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GPS compass

Jan 1, 2003

The latest twist in determining a sailboat's heading ignores the earth's magnetic field and makes use of GPS satellites. By processing these signals, new GPS compass units can very accurately calculate the heading of your boat. While not yet an item for the small- to medium-sized vessel, GPS compass units have some definite advantages and should begin to be seen on larger boats.

Of course, there are some larger yachts and commercial vessels that carry north-seeking marine gyrocompasses, but the vast majority of voyagers rely upon a magnetic sensor to determine the heading of their boats. This magnetic sensor may be a standard mechanical compass, or it could be an electronic magnetic sensor, such as a flux gate unit. In all cases, the heading information is based on detection of the Earth's magnetic field, displayed with reference to magnetic north.

There is nothing wrong with this way of navigating; it has served mariners ever since the discovery of the magnetic compass, attributed to the Chinese in about A.D. 1090. However, although ubiquitous, the magnetic heading sensor does have a number of significant limitations, primarily caused by the nature of the Earth's magnetic field.

A large vessel's gyrocompass avoids the shortcomings of the magnetic compass, albeit at a substantial initial purchase cost, followed by significant annual maintenance expense. Further, the gyrocompass requires an uninterruptable source of power and a warm-up/alignment period that can extend to and beyond an hour before it can be relied upon. Gyrocompasses also have a few notable shortcomings, especially when used at very high latitudes.

A GPS receiver typically computes a vessel's course over ground (COG) and speed over ground (SOG) using successive comparisons of geographic position information. Although the COG and SOG data is usually very accurate, it is valid only when the vessel is moving. Accuracy decreases when the vessel is moving very slowly and becomes indeterminate when motion stops.Satellites point the way

A new and innovative means for determining true heading is now available, courtesy of the same GPS satellites that have so totally changed our concept of precision navigation. Two new systems for determining true heading have recently entered the market. Both systems, and others sure to follow from other manufacturers, determine true heading by analyzing the way in which signals from the orbiting GPS satellite constellation arrive at their antennas. The concept seems simple: By comparing the time delay between the arrival of a particular point on a satellite's signal at one antenna with its arrival of that point at an antenna a short distance away, it is possible to determine the relative bearing between the two antennas and the satellite that transmitted the signal.

JRC Marine Electronics recently introduced a two-antenna GPS compass called the JLR-10. In this system, a phase comparison process applied simultaneously to at least five satellites will provide remarkably accurate true heading information. The two antennas need be separated by only a few wavelengths — a short distance, since the wavelength of the GPS signal is a little more than 7 inches. The JLR-10 twin antennas are mounted on a common rail, keeping the distance between them fixed and simplifying installation. The JLR-10 has an accuracy of 0.7°. It outputs data in several formats of NMEA 0183 for display, radar and other nav-aid integration. And with two GPS receivers, the JLR-10 also obviously provides position information.

As you might suspect, if two of something is good, three may be better. Furuno has taken that approach by using three GPS antenna/receivers in their new SC-60 and SC-120 GPS compass systems. Using their three antennas, the compasses compute true heading from the GPS satellites to an accuracy of 0.8° for the SC-60 or 0.5° for the slightly larger SC-120 system. In both systems, the GPS receivers provide signal-carrier information to very cleverly programmed data processing hardware that calculates true heading. With three antennas, the system also computes the roll and pitch angle of the vessel. Both Furuno systems also provide triple-redundant GPS position information for use with navigation equipment.

Navigation practice will change significantly for vessels using the GPS compass. Although the system can deliver magnetic direction, heading information is referenced to true north. Some may lament today's ready opportunities for correction from magnetic to true and then back to magnetic. We might even forget our favorite aide memoire — TVMDC — True Virgins Make Dull Companions.

