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GPS & PPM timing

Discussion in 'General Electronics Discussion' started by thames, Jul 19, 2010.

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  1. thames


    Jul 19, 2010
    Anybody knows the details on how GPS modules handle PPS timing?

    I have implemented a PPS based clock. My uP software basically waits for the next PPS pulse from the GPS module and uses the relevant UTC time message (NMEA or SiRF) to obtain the actual UTC time. This usually arrives 300-400mS later.

    One problem I have found is that after power on, the GPS module's coded time message (SiRF or NMEA) is exactly one second off. And that after a few seconds (which can be up to a few minutes in cases) it suddenly falls into "place" i.e. it displays correctly.

    I have found the same problem with the two GPS modules I've tried: RF solutions GPS-310FA and Sequioia SGPS25LP.

    The main problem I have is that the GPS module doesn't say when the time data is correct (i.e. not one second off). So I have to just wait the 10-15 minutes and assume the data will be correct by that time.

    Any ideas what is going on, or how I can get around the problem?
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