GB2224176A

GB2224176A - Radar range finder

Info

Publication number: GB2224176A
Application number: GB8923358A
Authority: GB
Inventors: Richard John Lefroy Pardoe; Richard Charles Prime
Original assignee: Thorn EMI Electronics Ltd
Current assignee: EMI Group Electronics Ltd
Priority date: 1988-10-21
Filing date: 1989-10-17
Publication date: 1990-04-25
Anticipated expiration: 2009-10-17
Also published as: GB8923358D0; GB8824653D0; GB2224176B

Abstract

A radar range finder suitable for use as a radar altimeter is described. The radar range of the received signal is determined by a tracking gale 9 whose output is stored in a tank circuit 26. The energy in the tank circuit is then allowed to decay away with time and the amplitude of the signal is sampled at some time after the initial injection of energy. This sampling time is fixed with respect to the original transmit pulse time of the radar thus causing near range echoes to decay more in the tank circuit than distant echoes which are more delayed before they enter the tank circuit. The output from the sampling gate which provides this range compensation is then filtered and amplified by the receiver system. <IMAGE>

Description

RADAR RANGE FINDER This invention relates to a radar range finder and it relates particularly to a leading edge tracking radar altimeter having an altitude compensation circuit.

A known leading edge tracking radar altimeter, illustrated schematically in Figure 1 of the accompanying drawings, comprises a transmitter 1 arranged to transmit radio frequency pulses via a transmit antenna 2, and returns, in the form of long-tail pulses, are received by a receive antenna 3. A typical return pulse is shown, by way of illustration, in Figure 3c of the accompanying drawings, the elongated pulse tail being due to reflections at the ground which occur over a range of different angles. The leading edge of the return pulse gives a true representation of altitude.The received pulses are combined in a mixing circuit 5 with the output from a local oscillator 4 giving rise to an i.f. output which is amplified by an amplifier 6 and detected by a detector 7 to give negative pulses, which have random amplitudes due to incoherent doppler modulation, the doppler frequency being much lower than the pulse repetition frequency. These negative pulses are amplified by the video amplifier 8. A tracking gate 9 is closed at the start of each transmitted pulse and is opened for a fixed short period by the comparator 10 when the output from a ramp generator 11, which is synchronised with the transmitter, equals the output from the integrator 12.In the absence of a reflected signal, the positive reference 13 drives the integrator output more positive with each successive transmitted pulse, effectively delaying the opening of the tracking gate 9 more and more until a return signal is detected, thereby rendering the integrator sensitive to progressively increasing ranges. The integrator output reaches a stable value when the positive reference 13 and the gated negative return signal cancel each other. Any change in altitude will alter the tracking gate timing and the integrator output will restabilise. If the altitude increases, the return pulse is delayed more, hence less signal passes the gate, the integrator output goes more positive and the gate is opened later. If the ramp voltage is linear with time, the altitude output 14 will be linear with altitude.

In order to correct for variations, with altitude, of the return signal an automatic gain control circuit is provided having a control input 15 to amplifier 6. Conveniently, the AGC voltage may be swept in synchronism with the ramp generator, or alternatively, the AGC voltage may be given a value dependent on the time position of the tracking gate; or it can be provided using a wide range gate after the video amplifier 8, using most of the ground return pulse.

It is an object of the present invention to provide radar range finder having a more convenient means of range compensation.

Accordingly there is provided a radar range finder suitable for use as a radar altimeter comprising a transmitter to transmit a series of radar pulses, a receiver to receive corresponding returns and to derive a series of video pulses from the returns, and a processing circuit operative on the video pulses to generate therefrom a measure of altitude, wherein the processing circuit includes a range compensation circuit having means to cause each video pulse to decay and means to sample the decaying pulse at a predetermined time relative to the start of the corresponding transmitted pulse.

In a preferred embodiment of the invention the processing circuit includes a gating circuit connecting the receiver to the range compensation circuit, a detection circuit to derive a series of detection pulses, all having the same polarity sign, from the sampled pulses, an integration circuit operative on said detection pulses and on a reference voltage of the opposite polarity sign and means to control the times during which said gate circuit is open so that the output of the integration circuit is directly related to altitude.

The control circuit may comprise a ramp generator synchronised to said transmitter and a comparator arranged to compare the output of the ramp generator and the output of the integrator.

In order that the invention may be carried readily into effect an embodiment thereof is now described by reference to the accompanying drawings in which Figure 1 shows, in block schematic form, a known leading-edge tracking radar altimeter.

