Features February 1, 2001 Issue

WAAS Up? New Acronym Stands For Better GPS Accuracy

Raytheon's Raychart 320, with its improved satellite-based differential system, is consistently more accurate than two conventional DGPS units from Raytheon and Garmin—and it's less expensive.

Whatever limitations electronic position finding may have suffered during its relatively brief existence, a lack of confusing acronyms hasn’t been one of them.

The Raychart 320 was only off by 5.6 feet in
our tests, compared to 16.8 feet for the Garmin
215D and 14.2 feet for the Raychart 420D.

Following RDF through LORAN-C, SATNAV and GPS and DGPS, we now have WAAS (Wide Area Augmentation System).

The Federal Aviation Administration (FAA) hopes WAAS will bring a new level of precision to navigation that will ultimately usher in a new era in traffic management. Unfortunately, the primary contractor, Raytheon, has experienced serious delays and cost overruns, and some indicators are that WAAS may never be as precise as hoped. For marine use, however, it is a major leap forward.

Background
When the Global Positioning System (GPS) was first set up, it was strictly a military system. When it was released for civilian use, the U.S. Department of Defense, presumably nervous about some international bad guys using it to direct missiles at strategic targets, set up Selective Availability (SA)—an intentional satellite-clock error that reduced positioning accuracy unless a special military-only receiver was used. The value of SA was hotly debated—the importance of having a modern bomb miss by a couple of hundred feet isn’t instantly obvious. The risks of relying upon an incorrect indicated position were severe enough that the Coast Guard developed an error-correction system called Differential GPS (DGPS).

The idea behind DGPS was simple enough: If you have a receiver at a known position, you can correct for the errors introduced by SA at that location. That correction information is then broadcast to GPS receivers situated reasonably close to the fixed receiver, with the assumption being that the same corrections will apply. The original government-sponsored differential correction scheme uses existing Coast Guard-operated radio beacon transmitters (the old RDF system). These transmitters are located throughout the country and operate in the 300 kHz range. This system works well but has several drawbacks.

First, range is limited to about 100 miles of the coast under ideal weather conditions. Second, the signals used to transmit the differential corrections are subject to interference from other electrical equipment, weather conditions and propagation problems. To receive differential corrections an additional receiver and antenna must be connected to a GPS unit, which is built to accept these signals. This complicates installation and, more importantly, adds considerable cost.

Adding differential correction to a differential-ready GPS costs a minimum of $300, plus installation.

This entire system was built as a quick-and-dirty fix primarily to overcome the effects of SA while the government researched a better solution for transmitting differential corrections. With all its drawbacks, though, DGPS worked (and still does, for that matter). Position errors, even with SA in effect, are typically reduced from several hundred feet to about 30 feet.

The introduction of DGPS made the use of SA seem even more odd. Here was the Coast Guard setting up a system specifically to defeat SA. And after a dozen or so years of complaints, President Clinton pulled the SA plug. Suddenly, the accuracy of all GPS receivers, including DGPS, improved dramatically. GPS units without the differential feature could locate within 30 feet or so; DGPS units cut that error almost in half.

While this kind of accuracy is certainly good enough for many boaters, it was still inadequate for many more demanding applications: aircraft, for instance, where an error of 15 feet could be devastating to pilots landing on a narrow runway, boaters seeking to locate a narrow channel under poor visibility, or fishermen wanting to return to a particular spot.

How does one make a more accurate GPS? Let’s look at the sources of error.

Correcting Errors
GPS is based on the ability of the satellites to know their own locations and your receiver’s capacity to maintain synchronization with the satellites. The satellites tell your receiver where they are in space. Your receiver calculates the time it takes for each satellite’s signal to reach it, and therefore can determine how far it is from a satellite. Do this for at least three satellites, add some math, and you will know where you are. But there are some intrinsic sources of error found in GPS, even without SA’s intentional error.

Satellites don’t always have predictable orbits; clocks are not completely accurate and signals from satellites can be subject to bounce and other distortion caused by the atmosphere. Fortunately, all of these errors can be corrected. That’s where DGPS, and now WAAS, comes in.

