reasonably accurate knowledge of any of these parameters will increase the TTFF.  The

amount of increase is dependent on the particular quantity and level of uncertainty.

7.3  AIDING TO TRANSLATE NAVIGATION SOLUTION

The navigation solution of an unaided GPS receiver is referenced to its antenna position. 

An aided GPS receiver can reference its navigation solution to another location. For

example, the GPS navigation state can be resolved at the IMU instrument axes center in

the case of an INS.  To perform the calculations, the receiver needs to be aided with

attitude information and a lever arm vector.

The attitude information in the form of roll, pitch and heading is provided in most cases by

an INS or AHRS.  A GPS receiver usually does all internal calculations in ECEF before

carrying out any coordinate transformations.  Using latitude and longitude in conjunction

with attitude, the transformation between the GPS ECEF navigation frame and the HV body

frame can be determined.  Onboard ships, attitude aiding is also used to compensate for

antenna motion and, together with water speed information, to do relative course and

speed calculations.

A lever arm vector is provided to the GPS receiver as a vector between the GPS antenna

and the HV reference point.  If attitude aiding is removed from the GPS receiver, the

navigation solution should revert back to the GPS antenna location.  Often, more than one

set of lever arm corrections may be stored in the GPS receiver.  This is useful for

installations having more than one INS aiding source or, in the case of big ships, where

position and/or velocity information for different locations onboard may be of interest. 

However, only one attitude aiding source should be used by the GPS receiver at any one

time.  Hence the propagated navigation solution will only incorporate the one set of lever

arm corrections applicable to the particular aiding source that is providing aiding data to the

GPS receiver.  Should the aiding source be changed, the lever arm corrections will change

accordingly.

7.4  AIDING TO REPLACE A SATELLITE MEASUREMENT

During normal receiver operations, four satellite measurements are required inputs to solve

the equations for position (U

x

, U

y

, U

z

) and clock offset Dt.  In case of limited satellite visibility

or poor satellite geometry, one or more of the four satellite inputs may be replaced by

inputs from an external aiding source.

When the GPS receiver is shipborne, or has barometric altimeter aiding or has a known

height, then only three satellites are needed.  Additional aiding by a precise clock can

supplement the measurements in a two satellite situation. 

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