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GPS plus loran

Dec 14, 2009

'' National Oceanic and Atmopheric Administration nautical charts carry this note; "WARNING ” The prudent mariner will not rely solely on any single aid to navigation, particularly on floating aids. See U.S. Coast Guard Light List and U.S.


National Oceanic and Atmospheric Administration nautical charts carry this note; "WARNING — The prudent mariner will not rely solely on any single aid to navigation, particularly on floating aids. See U.S. Coast Guard Light List and U.S. Coast Pilot for details." Anyone who has encountered a missing or misplaced aid to navigation knows the truth and the worth of this advice. Having once encountered a misplaced buoy or a missing daymark or beacon we habitually verify that the aids to navigation we see are where they should be. Yet we willingly assume, without any independent verification that the GPS fix displayed on our chartplotter is correct to within the three to perhaps 10-meter precision to which we are accustomed. (The GPS receivers used in aircraft and on ships that must comply with International Maritime Organization standards include an automatic receiver autonomous integrity monitor that continuously verifies the accuracy of the data presented to the user).

In general, our trust in the precision and reliability of GPS information is well founded, however the system can and has often been interfered with, creating undetected and occasionally significant errors or depriving us of a usable signal. A prudent mariner wants additional sources of navigation information, preferably one that is fully compatible with the chartplotter. Fortunately, the existing long range navigation system (loran) provides precisely what we need; a robust source of navigation position and timing information that is compatible with chartplotters (and other systems equipment that rely on the GPS). We might obtain the desired second opinion by comparing the GPS data to the readout on a loran receiver's display. However, thanks to the work done by some very clever engineers, we now have access to combined GPS/loran receivers that will satisfy our need for a source of navigation information that is independent of GPS, assure superior navigation data when both GPS and loran are operating normally, and improve the performance of the vessel's autopilot and radar overlay image on the chartplotter by delivering true heading information regardless of whether the boat is in motion.

Two combined units available

Two combined GPS/loran receivers are currently available; the Si-Tex Marine Electronics e-Loran Integrated GPS/Loran Receiver Sensor and the CrossRate eLGPS 1110. The Si-Tex receiver must be connected to a Si-Tex C-Map Max-capable chartplotter or their P-Sea WindPlot software for Windows computers. The CrossRate eLGPS 1110 receiver provides a standard NMEA 0183 output data stream that is compatible with virtually all chartplotters and computer-based charting software, including the MacENC and GPSNavX programs used in preparation of this article.

The specifications for the 20-channel Wide Area Augmentation System (WAAS)-enabled GPS receiver in the eLGPS 1100 include a receiver reliability of 99.994 percent and position accuracy of two meters. The accuracy of the true heading data delivered from the eLGPS 1100 is specified at better than one degree, even when stationary. (The system accuracy specification assumes the availability of GPS, WAAS and the eLoran signal with 9th pulse information). As with a GPS compass, the accuracy of the heading data is independent of the influence of magnetic fields, including magnetic dip errors and the motion-induced artifacts often seen in flux gate, rate gyro-aided heading sensors. Accurate, stable heading information will improve the performance of a chartplotter operating in a radar-overlay mode, assuring that the radar and chart images remain in register as the vessel moves. Performance of the vessel's autopilot, especially in a following-sea condition, will be improved by using the heading information from the eLGPS 1100 in place of the normal flux gate magnetic sensor/rate gyro source.

Both the Si-Tex and CrossRate integrated GPS/loran receivers operate on a "chainless, all in view" basis, using information from any available loran transmitter. There is no need to select a Group Repetition Interval, in fact there are no operator controls. The CrossRate receiver is contained in a 7.4-inch diameter, 3.9-inch high, 2.75-pound radome shaped enclosure. Required DC operating power is five watts from a nine- to 30-volt source. The RS-422 configured NMEA 0183 bit stream includes position, speed over ground, course over ground, true heading, date and time and many more navigation parameters, at speeds up to 115.2 kbps.

The CrossRate receiver was tested on the CrossRate company boat in Casco Bay, near Portland, Maine, with data displayed on a Raymarine C120 plotter. I also tested a receiver on my sailboat in St. Petersburg, Fla., with the eLGPS 1100 connected to the USB port of a MacBook Pro computer with MacENC software.

Loran-only mode

I was interested in checking the performance of the loran portion of the receiver and, lacking a way to switch-off the receiver's GPS circuits, I operated the receiver inside a building that prevented the reception of GPS signals. Position information displayed on the chart was generally within 100 meters of the known position despite the extremely unfavorable loran reception conditions.

The combined GPS/loran receiver will be a welcome addition to any well-equipped boat. Its immediate value will primarily be seen in improved navigation data stability and in the performance of the vessel's autopilot and radar overlay chartplotter. The ultimate benefit of the combined sensor receiver will become evident when the GPS signal is interfered with or is entirely unavailable. While most recreational mariners have not experienced a loss of GPS coverage, such events are not infrequent and can result from "innocent" signal interference (the Coast Guard has issued warnings in Notices To Mariners about interference from some types of amplified TV antennas and has warned of possible interference from some types of fluorescent lamps) and from inadvertent operation of powerful radio transmitters. An inadvertent signal from a Navy transmitter eliminated GPS service and most cellular telephone service in the San Diego area (at the time, most cellular telephone systems were totally dependent on GPS timing information, only the few capable of receiving timing information from both GPS and loran remained in service).

Jamming GPS

The vulnerability of the incredibly weak GPS signals to intentional interference and "spoofing" (causing a normal appearing position fix to be in error over distances of miles) has been verified. A test conducted in the U.K., using a transmitter that was emitting less than two watts, interfered with the usefulness of GPS at more than 50 miles from the transmitter. In contrast, terrestrially-based loran signals are far more robust and have superior ability to resist jamming and spoofing.

While we may expect that intentional jamming or spoofing of the GPS signal is unlikely in the waters in which we sail, we should not assume that the interference created by solar radiation won't deprive us of navigation information at a critical time. The level of the solar activity that adversely effects radio communication and navigation signals has been unusually low during recent years, but is forecast to increase in the coming years with the beginning of the 11-year solar cycle in 2010. (The Great Solar Tempest of 1859, 150 years ago, interfered with telegraph communications, started fires where wires created sparks and created visible aurora in areas far south of where this evidence of solar energy is not normally visible).

As this article is written, the future of the U.S. loran system and its evolution into eLoran is not yet certain. The necessity for a dual, U.S.-based position, timing and navigation (PTN) system appears to have been clearly recognized by the Congress and they assure that adequate funding is provided both for the existing loran system and its evolution into eLoran. The Coast Guard's excellent work in automating a number of loran stations has begun to reduce the annual operating cost for this vital PTN resource. Other countries, especially Great Britain and some others in Europe are working on improvements in their loran systems and are active participants in the definition of the eLoran standard.
 

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