11.3.5.2  PDOP

The attitude determination accuracy is influenced by the satellite geometry the same

way as position accuracy.  Poor satellite geometry results in less accurate position and

attitude determination.

11.3.5.3  Antenna Location

Antenna location errors are a minimum when the two position difference vectors are

orthogonal.  Simulation results indicate that acceptable performance can be obtained

when the vectors intersect at an angle between 45 degrees and 135 degrees.  The

performance deteriorates rapidly outside this domain.

11.3.5.4  Antenna Position Difference Uncertainty in the Body Frame

Uncertainty of the difference vectors in body  coordinates is a function of two primary

factors: body flexure and errors inherit ed from the calibration process.  Body flexure

alters the relative position between antennas, hence causing errors.

11.3.5.5  Measurement Accuracy and Error Budget

The most significant factor influencing the feasibility of GPS attitude measurement is

the accuracy of the range difference measurement.  The error sources affecting

accuracy are as follows:

1.

Atmospheric delays

2.

Multipath effects which can be quite significant, but can be largely negated

by proper antenna placement

3.

Phase difference measurement accuracy

4.

Transmission delay stability

Very little is published about what accuracies can be expected when using GPS for

attitude reference, but one manufacturer claims a heading accuracy of 0.3 degree, roll

and pitch accuracy of 1.0 degree, updated at a 20 Hz rate with no practical speed limit,

a maximum acceleration of 10 G, and a maximum angular velocity of 30 degrees/sec. 

The same manufacturer claims a heading accuracy of 0.05 degrees within 5 minutes in

a stationary mode.  This performance is with a C/A code receiver, using 3 antennas

placed in a triangle with 57 cm baseline between the antennas.  Generally, the

measurement accuracy depends on baseline length and measurement time.  Longer

baselines and longer measurement times will improve the accuracy.

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