Figure 1 6.  Satellite Signal Modulation

The BPSK modulation spreads the RF signals by the code bandwidth.  The result is a symmetrical

spreading of the signal around the L1 and L2 carriers.  The C/A code spreads the L1 signal power

over a 2.046 MHz bandwidth centered at 1575.42 MHz.  The P(Y) code spreads the L1 and L2

signal powers over a 20.46 MHz bandwidth centered about 1575.42 MHz on L1 and 1227.6 MHz

on L2.  Figure 1 7 shows the L1 and L2 signal spectrum as it appears at the 0 dB gain receiver

antenna at the Earth s surface.  The C/A code component of L1 signal has a power of  160 dBW

(decibels with respect to one watt), the L1 P(Y) code signal has a power of  163 dBW, and the

L2 P(Y) code signal has a power of  166 dBW.

1.4.2  GPS Receiver Operation

In order for the GPS receiver to calculate a PVT solution, it must:

Search for a PRN C/A code lock

C/A code track, carrier track

Obtain bit synchronization with the navigation message

Obtain frame synchronization, ie obtain HOW  and Z count

Decode GUV or CVw

Remove SA

Transition to P(Y) code,  code lock,  carrier lock

Data lock on P(Y) code

Search, acquire and track 2nd to 4th SVs, up to all in view

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