A Sense of Direction
The strange and tortuous search for a way to improve GPS reliability.
Last December CrossRate Technology introduced its first (and so far only) product: a combined GPS and Loran receiver called the eLGPS1110. Two months later, Loran was switched off. CrossRate’s timing could have been better, but their concept was brilliant.
GPS provides an accurate position, but its signals are vulnerable to blocking, interference, and jamming. Loran isn’t as accurate, but it’s very reliable. By using the GPS to provide up-to-date corrections for the Loran, CrossRate ended up—for a while— with the best of both worlds: an integrated system that was as accurate as GPS and as reliable as Loran.
And this wasn’t the only clever idea. The eLGPS1110 also included a radio compass. There are plenty of us around who remember using these devices to work out where we were by measuring the bearing of distant radio beacons, but the eLGPS1110 stood that principle on its head. Knowing where it was, the unit could figure out the direction of the available Loran transmitters and then use directional aerials to identify the direction from which the Loran signals were being received. From that, it could to work out the boat’s true heading to an accuracy that was claimed to be better than one degree.
But without Loran, the eLGPS1110 is just a rather bulky GPS, whose original list price of $1,300 seems hopelessly optimistic.
Or is it?
Over the thousands of miles of the U.S. coastline, magnetic variation is between ten and 20 degrees. That’s bigger than almost anywhere else in the world outside of the Arctic and Antarctic and makes the idea of a radio compass—which is immune to both variation and deviation—seem particularly appealing. And there’s no reason why a radio compass needs Loran. So as far as the compass function of the eLGPS1110 was concerned, Loran was simply a strong, continuous radio signal from a fixed source. There’s no fundamental reason why the unit couldn’t be modified to work just as well with any other long-range, low-frequency continuous transmitter, such as WWVB—the time signal that regulates radio-controlled clocks all over the United States.
Unless and until someone revives the idea of the radio compass, there are several alternative ways of getting heading data into an instrument system. The most popular is the fluxgate. But the snag with a fluxgate is that it works by sensing the Earth’s magnetic field, just like a swinging card compass, so it is just as badly affected by variation and deviation. And fluxgates have their own, additional source of error.
The trouble is that the Earth’s magnetic field has a vertical component (acting towards the center of the Earth) as well as the north-south component that the compass uses. As long as the fluxgate is perfectly horizontal, this doesn’t present a problem. But when have you ever seen anything on a boat at sea that was always perfectly horizontal?
At the mid-latitudes around the United States, tilting a fluxgate by one degree creates two degrees of heading error. And it gets worse as you move further north: in the Great Lakes one degree of tilt produces ten degrees of heading error. In a steering compass or autopilot, that might be okay—it’s similar to the swing of a swinging card. But on a north-up radar display it translates to a smeared and smudgy picture.
Fluxgate manufacturers traditionally minimize the problem by mounting their heading sensors on gimbals or in oil-filled bowls, but neither is a complete solution. They also use mathematical processing techniques to average out tilting errors, but doing so inevitably makes the compass less responsive so that the smudgy radar picture is caused by the compass failing to respond to the movement of the boat rather than by over responding to tilt.
A more drastic solution is to use a rate gyro. Unlike the heavy and expensive gyro compasses used on ships, a rate gyro is a solid-state device that works by sending beams of laser light around a coil of thousands of feet of fiber optic cable. It’s no good at sensing direction, so it’s useless as a compass. But it is very good at telling the difference between genuine changes of direction and the fluctuating tilt errors that affect a fluxgate.
Together, a fluxgate and a rate gyro cancel out each other’s shortcomings, just as the combination of GPS and Loran did in the eLGPS1110. In fact they make such a powerful combination that all the mainstream electronics manufacturers include a gyro-stabilized fluxgate in their catalogs.
But adding a rate gyro cannot get rid of magnetic variation. For that, you need to look for something that doesn’t depend on the Earth’s magnetic field. One solution that has been growing in popularity is the GPS compass, consisting of two (or three) GPS antennas, mounted on a horizontal bar and feeding their signals into a single processing unit.
Of course, both antennas receive the same signal at the same time—give or take an immeasurably tiny delay if one antenna is further from the satellite than the other. But although the time difference between the signals received by the two antennas is too small to measure, it’s relatively easy to measure the phase difference between them and to use this to work out which antenna is closest to the satellite and by how much.
The GPS compass knows where the signals are coming from and can calculate its own orientation relative to the satellite by measuring the phase difference between the signals received by its two antennas. By combining the two pieces of information, it can calculate the vessel’s heading.
GPS compasses involve a combination of sophisticated technologies and they don’t sell in huge numbers, but with street prices now nudging down towards $2,000, they are a very real alternative to fluxgates—at least until a new version of the radio compass comes along.
This article originally appeared in the June 2010 issue of Power & Motoryacht magazine.