Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 545

Doppler bins assigned was computed solely as a function of the maximum expected
host velocity. This calculation ignores the fact that Earth-borne host velocities are
largest in the horizontal plane; a more realistic assignment of search range could
therefore have been made using the elevation angle of the to-be-acquired satellite, E ,
as indicated in the following equation:

∆ Dv = cos Ev + sin E (9.50)

H max z max

Since maximum vertical velocities (i.e., v z max above) are expected to be small relative
to maximum horizontal velocities (i.e., v H max above), smaller search ranges would
generally be assigned to higher elevation satellites using (9.50).
Providing the mobile receiver with an approximate location is most useful for
acquisition assistance when combined with either ephemeris or almanac data for
the satellites expected to be visible. The position provided by the network is gener-
ally either the location of the serving BS or the center of the service area; it is there-
fore expected to be within 20 km of the mobile's actual location. Given this
position, and either an ephemeris or almanac representation for each satellite,
Doppler and Doppler rate information can be computed by the mobile with satis-
factory accuracy (the sensitivity of Doppler prediction error to position error is gen-
erally less than 1 Hz/km [74]), the value of which for acquisition assistance has
already been discussed.
As referenced in the preceding paragraph, satellite ephemeris or almanac infor-
mation enables accurate Doppler prediction, given relatively coarse position infor-
mation. In addition, if GPS time is known such that the satellite positions can be
accurately computed (a 1-second error in knowledge of GPS time translates to 1 km
of ranging error in the worst case), an accurate range to the GPS satellite can be
determined. If, additionally, the handset has been time-synchronized to GPS time
and the error of each satellite's time relative to GPS time can be accurately predicted
(i.e., via the satellite clock correction polynomial coefficients that enable this, which
are also provided in the navigation model assistance data), prediction of the satellite
code phase can be made as described in (9.49) to substantially reduce the range of
code phases to search. For example, if the local oscillator has been synchronized to
GPS time, and GPS time-of-week is known to 1 second, the relative code phases can
be resolved to roughly 140 half-chips (i.e., 21 km of ranging error) after finding a
first satellite, representing a significant savings relative to a full code phase search of
2,046 half-chips.
The information, provided to assist acquisition, can also increase sensitivity
(i.e., enable acquisition of weaker signals). This is because the assistance informa-
tion is likely to reduce the search ranges in Doppler and code phase such that the
receiver has sufficient correlators to cover all cells in a parallel search and thus can
spend more time searching the remaining space.
Independent of the acquisition assistance types already discussed, the primary
form for actual sensitivity-increasing assistance data is the provision of navigation
data bits over the cellular network. Given that the navigation data bits can be syn-
chronized with the knowledge of the data bit edges for each satellite for which
acquisition is attempted, the PDI can be extended beyond one navigation data bit:
each doubling of the coherent integration time lowers the acquisition threshold by 3
dB. However, each doubling of the coherent integration period requires a corre-