A Degree of Difference
From a GIS mapping perspective, the Wide Area Augmentation System (WAAS) commissioned by the Federal Aviation Administration (FAA) has been quickly emerging as a significant source of accurate GPS data. The system began initial broadcasting in August 2000; since July 2003, when it was declared Initial Operating Capable (IOC), it has been utilized by many users. In the past decades, many (including me) have relied on other data options for differential correction for mapping-grade L1 GPS systems when autonomous data was not sufficiently accurate. These options include post-processing, subscribing to services such as OmniSTAR (Fugro; Houston) and using DGPS beacon receivers to access the free DGPS signal broadcast by the U.S. Coast Guard (USCG).
WAAS is an alternative to these options that requires low overhead. There is no need for a special receiver. There is no need for a special antenna. And no additional hardware or software is needed as long as the receiver used is WAAS-enabled, which is a feature of many professional and consumer grade units sold today. The WAAS signal is free, and the WAAS-enabled GPS receiver required can be the size of a PDA. Best yet, the system can deliver accuracy to the meter level.
But before you run out to buy that $200 WAAS-enabled GPS receiver, it’s prudent to have a little background knowledge of the system.
How It Works
WAAS is an FAA system designed for aircraft navigation. When the FAA commissioned WAAS for aviation use on July 10, 2003, it was ready for primetime use. Much like the Department of Defense designed and paid for the Global Positioning System, the FAA designed and paid for WAAS.<P>
In a nutshell, the WAAS system is compromised of 25 GPS ground stations, two master control stations, three uplink stations and two geosynchronous Inmarsat (Arlington, Va.) satellites. Following data collection and processing from the ground and master stations, the WAAS correction is sent to users via the uplink stations and Inmarsat satellites.
WAAS, like GPS, is a line-of-sight technology. In order to use WAAS, one of the two Inmarsat satellites has to be visible by the user’s GPS receiver. This means that blockage by tree canopy, buildings and other obstructions can prevent corrections from being received.
All of the United States is covered by one of the Inmarsat satellites, with limited areas benefiting from dual coverage. Specifically, the POR (Pacific Ocean Region) Inmarsat satellite is located at 178° east longitude, about 20° west of Hawaii. The AOR-W (Atlantic Ocean Region - West) Inmarsat satellite is located at 54° west longitude over northern Brazil.
The Inmarsat satellites are located directly above the equator so in the United States, users must realize that the WAAS satellites are at a relatively low elevation angle above the horizon. The elevation angle depends on where the user is located. This angle varies from about 48° above the horizon in Miami, 34° in Houston and 12° in Los Angeles to 38° in Boston, 26° in Minneapolis and 11° in Seattle. In Hawaii, the elevation angle is quite good at 53° in Honolulu. However, it is far removed from the network of WAAS stations in the continental U.S. (CONUS), so accuracy is degraded by 20-40 cm compared to WAAS in the CONUS.
In Alaska, the WAAS satellite visibility is worse. In Juneau, it is 12° and in Barrow it is 8°. In terms of accuracy, Alaska is closer to the CONUS network of WAAS ground stations and also has two of its own, as well as four more WAAS ground stations planned. Also, the launch and repositioning of future WAAS satellites will improve the WAAS satellite visibility issues in Alaska.
Remember that WAAS was designed for aviation use. So if POR or AOR-W is only 10° above the horizon in a particular area, it may be fine for an airplane but you will not have line of sight to it continuously during a mass-production GPS mapping project. Spotty WAAS coverage, as any user who runs Real-Time Kinematic or USCG beacon receivers knows, is no way to run a GPS mapping effort. To address this problem, some innovative GPS receiver manufacturers have developed proprietary technology that models WAAS corrections if AOR-W or POR are temporarily out of sight. These receivers can continue providing WAAS-like accuracy for up to 40 minutes after an AOR-W or POR satellite is out of view.
The System's Accuracy
Much of the debate about WAAS concerns the system’s accuracy. A search on the Internet turns up a myriad of WAAS test reports that are all over the map (so to speak)—and these include documents published by the FAA itself. Possibly the most objective and informative report (and least publicized) is published by the National Satellite Test Bed (NSTB). This group’s latest periodic report is dated May 2005 and covers the period of Jan. 1, 2005 through March 31, 2005.1 The report includes WAAS error data collected at a number of WAAS and NSTB reference stations throughout the continental United States, Alaska and Hawaii. The objective of the report with respect to WAAS is to evaluate and monitor the ability of WAAS to augment GPS by characterizing important performance parameters, investigate anomalies and create an archive of performance for future evaluation.
Table 2-2 of the report (click HERE to access the report in PDF format) shows the 95 percent confidence levels for horizontal and vertical accuracies calculated by these receivers over a three-month period (about seven million samples) when all WAAS corrections (fast, long term and ionospheric) for at least four satellites are available. This information is in reference to Precision Approach (PA mode)2. The report shows that all horizontal values except for Los Angeles are submeter.
The value of this FAA report to the GIS and surveying community is that it illustrates the potential of WAAS and not necessarily the results users can expect. The receivers, antennae and test sites used are the ideal; the receiver and conditions used by most users will most likely be less than ideal. Nonetheless, Trimble (Sunnyvale, Calif.), Geneq (Montreal, Quebec, Canada), Thales Navigation (Santa Clara, Calif.), CSI Wireless (Calgary, Alberta, Canada) and Leica Geosystems (Norcross, Ga.) have introduced affordable products that claim to be capable of submeter positioning using WAAS.
