Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 541

tors to code-phase uncertainty. There are methods to remedy this common-mode
frequency and time problem that are unique in a cellular handset.

Time. Some types of handsets, such as CDMA, have precise knowledge of GPS
time internally as long as the handset is monitoring at least one paging channel.
CDMA cell towers are synchronized in time using GPS receivers in each cell tower.
The handset uses the precise time information when handing over from one cell
tower to another so that it can align the cell signal spreading code phase and main-
tain seamless communication as the user moves from one cell to the next. By trans-
ferring the precise time information into the GPS function, it becomes possible to
substantially reduce the contribution of time error as it reflects into the code phase
uncertainty dimension, leaving (mostly) the contributions to position uncertainty as
shown in Figure 9.41.
Certain types of handsets, such as GSM, do not have precise time information
available internally. As such, methods have been devised by which precise time can
be delivered to a GSM handset via the network to handset messaging protocol so
that it too can be time synchronized as in CDMA.
The GSM over-the-air protocol is time-division multiplex—each handset is
assigned a timeslot in which it receives and transmits packets of data between itself
and the network. To accomplish precise time transfer in the asynchronous GSM
network, an additional hardware element is installed in the network called a loca-
tion measurement unit (LMU). The LMU contains a GPS receiver for time synchro-
nization. It also contains a GSM phone receiver that it uses to measure the absolute
timing of certain data packets that it receives from each cell tower it can “hear”—in
effect, time tagging the bits received with GPS time. The LMU measures the time
shift or time offset of each cell tower signal that it can hear and makes this time-shift
information available to the cell network for delivery to those handsets desiring pre-
cise time correction. The handset accepts parameters via a network-to-handset mes-
sage that allows it to instantiate a particular portion of the network-to-handset
message with a precise time tag. As such, when the handset receives the particular
portion of the network-to-handset message, it can associate the event of receiving
the bits with the precise time tag (derived from the LMU), thus providing a method
of time transfer that is much better than 1 ms, or 1 GPS PRN-code time period.
Installing LMUs into a GSM network is a rather expensive proposition, so not
all GSM networks will have LMUs. Network operators prefer a lower cost alterna-
tive—to deliver an approximate time estimate to the handset via a standard net-
work-to-handset message. Network latencies in delivering the message to the
handset establish the best possible accuracy of no more than 2 seconds; thus,

approximate time is useful in computing satellite Doppler when satellite ephemeris
and approximate position is available, but it is generally useless in computing pre-
cise code phase estimates for each satellite so as to avoid searching the entire code
phase space.
All is not lost, however, because as described earlier, most receivers take advan-
tage of the common-mode nature of time uncertainty once one satellite is detected.
After detecting a first satellite generally using a full-code phase scan, the code phase
uncertainty region for the remaining satellites is reduced substantially because the
measured code phase from the detected satellite can be differenced with the pre-

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