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Craving Direction Page 2

Electronics - January 2002 - GPS Gyro Compasses
Electronics January 2002
By Ben Ellison


Craving Direction
Part 2: The dependence of GPS gyros on the satellites can be seen as a weakness.
   
 
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• Part 1: Craving Direction
• Part 2: Craving Direction continued


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These GPS gyros, also known as "satellite compasses," cannot simply compare latitude/longitude positions over such short distances to derive a heading; they have to go deeper into the GPS signals. Furuno, in language that seems designed to impress a Nobel prize committee, describes the process as decoding and comparing "phase data in the carrier frequency" with "advanced kinematic technology," which even includes something called the LAMBDA algorithm licensed from a Dutch professor. While acknowledging that the basic GPS gyro scheme can be done with two receivers, Furuno adds a third GPS to reduce the influence of pitch and roll and samples five satellites for further checks on 3D data. The SC60 also incorporates a rate gyro, able to provide inertial navigation while a vessel passes under a bridge and adding another level of "confidence checking." Furuno product manager Bill Haines says, "A lot of engineering went into this device."

JRC’s unit only uses two receivers, and its product literature is much less verbose, but the company claims results as good as Furuno’s–which is to say, darn good. These machines are said to be accurate to less than one degree and supposedly handle rapid turn rates and extreme sea motion with aplomb.

Both systems have separate display and processor boxes, the latter sporting remarkable numbers of output ports. The idea is to feed not just radars, autopilots, chartplotters, and PCs, but also heading repeaters, sonars, true wind and current gauges, and gizmos yet to be developed. Unfortunately such interfacing is not always simple. One of the oddities of the heading business is the large number of protocols that have grown up over the years, largely as manufacturers competed to quickly feed data to their autopilots and radars. Thus the JRC and Furuno units each support their own data types, respectively called NSK and AD-10, as well as slower NMEA 0183. Separate converters exist that can translate the NMEA output to Sin/Cos, Robertson, Raymarine SeaTalk, and other languages.

The display screens of these GPS gyros, of course, show heading information in various formats and, at least in the case of the SC60, pitch and roll values. They can also show all the position, COG, SOG, and other data familiar to regular GPS units, which in fact they are. (Note for the confused: A regular GPS is not a compass, despite any compass-like screens it may have. The technique of comparing positions over time to determine heading will work to some degree for many of the functions I’ve discussed, but it is prone to error and fails altogether when a boat stops moving.)

The dependence of GPS gyros on the satellites can be seen as a weakness; if your position and heading are both based on GPS, and somehow the "birds" go out, you have a problem. Aside from that, it’s hard to find fault with this new "compass" technology, unaffected as it is by the vagaries of magnetism (a subject I’ve actually barely touched on), and free as it is from the famously expensive maintenance needs of real gyros full of precision moving parts. The Furuno and JRC units do sell for about $5,000 apiece–a bargain relative only to gyros–but representatives of both companies are sure that less expensive units will evolve rapidly. After all, these systems are built of common GPS units, processors, and software–all susceptible to marvelously shrinking manufacturing costs. Then there’s a whole other technology, solid state Fiber Optic Gyro, showing great promise further out in development time. Suffice it to say that our heading-hungry bridges will be well fed eventually.

Ben Ellison has been a delivery captain and navigational instructor for nearly 30 years and was recently editor of Reed’s Nautical Almanacs.

Furuno USA Phone: (360) 834-9300. Fax: (360) 834-9400. www.furuno.com.

JRC Phone: (206) 654-5644. Fax: (206) 654-7030. www.jrc.co.jp/index-e.

Previous page > Craving Direction, Part 1 > Page 1, 2

This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.

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