Principle Of Operation - Raytheon STD 21 GPS Operation Manual

1.3

Principle of Operation

The Raytheon Marine/Anschütz STD 21 GPS compass uses continuous carrier phase
measurements of two GPS antennas aligned to the ship's longitudinal axis for relative
positioning. Relative positioning means that state of the art algorithms exploit the carrier
phase measurements and the broadcast satellite ephemeris for precise computation of
the baseline vector pointing from the aft antenna to the fore antenna. Then, the baseline
vector's continuously computed north, east, and down coordinates can easily be con-
verted to polar co--ordinates, that is, pitch and heading signals.
In order to bridge over short--term satellite signal looses, e.g. signal blockage or electro-
magnetic interference, the STD 21 GPS--/INS compass incorporates gyros. These hard-
mounted devices measure the ship's three--dimensional angular rates for continuous in-
tegration of the ship's roll, pitch, and heading angles. The integration is also executed
during GPS satellite signal reception. Thus, the differences between the integrated and
GPS--derived pitch and heading angles may be computed. When combining these angu-
lar differences with a gyro model through an optimal estimation algorithm, the gyros be-
come precisely calibrated, thus allowing a longer and more accurate bridging of satellite
signal outages.
Availability of STD 21
A satellite compass is not an autonomous device as is a gyro compass. Thus, the STD
21 satellite compass does not have the availability of the STD 22 gyro compass.
The reasons for the slightly reduced availability which holds for any satellite compass
are:
" Exploitation of extremely low--power satellite carrier signals,
" Use of statistical methods within the digital signal processing path,
" High sensitivity to multipath and signal blocking effects that adversely affect unin-
terrupted satellite signal reception at the Antenna Unit.
Nevertheless, the STD 21 has a proven -- through long term testing -- availability of
99.9% if installed in accordance with this manual.
Installation of the STD 21
A satellite compass like the STD 21 exploits the carrier phases of GPS satellite signals
when computing the heading angle based on the principle of interferometry.
3717/110--850.DOC020002
Satellite Compass
STD 21 GPS
4
Edition: Feb. 05, 2007