GB2286306A

GB2286306A - Direction finding apparatus

Info

Publication number: GB2286306A
Application number: GB9401828A
Authority: GB
Inventors: David Rodney Thwaites
Original assignee: Siemens Plessey Electronic Systems Ltd
Current assignee: BAE Systems Defence Systems Ltd
Priority date: 1994-02-01
Filing date: 1994-02-01
Publication date: 1995-08-09
Anticipated expiration: 2014-02-01
Also published as: GB2286306B; GB9401828D0

Abstract

The direction finding apparatus comprises at least two spaced antennas 1, 2 each being connected to a respective receiver with its own local oscillator 8, 20. Each receiver is provided with means 16, 28 for measuring the instantaneous phase of a desired signal or component thereof over a first sampling period. A crossover switching arrangement 32 is provided for interchanging the antennas so that the measuring means can measure the instantaneous phase over a second sampling period from which a value of the instantaneous signal or component thereof is computed. These values are then used by a phase calculation circuit 30 to determine the bearing angle. The apparatus provides a simplified approach to direction finding on frequency hopping signals as well as conventional signals. The use of phase match receivers or receivers with coherent local oscillators is avoided. <IMAGE>

Description

DIRECTION FINDING APPARATUS The present invention relates to direction finding apparatus, and in particular may be used on frequency hopping signals.

A commonly used technique to provide direction finding on communication signals is to measure the phase difference at the radio frequency (RF) between two or more spaced antennas excited by the incoming signal of interest. A significant problem with this known technique is that the antennas must be provided with closely phased matched receiver channels. Furthermore, the receiver channels must be phase coherent, i.e., driven from a common set of local oscillators which implies that standard signal channel receiver designs cannot be used, leading therefore to expensive hardware implementations.

An object of the present invention is to provide direction finding apparatus which avoids the need for phase match receivers with coherent local oscillators.

According to the present invention there is provided direction finding apparatus comprising at least two spaced antennas each being connected to a receiver having an independent local oscillator, each receiver being connected to measuring means arranged to provide an instantaneous phase value of a desired signal or component thereof over a first sampling period, means for interchanging the antenna connections to the receivers so that said measuring means provides the instantaneous phase value of the desired signal or component thereof over a second sampling period.

Preferably the values of the instantaneous phase of the signal or component thereof, for each receiver, and for the two sampling interval periods are applied to an input of a phase calculation circuit arranged to generate at an output thereof a signal indicative of a bearing angle.

The measuring means may comprise for each receiver, an analog to digital converter arranged to sample the output from the respective receiver and provide a digital output signal which is applied to a digital signal processor arranged to perform a complex fast fourier transform giving a complex spectrum of the intermediate frequency passband.

The output signals from each analog to digital converter are stored in storage means prior to application to the digital signal processor.

An embodiment of the present invention will now be described with reference to the accompanying drawing, Figure 1, which shows a block diagram of direction finding apparatus, together with a sampling timing diagram.

Referring to Figure 1, a first antenna 2 is connected via a switching arrangement 32 to an input of a first radio receiver 6.

The radio receiver 6 is connected to and driven by a local oscillator s. An output from the radio receiver 6 is connected to an input of an analog to digital converter 10, which is connected to and driven by a clock circuit 12 which is arranged to sample the converter 10. The clock circuit 2 is controlled by control signal C2.

An output from the converter 10 is connected to an input of a digital memory 14, the output of which is connected to an input of a digital signal processor 16. The output of the processor 16 is connected to an input of a phase calculation circuit 30.

A second antenna 4 is connected to an input of a second radio receiver 18 by way of the switching arrangement 32. The second radio receiver 18 is connected to and driven by a local oscillator 20. An output from the receiver 18 is connected to an input of an analog to digital converter 22, which is connected to and driven by a clock circuit 24 which is arranged to sample the converter 22. The clock circuit 24 is controlled by the control signal C2. An output from the analog to digital converter 22 is connected to an input of a digital memory 26, the output of which is connected to an input of a digital signal processor 28. The output of the processor 28 is connected to a further input of the phase calculation circuit 30.

The switching arrangement 32 is arranged to be controlled by a control signal C1 and when operated causes the antenna 2 to be disconnected from the radio receiver 6 and connected to the radio receiver 18, and similarly causes the antenna 4 to be disconnected from the radio receiver 18 and connected to the radio receiver 6.

The antennas 2, 4 are spaced by less than half a wave length and respectively provide the RF signals to the radio receivers 6, 18. The receivers 6, 18 need not be phase matched. Each of the receivers are connected to measuring means for providing the instantaneous phase of a desired signal or a component thereof.

