Furuno Sc 50 manual Principle of Satellite Compass, AP-13

APPENDIX

6. Principle of Satellite Compass

Own ship's heading can be determined by decoding the data in the carrier frequency in addition to ordinary GPS parameters. In principle, a pair of two antennas A1(ref) and A2(fore), each connected with an associated GPS engine and processor, are installed along the ship's fore-and-aft line. GPS systems at A1 and A2 calculate the range and azimuth to the satellite. Difference in range between A1 and A2 is ∆λ + nλ where λ is 19 cm. “n” is automatically found during the initialization stage by receiving three satellites. A fraction of a carrier wavelength, ∆λ , is processed by FURUNO’s advanced kinematic technology in geographical survey, thus determining a vector (range and orientation) A1 to A2.

In reality, a third antenna is used to reduce the influence of pitch, roll and yaw, and five satellites are processed to process 3D data. If the GPS signal is blocked by a tall building or the vessel is under a bridge, the 3-axis solid-state angular rate gyros in the processor unit take place of the satellite compass, maintaining the current heading continuously.

Antenna A3

Heading		line
	-aft
	-and
θ	Fore

∆λ

nλ

λ

Vector to

decide

heading Antenna A2

Difference between the range from satellite to antenna 1 and the range to antenna 2.

Antenna A1

AP-13