Chuck Husick: Techno-Talk, September 2002 BoatUS Magazine - Updated April 2009
It is a safe bet that everyone reading this column is familiar with
the magnetic compass. Powered by the Earths magnetic field, it
is the ultimately reliable navigation instrument, continuing to work
if the boats 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 boats motion, making the autopilots performance deteriorate
in a seaway. A stable source of true heading information would be welcomed
by many boat owners.
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
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.
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
current manufacturers of GPS compass systems have designed their equipment
somewhat differently. The JRC JLR-10 systems two antennas are
mounted on a rail, 20 inches apart. The systems display screen
includes a full capability GPS navigation system. The Furuno SC-60 systems
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. Furunos 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. Furunos 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.
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 boats 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.
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.
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 Earths
magnetic field does not reverse its polarity.