Figure 2 shows, in block schematic form a leading-edge tracking radar altimeter in accordance with the present invention.

Figure 3 shows signal waveforms useful in understanding operation of the altimeter shown in Figure 2, and Figure 4 shows a circuit used in the circuit of Figure 2.

Referring now to Figures 2 and 3, a pulse transmitter 1 comprises a pulse modulator 21 and a Gunn source 22, providing radio frequency pulses of sufficient width to overlap return pulses at ranges of the order of 10 m to 100 m, as illustrated in Figure 3a of the drawings. These pulses are fed via the coupler 23 to a T/R antenna 24 and, after reflection at the ground, long-tail pulses shown at Figure 3c are received by the T/R antenna 24 and are routed, via coupler 23, to a mixing circuit 5. A sample (Figure 3b) of each transmitted pulse is also supplied to the mixer 5, as a local oscillator signal, by means of a link 25.Mixing circuit 5 produces coherent doppler modulated long-tail pulses to a video amplifier 8 and tracking gate 9 assumes the open condition at the beginning of each transmitted pulse, as represented in Figure 3d, and is closed in response to an output signal produced by a comparator 10 whenever the output from a ramp generator 11 equals the output from integrator 12. In the absence of a return signal, the positive reference 13 holds the integrator output positive, and thereby maintains gate 9 in the open condition. However, if a return signal is received, a detector 7, which is connected to the tracking gate by a tank circuit 26 and a range compensation gate 27, produces a corresponding pulse of negative polarity sign making the output of the integrator less positive and causing the tracking gate 9 to close.The integrator output stabilises when the positive reference 13 is cancelled by the negative return signal provided by the detector 7. The tracking gate 9 then follows any changes in altitude. The greater the altitude, the later will be the return pulse and hence the greater the ramp voltage and the greater the integrator output needed to close the gate. If the ramp voltage is linear with time, the altitude output 14 will be linear with altitude.

Compensation for the variation of signal level with range is achieved by a range compensation count which includes a tank circuit 26 and range compensation gate 27. Each pulse passing the tracking gate 9 is stored in the tank circuit 26 where it slowly decays as illustrated in Figure 3f. It is sampled and held (Figure 3g), the range compensation gate 27 being closed by a gate drive 28 at a fixed time after the start of the transmitted pulse. Thus the longer the range, the later the pulse reaches the tank circuit 26 and therefore the less it decays before the range compensation gate 27 is closed.

In a particular embodiment of the invention a combined tank circuit and range compensation gate may be provided, as shown in Figure 4 of the drawings. In this configuration the output from the tracking gate 9 is buffered by amplifier Al before charging the tank circuit. The tank circuit consists of the resistor R and the capacitor C. The capacitor is on the output side of the sampling gate G but it still acts as the tank circuit because in this application gate G is always conducting when the tracking gate 9 is conducting. This means that the capacitor stores the tank circuit energy when G is conducting and holds the energy throughout the interpulse period when G is insulating. The following amplifier stage A2 has a high input impedance so as not to significantly discharge the capacitor C.

Returning to Figure 2, successive pulses are coherently doppler modulated (at a frequency low compared with the pulse repetition frequency) so a doppler filter and amplifier 28 effectively reconstructs the doppler waveform. After amplification the doppler signal is routed to the detector 7 which provides a negative signal to the integrator 12.

Claims

ClAIMS

A radar range finder suitable for use as a radar altimeter comprising a transmitter to transmit a series of radar pulses, a receiver to receive corresponding returns and to derive a series of video pulses from the returns, and a processing circuit operative on the video pulses to'generate therefrom a measure of altitude, wherein the processing circuit includes a range compensation circuit having means to cause each video pulse to decay and means to sample the decaying pulse at a predetermined time relative to the start of the corresponding transmitted pulse.
2. A radar range finder according to Claim 1 in which the processing circuit includes a gating circuit connecting the receiver to the range compensation circuit, a detection circuit to derive a series of detection pulses, all having the same polarity sign, from the sampled pulses, an integration circuit operative on said detection pulses and on a reference voltage of the opposite polarity sign and means to control the times during which said gate circuit is open so that the output of the integraton circuit is directly related to altitude.
3. A radar range finder according to Claim 2 in which the control circuit comprises a ramp generator synchronised to said transmitter and a comparator arranged to compare the output of the ramp generator and the output of the integrator.
4. A radar range finder substantially as hereinbefore described with reference to, and as illustrated in, Figures 1, 2, 3 and 4 of the accompanying drawings.