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If the satellite moves around the orbit at a velocity, the pseudorange between the receiver and the satellite should be changing right? If so, how does a receiver lock onto a satellite signal if the propagation delay keeps changing as well?

Shouldn't the receiver would need to constantly readjust to find the correlation of the signal, or am I getting the wrong idea here?

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  • $\begingroup$ The GPS signals are derived from a base clock signal of 10.23 MHz. To compensate all relativistic effects, the base clock is 10.23 MHz - 4.55 mHz. The received signals on Earth are then derived from a 10.23 MHz,. Not the receivers are corrected but the satellites. With the correction, locking of a receiver onto a satellite is possible. $\endgroup$
    – Uwe
    Commented Sep 29, 2022 at 4:30
  • $\begingroup$ @Uwe Does a receiver remains locked onto a satellite as long as the signal is visible? $\endgroup$
    – newlearner
    Commented Sep 29, 2022 at 7:43
  • $\begingroup$ Shouldn't the locked signal keeps getting interrupted if the satellite keeps moving thus affecting the time it takes for the signal to reach receiver? $\endgroup$
    – newlearner
    Commented Sep 29, 2022 at 7:46
  • $\begingroup$ @Uwe The satellites can't correct for the shift due to their orbital speed. This depends on the direction of the receiver and is different for every single receiver. $\endgroup$
    – asdfex
    Commented Sep 29, 2022 at 9:58
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    $\begingroup$ @Uwe The receivers are constantly correcting themselves, not only for the changing frequency of the received signal due to Doppler shift, but also for the changing frequency of the receiver's commercial off-the-shelf oscillator. The GPS satellites have top-notch oscillators; they have atomic clocks onboard. Commercial receivers do not use atomic clocks internally, so their oscillators vary with manufacturing variances, temperature, etc. Dealing with these variations is a solved problem. $\endgroup$ Commented Sep 29, 2022 at 15:06

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Some GPS receivers use a phase lock loop to lock onto a signal from a GPS satellite, others use a frequency lock loop, and yet others use a hybrid of the two. Phase lock loops and frequency lock loops are the two of leading contenders for locking onto a signal whose frequency changes slightly due to changes in the incoming signal and due to using an imperfect oscillator in the receiver.

To be commercially viable, GPS receivers almost always use inexpensive, off-the-shelf oscillators, which means the phase lock loop / frequency lock loop has to contend not only with the Doppler shift in the incoming satellite signal, but also with the inherent imperfectness of the oscillator. Phase lock loops and frequency lock loops use something akin to a Kalman filter to adjust oscillator behavior based on phase error or frequency error.

Locking onto the carrier signal is just the start. The next step is bit synchronization, which converts the analog signal embedded in the carrier signal into a bit stream. The next step after that is frame synchronization, which uses a regularly repeated pseudo noise signal embedded in the bit stream to identify the start of a transmission frame. It's only after signal lock, bit sync lock, and frame sync lock that a receiver of a satellite signal can make sense out of the incoming signal.

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  • $\begingroup$ If the receiver is locked onto the satellite, how does it continues to measure the time delay (the satellite continues moving even if the receiver is not right?) if both signals are 'synchronised'? $\endgroup$
    – newlearner
    Commented Sep 30, 2022 at 10:00
  • $\begingroup$ @newlearner A GPS receiver doesn't have a reliable clock. A GPS receiver doesn't measure the time delay. It estimates position -- and time. That's why a GPS receiver needs to be receiving signals from at least four GPS satellites. $\endgroup$ Commented Sep 30, 2022 at 12:52

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