Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 229

The value for P fa is then substituted into (5.43) to compute the threshold, V t
(assuming the signal is absent). V t sets the lower limit of the integration when (5.40)
is substituted into (5.37) as follows:

2

z

∞
∫ σ

− + CN

zCN dz 2

z

2

2 σ

P = e I (5.51)

n

d 0

2

σ

n

V n

t

σ n normalized to unity. Finally,
P D is computed for the M and N values using (5.49). The search speed for this exam-
ple, assuming a dwell time of 5 ms, is R S =

Using (5.51), P d is evaluated for each C / N with

1/( N 2 T ) = 12.5 chips/s, which is more

than five times slower than the Tong search speed.

5.8.3 Direct Acquisition of GPS Military Signals
While P(Y) code was designed for acquisition through C/A code, efforts have been
made to develop direct P(Y) acquisition capabilities, and such capabilities are pro-
vided in some modern P(Y) code receivers. Direct P(Y) code acquisition is used if the
receiver can accurately predict the satellite transmit time so that less time is required
to acquire the P(Y) code by direct sequence than to perform a C/A code search and
handover. The direct P(Y) code acquisition condition is satisfied if the receiver has
previously acquired four or more satellites and its navigation solution has con-
verged. Under certain jamming conditions, it may be impossible to acquire the C/A
code but possible to acquire P(Y) code. Then, direct P(Y) code acquisition is essen-
tial and can be supported if the navigation state has been transferred to the receiver
with sufficient precision and the ephemeris data for all SVs to be acquired are pres-
ent. The most sensitive navigation state parameter is precise time. Reference [6]
describes a multiple correlator/search detector architecture that supports rapid
direct P(Y) code acquisition.
In contrast to the P(Y) code signal, the M code signal was designed so that direct
acquisition would be the primary means of acquisition, drawing on advances in
acquisition algorithms and integrated circuit technology. The BOC s (10,5) modula-
tion allows separate acquisition processing on upper and lower sidebands (with
processing at the 5.115-MHz spreading code chip rate) and noncoherent integra-
tion of the results from the two sidebands, as illustrated in Figure 5.41 [20]. Note
that the two sidebands may be selected and processed at the digital IF part of the
receiver rather than by two L-band downconverters. Also note that the replica M
code does not contain the square wave component. This approach suffers only a
fraction of a decibel in performance compared to coherent processing of both
sidebands.
Interestingly, when the sideband acquisition processing approach is used, M
code signal direct acquisition processing uses approximately half the arithmetic
operations and half the storage of Y code signal direct acquisition processing [20].
An integrated circuit based on this processing approach has been designed, built,
and tested, displaying the ability to acquire even with relatively large initial time
uncertainties in significant levels of jamming [21]. The processing architecture is
based on computation of short-time correlations, followed by FFT backend pro-
cessing for parallel search of multiple frequency values.