VOR waveform synthesis and calibration

The VHF omnirange radial system (VOR) is used extensively as an aircraft navigational aid, and the need for greater utility of VOR has led to a requirement for greatly improved standards and traceable calibration facilities in this parameter. The paper presents the development and assessment of two standard VOR waveform synthesisers and a VOR waveform decoder. The synthesiser designs use the technique of regeneration of a wave shape which has been ?fixed? by digital methods in an addressable memory. The technique has been refined and brought to a state of high precision. One synthesiser is a complex desk-top computer-controlled system which, as well as being a standard, is excellent for producing imperfect waveforms. The second synthesiser standard is a compact portable unit suitable for field system testing and standards dissemination. The decoder design is based on digital sampling and storage of one cycle of the composite ?audio? waveform: this data is then Fourier transformed and analysed to determine the FM modulation depth and the differential phase of the two 30Hz modulating signals?the VOR bearing angle. The standard synthesiser and decoder standards have demonstrated uncertainties not exceeding ï¿¿ 0.03ï¿¿ (95% confidence level). An extensive and detailed analysis of the sources of uncertainty is presented, particularly with regard to a ?digital? decoder operating with a signal produced by a ?digital? synthesiser. The uncertainties are tackled on two fronts: first by computer simulation not involving any hardware or genuine data transfer, and secondly by analysing data transferred from synthesiser to decoder. By these means, various sources of uncertainty are identified and assigned. The sensitivity of the detection method to digitisation uncertainties, asynchronism between decoder and generator, anharmonic 9.96 kHz subcarriers and random amplitude and phase noise was investigated. Measurements are also made on transmissions from local VOR beacons.