WAAS differs most dramatically from DGPS in that it replaces the old-fashioned land-based 300 kHz transmitting stations with a network of low-level satellites that broadcast over the same wavelength as the GPS satellites. The satellites are in geosynchronous orbit. They remain in fixed positions relative to the earth’s surface (if anyone’s still suffering from acronym deprivation, these are referred to as GEO—Geosynchronous Earth Orbiting satellites). This satellite system covers a much wider area than the older DGPS towers, and the use of a high-frequency signal means less interference from weather and stray electrical fields, as well as obviating the need for a separate antenna/receiver (which DGPS requires).

WAAS-capable receivers extract the data received from the GEO satellites and use it in combination with the GPS signals to calculate a position very accurately.

The information transmitted by these satellites comes from ground reference stations that collect signals from the GPS satellites and send them to a Wide Area Master Station (WMS), which in turn calculates the necessary correction data and checks for system irregularities. The correction message is then prepared at the WMS and sent to the GEOs. Each WRS station contains three complete sets of ultra-accurate GPS receivers to assure redundancy.

All of these factors combine to make a system that is not only more accurate than DGPS but covers a much wider geographical range. (European and Asian countries are now developing compatible systems that will insure nearly worldwide coverage.)

From the boater’s viewpoint, the main point here is that the differential correction signals are sent on the same frequency as the standard GPS signals. This means that a separate differential receiver is not needed. In fact, this feat is performed primarily by software within the GPS receiver. Therefore, little if any cost is added to the price of a WAAS GPS receiver.

What We Tested
At the time this article was prepared, only two models of WAAS GPS receivers were available: The Raytheon RayNav 300 and the cartographic Raychart 320.

The Raystar 120, a GPS satellite sensor that will upgrade electronic equipment (and not just from Raytheon), will be in stores in January. It comes in two versions: "Sea Talk" for Raytheon products and an NMEA version for devices from other manufacturers. Retail prices for both are $650.

We used the Raychart 320 in our tests, comparing it to two conventional DGPS units: the Garmin GPSMAP 215D and the Raytheon Raychart 420D. We tested them by repeatedly placing each model’s antenna directly on top of a USGS (US Geologic Survey) survey mark at a known location (39° 49' 14.98266" N, 075° 37' 01.37744" W). Location readings were logged, and the distance between these readings and the known location calculated. The test results are shown in the table above. Note that only the position accuracy of each model was tested; we did not compare features or other abilities.

What We Found
The Raychart 320 consistently outperformed both other models, and is much less expensive. To further our comparison, the table on this page gives prices for other high-end non-DGPS and DGPS receivers in the same price range as the Raychart 320 and RayNav 300. Both of these models provide much more value for only a few dollars more than a conventional GPS and less money than a DGPS. The WAAS receiver is the hands-down best buy.

Recommendations
WAAS works as advertised, providing the best absolute and repeatable accuracy. And if that isn’t enough, WAAS receivers are less expensive than DGPS receivers and are easier to install. If you already have a DGPS unit, WAAS can provide some improvement in accuracy, though probably not enough to justify scrapping your existing GPS receiver.

If you already have a GPS (without the differential feature), your first question should be: “Is my present unit accurate enough for my needs?” We suspect that in the majority of the cases the answer is “yes.” If it’s “no” (or if you just like to have the most cutting-edge equipment available), the question becomes “DGPS or WAAS?”

This is a no-brainer. If you are shopping for a DGPS, we think you should buy a WAAS model. Additional models will be available from other companies very soon. WAAS, designed using taxpayer money, is not a patented product. The technology is freely available to any manufacturer. In fact, there is no technical reason why a WAAS-capable handheld GPS receiver cannot be produced, because of the simplicity of WAAS and the fact that it is a software-based upgrade. It’s logical to predict that within several years all GPS receivers will include WAAS. What’s the only reason not to buy the Raytheon models now? By summer, increased competition will result in lower prices.

At this accuracy level, even more features can be added to GPS receivers, especially cartographic models. Receivers could be programmed to locate the nearest NAVAID and tell you what and where it is. For example, “West Channel, Red #9, 112', bearing 042°.” Even depth data could be correlated to your position, adjusted according to stored tide tables.

It’s about time that something exciting happened in marine electronics—and WAAS is it.

Also With This Article
Click here to view Performance: WAAS vs. DGPS.
Click here to view Market Scan: WAAS vs. DGPS GPS.

 

Contacts- Garmin International, 1200 East 151st, Olathe, KS 66062; 913/397-8200. Raytheon Marine, 22 Cotton Rd., Nashua, NH 03063; 603/881-5200.

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