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copied!<p>Up until version 2.3 of the NMEA protocol, there was no way of including information about the source of the GPS information, however due to input from the FAA this has since been added to a number of the NMEA message types including GLL and RMC which are commonly used for general purpose positioning (see <a href="http://gpsd.berlios.de/NMEA.txt" rel="nofollow">this document</a> for more information).</p> <p>This new field is added to the the NMEA message as an optional field following all of the older data fields.</p> <p>The new FAA mode indicator (usually referred to as the mode) fields include the following:</p> <ul><li>A = Autonomous mode (pretty much the default)</li> <li>D = Differential mode (when using differential correction)</li> <li>E = Estimated (via dead reckoning or similar techniques)</li> <li>M = Manual input</li> <li><b>S = Simulated</b></li> <li>N = Invalid</li> </ul> <p>I do not know much about the FakeGPS application, but if it just takes a text file of NMEA messages you may be able to add this field (or if it already exists, just change the value to "S" for simulated) - just remember to change the checksum at the same time.</p> <p>When parsing the data you just need to look for this field to differentiate between the simulated messages and true GPS based information where this field will be most likely either A, or missing (depending on the GPS receiver and version of the NMEA protocol supported). If you've developed your own parser then adding this functionality is really quite simple, if you're using a third party parser then this could be a little more difficult (hopefully it's already supported).</p>

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