is a final phase rotation circuit that enables the doppler in the satellite signal to be precisely

tracked.

1.4.2.4  Code Tracking

The code tracking loop is used to make pseudorange measurements between the GPS satellites

and the GPS receiver.  The receiver s code tracking loop generates a replica of the C/A code of

the targeted satellite.  The estimated doppler is removed by the phase rotation circuit prior to the

correlator.

In order to align the received signal with the internally generated replica, the internally generated

code is systematically slewed past the received signal. Typically the output of the correlator is

integrated over 1 to 10 ms. If correlation is not detected the phase of the internally generated

code is advanced by one chip.  If correlation is not detected after the whole code has been

searched the doppler is adjusted and the process repeated until correlation is achieved. Code

synchronization is initially maintained by also correlating the received signal with half chip early

and late codes. A simple feedback system keeps the prompt ("on time") code correctly positioned.

 To extract the carrier which is still modulated by the navigation message,  the prompt code is

subtracted from the incoming signal.  The delay that the receiver must add to the replica code to

achieve synchronization (correlation), multiplied by the speed of light, is the pseudorange

measurement.  Once the carrier is reconstructed, the center frequency of the replica code is

adjusted using Doppler measurements from the carrier tracking loop to achieve a precise

frequency lock to the incoming signal, thereby allowing more precise pseudorange measurements.

The bandwidth of the code tracking loop is typically 0.1 Hz, which implies that independent

measurements are available at approximately 10 s intervals.

1.4.2.5  Carrier Tracking and Data Detection

The receiver tracks the satellite carrier by adjusting the frequency synthesizers to produce a

stationary phase at the output of the code tracking loop.  The inphase and quadrature components

are used to calculate the carrier s phase and doppler.  A data bit is detected by a sudden change in

the phase of the detected signal. The bandwidth of the carrier tracking loop is typically 6 Hz for a

military airborne receiver, resulting in independent measurements being available every 150 ms.

Doppler is measured to provide an estimate of the relative velocity between the receiver and the

satellite.  These measurements are typically termed pseudorange rate measurements or they can be

integrated over regular time intervals to produce deltarange measurements. 

The receiver uses the doppler measurements from four (or more) satellites to determine the

receiver velocity (in three dimensions) plus the receiver s master oscillator frequency bias.  The

deltarange measurements of the carrier tracking loop are also used to aid the code tracking loop

to ensure code tracking is maintained during dynamic maneuvers where the simple  code tracking

system would be unable to maintain lock.

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