GB2243738A - Direction finding - Google Patents

Abstract

A radio direction finding system in which one received signal is offset in frequency by a predetermined amount in a phase shifter (9) before combination (10) with a second received signal. The combined signal is amplified and the envelope detected by a narrow band, communications receiver (8). The phase difference between the detected envelope and a phase shifter control wave is measured in a microprocessor (12), which phase difference corresponds to the phase difference between the two received signals, and then converted into a bearing value by another microprocessor (13). <IMAGE>

Description

Direction Finding This invention relates to direction finding and in particular to phase difference measurements therefor.

According to one aspect of the present invention there is provided a radio direction finding system comprising two receiving antennas; phase shifter means for offsetting one received signal in frequency by a predetermined amount; phase shifter control wave means serving to rotate the phase shifter phase at a predetermined reference rate; means for combining the offset signal with the other received signal; means for amplifying the combined signal and detecting the envelope thereof; means for measuring the phase difference between the phase shifter control wave and the detected envelope, which phase difference corresponds to the phase difference between the two received signals; and means for converting said phase difference into a bearing value.

According to another aspect of the present invention there is provided a signal processor unit and receiving antenna subsequently for use with a narrow band, communications receiver, which processer unit provides direction finding information in response to signals received from two antennas, the processer unit including phase shifter means for offsetting one received signal in frequency by a predetermined amount; phase shifter control wave means serving to rotate the phase shifter phase at a predetermined reference rate; means for combining the offset signal and the other received signal; an output whereby in use of the sub-assembly the combined signal is applied to the receiver for amplification and detection; an input whereby in use of the sub-assembly the receiver detected signal is input to the sub-assembly; means for measuring the phase difference between the phase shifter control wave and the envelope of the receiver detected signal in use of the sub-assembly, which phase difference corresponds to the phase difference between the two received signals, and means for converting said phase difference into a bearing value.

According to a further aspect of the present invention there is provided a method of radio direction finding using two antenna elements comprising the steps of employing phase shifter means to offset one received signal in frequency by a predetermined amount; rotating the phase shifter phase at a predetermined reference rate by phase shifter control means; combining the offset signal with the other received signal; amplifying the combined signal and detecting the envelope thereof by means of a narrow band communication receiver; measuring the phase difference between the phase shifter control wave and the detected envelope, which phase difference corresponds to the phase difference between the two received signals, and converting the said phase difference into a bearing value.

Embodiments of the invention will now be described with reference to the accompanying drawing which illustrates one embodiment in block diagram form.

The angle of arrival (bearing) of a received radio frequency signal may be determined from the phase difference of the signal as received by two or more adjacent antennas. This is a commonly employed radio direction finding technique. The equipment generally employs a specific receiver which performs the necessary channel selection, signal amplification and detection as well as the processing required to determine the phase difference and bearing.

The present invention is directed to providing suitable processing equipment which enables an existing receiver that is unmodified to be used, which existing receiver will perform the necessary channel selection, signal amplification and detection. Specifically the existing receiver can be an airborne narrow band, communications receiver with a requirement to provide unproved direction finding of sonobuoys, for example.

In the example illustrated in the drawing there are five spaced apart antennas 1 which are each linked to a commutator unit 2 preferably by matched cables 3.

The diodes 4 in the commutator unit 2 may be energised to connect each antenna to either one or the other of the phase measuring lines 6 between the commutator unit 2 and a processor unit 7. In order to avoid requirements for stable or matched cables to the processor unit 7, and to eliminate the effects of receiver delay, the antennas need to be switched to the phase difference measurement circuits in the processor unit according to a set sequence. As will be appreciated the antennas are taken in pairs and the differential phase between the antennas of a pair is determined. It is possible to determine the differential phase between all five antennas if five measurements between pairs are made. By this means unwanted delays from all points after the commutator will be eliminated.

The processor unit 7 is coupled to the existing narrow band, communications receiver referred to above which is illustrated schematically at 8. This receiver is such as to detect a signal which is carrying its information as envelope modulation, the phase of which denotes the parameter to be measured. The processor unit 7 includes means to implement envelope modulation of this form by applying successive phase shifts to one of the signals coupled to the processor unit by the commutator unit 2, using a 4 bit phase shifter 9 in the illustrated version, such that this signal is effectively shifted in frequency by a small amount, 5Khz in the illustrated version, prior to being combined, in a hybrid 10, with the other signal and fed through the receiver 8 for amplification.An analogue to digital converter 11 in the processor codes the 5Khz receiver detector output, for example by taking 10 bit samples every 4.0 usec, and two microprocessors 12 and 13 extract bearing information. Microprocessor 12, which comprises these elements within the dashed box, is continuously involved in element phase measurement thereby achieving maximum correlation times, and controls via inputs 5 which diodes are energised and thus which antennas are sampled, whereas the microprocessor 13 serves to apply the outputs of microprocessor 12 and frequency data from the receiver to a beam forming algorithm and to thus extract bearing information. The processor thus serves to add, in the hybrid 10, the radio frequency components, one of which is phase shifted by the offset generator (phase shifter 9), thereby generating a waveform the envelope phase of which is determined by the phase of the off-set generator, which is known, and the phase difference between the two original waves, which is that to be measured. The microprocessor 12 has as its function to measure the phase difference between the reference 5Khz control and the resulting 5Khz detected tone derived by detection in the receiver 8. As will be appreciated from the drawing, the microprocessor 12 employs conventional sine/cos (I/Q) techniques to produce the required phase difference between antennas for the bearing determining microprocessor 13.

