GB2069282A - Simulator for Doppler VOR signals - Google Patents

Abstract

A simulator for Doppler VOR signals includes sources for carrier frequency (1), 30 Hz reference frequency (2), variable phase 30 Hz frequency (3) and baseband frequency (4). Programmable read only memories (PROM's) (8a, 8b) control up/down counters (5a, 5b) to generate antenna phase jump functions which in turn drive sinusoid function generating PROM's (6a, 6b) the outputs of which modulate a proportion of the carrier frequency to generate simulated upper and lower sidebands. The sidebands are further modulated by outputs by PROM's (20a, 20b) which impose blending laws on the sidebands before the sidebands are combined (25) with the rest of the carrier and the 30 Hz reference signal. The variable phase 30 Hz signal controls a phase locked loop including counters (17a, 17b) from which stepping control signals are derived for the phase jump PROM's. <IMAGE>

Description

SPECIFICATION Simulator for doppler VOR signals This invention relates to simulators for Doppler VOR signals, and in particular for alternating sideband (ASB) Doppler VOR.

The Doppler VHF omnidirectional range (VOR) is an internationally accepted radio navigation aid for air traffic control systems. Both the basic Doppler VOR and a modification thereof known as ASB Doppler VOR are well documented, for example in "Radio Navigation Systems for Aviation and Maritime Use", edited by W. Bauss, published by Pergamon Press, 1963, in Chapter 2.04, and in Electrical communication, Vol. 50 No.4, 1975 at pages 245248. Briefly the basic VOR beacon operates with a carrier frequency in the range 108 to 118 MHz and radiates a variable phase amplitude modulated signal together with a reference signal. The phase difference between these signals corresponds to the azimuth and is measured by the receiver in the aircraft.

A 30 Hz signal, generated by rotating a VHF figure-of-eight directional pattern at 30 revs/sec., provides the variable phase component. This directional pattern is superimposed on the omnidirectionally radiated carrier, thus producing at the receiver a signal that is amplitude modulated at 30 Hz, with the phase of the modulation related to the azimuth. The reference signal is also a 30 Hz waveform, but it is used to frequency modulate a 9960 Hz sub-carrier with a frequency shift of + 480 Hz. The carrier frequency is then amplitude modulated by the 9960 Hz sub-carrier. This ensures adequate decoupling between the variable signal and the reference signal which have the same frequency. In addition to carrying the reference signal the carrier is amplitude modulated with the identity signal (1020 Hz), and possibly with speech (300 to 3000 Hz).

In the Doppler VOR the functions of the two 30 Hz signals are interchanged with respect to the basic VOR. This means that the 30 Hz which amplitude modulates the VHF carrier is now the reference signal and the variable 30 Hz signal frequency modulates the 9960 Hz sub-carrier. The signals are radiated from a static aerial array which simulates electronically a mechanically rotating aerial. A number of individual antennas arranged in a circle are sequentially fed with the sideband signal in such a way that the radiation centre moves round the circle at the prescribed speed. In a double sideband version two sidebands, the radiation centres of which move in the same angular direction, are radiated by opposite points on the circle of antennas.

A modified double sideband method is also used which is known as the alternating sideband method (ASB). This is derived from the double sideband method by omitting every second sideband antenna, on the assumption that the resulting number of antennas is odd. The simulated rotation of the sideband radiation is identical to that of the double sideband method, although the radiation transition is not from one antenna to the adjacent antenna, but to an antenna which is approximately opposite on the circle and which is radiating the other sideband.

It is obvious that a crucial factor in the successful and safe operation of the Doppler VOR system is proper and accurate alignment of the aircraft receiver. This can only be achieved using eitherthe signals from a beacon or from a simulator. One known form of simulator uses a costly radio frequency commutator and a multiplicity of coaxial cables which are accurately cut to different lengths, each cable representing one of the antennas of the circle. This simulator is expensive and bulky and is awkward to set up.

According to the present invention there is provided a simulator for Doppler VOR signals including means for generating a VHF carrier signal, means for generating a 30 Hz reference signal, means for operating a 30 Hz signal having a variable phase with respect to the 30 Hz reference signal, means for generating a baseband signal the frequency of which is both an integral multiple of 9960 and an integral multiple of 30n, where n is the number of or a multiple of the number of antennas in the circle of a DVOR beacon to be simulated, programmable logic means which is fed with the baseband signal, means for deriving from the variable phase 30 Hz signal a programme stepping control signal the frequency of which is 30n Hz which control signal is applied to the logic means which is programmed to generate sideband modulating signals having prescribed phase steps and blending laws at the baseband frequency, means for modulating a proportion of the VHF carrier signal with the sideband modulating signals, means for modulating the remainder of the carrier signal with the 30 Hz reference signal and means for combining the modulated carrier signals to provide the stimulated DVOR signals.

A preferred embodiment of the invention includes two separate programmable logic means which are stepped alternately to generate separate upper and lower sideband modulating signals, the baseband signal frequency being both an integral multiple of 9960 and an integral multiple of 30n, where n is twice the number of antennas in the circle, said number of antennas being an odd number.

An embodiment of the invention will now be described, with reference to the accompanying drawing which depicts in schematic form a simulator for an alternating sideband (ASB) Doppler VOR.

The simulator requires, as does the normal VOR, a carrier frequency source 1 and a 30 Hz reference source 2. In addition it requires a 30 Hz source 3 the phase of which is variable relative to the 30 Hz reference.

The simulator is assumed to be simulating an ASB DVOR beacon having a circle of 39 antennas. A source 4 generates a baseband signal the frequency of which is 9960 x 2 n Hz, where n is the number of antennas in the circle. In the present example source 4 generates a signal of 776,880 Hz.

