Zusammenfassung
Acquisition of very weak GPS signals requires long coherent and incoherent
integration. Increasing the coherent, or predetection, integration
time (PIT) improves sensitivity, hut is limited because of the unknown
data and hit edges. Incoherent integration is less sensitive, because
of squaring loss. Two algorithms are developed to use a PIT which
is a multiple of one data bit interval, without requiring assisting
information. Both algorithms estimate the most likely data bit combination
and use this to subsequently reverse the data signs over each PIT
and synchronize the start of each PIT. The algorithms differ in the
method of coherent integration. One uses circular Correlation (FFT/IFFT),
the other uses double block zero padding (DBZP). Circular correlation
requires larger numbers of Doppler bins as the PIT is increased.
In contrast, DBZP does not use Doppler bins, hut suffers from loss
because it does not account for the Doppler effect on the code duration.
Consequently, the varying delay between the received signal and the
replica will cause subsequent incoherent integrations to be added
at the wrong delay. Solutions have been developed for each of these
problems. In the case of circular correlation, a small PIT is used
initially and after few steps the Doppler bins which have the lowest
likelihood are eliminated. Thus, the PIT can be increased without
increasing the total number of Doppler bins. In the case of DBZP,
an approach is developed to compensate for the change in the code
duration and relative delay, with only a small increase in the processing
requirement. Simulations indicate that both of these algorithms can
acquire signals below 15 dB-Hz. The problem of acquiring weak signals
in presence of strong interfering signals is also addressed.
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