— January 2002
By Ben Ellison
|GPS gyro compasses feed the heading-hungry bridge.|
More and more components at the modern helm want to know your boat’s heading, want it accurate, and want it now. For instance, the various radar/chartplotter devices and PC programs that can overlay radar targets on digital charts need fast, dead-on heading data to synchronize the imagery. Without it, a wonderfully useful display can become a mess.
MARPA, or Mini Automatic Radar Plotting Aid, is a neat radar function proliferating in large yachts, enabling a user to lock onto and plot a moving target. It can be a major help in a busy channel but will go wacky if the boat’s heading data is unstable, as can happen when a fluxgate compass bounces around in a seaway. Autopilots have always needed the best possible heading data to do their work, and so have auto tracking satellite TV antennas that demand pitch and roll information as well as heading. I could go on.
Electronics manufacturers have been busy trying to feed the heading hunger. KVH’s GyroTrac combines "three-axis gyro stabilization" with a fluxgate electronic compass to produce what it calls "the world’s finest magnetic compass." Raymarine recently introduced its G Series Course Computers, which similarly add "integral Rate Gyro sensors" to its regular electronic compass for what the company says is solid heading information throughout a Raymarine network. Both companies, and others like Simrad, are using motion detection to ameliorate the problems a magnetic sensor can have with acceleration and centrifugal forces.
It’s worth noting that these gyro applications are not the same gyros that have been the crème de la crème of heading sensors on ships for some years and cost about as much to purchase and maintain as a good automobile. A real gyro has nothing to do with the magnetic flux of the Earth but instead uses the tendency of a mass in gyroscopic motion to align itself with the Earth’s rotational axis. A real gyro delivers true heading data, cares not a whit about any magnetic deviation present on a vessel, and is very stable relative to the vessel’s motion. It’s these qualities that understandably lead marketing folks to call a whole new heading technology "GPS gyro."
When I first heard about the GPS gyro concept, it seemed pretty obvious and pretty neat. Put one good GPS on the bow and one on the stern, and a simple box can compare their positions and compute the heading. Silly me. The actual devices, first introduced by JRC and now also available from Furuno, accomplish the mission with GPS units that are quite close together, but the processing required is formidable. JRC’s JRL-10 antenna unit uses two seven-inch GPS receivers lined up about 20 inches apart on the ship’s lubber line, while Furuno’s SC60 puts an array of three receivers inside a dome about 26 inches across and six inches high.
This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.