Whilst satellite navigation will no doubt one day become our principal means of getting from one place to another, the current GPS system is fallible and should be used with knowledge and caution, not blind faith. The satellite clock may drift off time, the satellite may stray from its orbit, or its transmitter may simply fail. It can take up to two hours for such failures and errors to be resolved. Position errors up to 2 km have been reported. Also, the GPS signal received from the satellite is at very low power and is vulnerable to interference.

Equipment permanently installed in an aircraft must be fitted in a manner approved by the CAA. Where a hand-held unit is carried it, its antenna and any leads and fittings should be secured in such a way that they cannot interfere with the normal operation of the aircraft’s controls and equipment, and do not inhibit the pilot’s movements or vision in any way.

Before using GPS equipment in the air, pilots should learn about the system in detail. If no suitable instructor is on hand, practise using the equipment on the ground in advance.

Plan the flight and prepare a map and log in the normal way. Enter route information from the log directly into the receiver as a ‘Flight Plan’. Deleting/moving/changing the names of existing User Waypoints should be prohibited if the GPS is operated by more than one pilot. If there is no map, or it is too small to be of practical use, compare each individual track and distance as displayed on the GPS screen with your previously prepared flight log.


The GPS system should NEVER be used in isolation. The risk of loss/degradation of the signal, with the attendant possibility of a position error, is genuine. More importantly, the risk of human error in data input and display reading is high & such errors can go unnoticed until it is too late. To avoid becoming totally dependent on the GPS, ask yourself two questions regularly throughout the flight:
·         Does the GPS agree with at least one other independent source of navigation information?
·         If the GPS quits completely, right now, can I continue safely without it?
If the answer is “Yes” to both questions, you can continue to use the equipment for guidance. However, if the honest answer to either one of the questions is “No”, then you should establish navigation by some other means.

Re-programming in the air is likely to produce human error. To avoid the need, pre-plan possible route changes, for example around controlled airspace in case you cannot obtain clearance, or around high ground in the event of bad weather. Note the ICAO designators of all suitable diversion aerodromes.

If you do need to change your planned route in flight, make at least a rough set of mental calculations (and note them down) BEFORE you turn onto the GPS track. Then if your new heading does not agree with your mental calculations, you will know there is an error somewhere for correction. Check the new route on a map for terrain, any NOTAM activity, and controlled or restricted airspace.

Pilots have been known to produce and follow their own approach procedures using GPS information. User-defined approaches can be dangerous and are not authorised.

Tony BirthComment