GPS signals

Discussion in 'General GPS Discussion' started by shireesh, Oct 28, 2017.

  1. shireesh

    shireesh

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    Hello everyone. I have question related to gps signals. At antenna signal from different satellites will be mixed signal or not? If yes how satellite despreads and identify individual signal from that mixed signal?
     
    shireesh, Oct 28, 2017
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  2. shireesh

    Nuvi-Nebie Moderator

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    My understanding is that although all satellites in a constellation are transmitting on the same radio frequency i.e. 1575.42 MHz for the GPS L1 carrier, that each satellite is allocated a unique timeslot, so that only one satellite transmits at any one time
     
    Nuvi-Nebie, Oct 28, 2017
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  3. shireesh

    jimh

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    GPS signals are on a common frequency. The signals are transmitted using a code division multiple access or CDMA method. They DO NOT use a time division multiple access or TDMA method. For more details about CDMA see

    https://en.wikipedia.org/wiki/Code-division_multiple_access

    and

    https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum#Uses

    and

    https://en.wikipedia.org/wiki/Global_Positioning_System#Demodulation_and_decoding

    A further consideration is that at a particular receiver location, the frequency of the signals from GPS satellites will be altered by their apparent motion to the receiver. This is known as Doppler Shift. Thus the actual frequency received for each satellite will be slightly different due to the effect of Doppler Shift.
     
    Last edited: Nov 4, 2017
    jimh, Nov 4, 2017
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  4. shireesh

    Nuvi-Nebie Moderator

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    Ah ok, I don't know much about CDMA, I'm still trying to get my head around how it can find one of 12 codes that may be present when they are superimposed on top of one another.

    Are you saying that the receiver can make use of the differences in frequency that is sees due to the doppler effect for separating the CDMA codes, or is it used in RTK?
     
    Nuvi-Nebie, Nov 4, 2017
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  5. shireesh

    jimh

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    The GPS satellites all transmit their L1 signal on the same frequency, 1575.42-MHz. At any particular receiver location, due to the apparent motion of the satellite relative to a stationary receiver, the signals arrive at slightly different frequencies due to Doppler Shift. A GPS receiver must be able to be slightly tunable so it can adjust to the changed frequency of the arriving signal.

    A GPS receiver must be capable of simultaneous reception of at least three or four signals. Modern receivers can receive dozens of signals simultaneously.

    The CDMA method is a form of a general method called SPREAD SPECTRUM. The signal is not on just one frequency, but it is spread across a wide band of frequencies. By careful selection of the CDMA modulating digital codes, all the GPS signals can be sent on one frequency and not interfere with each other.
     
    jimh, Nov 6, 2017
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  6. shireesh

    Nuvi-Nebie Moderator

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    It sounds like you are saying that a GPS receiver has to tolerate this frequency shift, rather than being able to use it

    These two statements appear to contradict each other, I'm guessing you mean there is a single carrier frequency of 1575.42 MHz, but that this is the centre frequency of a 'band' which has content above and below this centre point and that the 'codes' are spread over the band
     
    Nuvi-Nebie, Nov 6, 2017
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  7. shireesh

    jimh

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    There is no contradiction in what I said. You should undertake a study of radio signals. In brief, a radio signal consists of a carrier wave and modulation applied to the carrier wave. Any radio signal is not a phenomenon of a single frequency. Even the on-off keying of an unmodulated carrier causes spreading of the frequency. When a carrier wave is modulated by a wideband modulation signal, the resulting signal is spread over a bandwidth that is proportional to the modulating signal's bandwidth. The GPS L1 signal is modulated by a wideband signal, so the resulting signal is wideband.

    If you study radio signals, modulation, and CDMA methods, you will be able to understand this better. A good textbook on radio and various forms of modulation will give you a background that will allow you to understand how my statements that you have taken out of context are not contradictions.

    The GPS receiver adjusts its center frequency to match the frequency of the signal that is arriving. You would have to do the same thing in your car radio if your car were travelling toward a broadcast station at a speed of 14,000-kilometers-per-hour. To receive a station that was sending on 100.000-MHz, you would have to tune slightly higher in frequency to receive it due to Doppler shift. The signal would appear to you to be arriving at 100.131-MHz.

    You can calculate Doppler Shift with this handy calculator:
    http://www.calctool.org/CALC/phys/default/doppler
     
    Last edited: Nov 7, 2017
    jimh, Nov 7, 2017
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  8. shireesh

    jimh

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    Also regarding GPS signals:

    The signal level of the L1 signal from a GPS satellite as received on Earth is extraordinarily low, and would generally be impossible to receive and demodulate if it were not for the property of CDMA demodulation known as CORRELATOR GAIN. The detection of GPS signals in the receiver with a correlator provides a substantial improvement in signal-to-noise ratio, an improvement of more than one-hundred times, and this correlator gain is crucial in the detection of the the signal and demodulation of it.

    The more one studies the technology, science, and mathematics involved in the design of the GPS system, the more beautiful it becomes.
     
    jimh, Nov 7, 2017
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  9. shireesh

    Nuvi-Nebie Moderator

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    O.K. thanks for that, I find there is always a problem trying to explain something to someone without using the kind of terminology that they are obviously not familiar with. I'm sure Shireesh has some valuable information to answer his original question
     
    Nuvi-Nebie, Nov 7, 2017
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  10. shireesh

    jimh

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    Also, it should be noted that the Russian GNSS system called GLONASS works differently. In GLONASS, all satellites send the SAME code for their CDMA modulation, but each satellite operates on a different carrier frequency. The identity of the satellite is established by the carrier frequency, not by the CDMA code.
     
    jimh, Nov 8, 2017
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