GB2061051A - Direction finding system - Google Patents

Abstract

A direction finding system wherein the sensor signals are multiplexed and wherein direction of arrival information is derived directly i.e. without demultiplexing, from the baseband and sideband of the multiplexed signal. <IMAGE>

Description

SPECIFICATION Improvement in or relating to direction finding systems The invention relates to electronic direction finding systems and homing systems.

There are many varieties of multi-channel direction finding (DF) receivers in use today, particularly for radar application. They do, however, have a number of problems associated with them. For example: non-multiplexed systems are complex, expensive and are subject to errors in direction of arrival (DOA) information, this being caused by receiver channel imbalance.

Systems that do employ time-division multiplexing are in general simpler, less costly and exhibit lower bias errors but, depending upon how the multiplexing is applied may require more than one measurement time slot in order to derive 2-plane DOA information. Even this can be avoided if the multiplexing is achieved by high speed switching between channels, providing the switching rate is greater than the incoming signal duration and is no slower than that dictated by the Nyquist sampling theorem. To date, however, this technique uses synchronous demultiplexing to reconstruct the signal into its pre-multiplexed form, whence standard techniques are used to extract DOA information.

Such a system reduces bias errors and potentially offers small savings in complexity and cost. Larger savings could be achieved if a simpler technique were used for the extraction of directional information from the multiplexed waveform.

Broadband receivers from which accurate directional information is required, have mainly been limited to passive surveillance or support applications (ESM). These systems are used for information only and are, as a rule, complex, powerful in their analysis capability and expensive. Conventional techniques such as those described above are used for error reduction. To fit such receivers into missiles as homing heads has not been cost-effective to date, although this may be desirable for some applications.

Passive surveillance 'nets' whereby a number of ESM receivers are connected to a central computer have been produced primarily for military applications. Here the parameters of received emissions and their DOA information relative to each receiver is fed to the central computer. It is usual to analyse each emitter in every ESM receiver before passing this information on via a data highway. This results in a very expensive system.

According to one aspect of the invention, there is provided a direction finding system comprising electromagnetic radiation sensor means for responding to received electro-magnetic radiation to provide at least two alternating signals having a relationship indicative of the direction of arrival of said radiation at the sensor means, multiplexing means for switching between one and the other of said least two signals to form a composite signal having sidebands carrying information indicative of said direction of arrival and demodulator means for receiving said composite signal and for extracting said information.

According to a second aspect of the invention there is provided a demodulator for use in the system above, the demodulator comprising means for measuring the phase of said composite signal to give a fine direction of arrival measurement and, to give a coarse measurement, means for detecting the envelope of the composite signal, means for extracting from the detecting means output the fundamental and a sideband signal, and means for combining these two signals.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figures 1 and 2 are respective simplified diagrams of two D.O.A. modulators, Figure 3 is a simplified diagram of a demodulator, Figures 4 and 5 are respective diagrams illustrating various arrangements of the direction finding system.

The systems to be described comprise two parts: 1. A direction of arrival signal modulator - this is coupled to a direction sensing head.

2. Direction of arrival demodulator - this is coupled to a receiving system.

1. The direction of arrival modulatortakes M signals from a direction sensing head and combines them into M/2 signals. The present system was designed to operate on the output of a single plane RF monopulse antenna system, although the invention is applicable to outputs from other forms of head covering a different part of the electromagnetic spectrum, assuming that the switching speed exceeds twice the spectral occupancy of the signal.

Very high speed switching is used to multiplex the sensor outputs to produce a signal which has upper and lower spectral sidebands impressed on the original signal. The information contained in the modulation upper and lower sidebands effectively labels the received signal with its direction of arrival both in terms of amplitude and phase interferometer information.

The original system was designed for radar signals with bandwidths fp Hz (dependent on pulse duration), Figure 1. outputs Ap and Bp from the direction sensors are filtered before the high speed switching to eliminate signal interference due to switching harmonics. In low signal density environments and or fixed frequency applications the pre-switching filters could be fixed but in the case of wide frequency coverage systems in high signal density environments narrow turnable YIG RF filters are to be used. The high-speed switching (modulation) is done using either p-i-n switches or F.E.T.

devices. A high-pass filter was included after the switches to remove switch drive break-through and any other troublesome low frequency components which would affect down conversion. One of the features of the original modulator was an output at a constant intermediate frequency.