Thus, the phase difference between the radio frequency signals received at two antennas is established using a narrow bank, communications receiver together with a processor unit. One radio frequency signal is offset in frequency by a small amount using a phase shifter, in the processor unit, capable of 3600 shift. The offset signal and the other radio frequency signal are added in the processor unit and fed through the receiver for amplification. Detection of the envelope, by the receiver, which represents a single side band pair yields a low frequency component whose phase is defined exactly by the phase difference of the original signal pair and the phase shifter control wave. Measurement of phase difference between the reference control wave and the receiver output wave gives the required information. Any effects due to receiver delay may be eliminated by interchanging the original signal components as applied to the processor unit. There is thus provided a means of measuring differential phase and thus bearing without imposing a difficult requirement on the mechanism of the receiver and in fact allowing a conventional receiver to be used whilst providing an improved direction finding result.

Since the receiver tends to be the expensive part of the overall system, the ability to use the above described processing arrangement with a conventional receiver is particularly advantageous.

Whereas the specific embodiment involves five antennas it will be appreciated that in general two, or more, antennas are involved and these are taken in pairs, and the phase difference from one or more pairs of antennas is converted into a bearing value.

Claims (1)

1. CLAIMS.

   1. A radio direction finding system comprising two receiving antennas, phase shifter means for offsetting one received signal in frequency by a predetermined amount; phase shifter control wave means serving to rotate the phase shifter phase at a predetermined reference rate; means for combining the offset signal with the other received signal; means for amplifying the combined signal and detecting the envelope thereof; means for measuring the phase difference between the phase shifter control wave and the detected envelope, which phase difference corresponds to the phase difference between the two received signals; and means for converting said phase difference into a bearing value.

   2. A radio direction finding system as claimed in claim 1 wherein said amplifying and detecting means is comprised by a narrow band, communications receiver.

   3. A radio direction finding system as claimed in claim 2 wherein the phase shifter means, phase shifter control wave means, combining means, phase difference measuring means and phase difference converting means comprise a signal processor unit which is coupled to said receiver-.

   4. A radio direction finding system as claimed in claim 3 and comprising a plurality of receiving antennas from which said two receiving antennas are selected by a commutator unit under the control of the processor unit.

   5. A radio direction finding system as claimed in claim 4 wherein the phase difference measuring means and the phase shifter control wave means are comprised by a first microprocessor.

   6. A radio direction finding system substantially as herein described with reference to the accompanying drawing.

   8. A signal processor unit and receiving antenna subsequently for use with a narrow band, communications receiver, which processer unit provides direction finding information in response to signals received from two antennas1 the processer unit including phase shifter means for offsetting one received signal in frequency by a predetermined amount; phase shifter control wave means serving to rotate the phase shifter phase at a predetermined reference rate; means for combining the offset signal and the other received signal; an output whereby in use of the sub-assembly the combined signal is applied to the receiver for amplification and detection; an input whereby in use of the sub-assembly the receiver detected signal is input to the sub-assembly; means for measuring the phase difference between the phase shifter control wave and the envelope of the receiver detected signal in use of the sub-assembly, which phase difference corresponds to the phase difference between the two received signals, and means for converting said phase difference into a bearing value.

   9. A sub-assembly as claimed in claim 8 and comprising a plurality of receiving antennas from which said two receiving antennas are selected by a commutator unit under the control of the processor unit.

   10. A signal processor unit and receiving antenna sub-assembly, for use with a narrow band, communications receiver, and substantially as herein described with reference to the accompanying drawing.

   11. A method of radio direction finding using two antenna elements comprising the steps of employing phase shifter means to offset one received signal in frequency by a predetermined amount; rotating the phase shifter phase at a predetermined reference rate by phase shifter control means; combining the offset signal with the other received signal; amplifying the combined signal and detecting the envelope thereof by means of a narrow band communication receiver; measuring the phase difference between the phase shifter control wave and the detected envelope, which phase difference corresponds to the phase difference between the two received signals, and converting the said phase difference into a bearing value.

   12. A method as claimed in claim 11 and using a plurality of antenna elements, further including the step of selecting two said antenna elements from said plurality prior to said offset and combining steps.

   13. A method of radio direction finding using two antenna elements substantially as herein described with reference to the accompanying drawing.