The quality of the GPS receiver’s signal processing has a lot to do with the results users will achieve. Some WAAS-enabled receivers on the market will perform no better than 3 m to 5 m with 95 percent confidence. Users who perform professional mapping tasks shouldn’t expect to use Garmin or other consumer-grade products to achieve optimal results using WAAS. Additionally, antenna quality can make a significant difference. A $20 consumer-grade helical antenna is not going to perform as accurately as a well-designed, high-performance patch antenna.
Finally, it’s interesting to note that the FAA’s primary design function for WAAS was not accuracy, but rather integrity with a high degree of availability. Referring again to Table 2-2 of the NSTB report, the percentage time in PA mode availability is 99.9 percent. Accuracy is actually a by-product of reliability.
The ITRF00 Datum
The emergence of WAAS as a primary correction source has helped bring to light the difference between WGS 84/ITRF00 and NAD83 for mapping-grade GPS users. Since WAAS works in the ITRF00 reference frame, users of high-performance WAAS receivers need to pay attention to its difference from NAD83. ITRF00 is more than a meter different than NAD83 in some areas of the country. For example, at the WAAS station in Leesburg, Va., the difference is .892 meters. In Dallas, Texas, it’s .954 meters. In Oakland, Calif., it’s 1.162 meters.
Since very few maps are maintained in ITRF00 or WGS84 datums, it’s essential to understand the transformation methods and tools available to transform WAAS-corrected data to the datum a user is working with. The easiest way to accomplish this is in the GPS data collection software from the manufacturer. GPS data collection software is beginning to address this issue where it hasn’t in the recent past. The newer software versions are now including a GPS input datum setting for ITRF00 that automates the transformation to NAD83 and other datums so the transformation is automated for the user. Users should check with their software vendors for the latest version. Alternatively, the National Geodetic Survey (www.ngs.noaa.gov) publishes a software program called HTDP (Horizontal Time Dependent Positioning) that will transform ITRF00 coordinates to a number of datums.
The Future of WAAS
There is no doubt that WAAS is the cornerstone of U.S. aviation navigation for the foreseeable future. Non-aviation users such as surveyors and other geomatics professionals will continue to benefit from the FAA’s effort and continued system enhancements. There are two near-term enhancements that are expected to improve our visibility of WAAS broadcast satellites, according to Tom McHugh, WAAS technical director at the FAA.
The first is moving AOR-W 48° to the west from over northern Brazil to a longitude that nearly splits the continental United States. The result will be stronger nationwide visibility to AOR-W, especially in the central and western United States. This is scheduled for January 2006.
Secondly, the FAA announced that it signed contracts for two additional satellites to broadcast the WAAS signal: the Telesat ANIK F1R and PanAmSat Galaxy XV. Telesat ANIK F1R was launched Sept. 8, 2005, and will be positioned at 107° west. At press time, PanAmSat Galaxy XV was scheduled for a mid-October launch and will be positioned at 133° west. McHugh says it will take about one year for the satellites to become fully tested and operational. Although not considered operational until late 2006, the test signals will be available to users as early as February 2006, according to McHugh.
The FAA’s leases for the existing Inmarsat-III satellites (POR and AOR-W) expire in July 2007. Depending on budget constraints, the leases may or may not be renewed. If they are not renewed, the new WAAS broadcast constellation will still have a much better broadcast footprint than what we have today, with dual coverage throughout the continental United States.
Other WAAS system enhancements include adding four more ground reference stations in Alaska, adding four stations in Canada and five in Mexico for enhanced continental U.S. and Alaska coverage, and improving “the utility of WAAS for the other countries in concert with the FAA Administrator’s international goals.”3
The success of the WAAS concept has spun off similar systems in other parts of the world. EGNOS (European Geostationary Navigation Overlay Service) is being deployed throughout Europe and MSAS (Multifunctional Transport Satellite-based Augmentation System) is being deployed in Japan.
A Viable Alternative
The simplicity of WAAS-enabled GPS receivers makes it very attractive from an ergonomic and cost standpoint. The accuracy and reliability of WAAS has made it a viable alternative to other forms of real-time DGPS and post-processing as long as the ITRF00/NAD83 datum transformation difference is recognized and reconciled.
For non-aviation users, WAAS is in its infancy. Just as manufacturers focused their R&D efforts in optimizing code phase receivers throughout the 1990s, the same will happen with WAAS-enabled receivers this decade as manufacturers choose to optimize their receivers to exploit the investment made by the FAA in WAAS. The result will be smaller, lighter and less expensive GPS receivers for mapping-grade work.
In the Pacific Northwest, where POR is located about 12° above the horizon, Portland General Electric (PGE) has 14 WAAS-enabled receivers it uses for mapping utility poles. Although POR is visible most of the time in PGE’s operating region, there are times during the day when tree canopy or topography obstructs view for several minutes at a time. During this time, the receiver’s proprietary algorithm kicks in and PGE personnel experience WAAS-like accuracy until the POR satellite is back in view. The result is that the equipment operators have the ability to continue working without interruption.
“It is really slick,” says Bill Tierney, business development manager of PGE’s Utility Asset Management division. “We initially tried six compact flash WAAS receivers because the price point was attractive, but the results were disastrous. Some poles were more than 60 feet off. With the GENEQ SXBlue [GPS receiver; GENEQ Inc., Montreal, Quebec, Canada], accuracy hasn’t been an issue at all."