to one of Claims 1 to 12, characterised in that the cross-echo Doppler is intended for monitoring the carrier frequencies of the carrier frequency oscillators (21, 22) as a diagnostic function.

14. Device according to one of Claims 1 to 13, characterised in that cross-echo transmitter identification is provided with the aid of estimated carrier frequency differences, which are based in particular on estimates of the current image frequency, estimates of the integer part of the ratio of the carrier frequency difference and the pulse repetition rate, and knowledge of the current pulse repetition rate.

15. Device substantially as hereinbefore described with reference to the accompanying drawings.