3.5.3  Almanac Collection

The first time a receiver is operated, it must perform an iterative search for the first

satellite signal unless it can be loaded with a recent satellite constellation almanac,

the approximate time and the approximate receiver location. The almanac gives the

approximate orbit for each satel lite and is valid for long time periods (up to 180

days). The almanac is used to predict satel lite visibility and estimate the pseudo 

range to a satellite, thereby narrowing the search window for a ranging code.  Once

the first satellite signal is acquired, a current almanac can be obtained from the

NAV msg. It takes up to 12 1/2 minutes to collect a complete almanac after initial

acquisition. An almanac can be obtained from any GPS satellite.  Most modern

receivers can update the almanac periodically and store the most recent almanac

and receiver position in protected memory.  A clock can also be kept operating

when the receiver is off or in standby mode, so as to minimize initial acquisition

time for the next start up.

3.5.4  Initial Uncertainties

The initial uncertainties associated with a GPS receivers initial position, velocity,

acceleration, jerk and time inputs must be specified when satellite acquisition times

are being tested.  Acquisi tion and reacquisition times will vary depending on the

accuracy of the receiver initial ization. Some military TTFF requirements that include

jamming and other sensitive para meters in the start up scenario may be classified.

3.5.5  Ephemerides Collection

Ephemeris data forms part of the 50 Hz NAV msg transmitted from the GPS

satellites. Unlike almanac data which can be obtained for the whole constellation

from a single satellite, ephemeris must be collected from each satellite being

tracked on acquisition and at least once every hour. Ephemeris information is

normally valid for 4 hours from the time of transmission, and a receiver can

normally store up to 8 sets of ephemeris data in its memory. Acquisition and

reacquisition times for a receiver will vary, depending on whether valid ephemeris

data is already available to the receiver.  When testing acquisition time it is

necessary to specify whether a valid set of ephemerides is resident or not within the

receiver. Depending on the NAV msg collection scheme employed in a particular

receiver, it can take between 30 seconds and 3 minutes to collect the ephemeris

information.

3.5.6  Enhanced Acquisition Techniques

A number of enhanced acquisition techniques have been developed for modern

receivers. TTFF performance can be significantly improved by the use of multi tap

correlators and multi channel search algorithms.  Multi tap correlators are

essentially multiple correlators in the same package which greatly enlarge each

search window for code correlation.  Similarly, using all available receiver channels

in the search for the first satellite can reduce TTFF  by maximizing the effective

search window of the receiver.

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