The measuring means comprises, for each receiver an analog to digital converter 10, 22 which is arranged to digitise the IF signal from each receiver. Output signals generated by the analog to digital converter 10, 22 are temporarily stored in a digital memory 14, 26 respectively and are then applied to a digital signal processor 16, 28 respec.ively. The digital signal processors 16, 28 are arranged to perform a complex fast fourier transform giving the complex spectrum of the IF passband. The parameters of the digitisation process are such that the sampling rate is above the Nyquist rate for the receiver bandwidth respectively, and is extended over a sampling time period long enough to provide spectral resolution capable of separating the signal of interest.For typical communications signals in the VHF band, for instance, a spectral resolution of about SkHz would be appropriate so that the sampling time period would be in the region of 200 microseconds.

The digitisation of the signals at the receivers 6, 18 is started substantially simultaneously and the control signal C2 performs this function. After the end of a fixed sampling time period, the digitisation is stopped and the second control signal C1 causes the switching arrangement 32 to operate, which interchanges the connection of the antennas 2, 4 so that the antenna 2 is now connected to the receiver 18 and the antenna 4 is now connected to the receiver 6. Following a short delay period to allow for switch settling, a second sampling time period is started.

This would not necessarily require the use of another control signal as the timing for the second sampling period could be derived from the control signal C2.

After each of the sampling time periods, a value of the instantaneous phase of a desired signal or component thereof is computed for each of the receivers 6, 18. The value is computed as the arctangent of the ratio of the imaginary spectral component to the real spectral component. Thus for the first sampling time period, two values Phil and Phi2 would be obtained. In reality these values are directly effected by each receiver's local oscillator phase so that the two receiver channels would produce quite different phase measurements. The difference between Phil and Phi2 would therefore depend on the local oscillator phase of both the receivers 6, 18 as well as the antenna separation and the signal bearing.The difference between the second pair of phase measurements Phi3 and Phi4 would also depend on the same parameters except that, because of the antenna switching process between the sampling period, the sign of the contribution due to signal bearing would be reversed. This presumes that the relative phase of the two local oscillators changes by an insignificant amount over the time interval between the two sampling intervals. In practise the phase changes will amount to many cycles but for crystal controlled oscillators, the difference incurred over the short interval will be small.

Hence, the difference (Phil - Phi2) - (Phi3 - Phi4) gives twice the phase difference between the two antennas, from which the signal bearing can be calculated as in a standard phase difference based direction finding system using the phase calculation circuit 30.

Because the antenna phase difference measurement is made by two consecutive pair of instantaneous phase measurement using incoherent receivers, the subsequent calculation of the antenna phase difference is obtained by a calculation based on the instantaneous phase values which removes the effect of the local oscillator phase differences.

It will be appreciated by those skilled in the art that the above description has been of one implementation of the invention and alternative implementations are possible within the scope of the present invention. It will also be appreciated that although the invention may be used on frequency hopping signals, it may also be used on conventional signals.

Claims

1. Direction finding apparatus comprising at least two spaced antennas each being connected to a receiver having an independent local oscillator, each receiver being connected to measuring means arranged to provide an instantaneous phase value of a desired signal or component thereof over a first sampling period, means for interchanging the antennas so that said measuring means provides the instantaneous phase value of the desired signal or component thereof over a second sampling period.

2. Direction finding apparatus as claimed in claim 1, wherein the values of the instantaneous phase of the signal or component thereof, for each receiver, for the two sampling periods, are applied to an input of a phase calculation circuit arranged to generate at an output thereof a signal indicative of a bearing angle.

3. Direction finding apparatus as claimed in claim 2, wherein the measuring means comprises for each receiver, an analog to digital converter arranged to sample the output from the respective receiver and provide a digital output signal which is applied to a digital signal processor arranged to perform a complex fast fourier transform giving a complex spectrum of the intermediate frequency passband.

4. Direction finding apparatus as claimed in claim 3, wherein the output signals generated by each analog to digital converter is temporarily stored in a digital memory prior to application to the digital signal processor.

5. Direction finding apparatus as claimed in claim 4, wherein the output signals generated by the digital signal processors are applied to inputs of the phase calculation circuit.

6. Direction finding apparatus as claimed in any preceding claim, wherein the means for interchanging the antennas comprises a cross over switch.

7. Direction finding apparatus substantially as hereinbefore described with reference to the accompanying drawing.