Unlike the GPS position fixing process that depends on the information transmitted on the RF carrier wave from each satellite, the GPS compass operates by comparing the phase difference of the 1,575-kHz GPS carrier signal as it arrives at two or more antennas.An audio analogy

The operating principal of the GPS compass can be illustrated initially with aid of an audio analogy: Assume that a distant lighthouse is equipped with a large gong. Two microphones are mounted a few feet apart on a horizontal wooden beam that can be rotated about its center. The microphones are connected to an electronic timing device so that the arrival of the sound from the gong at the lighthouse at one microphone starts a timer that is in turn stopped by the arrival of the gong's sound at the second microphone. The time delay between arrival of the gong's sound at the two microphones will be a maximum when the axis of the microphones is aligned with the direction to the lighthouse. It will be zero when the board on which the microphones are mounted is set at 90° to the direction to the lighthouse. The maximum length of time recorded by the timer depends on the distance between the microphones and the speed at which the sound wave travels.

If the direction to the one lighthouse was all we wanted to know, we would rotate the microphone-equipped wooden beam until the timer provided the largest time-difference reading, at which time the centerline of the beam would be pointing directly at the lighthouse. We could verify the data by performing another check at the 90° point where the timer readout would be zero. The presence of additional gong-equipped lighthouses, each using a different frequency gong, would allow us to determine the true bearing to each, enhancing the accuracy of our aural relative bearing system. With precise knowledge of the position of each gong-equipped lighthouse, we could measure headings referenced to true north.

As with any system, there would be some complications, aside from the anticipated interference noises and the rather limited range of sound signals. Perhaps the largest problem would arise when the incoming sound from the gong reflected off a nearby object — a tree, a building or a part of our microphone assembly. This reflected signal would necessarily arrive somewhat later than the direct sound, creating confusion in determining precisely when to turn the interval timers on and off. The addition of a third microphone would likely allow us to reduce the errors caused by this multipath effect.

The GPS compass uses the basic GPS satellite carrier wave to determine the relative bearing to a distant satellite. Fortunately, the GPS system broadcasts on a frequency of 1,575.42 kHz, making the wavelength quite short, 19 cm (7.6 inches).

The similarity to the audio analogy can be illustrated by a system where the two GPS antennas are separated by a distance of one wavelength at the GPS carrier frequency of 1,575.42 kHz. With this antenna separation, a signal arriving from a GPS satellite aligned with the axis of the two antennas would be precisely 360° apart in phase angle. The first wave would be arriving at the more distant antenna at the instant the second wave was arriving at the antenna closest to the satellite. As with the audio system, the signals would arrive precisely in phase when the axis of the two antennas was at 90° to the arrival path of the signal from the satellite. As noted, increasing the separation between the receiving antennas can improve the measurement geometry; therefore, the GPS antennas are separated by more than one wavelength. The antennas in the SC-60 GPS compass (±0.8° accuracy) are separated by about 20 inches; those in the more precise SC-120 (±0.5° accuracy) are 33.9 inches apart.Rejecting multipath

The interference referred to in the audio analogy is present when receiving GPS signals, with the signal from the satellite bouncing off nearby objects, including parts of the vessel. As with the audio system, adding a third GPS unit will greatly diminish the effects of multipath reception of satellite signals. Of equal importance, the availability of numerous satellites, all of whose positions are precisely known, will allow the development of what is called an overdetermined least squares solution to the problem. In plain language, we can solve the heading determination problem in so many ways that we can eliminate the majority of the possible errors. The GPS compass system makes simultaneous use of the signal information from as many as five satellites to process 3-D data.

In addition to its primary function of determining heading, a three-GPS-receiver heading determination system is inherently capable of measuring the angular position of the receiving antenna array about all three axes; roll, pitch and yaw (in this instance yaw = heading). (A GPS compass system has worthwhile application in aviation. Mounted on the top of an aircraft fuselage, in direct sight of the overflying satellites and generally well removed from the reflecting surfaces that create multipath interference, it can serve as an artificial heading and attitude reference system, in aviation terms, an AHARS.)

On a vessel, roll and pitch information can be used to ensure that proper trim is maintained as fuel is consumed from various tanks and that trim tabs are set appropriately. In a practical GPS compass system, increasing the distance between the individual GPS receivers will improve the ultimate accuracy of the computed heading information.

