To the editor:
Sven Donaldson’s article on autopilots (“The inner life of an autopilot,” Issue 120, March/April 2002) is quite educational and well-written. However, his assessment of future GPS-based heading devices is not indicative of the commercially available GPS heading products that already exist or the technology that they incorporate to derive a vessel’s heading.
Donaldson stated that GPS positional updates, in combination with rate gyros, will be used to periodically correct DR heading errors for GPS-based heading systems. In reality, current GPS-based heading products don’t use GPS positional updates to derive heading information. Instead, they all utilize real-time GPS carrier-phase relationships to derive a vessel’s heading in real time (see “GPS compass,” Issue 122, May/June 2002).
Multiple geometrically fixed GPS antennas and receivers are utilized to compare GPS carrier signals between them. Because each GPS receiver determines the range and azimuth to each satellite and the relationship between each antenna is fixed, a sophisticated algorithm is used to calculate the time difference between each antenna’s reception of the same satellite carrier signal. Once the system knows the time each antenna receives the same signal, it uses the received signal time difference to compute a vector from one antenna to another antenna in real time. If these antennas are arranged in a fore/aft position on the vessel, this vector is the true heading of the vessel.
Three-axis solid-state rate gyros are used to compensate the heading data for pitch, roll, yaw or short periods of signal blockage. In practice, root mean square accuracy of better than 0.5° anywhere on earth is achieved. Follow-up rates better than 25° per second are also possible. It is interesting to note that WAAS or DGPS fixes do not contribute to increased heading accuracy because the GPS compass-heading computation relies purely on the phase relationship of the signal and not the P or C/A code GPS data stream that is used to calculate positional fixes.
The benefits of this technology are far-reaching: Power consumption is typically less than 12 watts (1 amp at 12 volts DC) and GPS navigational data (lat/long, COG, SOG, time) is a by-product of these systems. All issues related to the earth’s magnetism are eliminated, and installation on any vessel of any material is possible. There are no recurring costs for overhauls or sensitive elements, as is the case with all gyrocompass products. There are also no latitude/speed errors or correction factors required, and the rated accuracy is maintained at any latitude, heading or speed of the vessel, unlike a gyrocompass.
Eric R. Kunz, who holds a BSEE, is a product manager at Furuno USA Inc. in Camas, Wash.
