more complex receiver and receiver processing.  The all in view solution is also more robust,

since the temporary loss of a satellite signal (for example due to a physical obstruction near the

receiver) does not disrupt the flow of PVT data while the receiver attempts to reacquire the lost

signal.  Many receivers will track more than four satellites, but less than all in view, as a

compromise between complexity, accuracy, and robustness.  Receivers that select a "best" subset

do so based on geometry, estimated accuracy, or integrity.  More detailed discussion of specific

satellite selection criteria is provided in Chapter 6.

1.4.2.2  Satellite Signal Acquisition

The satellite signal power at or near the earth s surface is less than the receivers thermal (natural)

noise level, due to the spread spectrum modulation of the signal, orbital height and transmitting

power of the satellite.  To extract the satellite signal the receiver uses code correlation techniques.

An internal replica of the incoming signal is generated and aligned with the received satellite

signal. The receiver shifts the replica code to match the incoming code from the satellite. When

the codes match, the satellite signal is compressed back into the original carrier frequency band. 

This process is illustrated in Figure 1 8.

Figure 1 8.  Spread Spectrum Generation and Reconstruction

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