The system data output for the Furuno units is available in NMEA 0183 ver. 1.5 and 2.0 format, RS 422 or RS 232C, as well as in the 25-millisecond update, AD-10 format. Output connectors include a D-sub 9P type, suitable for connection to a computer. The control/display module resembles Furuno's familiar GPS 30, 31, 35 and 36 units, with data displayed on a 4.5-inch-diagonal, sunlight-readable, monochrome LCD display screen.

Four data-display formats are available. Heading display, including GPS time and date, true heading, SOG, COG and position fixing method (usually 3-D). Nav data display substitutes position information (lat/long) for heading. Steering display shows GPS time and date, position fixing method, heading, a moving tape bearing scale, COG, SOG and, in addition, vessel pitch angle and roll angle. The compass display provides an analog head-up version of heading plus GPS time, digital heading, pitch and roll, and position fixing method. The system incorporates a World Magnetic Model and can output magnetic heading for use with other systems. Selective smoothing adjustments can be set for vessel position, course and speed information.

Any interruption in receipt of satellite signals, such as might occur when a vessel passes under a bridge, would cause a dropout of heading information. Furuno has addressed this problem by incorporating a three-axis, solid-state gyro assembly that will continue to provide precise true heading information for up to 30 seconds after the interruption of satellite reception and progressively degrading heading information for several minutes.

In addition to the compass function, the SC-60 and SC-120 systems provide a triple-redundant GPS or DGPS system suitable for use with equipment such as Furuno's NavNet chart plotter/radar/sounder system. The DGPS sensor is an option, since determination of heading is independent of the accuracy of the position fix information supplied from the three GPS engines. When used, the DGPS antenna is mounted in a conventional manner, and the beacon receiver is easily installed within the system's main electronics module.Turning errors eliminated

The advantages provided by the GPS compass system are significant when compared to a conventional magnetic compass or a flux gate system. The northerly turning error &mdash the effect that causes a conventional magnetic sensor to lag behind actual turn ratesy�when navigating on headings close to north or to overshoot turn rates when on headings close to south are eliminated. Acceleration errors that create the illusion of a turn to the north when the boat accelerates on headings close to east or west and the false illusion of a turn to south when decelerating are eliminated. Elimination of these errors will result in improved autopilot performance. The improvement can be most apparent when using radar overlay on a chart image. In such setups, any lag in compass response can create an annoying misalignment between the radar information and the chart image that requires some settling time before it is resolved.

Also, high-latitude problems common to conventional compasses are avoided with use of the GPS compass. Vessels sailing from one hemisphere to another will enjoy consistent heading accuracy and stability. The errors that can affect gyrocompasses in high latitudes don't bother the GPS compass. Navigating with true-north-referenced heading information simplifies all navigation, including calculation of current set and drift. Radar systems, especially those equipped with automatic radar plotting aids (ARPA), will benefit from use of stable, true-north-referenced data.

System power requirements are modest: 1.1 amps at 12 VDC or 0.5 amps at 24 VDC.

The JRC and Furuno GPS compasses are not cheap. The JLR-10 has a list price of $4,295, while the Furuno SC-60 model lists for $4,395 and the larger SC-120 model is $7,995. However, these prices are very attractive when compared to the only other reasonable alternative &mdash the traditional, mechanical north-seeking gyrocompass.

Regardless of all of the above, you will still go to sea equipped with a conventional, mechanical magnetic compass. The unique value of the conventional compass lies in its ability to work, regardless of any foreseeable onboard or support system failure. The card will continue to rotate under the influence of the earth's magnetic field, even if the damping fluid leaks out. It may stick a bit when deprived of its normal buoyancy, but with a few light taps from a pencil to overcome friction, it will still rotate.

In extreme cases, you could remove the compass card from its housing and either balance it on a pin, suspend it from a thread or float it in a bowl of engine oil. You might even reinvent the original Chinese version of the magnetic compass, but you would at least be able to navigate to shore before the food and beer ran out.

Contributing Editor Chuck Husick is a sailor, photographer, electrical engineer, pilot and Ocean Navigator seminar instructor.