Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Patent
1990-01-04
1991-05-14
Blum, Theodore M.
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
G01S 144
Patent
active
050160192
DESCRIPTION:
BRIEF SUMMARY
This invention relates to processing synthetic-aperture aerial signals and in particular to methods and apparatus for generating directional polar diagrams with such aerials. The invention has particular application to radio direction-finding (DF) systems for determining the azimuth bearing of radio emissions, e.g., from aircraft, and, reciprocally, to radio beacons for enabling e.g., an aircraft to determine its azimuth bearing from a fixed ground transmitter. However, the invention is also applicable in other instances where a directional polar diagram is required, e.g., in aircraft short-range approach and/or landing aids.
The performance of DF and beacon systems depends on generating polar diagrams of sufficient directivity to discriminate against reflections from directions outside the direct line between the ground-station and e.g., the aircraft; such reflections can introduce what are termed "site errors" (even when remote from the ground-station) in the determined bearing. Present systems using synthetic-aperture aerials, such as VOR (VHF Omni-Range) beacons, give good site-error performance provided the wanted (direct) signal is greater than the unwanted reflections and provided the wanted signal exceeds a given amplitude level. If these two criteria are not met, the performance suffers a drastic decline, and this is also true of DF systems using such aerials. For this reason the term "synthetic-aperture" is not strictly appropriate in relation to such systems, as will be explained hereafter.
This decline in performance results from the inclusion of an amplitude-limiter in the signal-processing circuits of these systems. The present invention provides a method of processing the signals received by the aircraft (in the beacon case) or from the aircraft (in the DF case) which allows elimination of the amplitude-limiter and thereby alleviates the site-error problem at low signal amplitudes and/or when the unwanted signal exceeds the wanted signal, caused by its inclusion. Other advantages of its elimination will become apparent from the description.
According to the present invention a method of processing signals received by a synthetic aperture aerial (as hereinafter defined) from an external radiator, or received from said aerial at an external position, said signals including a carrier frequency which is phase-modulated at the scanning frequency of said aerial, comprises: and phase-modulated at the scanning frequency; which includes a selectable Bessel Function representing a directional polar diagram; configuration) of the scanning frequency, as applied to produce said phase-modulation of the generated signal, in order to maximise or minimise, as appropriate, a selected Bessel Function and thereby control the bearing direction of said polar diagram relative to the aerial.
The generated carrier may be shifted by an increment in order to average-out phase differences between the received and generated carriers.
The component of the multiplied output unmodulated by the scanning frequency may be selected, constituting a zero-order Bessel Function whose maximum value represents a polar diagram having its main lobe in the bearing direction. Alternatively or additionally the component of the multiplied output modulated by the scanning frequency or a harmonic thereof may be selected, constituting a first-order or higher-order Bessel Function whose minimum value represents a polar diagram having a null in the bearing direction.
The method may comprise adding two fixed scanning-frequency phase-shifts or amplitude-shifts to said controlling phase of the scanning-frequency in opposite senses, the two summed phases being applied to phase-modulate or amplitude-modulate the carriers of two generated signals which are separately multiplied by the received signal to derive outputs constituting two zero-order Bessel Functions whose maximum values represent a polar diagram having two overlapping main lobes which are symmetrical about the bearing direction.
The selected first-order or higher-order component may be
REFERENCES:
patent: 3886555 (1975-05-01), Royal
patent: 4078234 (1978-03-01), Fishbein et al.
patent: 4176351 (1979-11-01), Devita et al.
patent: 4617567 (1986-10-01), Chan
Blum Theodore M.
The Secretary of State for Defence in Her Britannic Majesty's Go
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