GB2214748A - Bearing measurement - Google Patents

Abstract

A bearing measurement system measures the distances to an object from two physically separated radar systems 2, 3 and then calculates the bearing of the object by triangulation. The two radar systems may be on the wing tips of an aeroplane, the object being on the ground. The transmitted pulses are pseudo-random code bi-phase modulated (4, 8) enabling demodulation of the echo signals (32, 34, 31) to effect pulse compression. The two ranges are fed to a logic and bearing calculation 38. Gates 17, 22, 24 in the transmission and reception paths determine the particular range band being investigated. <IMAGE>

Description

Bearing Measurement This invention relates to apparatus for measuring the bearing of a target or other feature relative to an aircraft or other vehicle.

The invention arose when considering mechanical problems of providing aircraft with radar antennas of the type which scan a field of view in order to detect a target and to identify its bearing. Such antenna may scan mechanically or electronically. The mechanically scanning antenna interfere with the airflow over the surface of the aircraft. Electronically scanned antenna can be made to conform with the surface of the aircraft, thereby avoiding this problem, but such antenna and the associated electronics may be complex and expensive as compared with a simple mechanical antenna and the shape of the surface of the aircraft may not be ideal for the purposes of providing the best possible characteristics of the antenna.

The ideal position for an antenna is usually on the nose of the aircraft so that no part of the aircraft obscures the field of view in front of it.

Unfortunately, siting the antenna on the nose causes particularly severe problems in that interference of the airflow caused by a mechanically scanned antenna will be very likely to change the flying characteristic of the aircraft and in that the nose is an area which is usually already occupied by other equipment.

This invention provides apparatus for measuring the bearing of a feature relative to a vehicle comprising two receivers spaced apart on the vehicle; timing means associated with each receiver for measuring the time for a signal from a source of radiation to be transmitted to the aforementioned feature and reflected to that receiver, and means for using the time measurements for calculating the bearing.

By employing the invention the need to scan the field of view can be removed, thereby enabling very simple antenna to be used which neither significantly obstruct the airflow nor require complicated electronic steering. Furthermore since it is beneficial to space the antenna apart by the maximum possible distance, to obtain the most accurate range measurement, the ideal positions for them are on for example the wing tips where space is more readily available than on the aircraft nose.

Whilst the invention arose in particular relation to the problems of aircraft-mounted radar it is also applicable to other vehicles such as spacecraft, boats and submarines.

The source of radiation is preferably mounted on the vehicle since a knowledge of the timing of the transmitted signal is required in order to make the required time measurement. It would however be possible for the source to be located for example at a base station providing some facility were provided to transmit to the vehicle the necessary information concerning the time of transmission.

Where the source is provided on the vehicle it is advantageous to use just one such source for the two receivers. This common source can conveniently share the same antenna as one of the receivers.

Because the accuracy of the bearing calculation depends on the accuracy of range resolution it is desirable to transmit pulses of radiation which are modulated so as to allow pulse compression in the receivers. Such pulse compression enhances the range resolution. The type of modulation applied to the pulses can take any conventional form. However a technique which is particularly advantageous is to use a pseudo random code to modulate transmitted pulses.

This enables great accuracy of range resolution to be achieved.

The radiation may be microwave energy but other forms of radiation such as light, infra red ad scund or ultrasonic radiation could be used. Light and infra red, e.g. from a pulsed laser, might be particularly applicable for systems on land vehicles and sound or ultrasound would of course be particularly applicable for underwater use.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows schematically a target and an aircraft carrying a radar system constructed in accordance with the invention; Fig. 2 is a block diagram of the radar system carried by the aircraft shown in Fig. 1; and Figs. 3A to 3C show waveforms at different parts of the circuit of Fig. 2.

Fig. 1 shows an aircraft 1 carrying a radar including a transmitting/receiving antenna 2 and a receiving antenna 3.

Referring to Fig. 2 the radar includes a pseudorandom code generator 4 which operates continuously until instructed to re-set. It is clocked at a frequency of 100 MHz by a clock signal supplied on line 5 from a controller 6. The pseudo-random code generator 4 generates a code as shown schematically in figure 3A. This code is presented on a line 7 to a biphase modulator 8 where it modulates a carrier frequency signal from an RF oscillator 9 to produce, on line 10, a bi-phase modulated signal. This is amplified at 11 and up-converted to an output frequency by mixing at 12 with a local oscillator signal provided from 13 via a splitter 14. The upper side band from mixer 12 is passed by a filter 15 and amplified at 16 before passing to a gate 17 where it is modulated by pulses provided on line 18 from the controller 6.The gate 17 thus produces pulses as shown in Figure 3B where the times when phase changes occur are indicated by the vertical lines.

