Data processing unit

Common reference oscillator

Another simpler alternative is to use one 8 channel P(Y)  code receiver and 3 antennas.

Four channels would operate as a sub  receiver and would be dedicat ed to one master

antenna.  The sub receiver and the master antenna read the NAV msg and provide

position and velocity information for use when processing the signals from the other

two antennas. This method can be used if the baselines between the antennas are

sufficiently short to assume the position and velocity measure ments are nearly the

same for all three antenna locations.  The two other antennas would only use 2 

channels each to do carrier phase measurements on two of the four satellites that the

"master" antenna was tracking.  Which of the two satellites all three antennas would be

tracking depends on the satellite geometry relative to the antennas.  The technique

requires that the measurement data is processed as if all satellite signals were

received at the same time.  Under high HV dynamics, the INS  derived angular

information may be better than GPS.

11.3.4  Use of Multiple Receivers and a Reference Oscillator

When multiple receivers are used for interferometry it is recommended that a common

external reference oscillator be used for all the receivers, otherwise the oscillators in

each receiver must be calibrated.  A common frequency reference would improve the

accuracy of attitude measurements because all phase measurements would be  done

using the same time reference and thereby eliminate "own clock errors".

11.3.5  Error Sources and Degradation of Performance

The most dominant error sources are as follows:

Absolute position uncertainty

PDOP

Antenna location

Antenna position difference uncertainty in the body frame

Measurement accuracy

11.3.5.1  Absolute Position Uncertainty

Errors in knowledge of the absolute position of the primary antenna can cause an

angular orientation error in the local  level frame with which attitude is referenced.  This

orientation error transforms into an equivalent attitude error.  Typical attitude errors of

less than 0.03 minutes of arc can be expected.

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