Doppler VOR

The acronym VOR stands for VHF Omni Range.

No, it's not a helicopter landing pad. It's a VOR antenna array

A VOR is a navigational aid that operates in the very high frequency part of the frequency spectrum (108 - 118 MHz). A VOR forms part of rho-theta (flq) navigational systems. Using these systems, a pilot can accurately and reliably determine his bearing and distance from a reference point. The VOR provides the bearing
(q) information. VOR stations are ground based and could be used for enroute as well as terminal purposes.

A VOR station is assigned a particular carrier frequency, to which the pilot would tune his airborne equipment This means that no two installations within interfering distance can have the same carrier frequency. Instrumentation in the aircraft is calibrated in 360deg. with Magnetic North being the 00 reference.

The VHF carrier is modulated by two 30 Hz signals. The phase difference between these signals carries the bearing information.
The carrier is also modulated by a 1020 Hz Morse code identification signal (for identifying a particular station) and by voice (300 Hz- -3,000 Hz).

The bearing information is extracted from the two 30 Hz signals by measuring the phase difference between them. The bearing of an aircraft with respect to the VOR station could be between 00 and 360deg. The phase difference between two signals could also be between 00 and 360deg . This means that the phase difference between the two 30Hz signals is a direct representation of the aircraft’s bearing.

The phase of one 30 Hz signal is kept constant all around the station. This signal is called the reference signal. The phase of the other 30 Hz signal is allowed to vary from 00 to 360deg. around the station. This signal is called the variable signal.

There are two types of VOR systems in use. The older Standard VOR and the newer Doppler VOR. In the Doppler VOR system, the carrier is amplitude modulated by the reference signal and frequency modulated by the variable signal. The Doppler Principle states that there is a change in frequency of a signal received when the distance between the source and receiver changes. When the distance decreases, the frequency increases. The opposite is true when the distance increases.

The reference 30Hz signal is obtained by amplitude modulating the VHF carrier with a 30 Hz sine wave signal. This signal is usually horizontally polarised for superior noise performance. The signal is radiated omnidirectionally so that its phase is independent of the aircraft’s position. The other signal has a phase that changes 1 degree for each degree change in bearing around the VOR station.

Because the 30 Hz signals have to be separated, a sub-carrier is introduced. This sub-carrier, in the form of two sidebands is 9960 Hz above and below the carrier frequency (Fc). SB1 known as the upper sideband is Fc + 9960 Hz and SB2 known as the lower sideband is Fc - 9960 Hz.

The sidebands are positioned on opposite ends of an imaginary arm. This arm is rotating at 1800 rpm (30 revolutions per second). To the observer in the far field, the frequency of the sidebands is changing at a 30 Hz rate (because of the Doppler effect). The sub-carrier is therefore frequency modulated at 30 Hz. The sub-carrier then space modulates the VHF carrier.

Because of the rotation of the sidebands, the phase of the variable signal is different for different positions of the aircraft. For an aircraft at magnetic north, the phase difference between the 30Hz signals is 00deg. This means that the two signals must be passing through their zero crossing at the same time. The rotation of the sidebands is done electrically.

The sidebands are radiated from a number of sideband antennas (usually 48). SB1 and SB2 are situated diametrically opposite each other on the imaginary rotating arm. If 48 antennas are used, then they are numbered from 1 to 48 (with antenna 1 at magnetic north) in an anticlockwise direction.
Energy is supplied to antenna 1 then antenna 2 until all 48 has been energised. When antenna 1 is being energised by SBl, antenna 25 is being energised by SB2.

Hope this helps, any questions welcomed.

73
Berni, G0IDA