Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 552

navigation or mapping to operate within the handset. To do so, the handset will
need a fresh copy of satellite ephemeris data, as it needs to know precise satellite
positions in order to compute range residuals and update its local estimate of user
position. Thus, one of the data elements the MS-based handset will need from the
cellular network is the real-time (current) ephemeris data. The ephemeris is also use-
ful in computing local acquisition assist data, given that the handset also knows
approximate position and time. Two other data elements that are obtainable by the
handset include this additional data.
The MS-based handset has a number of things it can request from the cellular
network. Table 9.15 describes the suite of assistance data elements that can be
requested by the handset. The handset can individually select each or all data ele-
ments listed in the table in one uploaded request to the network, thus, the handset
that is cold (i.e., no time, position, ionosphere correction, or ephemeris data) can
request the entire load in one uploaded message and then accept each assist data ele-
ment as it is delivered from the network message.
Each data element is formatted into a unique data message before being sent to
the handset. For a detailed description of each data element message, refer to [76].
The handset uses the assist data, transforms it into Doppler, code phase estimates,
and uncertainties, as described in Figure 9.41 and (9.48). The signals are acquired
and position determined using the locally stored ephemeris and ionospheric correc-
tion constants. The handset then can return the position to the network via one of
five different digital messages [81]. The messages contain the user position data
along with optional uncertainty and altitude. The optional messages include:

• Ellipsoid point;
•

Ellipsoid point with uncertainty circle;

Ellipsoid point with uncertainty ellipse;

•

Ellipsoid point with altitude;

•

Ellipsoid point with altitude and uncertainty ellipse.

•

The most general option, ellipsoid point with altitude and uncertainty ellipse from
[80], is detailed in Table 9.16.

Table 9.15 Fields in the GPS
Assistance Data Element
Parameter Presence

Reference time O

Reference location O

DGPS corrections O

Navigation model O

Ionospheric model O

UTC model O

Almanac O

Acquisition assistance O

Real-time integrity O

Source: [76].