Any navigator worth his dividers and parallel rule knows the value of true heading information. Until recently, the best way to get this data has been a north-seeking gyrocompass, which is commonly found on ships, is large and expensive, requires substantial annual maintenance, and needs about an hour of initialization each time it turns on.
In the past couple of years, though, navigators have been able to get precise true heading information via GPS compass systems, which compare the time of arrival of carrier wave signals from a minimum of five GPS satellites at two or more antennas separated by a known distance. Since the radio frequency used for GPS satellite transmissions is quite high, 1575.42 MHz, a wavelength is only 190 mm, allowing systems to be quite compact.
As with gyrocompass-derived heading information, the data from a GPS compass is free of the many errors inherent in all magnetic sensing systems. Unlike the gyrocompass, a GPS-based system requires no routine maintenance, consumes a minute amount of DC power and is ready for use only a few minutes from power-on. A GPS compass is also immune to gyrocompass errors, especially those that occur when navigating at high latitudes.
That makes these clever devices perfect for yachtsmen, the target audience for the new JRC America JLR-10. It combines a GPS-derived true heading system with a full-capability GPS navigator.
The JLR-10 uses two antennas, with their centers separated by 500 mm and positioned on a rail-like metal mounting bracket. The assembly weighs only 3.2 kg and mounts easily on a standard antenna mast.
Its electronics combine the known position of the GPS satellites with the difference in the time of arrival of their signals at the two antennas to compute the true heading of the longitudinal axis of the antennas. Although the information can be extremely accurate, a change in the position of the antenna caused by the boat's motion degrades it. In addition, momentary blockage of the signal path from a satellite or group of satellites can degrade or even interrupt the calculation of true heading.
For these reasons, the GPS-derived heading information is electronically combined with information about the motion of the vessel supplied from a set of solid-state gyroscopic sensors housed in the system's central processor module. This method of combining information from two complementary sources of data is well proven and widely used in other heading sense systems, including the gyro-assisted flux detectors used with many autopilots.
The system utilizes standard GPS information. An optional DGPS sensor can be plugged in, enhancing the precision of fixing your position, but at this writing, the system does not make use of WAAS information. The JLR-10 doesn't really need a differential correction or WAAS to provide precise heading data, but anyone who relies on it for navigational as well as heading data will appreciate the DGPS interface.
My evaluation of the JLR-10 included temporarily installing the system on my boat. Only three units comprise the system: the bar-like antenna, which is supplied with the two GPS antennas in place; the processing unit module, which must be installed in a protected location belowdecks; and the display/control unit, which is housed in a waterproof enclosure suitable for use in a totally exposed location.
The installation was fast and straightforward, with the exception of the unusual stiffness of the two coaxial cables that carry signals from the antennas to the processing unit. In addition, these cables are fitted with high-quality but large connectors and may be difficult to install on some boats. If you have to snake the cables through tight spaces, you will have to remove the connectors and reconnect them at their terminus. JRC explains this procedure in detail in the instruction manual that comes with the JLR-10.
A word of caution: The connectors are not inherently waterproof and must be carefully sealed against moisture, using self-bonding tape in accordance with the precise instructions in the manual. This is standard, since waterproof connectors are large and impractical for many yacht installations.
The 2.9-kilogram processing unit requires a space measuring 400 by 320 by 177 mm, including adequate access for cabling. Since the processor module contains the solid-state gyro sensors, the normal installation position is in line with the vessel's longitudinal axis, the connectors facing aft. However, almost any other orientation can be accommodated provided the installer enters the orientation information into the installation error page of the heading menu.
The control unit measures only 197 by 117 by 70 mm and may be flush-mounted or positioned on the quick-release, tilting mounting bracket that comes with the JLR-10.
A set of eight buttons and a rotary/push-to-enter knob flank the control unit's high-visibility monochrome LCD screen. All control functions are intuitive. You use the rotary knob to move through the setup pages and eight display modes-Heading, Position, Navigation, CDI, Plot, Waypoint, Route and Status S/F.
The JLR-10 calculates and displays ship's heading to one-tenth of a degree, and position information to three decimals. I found the mechanical detents of this control to be a bit light, making it fairly easy to overshoot the setting I wanted, especially when I was operating it in a seaway. This is common with rotary knobs, and resting the heel of your hand on the monitor to stabilize it generally solves the small problem.
With the exception of commanding a soft or hard reset of the navigation system, each control button performs a single function. The system can store as many as 499 waypoints defined by latitude and longitude or by bearing and distance from a known geographic location. Waypoints are stored by memory location number and may be assigned an identifier of up to five letters/numbers. Route navigation memory can store as many as 20 routes of up to 199 waypoints.
Although English is the default language of the JLR-10, it can display data in six additional languages. Heading data can be supplied to a companion JRC radar in NSK format at an update rate of 20 milliseconds, providing a performance improvement for the radar's ARPA function.
Provisions are also included for communication with an external computer, allowing unlimited expansion of the capacity for waypoint storage and track plotting. However, there is no built-in geomagnetic world map, the lack of which prevents GPS-derived heading information from being presented as magnetic headings. If you are accustomed to using true headings, this doesn't matter.
At the present time, only JRC and Furuno (Electronics, March) market a GPS compass in the United States, but other manufacturers are likely to follow suit as the marketplace warrants.
Price of the JLR-10: $4,295.
Contact: JRC America, Inc., (206) 654-5644; www.jrcamerica.com.