GB2099257A - Commutated doppler radar system - Google Patents

Abstract

This invention is a commutated Doppler radar system for detecting or tracking distant targets. It is a three- dimensional system which provides information about target range R, elevation h and bearing theta , by transmitting two signals (Fc + Fo) and (Fc - Fo) from a ground transmitter Tx via transmitting elements 2, 3. The double sideband signal reflected by a distant target 5 is received by a "mobile" receiver Rx (commutation) and contains Doppler shifts due to target and receiver motions. It provides a difference frequency component for determining the elevation and velocity of the target and a sum frequency component for detecting and correcting errors. A feature of the system is its use of four well-known techniques for reducing errors in the system which are frequency diversity, space diversity, time diversity and a large time- bandwidth products. A combination of all four techniques is used to obtain a better overall system performance. <IMAGE>

Description

SPECIFICATION Commutated doppler radar system I, Dr.Frank Robert Connor, a British subject resident at Flat 3, 10 Avenue Road, London SE25 4EA, do hereby declare this invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is a design of a commutated Doppler radar system for detecting or tracking a moving target.

It uses a double sideband signal for determining the annular position of a target, the radial or horizontal velocity of the target, its range from the ground transmitter and its height above ground. Furthermore, a comparative frequency measurement is able to distinguish between an approaching or receding target and by means of a 'beacon signal' of known design frequency, it is able to detect and correct multipath errors in the system.

In ground-based surveillance or tracking radar systems many different types of antenna structures are used for detecting or tracking moving targets. Most of these radars operate at microwave frequencies and are either two-dimensional systems which provide information about target range and bearing or three-dimensional systems which are capable of providing additional information about target elevation or height.

This invention falls within the latter category as it provides information about target range, elevation and bearing. Such three-dimensional radars have been built in the past and many different types are employed to meet specific requirements. Typical of these are frequency scanned systems and multiple beam systems which use reflectors or planar arrays. However, for various important reasons, the multiple beam system is superior to the frequency scanned alternative.

One important feature of this invention relates to the relative simplicity and low cost with which this design is capable of providing a multiple beam system in space for detecting or tracking a number of targets simultaneously. It achieves this by employing the well-known Doppler effect which results in a Doppler frequency shift whenever an observer or receiver is moving relative to a distant target. If, at a gound station, energy is radiated from two stationary radiators at suitable frequencies, the target can reflect back some of this energy to a mobile receiver on the ground, and Doppler frequency shifts due to target motion and receiver motion are both observed in the received double sideband signal.

Another important feature of this invention is its use of four well-known techniques for reducing errors in the system. Firstly, frequency diversity, by the use of two different transmitting frequencies; secondly, space diversity, by employing a baseline of a given length for both transmitter and receiver; thirdly, time-diversity, by the reception of signals at certain time instances only and fourthly, multiscan averaging, by the use of a large time-bandwidth product. A combination of all these techniques can lead to a better overall system performance in the detection or tracking of distant targets.

In the elevation configuration shown in Figure 1, the radar system employs a number of baseline receiving elements 1 and two transmitting elements 2 and 3. The elements 2 and 3 are place one at either end of baseline 1 and are energised by a transmitter Tx periodically or continuously with energy at suitable frequencies (fc+ fo) and (f,- f,) respectively, where fc is a carrier frequency and f0 is an offset frequency. The vertical receiving elements 1 are connected sequentially to a receiver Rx (commutation), using an upward or downward movement (bidirectional scanning), to simulate a receiver moving with uniform velocity v. The polar pattern of each element is designed to cover the required coverage volume and is shown as a dotted line in Figure 1.

As a result of commutation at the receiver, multiple beams are 'effectively' produced in space at different frequencies f1, f2 etc. which are stacked at various angular directions Oi, 62 etc. above ground level 4 and are shown in full lines in Figure 1. A distant target 5 is in the far-field of the antenna elements and is assumed to be approaching the ground-based radar with horizontal velocity v. The configuration shown in Figure 1 can be used to determine the angle of elevation of the target, while its angular position in azimuth is determined by a similar arrangement with a horizontal baseline of antenna elements. Alternatively, the elevation configuration can be mechanically rotated in azimuth as in a conventional radar system to provide omnidirectional coverage.

Figure 2 is a block schematic arrangement of the groundstation receiver which is illustrated for a single target. For multiple targets, a single receiver may be used in a time-shared mode with range-gates to separate different target returns or a single receiver may be used with separate narrowband channels for each target without the need for range-gates. Alternatively, a separate receiver may be used for each target returns or a single receiver may be used with separate narrowband channels for each target without the need for range-gates. Alternatively, a separate receiver may be used for each target return.

In the single target case, the received signals 6 are amplified in a preamplifier 7 and are then downconverted in a mixer 8 by means of the carrier signal 9 at frequency fc. Further amplification is provided by an IF amplifier 11 and the output is fed into suitable sideband filters 12 and 13. The separate sideband signals after product multiplication 14 yield a difference frequency signal and a sum frequency signal. The difference frequency signal is filtered by a narrowband tracking filter 15 (phase-locked loop), to yield the Doppler information fD or fd. The sum frequency signal is also filtered by a narrowband tracking filter 16 (phase-locked loop), which is centred on the known design frequency 2f0.

