As the GSG unit uses the heading and speed information of the RMC sentences, only one NMEA sentence is actually required to describe a simple, continuous movement.
For example, the following one-line trajectory specifies a continuous north-bound trajectory (as the heading field is set to 0.0 degrees) at a speed of 77 knots:
|RMC||Recommended Minimum Sentence C|
|111150||Fix taken at 11:11:50 UTC|
|A||Status [A = Active, V = Void]|
|6000.0000,N||Latitude: 60 deg 00.000' N|
|0100.0000,E||Longitude: 01 deg 00.000' E|
|77.000||Speed over ground: 77 knots|
|0.0||Heading: 0.0 degrees true|
|0.9,W||Magnetic Variation: 0.9 deg West|
|A||Positioning system mode indicator: [A = Autonomous]|
|*03||Checksum data, always begins with *|
One-line trajectories like this can be easily be made by manually creating the desired NMEA files: The example above can be taken as a baseline, then edit speed and/or heading fields as required.
To allow for testing the sentence's validity, the last 2 digits contain a checksum of the data (XOR of all bytes between $ and * symbols) – this checksum must be correct and can be calculated with e.g., this online tool: www.hhhh.org/wiml/proj/nmeaxor.html.
Note that the NMEA messages, including the checksums, are case sensitive and should be given in UPPERCASE even if the GSG unit (firmware version 3.00 and above) accepts messages in lower case.
To find out more about NMEA, and purchase a copy of the NMEA 0183 Standard, visit www.nmea.org.
To learn more about trajectories, see: Trajectories.