The delay in the receiver s code is a measure of the transit time of the signals between the satellite

and the receiver s antenna and hence, the range between the satellite position and receiver

position.  This measurement is called a pseudorange measurement, rather than a range

measurement, because the receiver s  clock bias  has not been removed.

Receivers typically use phase locked loop techniques to synchronize the receiver s internally

generated code and carrier with the received satellite signal. A code tracking loop is used to track

the C/A  and P code signals while a carrier tracking loop is used to track the carrier frequency. 

The two tracking loops work together in an interactive process, aiding each other, in order to

acquire and track the satellite signals.  A generic GPS receiver tracking system is illustrated in

Figure 1 9. 

ANTENNA

PSEUDO 

DELTA RANGE

CARRIER

MEASUREMENT

TRACKING

CHANNEL

PREAMPLIFIER

50 Hz

NAVIGATION

DATA

DOPPLER

EST.

IF SIGNAL

ON TIME

RF CONVERTER

EST.

CODE

TRACKING

PSEUDO RANGE

CHANNEL

MEASUREMENTS

LO IF

FREQUENCY

SYNTHESIZER

REFERENCE

OSCILLATOR

Figure 1 9.  Generic GPS Receiver Tracking System

1.4.2.3 Down Conversion

The received RF signal is converted, usually through two intermediate frequencies (IF), down to a

frequency near the code baseband, that can be sampled by an analogue to digital (A/D) converter.

 Inphase and quadrature digital samples are taken to preserve the phase information in the

received signal.  The samples are usually two bits to reduce conversion losses.  The sampling rate

must be higher than the code chipping rate for a non return to zero code, that is, greater than

10.23 MHz for the P(Y) code.  To ensure the phase of the received signal is maintained, all local

oscillators are derived from, and phased locked through, a series of synthesizers derived from the

receiver s master oscillator.  Following the A/D conversion there

1 12