Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 526

as make provision for some form of digital signal processing software or dedicated
hardware.
The reduction in search space allows the receiver to spend its search time focus-
ing on where the signal is expected to be, which in turn allows it to search at a much
narrower bandwidth, hence increasing signal detection sensitivity.
As referenced in this section, network assistance can refer to any one of three
forms:

• Acquisition assistance, intended to reduce the GPS receiver's time to generate a
fix—time to first fix (TTFF);

Sensitivity assistance, intended to help the GPS receiver lower its acquisition
thresholds;

•

Navigation assistance, intended to improve the accuracy or integrity of the
position solution generated by the GPS receiver.

•

Of course, certain types of information can qualify as more than a single type of
assistance: for example, supplying the GPS receiver with an initial, coarse position
estimate that represents the general location of the network infrastructure providing
the assistance data can assist both acquisition and navigation.
In this section, following a brief, historical view of GPS network assistance
through a discussion of example systems, the fairly recent FCC mandate [57] for cellu-
lar telephone emergency call location is reviewed. The FCC mandate is driving the
needs and development of network assistance to unprecedented levels: each type of cel-
lular network assistance is generally provided to optimize the TTFF and achievable
accuracy of the determined location. Following this review, the fundamental issues of
GPS receiver integration in cell phones as they relate to the use of network assistance
are reviewed and discussed. In the last section, example cellular network architectures
are reviewed, including discussions of their forms of network assistance, the applicable
standard for over-the-air protocol, and their current status and expected performance.

9.4.1 Historical Perspective of Assisted GPS
Examples of the earliest uses of network assistance predate the introduction of cellu-
lar telephones. Perhaps the earliest formal reference to the use of assistance informa-
tion is disclosed in [58]. In this patent, NASA inventors realized the potential benefit
of transmitting an initial almanac or ephemeris to a mobile GPS receiver to enable
prediction of satellite visibility and Doppler and eliminate the long data demodula-
tion time inherent in collecting the required data bits from the signals directly. An
Earth-based station in the vicinity of the mobile GPS receiver was used to send this
information through a geostationary satellite link. Of course, an initial, coarse esti-
mate of the mobile's location was required to generate the predictions. The very first
standard for sending ephemeris data over a wireless link was included as part of the
RTCM DGPS Standard [59]: message type 17 includes the ephemeris data for all
satellites visible to the DGPS reference station receiver. One of the earliest references
to sending measured pseudorange data over a wireless link to support location
determination external to the GPS receiver is described in [60]. It describes a vehicle
tracking system in which pseudoranges can be sent over a wireless link to a worksta-