This photo shows the Melbourne/Tullamarine T73A Doppler VOR (DVOR) on 24 April 2007. This navaid is located to the north of Runway 27 and east of Runway 34 and was commissioned in June 2001, replacing an earlier unit. The photo shows the antenna system, which is fed by dual (redundant) transmitter units in the green hut. One transmitter is active while the other is on 'hot standby', with automatic changeover in the event of an alarm or failure. The transmitter power is about 50W.

The single white antenna at left is an antenna for a co-located DME.

The DVOR equipment is an Interscan VRB-52D, the designation 'T73A' being in-house to Airservices Australia. An initial batch of four T73A DVORs were commissioned during 1998-2001, however Airservices Australia ordered another batch of 23 T73A DVORs for delivery during 2005-8 as part of its Australia-wide navaid replacement program. This unit replaced an earlier T64 conventional VOR which was installed about 1978.

The VOR system uses the phase relationship between a reference-phase and a rotating-phase signal to encode direction. The omni-directional, VHF carrier signal on 9960 Hz is modulated by two 30 Hz signals: a reference and a rotating variable signal. Measuring the phase difference between these signals enables direction from the station to be derived. The variable signal is derived from the rotation of a directional antenna array 30 times a second. Although older VOR antennas were mechanically rotated, Doppler VORs electronically rotate the signal around the ring of 48 white antennas at a rate which will generate a 30 Hz frequency shift at a point in space due to doppler effect, which achieves an equivalent result with no moving parts. (The structure under the antennas is a counterpoise to provide a good ground plane.) The Doppler VOR offers superior reliability, accuracy and less susceptibility to interference from terrain, buildings etc.

Interscan Navigation Systems is the descendant carrying on the aerospace business of Amalgamated Wireless (Australasia) and, as such, traces its heritage back to 1900. The company is headquartered in Sydney, but is currently (2008) wholly foreign-owned. In addition to the DVOR, Interscan manufactures DME and MLS equipment.

Left: A T73A DVOR transmitter on test in Airservices Australia's Melbourne Component Maintenance and Repair Depot in June 2008.

Click here to see photos of the T73A DVORs at Hobart and Launceston

(Photos: top-Phil Vabre; bottom-Ian Jager)

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