7.2  AIDING DURING INITIAL ACQUISITION

7.2.1  Position and Velocity Aiding

When a GPS receiver is first initialized for operation, approximate position and velocity of

the receiver are required to minimize satellite acquisition time.  The accuracy requirement

of the U.S. DoD program for position is < 100 km of actual receiver location, and for velocity

is < 100 m/s of actual receiver velocity, to ensure that satellite acquisition is within

specification.

7.2.2  Time Aiding

Time aiding can be used during the initialization process, similar to position and velocity

data.  The time accuracy requirement is < 20 seconds relative to UTC.  This is to ensure

that satellite acquisition time is within specification.

Time aiding, if sufficiently accurate, can also be used to enable a direct P(Y)  code

acquisition without first acquiring the C/A  code.  This type of time aiding is relevant to  HVs

such as submarines where minimum exposure time of the GPS antenna on the ocean

surface is of prime importance.  An atomic time standard is one way to enable direct

P(Y) code acquisition.

7.2.3  Almanac Data

Normal satellite acquisition requires the availability of a current satellite almanac, stored in

the receiver memory.  If there are no significant changes in the satellite constellation, then

the almanac is valid for several weeks. 

If no stored or valid satellite almanac data are available, the GPS receiver starts to search

the sky attempting to locate and lock onto any satellite in view.  Depending on the receiver

search strategy and on the actual satellite constellation, this process may take 15  60

minutes. When one satellite is being tracked, the receiver can download and read the

almanac information about all the other satellites in the constellation.

7.2.4  Effect On TTFF

Dependent on the type of integration (position, velocity and time) aiding data to the GPS

receiver during the initialization process are provided as follows:

a. Manually by the operator via the GPS CDU or HV CDU,

b. Automatically from INS/AHRS, PTTI or the HV mission computer (via 1553  bus),

c. Default by using the shut down values stored in the receiver memory.

Initial acquisition performance can be expressed by the TTFF.  In general terms, the TTFF

is the time from when the receiver attempts to track the satellite signals until a navigation

solution is determined.  Knowing the position and velocity of the receiver, current time, and

the positions of the satellites will all help to reduce the TTFF.  Conversely, a lack o f

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