The upper and lower sidebands (USB, LSB) are each generated by a 39 step up/down counter 5a, 5b to which the baseband signal is fed. Each up/down counter feeds a "Sin" programmable read only memory (PROM) 6a, 6b the output of which is fed to a D/A converter 7a, 7b for the generation of a (78 step) 9960 Hz sinusoid. The basic 39 step counters 5a, 5b are each reset in their normal sequence of 9960 Hz, 39 times within each period of 30 Hz to values which represent the phase jumps which occur as each antenna is switched. These jumps conform to the ASB law as defined by the preprogrammed "ASB". PROM 8a, 8b.

The resulting 9960 Hz component from each sideband counter is amplified and filtered before being converted to a single sideband of the carrier from source 1. A portion of the carrier is taken from a power splitter 9 and a fed to a phase shifter network 10 which is arranged to produce two carrier signals in phase quadrature. Each of the quadrature signals is fed to a respective balanced modulator in pairs of modulators 11 a0o, 11 a90o, 11 b0o and 11 B90o in the USB and LSB channels respectively. Modulator 11 a90o is also fed with the 9960 component from D/A converter 7a whilst modulator 11 ago is fed with the same signal which has been phase shifted by 90".

Similar modulation occurs in the LSB channel. The outputs of USB pair of modulators 11 ago and 11 a90o are summed in network 12a and the outputs of the LSB modulators 11 boO and 11 boo are differenced in circuit 12b.

The 776.880 KHz signal from source 4 is also fed, via a divide-byfour network 13 to an adder/subtrac- tor circuit 14 which forms part of a phase locked loop. The network 14 feeds a further divide-byeightythree network 15, the output of which is at 2340 Hz. This is then divided by two in divider 16 to produce a signal at 1170 Hz. The 1170 Hz signal is used to drive two 39 step counters 17a, 17b the counts from which are produced alternately.

These interleaved count outputs operate the two "ASB" PROM's -8a, 8b. The phase of the 39 step ASB sequence is locked by means of the phase locked loop to the 30 Hz variable phase signal from source 3. To achieve this the two counters 17a, 17b drive a decode logic circuit 18 to produce a 30 Hz signal the phase of which is compared with that of the 30 Hz signal from source 3. The output of the phase detector 19 is fed to the adder subtractor 14to complete the phase locked loop.

Blending law functions are generated by pre programmed PROM's 20a, 20b, driven by n true and inverted samples of 1170 Hz signals from divider 16 and clock signals. The outputs of PROM's 20a, 20b are fed to DIA converters 21 a, 21 b respectively and the analogue outputs are used to modulate, in PIN modulators 22a, 22b the USB and LSB components from the sum and difference circuits 12a, 12b respectively.

The remainder of the carrier output from splitter 9 is first phase adjusted by shifter 23 and then modulated by the 30 Hz reference signal in PIN modulator 24 before being combined with the USB and LSB components in combiner 25. It is necessary to modulate the carrier with the 30 Hz reference signal within the simulator as it is undesirable to use an already modulated carrier in the sideband gener ators.

To achieve double sideband Doppler VOR signals logic changes only are required.

Claims (6)

1. A simulator for Doppler VOR signals including means for generating a VHF carrier signal, means for generating a 30 Hz reference signal, means for operating a 30 Hz signal having a variable phase with respect to the 30 Hz reference signal, means for generating a baseband signal the frequency of which is both an integral multiple of 9960 and an integral multiple of 30 n, where n is the number of or a multiple of the number of antennas in the circle of a DVOR beacon to be simulated, programmable logic means whch is fed with the baseband signal, means for deriving from the variable phase 30 Hz signal a programme stepping control signal the frequency of whch is 30 n Hz whch control signal is applied to the logic means which is programmed to generate sideband modulating signals having prescribed phase steps and blending laws at the baseband frequency, means for modulating a proportion of the VHF carrier signal with the sideband modulating signals, means for modulating the remainder of the carrier signal with the 30 Hz reference signal and means for combining the modulated carrier signals to provide the simulated DVOR signals.

2. A simulator according to claim 1 including two separate programmable logic means which are stepped alternatively to generate separate upper and lower sideband modulating signals, the baseband signal frequency being both an integral multiple of 9960 and an integral multiple of 30 n, where n is twice the number of antennas in the circle, said number of antennas being an odd number.

3. A simulator according to claim 1 or 2 wherein the or each programmable logic means includes and updown counter having n steps to which the baseband signal is fed, first programmable read only memory for resetting the upidown counter times in each period of 30 Hzto represent phase jumps occurring in a DVOR beacon when each antenna is switched, second programmable read only memory driven by the upidown counter to generate a digital representation of a 9960 Hz sinusoid incorporating said phase jumps, a digital-to-analogue converter to which said digital representation is applied and balanced modulator means to which the converter output is applied, the modulator modulating the proportion of the VHF carrier.

4. A simulator according to claim 3 wherein the or each programmable logic means also includes a third programmable read only memory driven by a signal the frequency of which is 30 n Hz, said third memory being programmed to generate a digital representation of an amplitude blending lawfunc- tion, a second digital-to-analogue converter to which said blending law representation is applied, and means for further modulating the output of the corresponding balanced modulator by the output of the second converter.

5. A simulator according to any preceding claim including means for dividing the baseband signal to a lower frequency, a phase locked loop to which said divided frequency is applied, the other input to the phase locked loop being the variable phase 30 Hz signal, the phase locked loop including further frequency dividing means to divide the already divided baseband signal down to 30 n Hz, and a counter or counters having n steps driven by the 30 n Hz signal, theoutputs of said counter(s) being the stepping control signal applied to the programmable logic means.

6. Asimulatorfor Doppler VOR signals substantially as described with reference to the accompanying drawing.