Subsequently, the two signals 17 and 18 enter scan gates 19 and 20 and then individual digital counters 21 and 22. Their measured frequencies in binary code, are fed into a data processor 23 for storage, averaging and validation. The data processor is typically a dedicated microprocessor or digital computer which extracts true target information by rejecting unwanted targets due to ground clutter, chaff, rain etc. for a constant false alarm rate. The processed target information is presented on a visual display 24.

The operating principles of the system can be described with reference to Figure 2. For example, during an upscan, the signals received 6 after downconversion at the receiver 10 yield an upper sideband signal of frequency (fO+ fD+ 2fd) and a lower sideband signal of frequency (fO- fD- 2fd) where fD is the Doppler shift due to commutation at the receiver and 2fd is the two-way Doppler shift due to target motion.The Doppler shifts fD and fd are given by the expressions fD = L sin6=KsinO Th fd = VhCOS o/R = Va/ where K = UpTs is a design constant, L is the length of the baseline, Xis the Ts is the single scan time, va is the radial velocity of approach of the target and 0 is the angle of elevation of the target.

The doppler shift fD is directly related to the sine of the angular direction 6 and the Dopper shift fd is directly related to the radial velocity va of the target. After filter separation of the two sideband signals and product multiplication 14, we obtain a Doppler signal 17 at the difference frequency of (2fD+ 4fd) and a 'beacon' signal 18 at the sum frequency of 2four Because the Doppler frequency fD is reversed on scan reversal, by using bidirectional scanning, the two frequencies fD and fd can be determined separately.

Since the Doppler shifts fD and fd are additive for an upscan but subtrative for a downscan, when the target is approaching (or vice versa for a receding target), a preliminary comparison of the received signal over a bidirectional scan can distinguish between an approaching or receding target For an approaching target, the upscan frequency of (2fed+ 4fd) is greater than the downscan frequency of (2fD- 4fd), provided 2fD > 4fd by system design.

During normal surveillance or tracking operation, bidirection scanning is continuously employed and to separate the two Doppler frequencies fD and fd, the upscan frequency of (2fd+ 4fd) is measured separately from the downscan frequency of (2dD- 4fd). If these frequencies are summed, they yield a value of 4fD and when they are subtracted, they yield a value of 8fd. Thus, frequencies fD and fd can be individually determined and they are averaged over several scans to reduce random errors. The system is therefore capable of employing a large time-bandwidth product to provide maximum resolution.

The 'beacon' signal frequency of 2fo is twice the known offset frequency fO. In the presence of multipath effects on the transmitted signals, the beacon signal frequency will be altered from its true value 2fo by a multipath error. As the multipath error also occurs in the Doppler signal, the beacon signal 18 can be used to detect and correct the multipath error in any of the multiple beam frequencies f1, f2 etc.

Forthe target shown in Figure 1, the angle of elevation isO when it is at a slant range R from the ground transmitter. If the Doppler frequencies due to angular movement dO, at the beginning and at the end of a time interval T (typically 1 second) are fD and fD respectively, with fD > fD, the slant range R of the target and its height h above ground level 4 are given by the expressions R = vaT/loge(fD/fD) = kfdT/loge(fD/fD) h = RsinO=Rf/K where va, fo and K were defined earlier. The value of h is for a flat earth and may be modified to take account of the earth's curvature at long range.

Claims (12)

1. A radar system wherein means are provided for transmitting two signals from two stationary radiators positioned one at either end of a set of baseline receiving elements.

2. A radar system wherein alternative means are provided for transmitting two signals from a set of baseline elements, the elements being operated as transmittion or receiving elements.

3. A radar system wherein alternative means are provided for transmitting one signal from one half of a set of baseline elements and another signal from the other half of a set of baseline elements, the said elements being operated as transmitting or receiving elements.

4. A radar system as claimed in claim 1 wherein means are provided for receiving signals from a distant target by an element which simulates uniform motion along the said baseline of receiving elements.

5. A radar system as claimed in claims 1,2 and 3 wherein the two transmitted signals are one above and one below a carrier frequency by an amount equal to a lower offset frequency, or by an amount equal to different lower offset frequencies, the said signals having either sonar, radio, microwave or optical frequencies.

6. A radar system as claimed in claim 5 wherein means are provided for transmitting pulsed signals or continuous wave signals.

7. A radar system as claimed in claims 1,2 and 3 with a vertical baseline arrangement for use in elevation and a horizontal baseline arrangement for use in azimuth.

8. A radar system as claimed in claims 1, 2,3 and 7 with a vertical baseline arrangement for use in elevation and alternatively with mechanical rotation in azimuth.

9. A radar system as claimed in claims 1 and 4 wherein Doppler shifts are imposed on the two sideband signals received, due to target motion and commutation at the receiver.

10. A radar system as claimed in claims 7, 8, and 9 wherein means are provided for subtracting the two sideband signals to provide a difference frequency signal for determining the range, bearing and elevation of a distant target.

11. A radar system as claimed in claims 7,8,9 and 10 wherein means are provided for summing the two sideband signals to provide a sum frequency signal for detecting and correcting errors in the difference frequency signal.

12. A radar system as claimed in any of the previous claims and substantially described with reference to the drawings accompanying this application.