Chuck Husick: Techno-Talk, September 2002 BoatUS Magazine - Updated April 2009

GPS Compasses

It is a safe bet that everyone reading this column is familiar with the magnetic compass. Powered by the Earth’s magnetic field, it is the ultimately reliable navigation instrument, continuing to work if the boat’s electrical system fails. While of undisputed value, the information it provides must be corrected for deviation to obtain magnetic heading and then further corrected to yield the true heading information required for navigation. In addition, the magnetic compass, whether the conventional direct reading type or the electronic flux gate that provides guidance to the autopilot, is adversely affected by the boat’s motion, making the autopilot’s performance deteriorate in a seaway. A stable source of true heading information would be welcomed by many boat owners.

Until very recently the only practical source of true heading information was the north-seeking marine gyrocompass, usually costing $10,000 or more. A new device, the GPS compass was introduced a few years ago by Furuno and Japan Radio Corporation with similar devices available from a few other suppliers. These systems provide the navigator (and the autopilot) a source of accurate and motion stable true heading information.

As we are all well aware, the Course-Over-Ground (COG) information supplied from our Loran C or GPS navigators is not at all the same as heading data. Further, Loran C or GPS COG is computed from the changing position of the boat and becomes indeterminate when the boat stops or is drifting very slowly.

The GPS compass works by measuring the time difference between the arrival of signals from three or more (preferably five) GPS satellites at its antennas, two for the JRC system, three for the Furuno system. The system uses only the carrier wave from the satellite, it has no need to decode the navigation information contained in the carrier signal. The antennas of the GPS compass must be separated by a distance greater than the wavelength of the received signal. Fortunately the wavelength of the 1575.42 MHz GPS signal is small, 190 mm (7.497 inches), making the system practical for quite small boats.

GPS compass systems guard against short term interruptions in reception of GPS signals by incorporating solid state gyroscopes to provide a short-term memory capability, ensuring that heading information is maintained even if the vessel sails beneath a bridge or if signals are blocked by a nearby building.

The two current manufacturers of GPS compass systems have designed their equipment somewhat differently. The JRC JLR-10 system’s two antennas are mounted on a rail, 20 inches apart. The system’s display screen includes a full capability GPS navigation system. The Furuno SC-60 system’s three antennas are housed in a 25.6 inch-diameter low-profile radome-like enclosure while their model SC-120 places them further apart, on the ends of three arms. Furuno’s display shows only true heading, COG and speed data. The data accuracy from either of the GPS compass systems is typically on the order of 0.8 degrees or better, with a resolution of 0.1 degrees. Furuno’s three-antenna design enables it to provide roll and pitch information that can be useful in keeping a vessel properly trimmed as fluids are consumed from various tanks.

Additional models more recently introduced include the Furuno SC 110, with its 3 GPS antenna more widely separated than the radome model and the latest, SC30 antenna model often used with the Furuno NavNet3D chartplotter radar system.

Unlike a conventional gyrocompass that typically must operate for an hour or more before it can provide useful information, the GPS compass is usually ready for use in less than five minutes. Where the conventional gyro consumes a substantial amount of electrical power (most often 120 volt, 60 Hz AC) the GPS compass operates from the boat’s 12 volt DC bus drawing less than 2 amperes. The growing popularity of Radar and Satellite image overlay on chartplotter screens places an increased need for heading sensor precision and response speed. At this time the best results are achieved by using a GPS compass.

As with most electronic technology, the cost of newly developed products follows the classic learning curve common to most manufactured devices. For example the first Magnavox (now Leica) recreational/commercial boating GPS navigator sold for $10,000. Today a superior GPS receiver can be bought for about $100, a 100:1 cost reduction ratio. Although GPS compass systems currently sell for more than $2,300 we can expect significant price reductions as the technology and the hardware evolves over the next few years.

While we appreciate the many advantages of the GPS compass we will, along with the builders of even the largest ships, continue to install the familiar fluid-damped magnetic compass knowing it will work so long as the Earth’s magnetic field does not reverse its polarity.