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GPS 1 pulse-per-second signal

Discussion in 'Electronic Design' started by Richard Henry, Sep 24, 2007.

  1. Mnay GPS receivers have a 1 PPS output which is supposedly
    synchronized to UTC, given enough good satellite signlas. However,
    the pulse shape and timing seems to vary from one device to another.
    Does anyone have any advice on how to use this signal for
    synchronizing the operation of two or more objects spread over the
    Earth's surface?
     
  2. mpm

    mpm Guest

    I would check out how paging companies (remember them?) used GPS to
    simulcast their transmitters in a given area. The timing resolutions
    were much, much tighter than what you're looking for. I want to say
    there's a 10MHz carrier you can lock to, but I just don't recall. It
    was a long time ago....

    Glenayre was a major player in this technology so I would start there.
    Depeneding on how many you needed, I'll bet you can get this gear very
    cheaply in the used market. -mpm
     
  3. This is very true; the output pulse is different. Also, if a module is
    not specified as a timing source, there can be the significant jerks of
    phase on the 1 PPS signal.
    It depends on the accuracy. If you need a precise synchronization, you
    should use modules which are specified as the timing sources.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  4. James Arthur

    James Arthur Guest

    Use an ovenized local reference and lots of averaging...

    http://www.rt66.com/~shera/index_fs.htm

    Certain of Rockwell's Jupiter chipsets output a 10kHz reference
    signal--that allows much faster acquisition. Googling frequency
    reference + jupiter + gps gives some good examples.

    HTH,
    James Arthur
     
  5. Yep. The time between pulses is guaranteed to be exactly 1 second.
    However, the time of any particular pulse is NOT guaranteed to be
    synchronized with any particular time or event. If you take two GPS
    receivers, and compare the 1 pps outputs, they could easily be at
    quite different times. However, each succeeding pulse will be exactly
    1 second later.
    Sure. Get a receiver that is suitable for running a clock, not an
    interval timer. The complexity and cost will depend on how accurately
    you're trying to sychronize. Lacking numbers, I can't offer any real
    solutions.

    Maybe something like this will work:
    <http://www.gpsclock.com/specs.html>
    "The GPSClock 200 has an RS-232 output that provides NMEA
    time codes and a PPS output signal. About a half-second before,
    it outputs the time of the next PPS pulse in either GPRMC or
    GPZDA format. Within one microsecond of the beginning of the
    UTC second, it brings the PPS output high for about 500 ms."

    If you can live with some atmospheric and propagation drift, the
    cheapest solution is a WWVB 60KHz receiver and clock.
    <http://tf.nist.gov/stations/radioclocks.htm>
    The problem is that WWVB only works in North America.
     
  6. mpm

    mpm Guest

    Another interesting oddity about GPS is that the position error
    reported by two otherwise identical units will be dependent on which
    satellites have been acquired. This problem comes up a lot in
    differential GPS time & distance measurements.
     
  7. Hal Murray

    Hal Murray Guest

    What sort of things are you trying to do? How accurate do
    they need to be synchronized?

    The GPS units I've worked with all specify that the leading edge
    of the pulse will be very close to the second tick of UTC.

    If you need (or could use) a PC to do whatever you want to do,
    I'd plug the GPS unit into a PC running ntpd.

    If you only need time within a few 10s or 100s of ms, you
    may not need the GPS unit.

    Lots of info in either comp.protocols.time.ntp or
    http://www.ntp.org/
    There is a wiki at
    http://support.ntp.org/bin/view/Support/WebHome
    which has a page on setting up refclocks
    http://support.ntp.org/bin/view/Support/ConfiguringRefclocks

    The Garmin USB 18 LVC is popular because it's the lowest cost
    unit with a PPS signal. Some assembly required.
     
  8. Steve

    Steve Guest

    GPS timing receivers, such as Trimble Resolution-T or disciplined
    oscillators such as Trimble mini-Thunderbolt or Jackson Labs Fury will
    output a 1PPS whose leading edge is within +/- 15 nsec of UTC at any
    location on the earth (once fully locked). The pulse widths vary from one
    vendor to another, but the leading edges are the only significant data in
    the 1PPS outputs. Different vendor receivers have better or worse jitter
    specs, but the fact that all are aligned to UTC (or GPS master time which is
    an integral number of seconds offset from UTC) makes them suitable for
    aligning clocks that are scattered around the world.

    These timing receivers all make use of their GPS position in the world to
    account for propagation delays, and provide a stable pulse that is very
    accurate in frequency as well as time-aligned to a universal standard.
    Accuracy is lower if your clocks are moving.

    Steve
     
  9. Thanks for the responses and links.

    What I have learned so far from this thread, the links referred to,
    and a little studying on my own (please correct any errors I make
    here):

    There is an instant every second where UTC time transitions from one
    second to the next. The point of 1 PPS signal is to allow determining
    that exact instant so that devices placed remotely from each other can
    synchronize their operation without direct contact. An example use of
    such a system would be synchronizing a radio transmitter with a
    receiver.

    If a device knows its exact postion on the earth it only needs a good
    signal from one satellite to determine the second transition instant.
    If the device does not know its position, or knows that it is moving,
    it needs data from 4 satellites to determine the position and thus the
    accurate timing.

    Even though there is an indicator of the exact instant in the
    satellite's transmitted data, the signal cannot simply be received,
    decoded and used as is. The receiver must account for the estimated
    rf space and atmospheric propogation delays, a local estimate of
    antenna and cabling delays, snd some electronic circuitry delay. It
    can then adjust the received signal back to UTC second transition
    time, and compare that calculation to an estimate derived from an
    internal clock, adjusting the internal estimate accordingly.
    Hopefully the internal estimate will be good enough that the
    adjustment will never be more than one cycle of the reference clock.
    The estimate is what is used to generate the 1 PPS UTC-synch
    ronized pulse.
     
  10. Iwo Mergler

    Iwo Mergler Guest

    Watch the jitter on the pulse. Some GPS implementations do it
    in software - with a few microseconds of jitter. Some receivers
    generate the pulse in hardware and can achieve nanosecond jitter.
    The later ones are normally special designs for time keeping.

    It very much depends on your application. All GPS PPS
    implementations are accurate long term.

    To synchronize things, you need to watch the *specified* edge
    of the PPS signal (usually the 0-1 transition), which usually
    marks the moment which you'll find in the NMEA messages of the
    next second.

    In other words, the timestamps in the NMEA will give you time
    accurate to the second and the edge of the PPS signal takes it
    to fractions of a second.

    Kind regards,

    Iwo
     
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