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signal strength on a GPS

Discussion in 'Electronic Design' started by Jim Whitby, Sep 16, 2012.

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  1. Jim Whitby

    Jim Whitby Guest

    I'm asking this here in the hope someone knows something about GPS
    receivers.

    I'm a decent tech, but not an engineer...

    I have a Tomtom 2535 gps. Nice unit blah blah.

    Some ( software? ) engineer decided to change the gps info display. It
    now contains a pic of earth with dots for satellites instead of a bar
    graph. Ok. No biggie.

    Except now there is a signal strength percent! Percent of what???

    Full quieting? 20 db quieting? 10 db sinad? or ????

    I ask because at less than about "80%" I get incomplete data ( speed,
    direction, etc ). This is with 7 or more "locked" satellites.

    This occurs quite often and I'm trying to figure out if its bad or
    "normal" operation. If its weak signal ( 100% is 10 db sinad ), I can
    understand it. If 80% is 80% of full quieting, then it must be broke.

    Does anyone have a clue what 100% is relative to?

    Thanks

    Jim
     
  2. miso

    miso Guest

    sci.geo.satellite-nav
     
  3. dataBase?


    Best regards,
    Spehro Pefhany
     
  4. Jim Whitby

    Jim Whitby Guest

    On Sat, 15 Sep 2012 18:04:12 -0700, Jeff Liebermann wrote:

    Thanks will chk there.
    Not much help there, from what I've seen. Can't get to anyone at Tomtom
    that might know what the answer is ( I suspect (sh)he is locked up in a
    Faraday cage).

    Agin thanks for lead to sci.geo.satellite-nav

    Jim
     
  5. MrTallyman

    MrTallyman Guest

    You're an idiot.
     
  6. SoothSayer

    SoothSayer Guest

    They measure timing flag packet arrival times from three satellites.
     
  7. Guest

    If you use a 4-quadrant (Gilbert cell) multiplier (mixer) instead of a
    XOR gate, the despread signal will definitely have a positive SNR in
    the 1000/50 Hz bandwidth) and the absolute signal power should be
    easily measurable.
     
  8. qrk

    qrk Guest

    If it's Trimble, the measurement is relative to AMU (Arbitrary Mystery
    Units).
    http://gauss.gge.unb.ca/papers.pdf/SNR.memo.pdf
     
  9. Guest

    The GPS signal is an ordinary direct sequence spread spectrum (DSSS)
    signal. It can be received with a mixer/multiplier/xor gate by
    multiplying the received signal with the same chip clock (in this case
    1.023 MHz satellite specific PRN sequence) _synchronized_ with the
    transmitter modulator chip clock. After the demodulator the about 1
    MHz wide spread spectrum signal is despread to something about 1 kHz.
    At this point you could also make power measurements. After this, the
    50 bit/s data signal is extracted.

    The shift register and bit error detection you are talking about has
    to do with signal acquisition _before_ the receiver chip clock
    generator has been synchronized with the transmitter chip clock.
     
  10. G. Morgan

    G. Morgan Guest


    I didn't understand a damn thing in that document!
     
  11. G. Morgan

    G. Morgan Guest


    Do they x-mit a different freq. for military use?
     
  12. G. Morgan

    G. Morgan Guest

    Thanks.
     
  13. rickman

    rickman Guest

    If you don't know, why should we tell you.

    Rick
     
  14. Guest

    So in reality, you need four independent signal strength signals for
    the best (geometry) four satellites
     
  15. Guest

    You two do make quite a pair.
     
  16. MrTallyman

    MrTallyman Guest

    Try to tell us, since that is what *you* do for a living.

    As for myself...

    When I adjust the waveguide output channel of a high power satellite
    transceiver, my employer is quite comfortable with my understanding of
    logarithmic progression. Also when I adjust the noise source loop that
    we pump it with for the test.

    You are lost.

    As for you...

    When you adjust your mop head, be sure to bring it right up next to
    your face so your crabs and fleas can hop onto it and migrate to new
    horizons. Good job of spreading the vermin, you immature little retarded
    characterless honorless bastard. Go back to the kook group, dumbfuck.
     
  17. MrTallyman

    MrTallyman Guest

    ^
    SOME of your crabs and fleas. We wouldn't want you to go
    completely without friends.
     
  18. Guest

    Three variables require three equations; four, four. Don't forget time.
     
  19. mpm

    mpm Guest

    Well, it's not going to be SINAD since it's not an analog receiver.
    Your choices are most likely Bit Error Rate (BER), though a BER of 20% would seem completely unusable to me. The other option is C/I+N but again, that's used more for analog modulation schemes (or should be, though some companies think SINAD is acceptable for "digital modulation". These people should be tarred and feathered. You could write a book about modulation schemes and their effect on BER, and I'm sure some have. I've never found dBm lookup tables to be particularly good descriptors for BER. Your mileage mayvary.

    Bars and SINAD and I+N values aside, if the GPS fades too much, there's probably something wrong with the receiver or the placement of its antenna. Some antennas are powered (i.e., internal active preamps), so you might verify the batteries are fresh, or it's otherwise powered-up with the correct voltage.

    Shameless plug: We use a lot of Garmin Montana GPS's (650's and 650t's)forour field work. Expensive, but perfect for what we do. Of course, as with any early adopter there were some early software glitches; like "Continuefour-thousand one-hundred fifty miles, then make a U-turn", but they squashed that bug pretty quick. :)
     
  20. josephkk

    josephkk Guest

    Not entirely. The empherides for all the satellites is in flash and can
    be updated by data from the satellites which are in turn updated by ground
    support stations. Since a time solution is relatively in dependant of the
    location solution and the satellites found, it can be use to get the
    empherides data and thus know which satellites to look for the location
    solution. Of course the receiver remembers the last location it had lock
    and keeps time while powered off, the empherides is immediately useful in
    most cases.

    ?-)
     
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