Specifications

IntroductionGPS and Assisted GPS
10Operating Manual 1171.5248.12 ─ 15
C/A codes 1 to 37 selectable, 1023 chips per C/A code
Modulation BPSK (C/A code, P-code)
QPSK (C/A code and P-code)
Information data rate (navigation message) 50 Hz
Frame structure of navigation data 25 frames consisting of 5 subframes where 1 sub-
frame consists of 10 words, 1 word consists of 30 data
bits, 1 data bit consists of 20460 C/A code chips.
2.2 Assisted GPS (A-GPS)
Assisted GPS (A-GPS) was introduced to different mobile communication standards to
significantly reduce the Time To First Fix (TTFF) of a user equipment (UE) containing a
GPS receiver. This is achieved by transmitting information about the satellites directly
from the base station to the UE.
In order to obtain a valid position fix, a stand-alone GPS receiver performs the following
steps:
1. Find visible satellites by searching different C/A codes over different Doppler shifts.
(This process is also called acquisition.)
2. Read out ephemeris data from the satellite's signal in order to get the satellite's cur-
rent position.
3. Read out the complete navigation message from the satellite's signal in order to get
the ionospheric model and the positions of all satellites (almanac).
4. Perform measurements to get runtimes of the different satellite signals.
5. Calculate exact location based on the satellite's positions and the measurements
taken. (This process is also called navigation.)
Depending on previous knowledge, a stand-alone GPS receiver needs about 30-60 sec-
onds for a first fix and up to 12.5 minutes to get all information (almanac).
In A-GPS "UE based mode", the base station assists the UE by providing the complete
navigation message along with a list of visible satellites and ephemeris data. In addition
to this information, the UE gets the location and the current time at the Base Station and
that speeds up both acquisition and navigation processes of the GPS receiver and hence
reduces TTFF to a few seconds.
In A-GPS "UE assisted mode", the base station is even responsible for the calculation of
the UE's exact location, i.e. the base station performs the navigation based on the raw
measurements provided by the UE. Since the Acquisition Assistance Data provided by
the Base Station already serves speeding up the Doppler and Code Phase correlation,
i.e. the acquisition process, the UE only has to perform the measurements for getting the
runtimes of the different satellite signals. Complexity of the UE's GPS module is hence
reduced which leads to lower power consumption.
Assisted GPS (A-GPS)