NMEA
About NMEA
NMEA is an abbreviation for National Marine Electronics Association. NMEA 0183 is an interface standard, which define a data transmission protocol and sentence formats among other things for communication between marine instrumentation.
GPS receivers
Almost all GPS receivers support NMEA 0183, which means that they are able to transmit GPS related information formatted as NMEA 0183 sentences via the serial port.
Please notice that many GPS receivers support several different protocols, so you need to make sure, that you have selected NMEA 0183 V2.0 or a newer version.
Due to limitations in the NMEA 0183 interface standard description, the implementation of NMEA 0183 support in the various GPS receivers are often quite different. This causes problems for applications like Cetus GPS which are connected to many different GPS receivers.
Each GPS receiver has its own set of NMEA sentences and sentence transmission interval. The most common is that sentences are repeated each second or every two seconds. As described under Satellite fix GPS receivers also have different ways of handling the situation where a satellite fix is lost.
Cetus GPS
Information like the current position, altitude and time are available via different NMEA 0183 sentences. For normal GPS navigation it would be enough to read the position from any of the sentences which contains it. But for professional positioning and tracking some accuracy information is required along with the position and altitude. This limits the number of useful sentences.
A very common used sentence for GPS navigation is the Recommended Minimum Specific GPS Transit Data (RMC) sentence. This sentence contains no accuracy information though, so Cetus GPS only uses it for determining the current speed, course, magnetic variation and date.
Instead Cetus GPS obtains position and accuracy information from the Global Positioning System Fix Data (GGA) sentence. It contains the position, altitude, time of fix, number of satellites being tracked, HDOP and DGPS information.
If a GPS receiver fails to transmit the GGA sentence at least once every two seconds when a satellite fix is available, Cetus GPS will not work. There is no workaround for this, as no other sentence contains the necessary information.
If a GPS receiver fails to transmit the RMC sentence at least once every two seconds when a satellite fix is available, Cetus GPS will not display or store current speed and course, and the current time and date will be retrieved from the Palm OS clock instead of the GPS receiver.
Cetus GPS also reads the Satellites In View (GSV) sentence. The information is used to update the satellite map and signal strength table at the GPS page. If GSV sentences are not transmitted by the GPS receiver, the GPS page will not be updated. The rest of Cetus GPS will work fine though.
Some GPS receivers are configurable in a way that let you specify which sentences to transmit and the transmission interval for each of them. You should then configure your GPS receiver to transmit GGA, RMC and GSV sentences at least every two seconds. Information about configuring this is probably available in some kind of technical specification document, it is normally not included in the user manual.
At the Cetus GPS homepage you will find a list of GPS receivers that have been tested with Cetus GPS and hence transmit at least the GGA and RMC sentences.