Figure 11 1.  Interferometry Using GPS

The phase mismatch can be used to determine the relative orientation angle,

 q

.  Single

differencing can be defined as taking the instantaneous difference in phase between

the received signals from one satellite as measured at the two different antenna

locations. Double differencing is obtained by differencing the single differ ences for one

satellite with respect to the single differences for a second satellite.  One interferometer

can be used to determine the azimuth and elevation of the user.  Two inter ferometers

are required for 3 D attitude determination, (roll, pitch, and yaw).

The carrier wavelength ambiguity problem (determination of number of full cycles, n)

can be solved by using the best estimate of position and attitude information for ini  

tialization, use of multiple satellites to provide additional geometric information, use of

P(Y) code to reduce the possible number of carrier wavelengths, and jointly processing

on L1 and L2 carrier signals by the interferometer, known as widelanning. 

11.3.3  3 D Attitude Reference System

3 D orientation requires at least 3 independent antennas to define a geometric plane. 

Four non coplanar antennas could be used to define two planes and provide

redundancy. Orientation solutions of the planes containing the antennas are related  to

the vehicle and therefore allow the orientation of the vehicle to be determined.  The

preferred method of operation is to get L1 and L2 frequency observations from four

satellites continuously.  This would require one 4  channel P(Y) code receiver (or the

equivalent) dedicated to each antenna.  Optimum hard ware for a 3 D attitude reference

system is as follows:

antenna subsystems

1 GPS 12 channel receiver

11 4