Global Positioning System Reference
In-Depth Information
duce an adjusted frequency inside the phone receiver. The control registers of the
fractional-N synthesizer are translatable into a known frequency of operation of the
handset reference oscillator. This frequency is known to better than 0.05 ppm,
achieving the same goal as long as the synthesizer tuning parameters are made avail-
able to the GPS function. The latter method offers significant advantages over the
former, as the discrete jumps in frequency attributed to manually adjusting the ref-
erence oscillator frequency can cause data demodulation and tracking problems to
the GPS function. The frequency jumps cause instantaneous phase rotation of the
received GPS biphase modulated signal. If large enough, the instantaneous phase
rotation due to the frequency jump cannot be discriminated from the
180º phase
rotations due to signal PRN modulation or navigation data bit modulation, thus
confusing the data demodulation process and causing possible loss of lock.
As with time, using handset-based frequency aiding information is not abso-
lutely necessary in order to meet the acquisition time goals of cellular A-GPS. In all
cases, the battle can be won by having sufficient correlators available to search out
the uncertainty space in sufficient time. However, there is a cost and power con-
sumption penalty associated with a maximum correlator solution that is painful to
overcome, at least in the near term until IC technology evolves further. As with
time, one can take advantage of the common-mode nature of the reference fre-
quency uncertainty for applications by choosing to install a separate GPS reference
oscillator. In this case, most of the Doppler uncertainty is due to reference oscillator
uncertainty and can be solved for as soon as one satellite is detected. Thus, the total
uncertainty search space collapses significantly once a first satellite is detected and a
precise Doppler measurement to it is made.
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9.4.5 Types of Network Assistance
The specifics of assistance information that are available from a cellular network
are governed by applicable standards that can vary with cellular telephone technol-
ogy type (e.g., AMPS, CDMA, TDMA, GSM, or WCDMA). Generally, the individ-
ual standards messaging protocols are similar across the MAs, with the possible
exception of the CDMA standard in which additional assist data types and location
methods are included. In order to simplify the discussion, we will focus on the GSM
cellular standard, since it is the most widely deployed standard.
The possible forms for acquisition assistance include the following:
A list of visible satellites;
Predicted GPS satellite Dopplers and Doppler rates;
Azimuth and elevation angles for the visible satellites;
Local oscillator offset information (through the handset AFC function);
Approximate mobile location;
GPS satellite ephemeris information;
GPS almanac;
Satellite clock correction terms;
Approximate GPS time;
Precise GPS time;
 
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