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Satellites Aid Surveying, Aircraft

Land surveyors and airport landing systems in New Zealand are benefiting from sophisticated satellite technology, with the development of new, highly accurate global positioning systems (GPS).

GPS systems use a constellation of 24 US government satellites orbiting the Earth 20,000 kilometres up. Receivers on Earth pick up signals from these satellites and determine precise positions by comparing distances to several different satellites. Even though the satellite group has only just become completely operational, many large, diverse markets using this technology have already opened up.

Land surveying is made easier by the first commercially available, centimetre-accurate, real-time GPS survey system, from Trimble New Zealand, the local subsidiary of Silicon Valley company Trimble Navigation Ltd. The New Zealand company has been been producing software for surveyors since the early '80s and developed the data collection and processing software for Trimble's Real Time Kinematic (RTK) Site Surveyor GPS survey system.

GPS has already become the most cost-effective method for performing control surveys. With RTK, it can now provide centimetre accuracy for the full range of traditional land survey applications, from geodetic control and topographic mapping to road stake-out and navigation.

Trimble has also been involved in the first use of a GPS-based instrument landing system (ILS) at several New Zealand airports. The ILS system uses special techniques to overcome errors in the GPS system, enabling it to be made more precise for high-accuracy applications such as aircraft management. Errors are typically caused by ionospheric and tropospheric variations, inaccuracies in satellite atomic clocks and receiver clocks, and Selective Availability -- the intentional degradation of the system for strategic purposes by the US Department of Defense.

Recently at Auckland Airport, a Piper Navajo made several approaches with the aid of a GPS receiver and a differential reference station at Ardmore Airport 12 kilometres away. Differential GPS requires a reference station at a known location to broadcast correction data between its GPS and known positions to moving receivers in the area. All approaches were successful, both those manually flown and those operated by auto-pilot. Precision approaches were also set up at Ardmore, where the airport has no navigation aids, and has a row of hills to the east making a direct runway approach difficult. The results were excellent, and further successful demonstrations have been carried out at Wellington Airport.