Any private pilot who trusts his/her hand-held global positioning system satellite navigation system receiver implicitly is likely to have their confidence in the Navstar GPS severely dented by a recently issued UK Civil Aviation Authority Safety Sense Leaflet (SSL).

The CAA, while acknowledging that "the GPS system has shown exceptional reliability", also adds that "it has been known to suffer technical and human failure" and insists it must not be used as a sole means of navigation. Back-up can come from anything from visual position fixes to terrestrial navigation beacons.

The long list of potential accuracy reducers listed and explained by the SSL includes:

Availability and geometry - satellite relative position varies and the accuracy of fixes can vary accordingly. The variation away from high accuracy is known as dilution of precision; Failure/error - "The satellite clock may drift off time or its transmitter may fail. At such times, position errors up to 2km have been reported." This can go undetected even by receivers fitted with the receiver autonomous integrity monitor, and it can take up to 2h to rectify such failures/errors, says the CAA; Dynamic masking and terrain shielding - aircraft parts can attenuate signals depending on aircraft attitude and satellite positions, and if the aircraft is low, terrain can block signals; Multi-path reflections: signals can bounce off terrain and give range errors from the satellite. These errors are usually small, the CAA says, and can show themselves as a sudden apparent shift of position; Interference and jamming: signals are "highly vulnerable" to interference, either intentionally caused or from television signal broadcasting, harmonics of radio transmissions and navigation beacons. Notices to airmen will publish details of areas in which GPS is being intentionally jammed for military exercise purposes; Sunspots can affect signal transmissions, and their effects cannot be predicted; Selective availability - since the US Department of Defense owns the GPS constellation, it can move satellites to improve signal availability in a particular area to the detriment of other areas; General advice on equipment usage that the CAA gives includes: shared-use hand-held receivers should be checked by each user to ensure that waypoints previously entered have not been changed by other users; on start-up, check the status of the equipment and its battery; and before moving off blocks, check the indicated position against known position; in flight, regularly check GPS position against visual fixes or terrestrial navigation aid fixes.

Source: Flight International