Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 219

satellite's navigation message as the a f 1 term (see Sections 4.4 and 7.2.1). The mea-
surement also includes the receiver's reference oscillator frequency error. This error
is determined as a time bias rate correction by the navigation solution. For some
applications, it is also corrected for the differential ionospheric delay, but this is
usually a negligible error.

5.8 Signal Acquisition

There is a large amount of literature on PRN code acquisition in direct sequence
receivers. For an extensive historical survey and descriptions on time domain search
detectors, [15] is recommended. Reference [16] describes the use of modern fre-
quency domain search techniques for rapid acquisition. The following GPS signal
acquisition material is based on traditional time domain search techniques.
GPS signal acquisition is a search process. This search process, like the tracking
process, requires replication of both the code and the carrier of the SV to acquire the
SV signal (i.e., the signal match for success is two dimensional). The range dimen-
sion is associated with the replica code. The Doppler dimension is associated with
the replica carrier. The initial search process is always a C/A code search for C/A
code receivers and usually begins with a C/A code search for P(Y) code receivers.
The initial C/A code search usually involves replicating all 1,023 C/A code phase
states in the range dimension. The criteria for direct C/A code and direct P(Y) code
acquisitions were discussed in the previous section. If the range and Doppler uncer-
tainty are known, then the search pattern should cover the 3-sigma values of the
uncertainty. If the uncertainty is large in either or both dimensions, the search pat-
tern is correspondingly large, and the expected search time increases. Some criteria
must be established to determine when to terminate the search process for a given
SV and select another candidate SV. Fortunately, the range dimension for C/A code
search is bounded by the ambiguity of C/A code to only 1,023 chips total range
uncertainty, but it is essentially unbounded for direct P(Y) code search.
The following example assumes that a C/A code search is being performed and
that all 1,023 C/A code phases are being examined. The code phase is typically
searched in increments of 1/2 chip. Each code phase search increment is a code bin.
Each Doppler bin is roughly 2/(3 T ) Hz, where T is the search dwell time (the longer
the dwell time, the smaller the Doppler bin). The combination of one code bin and
one Doppler bin is a cell. Figure 5.34 illustrates the two-dimensional search process.
If the Doppler uncertainty is unknown and the SV Doppler cannot be computed
from a knowledge of the user position and time plus the SV orbit data, then the
maximum user velocity plus just less than 800 m/s maximum SV Doppler (for worst
case, see Section 2.5) for a stationary user must be searched in both directions about
zero Doppler.
As stated earlier, one Doppler bin is defined as approximately 2/(3 T ), where T

=
signal integration time per cell or dwell time per cell. Dwell times can vary from less
than 1 ms (Doppler bins of about 667 Hz) for strong signals up to 10 ms (67-Hz
Doppler bins) for weak signals. The poorer the expected C / N 0 , then the longer the
dwell time (and overall search time) must be in order to have reasonable success of
signal acquisition. Unfortunately, the actual C / N 0 is unknown until after the SV sig-