Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 544

• Predicted codephases;
•

Predicted codephase search window;

Navigation data bit timing information (bit number, fractional bit);

•

Navigation data bits (sensitivity assistance).

•

This list is redundant, as a mobile receiver will only use a subset of this informa-
tion in attempting to acquire the requisite number of satellites for a fix (e.g., pre-
dicted Doppler and Doppler rate information is not needed if an ephemeris, coarse
location, and coarse GPS time are provided). For example, the MS-assisted handset
could use visible satellite list, predicted Doppler (and sometimes rate), and predicted
code phase. The MS-based handset could use approximate position, ephemeris, and
approximate time. Both can transform these parameters into corresponding Dopp-
ler and Doppler uncertainty and code phase and code phase uncertainty, as dis-
cussed earlier. The GPS receiver then conducts its satellite search algorithm over the
search region of interest in a parallel manner if sufficient correlators are available (or
sequentially if not).
The visible satellite list is generated within the cellular network by simply report-
ing the visible satellites at a GPS reference receiver within or in the vicinity of the cel-
lular network. The reference receiver should be positioned to ensure an unobstructed
view of the sky. Because of the relative proximity of the network and the mobile with
which it is communicating (i.e., a maximum separation of 20-30 km is expected), the
visible satellite list is virtually the same for the reference and mobile receiver, except
possibly for a satellite very close to the horizon (i.e., less than the separation distance
divided by the radius of the Earth, or roughly a 0.2º elevation for a 20-km separation)
and with an azimuth opposite to the LOS between the reference receiver and the
mobile. Knowledge of the satellites that are potentially visible permits the mobile
receiver to focus its search and avoid wasting time searching for satellites that are not
visible, thus reducing its time to acquire sufficient satellites for a fix.
Because the GPS satellite Doppler has such a large dynamic range relative to the
Doppler attributable to Earth-borne vehicle motion, providing this information to a
mobile receiver drastically reduces the required number of Doppler bins (and so
correlators) required to cover the search space. The magnitude of the satellite Dopp-
ler can approach 4.2 kHz in the worst case. Assuming that a coherent integration
time of 20 ms (the largest value generally possible without knowledge of the naviga-
tion data bits) is required to acquire a weak GPS signal, the Doppler bin size must be
restricted to 50 Hz due to the well-known Doppler error modulation through the
sinc function. This Doppler bin size thus implies a requirement for 100 Doppler bins
per code phase bin for acquiring this satellite. In contrast, given knowledge of the
satellite Doppler and Doppler rate, the Doppler range can be restricted to a level that
is consistent with maximum expected host velocity (i.e., 250 Hz, or only about 5
Doppler bins). Because satellite Doppler rate is relatively small compared to the
Doppler itself [74], it is generally not needed as part of the assistance data. Only for
very long noncoherent accumulations is the satellite Doppler rate potentially signifi-
cant (e.g., a total integration time of 20 seconds can produce a maximum Doppler
error of 20 Hz in the worst case).
Satellite azimuth and elevation angles can be used by a mobile receiver in its
assignment of search ranges. For example, in the previous paragraph, the number of