Why have some AIS units stopped broadcasting valid position reports?
On February 27th, 2008 the GPS constellation increased to 32 satellites (PRN 32) thus providing a 5% increase in satellite availability and DOP (dilution of precision) world-wide. It has come to our attention that some (non-USCG type approved) AIS units—particularly old equipment which is non-compliant with the GPS interface standard (IS-GPS-200)—cannot recognize this additional satellite and subsequently are unable to calculate a position and broadcast a valid AIS Position Report. Note, the reported malfunctioning units do continue to receive position reports and are able to send and receive AIS text messages. Owners of AIS equipment denoted at http://www.navcen.uscg.gov/lnm/Saab_R3_AIS_prob.htm, however, should be aware that their internal GPS systems may not act as a proper—timing or position—back-up under certain circumstances, i.e. when in view of PRN32. AIS users must ensure their units have or are interfaced with a properly operating Electronic Position Fixing System at all time
