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What is the timing of NMEA messages output on the RS232 COM port?


Both the PosApp (SimGEN / SimREPLAYplus / SimREPLAY) and SimPLEX software applications allow output of NMEA messages over the COM port in the PC. However, the timing of the NMEA message output, with respect to the simulator 1PPS (GPS time), is subtly different between the two applications as a result of the way in which they process the scenario data during the test.

  • SimPLEX replays scenario data from pre-written files. As such it has all data in advance of the required time of output of NMEA messages. For this reason SimPLEX is able to output the NMEA message just before the 1s epoch for which it applies.
  • PosApp calculates scenario data in real-time at the scenario iteration rate (10Hz, 100Hz or 250Hz depending on hardware capability and user selection). Data for the 'next' iteration rate epoch is prepared during the epoch beforehand so that it can be applied 'on time'. Thus an NMEA message, output at only 1Hz (for example), will be broadcast either 'on time', at the 1s epoch for which it applies or at the next simulation epoch that follows.

Of course the chosen RS232 baud rate will also influence the reception time for the message.

This may be of primary concern to customers upgrading their products from SimPLEX capability to PosApp capability, e.g. from an STR4500 to a GSS6700, and have chosen to use the timing of NMEA messages to synchronise their UUT. Some small realignment of their test set-up may be required, or direct use of the simulator 1PPS output may be preferable.

Product : SimPLEX,SimPLEX45,SimREPLAYplus,SimREPLAY,SimGEN,PosApp,NMEA