On a hill just south of the Magaliesberg in Gauteng, at SANSA's space operations Hartebeesthoek facility, there is a short pole called Doris. To the casual observer it's just a simple white pole with chipped paint and some electric wiring down the side. Lost in the sea of imposing antennas and other equipment, it really doesn't look like anything special.
To the space operations industry, however, Doris the pole is a sophisticated piece of equipment vital to the aviation industry and worthy of daily monitoring and attention.
The biggest physical task for Doris is to stand still in a fixed position. This is not so easy in soil which naturally expands and contracts with temperature changes, so Doris consists of steel pipe filled with cement and sunk into a hole drilled into the bedrock. The steel pipe, perforated for the first four metres, has a 10mm thick wall and a diameter of 28cm. Once it was sunk into the six metre deep hole, it was filled with concrete and vibrated to let the concrete seep through the perforations, binding to the bedrock wall. Thus, an incredibly stable platform was created for the highly sensitive sensor.
Mounted on Doris is a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) transmitter system which works with satellite payloads to determine extremely precise orbit and ground locations. These transmitters, on poles just like this one around the world, send signals from their fixed locations to satellites in low Earth orbits. South Africa hosts another DORIS antenna on Marion Island, which was upgraded by Pierre Cilliers from SANSA in 2010.
The system was designed by the French space agency CNES to determine the precise orbit locations required for observing Earth's oceans. Doris data has also become valuable in geophysics, helping researchers to measure continental drift, monitor geophysical deformations, and determine the rotation and gravity parameters of Earth.
SANSA's Carlos de Oliveira looks after Doris, knowing that any malfunction would cause a blind spot in a global network which is used extensively in navigation.
The national mapping and surveyor agency in France, the IGN, uses data from Doris to contribute to the international reference system, and a Doris beacon on the Dorsal Glacier in East Antarctica is providing data on the movement of ice in this remote location.
The Doppler Effect and Malaysian flight MH370
The Doris system is based on the Doppler Effect. Austrian physicist Christian Doppler theorised that a quantifiable effect is observed when a source of light or sound waves is moving in relation to a stationary observer. This principle is now used in a variety of location-orientated applications. Recently, Inmarsat 4-F1 communications satellite engineers identified two paths that the missing Malaysian flight MH370 could possibly have taken by using the Doppler Effect and data from their satellite.