Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 551

represent the LMU-generated parameters that link the asynchronous GSM cellular
messaging protocol state at an instant in time to the corresponding GPS-TOW time
tag (i.e., the cellular messaging protocol state defined by these parameters existed at
the precise GPS-TOW time tag that enables precise time transfer). The use of the
time transfer parameters are optional, (marked “O” in the table), which means the
handset does not necessarily need to use the data, and, in fact, the parameters may
be missing from certain cellular networks if the network operator does not want to
deploy LMUs. Those elements marked “M” are mandatory.
The parameters “azimuth” and “elevation” provide the MS-assisted handset
with the ability to compute approximate HDOP (see Chapter 7) as it acquires satel-
lites. Without some form of geometry quality indicator, the MS-assisted handset
does not know when it has detected sufficient satellites for a good fix. Thus, being
able to compute HDOP after each subsequent new satellite detection enables the
MS-assisted handset to know when it has detected sufficient satellites for a quality
fix and to deliver the pseudorange measurement response message to the network.
When the MS-assisted handset acquires sufficient satellites for a good fix, it
returns the measured pseudorange data to the network via a measurement informa-
tion element response message, depicted in Tables 9.13 and 9.14. As before, Table
9.13 data is sent one time and Table 9.14 data is sent N-SAT times. Table 9.14 rep-
resents the actual receiver measured range data that the network-based SLMC will
use to compute the position of the handset.

MS-Based Exchange
Referring to Figure 9.35, recall that the MS-based method provides the position
computation element in the handset, enabling local applications such as personal

Table 9.13 GPS TOW Field Contents

Number
of Bits Parameter Resolution Range Units

Reference frame

16 — 0-65,535 Frames

GPS TOW 24 1 ms 0-14,399,999 ms

N_SAT 4 — 1-16 —

Source: [76].

Table 9.14 Measurement Parameters Field Contents

Number
of Bits Parameter Resolution Range Units

Satellite ID 6 — 0-63 —

C / N 0 6 1 0-63 dB-Hz

Doppler 16 0.2 6,553.6 Hz

±

Whole chips

10 1 0-1022 chips

Fractional chips

0-(1-2 −10 )

10 2 −10 chips

Multipath indicator 2 4 levels —

Pseudorange RMS error 6 3-bit mantissa

3-bit exp

0.5-112 m

Source: [76].