Global Positioning System Reference
In-Depth Information
The expressions (5.52) and (5.53) can be used to determine the number of
coherent integration times needed to achieve a specific detection probability at a
given false alarm probability.
The time (in seconds) to search the initial time uncertainty of
±∆
seconds and an
initial frequency uncertainty of
±Φ
Hz is then
∆Φ
T
T
=
N T
(5.54)
search
n
N
STC
where T is the coherent integration time, N STC is the number of short-time correla-
tions within the coherent integration time, and
x
is the smallest integer greater
than x .
5.9
Sequence of Initial Receiver Operations
The sequence of initial GPS receiver operations depends on the design of the receiver
and the past history of the receiver operation. Obviously, the first operation is to
select the satellites and then to conduct a search for the selected satellites. There is
usually strong emphasis on how fast the receiver will acquire the selected satellites.
To determine which satellites are visible and which constellation of visible satellites
is the most suitable, three things are needed: (1) an up-to-date almanac; (2) rough
estimates of user position and velocity; and (3) an estimate of user GPS time. If any
of these parameters are missing or obsolete, the receiver has no choice but to per-
form what is called a sky search , described later. If all are available, then using the
user position, the GPS time estimate, and the almanac, the SV positions and LOS
Doppler can be computed. Using the estimated user position and the SV positions,
the visible SVs can be determined. From the list of visible SVs and the user position,
typically the best constellation geometry for good dilution of precision is deter -
mined. The best constellation might be selected based on some criteria other than
dilution of precision depending on the application. When the constellation has been
selected, the search process begins. From the user velocity and the SV LOS Doppler,
the total LOS Doppler can be determined. This is used in the Doppler search pattern
for the SV. If the approximate time and position are known and the ephemeris data
has been obtained during a recent previous operation, the time to first fix can be
around 30 seconds for a typical multichannel GPS receiver if the signals are unob-
structed. It can require up to 30 seconds just to read the ephemeris data for the SV
following signal acquisition. If the ephemeris is not available for the first fix, the
almanac data is ordinarily used until the more precise data become available. Read-
ing the almanac data following signal acquisition takes 12.5 minutes. The almanac
data, used for SV selection and acquisition, is valid for several days, whereas the
ephemeris data, used for navigation, begins to deteriorate after about 3 hours. For
the best navigation accuracy, the ephemeris data should be updated each time
newer data is available from the space segment.
A critical piece of information is time. Most modern GPS receivers have a
built-in timepiece that continues to run even when the set is powered down. They
also have nonvolatile memory that stores the last user position, velocity, and time
 
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