Australia  caution re use of GPS and DGPS The Australian Maritime Safety Authority (AMSA) issued a notice reminding mariners that even GPS and DGPS are not infallible and should not be used as the sole means of position-fixing. Older GPS receivers lack certain capabilities. Further, GPS read-outs are based on the WGS-84 geodetic datum, with the result that GPS data may not translate accurately onto charts using other datums. Note: The inability of most GPS receivers installed before 1 July 2003 to conduct Receiver Autonomous Integrity Monitoring (RAIM) is known, but not well publicized. This means that the older receivers will continue to utilize signals from a GPS satellite even when the satellite is malfunctioning (newer receivers automatically discount such erroneous signals). Older GPS receivers also lack course over ground and speed over ground output and are less able to cope with signal interference. Marine Notice 29/2007 (12/21/07).

> -----Original Message----- > ............. Further, GPS read-outs are > based on the WGS-84 geodetic datum, with the result that GPS data may > not translate accurately onto charts using other datums. Lee, That was not completely accurate. The datum to be used is a function of a user setting in the receiver. While most GPS receivers come from the factory with WGS-84 set as the current datum, it is the user's responsibility to reset the datum if using the receiver with charts based on a different datum. All GPS users should check the datum in their receiver against the datum on the charts they are using. If you switch charts to one using a different datum, it is your responsibility to switch the datum in the receiver so that the proper Lat/Lon will be displayed for that chart datum. This is a common problem with US users bring a held help GPS receiver down to the Caribbean for a sailing charter vacation. The charts down there use datums that vary from chart to chart. Heck, some of the charts down there still reference surveys done by Capt. Cook in the 18th century! Kevin