Data from global-positioning-system (GPS) receivers are too unreliable to be used for sole-means navigation by aircraft, according to a study undertaken by the UK Civil Aviation Authority's National Air Traffic Services (NATS).
NATS made the claim after its own trials revealed problems with GPS "outages", availability and integrity, and possible software bugs. One study evaluated data derived from a GPS receiver installed on an in-service British Airways Boeing 747-400 between April 1994 and December 1995. A second study used data from four receivers located on the ground at London's Stansted Airport, for the 12 months to August this year.
NATS says that the two projects "-have illustrated clearly that there are far too many failures in the GPS receivers examined. A considerable reduction in such failures will need to be achieved before GPS can be considered a reliable sole-means system."
The 747 data revealed 28 "outages", each lasting an average of 81min, during which the aircraft would have travelled around 1,100km (600nm). NATS says that, while it cannot prove whether the problems were caused by avionics failures or so-called signals-in-space, "-what is certain is that an air-traffic-management system cannot be based on position reports in which more than 3% of the messages hold a useless GPS position report".
Once, the Stansted project recorded a simultaneous outage affecting two identical receivers, prompting NATS to say that adequate redundancy may have to be achieved using dissimilar receivers, or GPS receivers coupled with inertial platforms.
Source: Flight International
