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NMEA GPS Signal Generator

Discussion in 'Electronic Design' started by [email protected], Jan 28, 2013.

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  1. Guest

  2. Guest

    We're are building a GPS into one of our instruments. It may be a module ora PCB based one with an antenna. We'll we need to test this thing in manufacturing and we would like to test the performance of the antenna location in the instrument. GPS doesn't work in the building unless we're near a window and varies with what satellites are in the sky. So it is hard to compare the performance of different devices and even develop a simple pass fail test in manufacturing.

    What I would like is a device that sends a fake NMEA string with a range ofabout a couple feet. I could then create a manufacturing test stand that puts the DUT at a certain position and passes if it receives the string and fails if it doesn't. I could also test different antennas and antenna positions in the device by moving the device closer or farther away from the transmitter and find the minimum distance for communication.

    National Instruments and Agilent have test setups but they seem awfully expensive. You can get other RF dongles to do Wi-Fi and Bluetooth. I'd think transmitting GPS would be simpler than those. I'm surprised there isn't somesimple tester available with a satellite code that indicates it is a fake and only for testing.
  3. You are under several misapprehensions. The satellites do not send NMEA
    strings to be picked up by a receiver. The receiver decodes a very
    complex set of signals from several sats, and computes the local
    position from that, then outputs the NMEA string ( usually serially )
    based on that computed data.

    Its very easy to generate NMEA strings, even a very simple script
    running on a PC terminal program will do that. But that does not help
    you test a GPS reveiver front-end.

    For that you need a pseudolite generator, ie a 'thing' which generates
    the satellite signals. Available, but very expensive, as you found.

    Why not use one of the many active antennas, set up as a repeater, so
    you mount that on the outside of your building, with a clear view of the
    sky, and hence a reliable source of the existing sat signals. Feed the
    signal inside, to your GPS receiver, or even just another antenna acting
    as a local transmitter to your DUT ?
  4. Guest

    That explains why I can't find what I'm looking for. I don't know what I'm talking about.
  5. Guest

    Great Info. I'll definitely look into this.

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