The output of the gate 17 is passed through a circulator 19 to the antenna 2 from which it is transmitted to a region under inspection. A portion of the transmitted signal passes along path 20 to a target 21 in this region and is reflected back along path 20 to the same antenna 2. The signal received at 2 is passed through the circulator 19 to a second gate 22.

Energy is also reflected from the target along a path 23 to the second antenna 3 and is passed to a third gate 24. Gates 22 and 24 are operated by gating signals on lines 25 and 26 respectively supplied from the controller 6. The gates 22 and 24 are thus caused to pass the received signal during received periods P as shown in Figure 3C. The processing of return signals is the same for both antennas, so the same reference numerals will be used. The output of gates 22 and 24 are amplified at 27 and down-converted to an intermediate frequency by mixing at 28 with the local oscillator signal. The lower side band is passed by a filter 29 and amplified at 30 to a suitable level for correlation in a mixer 31.

Pseudo-random code generators 32, identical to the generator 4 are clocked by clock signals on line 33 from the controller 6. In this way the pseudo-random code generators 32 are caused to operate continuously like the generator 4. Their starting time is however delayed relative to the time when the generator 4 starts. This delay is selected, by the controller 6, so as to be equal to the round trip time of the signal to be transmitted and received from a target at a range currently selected by the controller for inspection.

The output of the pseudo-random code generator 32 is used at 34 to bi-phase modulate a signal from the oscillator 9, which signal has been offset in frequency at 35 by a variable amount selected by the controller 6 depending on the velocity of targets which it is, for the time being, desired to inspect.

The output of the modulator 34 is mixed at 31 with the returned signals from 30. The output of the mixers 31 are passed through low pass filters 36 and the returns from successive pulse periods are integrated at 37. The integrator 37 can be a bandpass filter with a bandwidth which is the reciprocal of the integration time. Alternatively it can be a digital fast Fourier transform device. The components 31, 36 and 37 constitute a correlator by which an output waveform is produced. The magnitude of this output waveform indicates the strength of the return signal; and its frequency components indicate the Doppler components of the received signal. The logic and bearing calculator 38 uses the signals produced by the two integrators 37 to calculate the bearing of the target.

It would, in theory, be possible to use just one code generator instead of the three shown at 4 and 32 in Figure 2. A digital delay would then be needed to connect the output of the single code generator to the modulators 34. Such an arrangement would be difficult in practice using existing technology since delays of perhaps 10,000 digits may be required.

It should be noted that the chcice of the periods when the gates 17, 22 and 24 are open is made purely for the purpose of preventing leakage of the transmitted signal into the receiver and of preventing returns being received from close-in clutter. These "pulse" periods are of no significance in determining range.

In order for a system in accordance with this invention to have all-round coverage a plurality of transmitters and receivers will be needed, each covering an arc.

A radar system as described would be especially suitable for use as an aircraft landing aid, with an aircraft calculating its position relative to the runway by reference to the range and bearing of a plurality of passive reflectors. This would have the advantage that the nearer the aircraft was to the runway the more accurately its position would be known.

Claims (8)

1. Apparatus for measuring the bearing of a feature relative to a vehicle comprising two receivers spaced apart on the vehicle; timing means associated with each receiver for measuring the time for a signal from a source of radiation to be transmitted to the aforementioned feature and reflected to that receiver, and means for using the time measurements for calculating the bearing.

2. Apparatus according to claim 1 in which the vehicle is an aircraft.

3. Apparatus according to claim 1 or 2 including a source on the vehicle.

4. Apparatus according to claim 3 in which the timing means associated with each receiver measures the time for radiation from a common source on the vehicle to be transmitted to the said "feature" and reflected to that receiver.

5. Apparatus according to any preceding claim including a source which transmited modulated pulses of radiation and in which each receiver includes a pulse compression system.

6. Apparatus according to claim 5 in which the source includes means for generating a pseudo-random code and for using that code to modulate the pulses.

7. Apparatus according to any preceding claim including a source of microwave radiation and in which each receiver is a microwave receiver.

8. Apparatus substantially as described with reference to Fig. 1 of the accompanying drawings.