As a variant on the modulator format a straight signal transponder was envisaged which retransmitted at the input RF frequency. This variant would not contain a V.C.O. mixer system but would have a wide RF bandpass filter after the switch unit, Figure 2.

2. The direction of arrival demodulator unit is essentially two demodulators in one: a.) Signal envelope demodulator.

b.) Signal carrier phase demodulator.

a.) The signal envelope demodulator provides the coarse direction of arrival information which is used to resolve the ambiguities of the 'fine' phase direction measurements. Video detection is used to recover the signal envelope and the baseband and first sideband are recovered by low-pass and bandpass filtering at 0 to fp and f,+ fp Hz respectively.

The signal recovered from the first sideband is proportional to the difference between the two antenna ihput signal moduli 1Apl and 1 Bpl, and the signal extracted from the low-pass filter is proportional to the sum of the two signal moduli 1Apl + 1 Bpl. As usual the summation channel is used as an automatic gain control applied to the difference channel.

b.) The output from the phase demodulator gives high resolution direction measurement but with ambiguities every 2 x radians relative phase shift between inputs Ap and Bp. Filtering and hard limiting before the phase demodulator is used to remove amplitude modulation distortions. A differential phase demodulation technique is used with a hybrid to give 90 phase shift and a delay element of llfs seconds. This eases the requirement on the coarse amplitude comparison circuitry to give a directional accuracy equivalent to + zl2 electricai radians phase shift over the field of view of the sensors.

The invention can be applied to a number of different systems of which three in portant types are described below.

The first application is in non-gimballed surveillance receivers where accurate instantaneous DOA of observed electromagnetic radiation is required. This radiation can be either CW or pulsed in nature, the shortest workable pulse length being fixed by technological limitations in high speed switch manufacture, this being a fundamental component in the modulator. In this application, directional accuracy is often very important and errors in DOA introduced by receiver channel imbalance are minimized by the time-division-multiplexing action ofthe invention.

Such systems are typically employed in fixed homing-head missiles where both modulator and demodulator are located on board the missile. This is shown schematically in Figures 4 and 5 where modulator and demodulator are simply connected back-to-back. The invention is well-suited for this application since it results in medium-to-low complexity, low cost systems.

On the same basis, the invention can be used in direction finding electronic support measures (ESM) receivers. The principle is as for the above application but can be extended as shown in Figure 5(b).

Here, remote DF receivers, labelled 'units A', and situated at different physical locations, modulate observed emissions in real-time with their DOA information but without destroying the time and frequency fidelity of those emissions. The modulated signal is then re-transmitted to a central processor labelled 'units B' where demodulation and signal processing takes place. Such a system could be usefully employed in battlefield surveillance, a large area being covered with perhaps cross references for each emitter located. In addition, the complex processor could be well back from the front edge of the battle area (FEBA) and be safer from destruction.

Thirdly, the invention coUld be used in nullseeking gimballed systems. The output voltage ratios in a conventional null-seeking receiver are designed to have well defined nulls at boresight and incident radiation off boresight produces error voltages which are used to drive electro-magnetic gimbals. In this system accuracy is only required near boresight. The invention can be used in two ways hem. Firstly, it can be used after the conventional beam forming networks of an amplitude and phase monopulse system, switching between sum and difference channels. Alternatively, it can be used in a combined amplitude comparison/phase interferometer dual mode receiver where similar accuracies to that of the former technique are achieved near boresight but without the need for a beam forming network. Both these techniques could be used in missile/projectile homing heads and target-tracking radars.

Claims (4)

1. A direction finding system comprising electromagnetic radiation sensor means for responding to received electromagnetic radiation to provide at least two alternating signals having a relationship indicative of the direction of arrival of said radiation at the sensor means, multiplexing means for switching between one and the other of said at least two signals to form a composite signal having sidebands carrying information indicative of said direction of arrival and demodulator means for receiving said composite signal and for extracting said information.

2. A demodulatorfor use in the system of claim 1, the demodulator comprising means for measuring the phase of said composite signal to give a fine direction of arrival measurement and, to give a coarse measurement, means for detecting the envelope of the composite signal, means for extracting from the detecting means output the fundamental and a sideband signal, and means for combining these two signals.

3. A direction finding system substantially as hereinbefore described with reference to the accompanying drawings.

4. A demodulator substantially as hereinbefore described with reference to the accompanying drawings.