Europe passes GPS-approach milestone in Sweden

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A major step towards the use of satellite navigation for landings has taken place in Sweden with the successful completion of the first Global Positioning System (GPS)-guided approaches using a different technique from the US model.

The Swedish CAA has completed the first check-flight of a differential GPS-guided approach at Ångelholm airport as part of the European NEAP (North European Automatic Dependent Surveillance-Broadcast (ADS-B) Network Applications) programme.

A Raytheon Beech King Air 200 was used for the successful test-flight yesterday, and now two SAS Fokker F-28 regional jets will also take part in the trial activity.

Although the work is very much research and development, SAS would be particularly anxious to see a GPS approach at Ångelholm eventually, because the airport has a notoriously challenging non-precision approach procedure for one runway-end at present.

The basis of the Ångelholm work is similar to that of the DGPS Special Category I (SCAT-I) approach work already done, mostly in the USA, by vendors such as Honeywell. A ground reference station at a surveyed location measures the difference between its known position and its GPS-derived position and then datalinks the error corrections to the approaching aircraft.

As in the US work, the data is ultimately used to drive the pilot's conventional instrument landing system (ILS) instrument in what is known as an "ILS-lookalike" approach. At Ångelholm, Raytheon E-Systems provides the ground station, and SAS Flight Support produced special approach plates.

The Ångelholm project differs in three major ways from the US activity, however: the first is that the required aircraft trajectory is stored on board rather than being datalinked from the ground station.

The second, and more important, is that the datalink used is the STDMA (self-organising, time-division, multiple-access) technique with which NEAP is experimenting for a variety of purposes. The US projects have so far used the alternative DO-217 system.

The third difference is that the approach is strictly speaking an overlay of a non-precision approach (ie without glideslope information) with vertical guidance added - a concept under consideration in ICAO navigation panels.

The two SAS F-28s are already equipped with the Swedish developed GPS/STDMA transponders used in the wider NEAP (and the original NEAN) programme for the ADS-B work. SAS F-28 fleet engineer, Per Ahl, explains: "We are aiming to see the whole closed loop with ADS-B and to use it to broadcast the aircraft position all the way down to the ground, then to the apron and eventually gate-to-gate, and use the differential guidance to make this approach down to non-precision approach minima."

Ångelholm has ILS on runway 14 and an NDB approach to runway 32, and it is surrounded on three sides by terrain up to around 7-900ft. SAS has devised GPS procedures for both ends, but performs the trial work only in visual conditions and in any case only with a 650ft decision height. Ahl says that an operational DGPS procedure could eventually take that down to about 250ft.

The trial is also being used to examine other issues relating to the approaches, including the practical limits of the localiser intercept-angle, and the important question of whether or not to emulate, on the ILS-lookalike approach, the increasing sensitivity of conventional ILS guidance as the runway is approached.

SAS will now perform a small number of dedicated trial flights with volunteer crews using the procedure and will later gather data over a longer period as and when the equipped F-28s visit Ångelholm in normal revenue service.