98034 Radio Interference with GPS

MARS 98034 Radio Interference with GPS
Report No. 98034

I recently returned from a 7 month tour of duty aboard a 2,000 ton survey ship designed for shallow water surveying. The ship was civilian manned but had naval oceanographers on board and was capable of receiving military GPS with no selective availability error. After a refit in Italy last September, we noticed that in certain areas of the world on certain headings we would lose the incoming GPS signal on the military GPS and the regular non-military GPS. Although civilian GPS is prone to periodic "locking up" when the screen "freezes due to selective availability error, the military GPS is supposed to be immune to this. The mystery deepened when it was noticed that both systems were locking up and appeared not to be receiving any incoming signal, and that this failure was occurring simultaneously in both systems.

After much trial and error and experimenting, we determined that the GPS signals were not being received on those particular headings only when a voice telephone call via Inmarsat A was being conducted from ship to shore or vice versa. What makes this all the more odd is the fact that while the Inmarsat A "golfball" antenna is on top of the pilot house, the military GPS antenna is at the very top of the radar mast, approximately 50 feet above the Inmarsat A antenna.

Our only hypothesis is that when we are in certain areas of the world, the satellite is at such an elevation for Inmarsat that the signal from the ship is "beamed up" at a perfect angle to intercept the GPS antenna. On most headings this causes no problems, but if the satellite is behind the ship, the satellite antenna will be pointed right to the top of the mast and the GPS antenna.

Whilst this problem is not chronic or life threatening, it is annoying, especially to the surveyors, who cannot use any oceanographic data obtained whilst the signal is disrupted. It has also helped me to explain to the junior navigating officers why they should not rely solely on GPS for position fixing when near to the shore.