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Comparing Four Methods of Correcting GPS Data: DGPS, WAAS, L-Band, and Postprocessing

Postprocessed GPS Data

While DGPS reference stations transmit GPS error corrections in real time, it’s also possible to save the correction information in files that can be used to correct GPS data days, weeks, or months after the data has been collected.

Software designed to use these files can compare the time a GPS location was recorded with the error correction for that time and apply the appropriate corrections to all locations collected with the roving GPS receiver. The corrected locations are more accurate than the locations collected by the roving receiver because much of the error identified in table 1 has been removed.

The National Geodetic Survey, an office of the National Oceanic and Atmospheric Administration’s National Ocean Service, coordinates a network of continuously operating reference stations (CORS, figure 3) that provide GPS carrier phase and code range measurements for correcting GPS data throughout the United States and its territories.

Figure 3—Location of National Geodetic Survey continuously
operating reference stations (CORS) across the United States.

The correction files can be found at the Web site: http://www.ngs.noaa.gov/CORS/.

The USDA Forest Service also has a site with these files: /database/gps/clickmap/cbsmap.htm.

Test Results

DGPS Beacon Corrections—The U.S. Coast Guard beacon signal can usually be obtained through canopy even when the station is as far as 350 miles away. Accuracy does depend on the distance from the GPS receiver to the beacon station. A good rule of thumb is that there is 1 meter of additional error for each 160 kilometers the receiver is from the beacon station. Typically, that error will be opposite the beacon station’s direction from the receiver. Figure 4 shows the approximate magnitude and direction of errors for positions acquired using different beacon reference stations under an open canopy at the MTDC GPS test course in Missoula. Figure 5 shows the locations of the beacon stations with respect to the MTDC GPS test course.

Figure 4—The approximate magnitude and direction of
position errors when using different beacon stations
to correct a position collected in Missoula at the MTDC
survey point. The most accurate corrections will
usually be from the closest beacon station.

Figure 5—The location of DGPS beacon stations
that are closest to the Forest Service's GPS test
courses in Montana and Idaho.

WAAS—WAAS works well if the sky is unobstructed. In the northern United States, an obstruction to the south at an elevation more than 15 degrees above the horizon can degrade the WAAS signal reception. It can take up to 20 minutes to obtain a lock on WAAS satellites.

L-Band Corrections—Satellite-based L-band corrections may have the same problems as WAAS corrections, depending on their source. If a L-band service provider uses satellites in low-Earth orbits, reception may be better than from WAAS satellites orbiting at higher altitudes.

Postprocessing—Postprocessing provides the best accuracy if real-time information is not needed.

Conclusions

The most accurate results, in order, are:

• Postprocessed position data
• Beacon data
• L-band data
• WAAS
• Autonomous GPS receiver data

Additional Information Available Over the Internet

Canadian DGPS (http://www.cdgps.com/e/desc.htm)

National Geodetic Survey CORS (http://www.ngs.noaa.gov/CORS)

OmniSTAR (http://www.omnistar.com/home.html)

Dale DePriest’s Navigation and GPS Articles (http://www.gpsinformation.org/dale/dgps.htm)

U.S. Coast Guard (http://www.navcen.uscg.gov/ndgps/default.htm)

WAAS (Raytheon) (http://www.waasperformance.raytheon.com/sis/sisqa.html)

WAAS (General questions) (http://www.gpsinformation.net/waasgps.htm)

David L. Wilson’s GPS Accuracy Web page (http://users.erols.com/dlwilson/gps.htm)

About the Author

Dick Karsky has been program leader of forest health protection, GPS, and the air portion of the watershed, soil, and air program since the fall of 1999. Dick has been a project leader at MTDC in the resource areas of GPS, range, cooperative forestry, engineering, fire, reforestation and nurseries, residues, recreation, and forest health protection. He received a bachelor’s degree in agricultural engineering from North Dakota State University and a master’s degree in agricultural engineering from the University of Minnesota. He worked for private industry before coming to the Missoula Technology and Development Center in 1977.

Library Card

Karsky, Dick. 2004. Comparing Four Methods of Correcting GPS Data: DGPS, WAAS, L-Band, and Postprocessing. Tech Tip 0471–2307–MTDC. Missoula, MT: U.S. Department of Agriculture, Forest Service, Missoula Technology and Development Center. 6 p. Describes four methods used to correct global positioning system (GPS) data and compares their relative accuracies. The correction methods include three that operate in real time—differential GPS (DGPS), the wide area augmentation system (WAAS), and satellite service providers transmitting corrections on L-band frequencies—and one that requires correcting the GPS data after it has been collected (postprocessing). The best methods, from the most to the least accurate, are: postprocessing GPS data, using DGPS beacons to correct GPS data in real time, using satellite service providers transmitting corrections over L-band frequencies to correct GPS data in real time, and using the wide area augmentation system to correct GPS data in real time.

Keywords: accuracy, beacon stations, CORS, differential GPS, global positioning system, L-band, OmniSTAR, wide area augmentation system.

Additional single copies of this document may be ordered from:

USDA FS, Missoula Technology and Development Center
5785 Hwy. 10 West
Missoula, MT 59808–9361
Phone: 406–329–3978
Fax: 406–329–3719
E-mail: wo_mtdc_pubs@fs.fed.us

Electronic copies of MTDC’s documents are available on the Internet at:

/eng/t-d.php

For additional information, contact Dick Karsky at MTDC.

Phone: 406–329–3921
Fax: 406–329–3719
E-mail: rkarsky@fs.fed.us

Forest Service and Bureau of Land Management employees can search a more complete collection of MTDC’s documents, videos, and CDs on their internal computer networks at:

http://fsweb.mtdc.wo.fs.fed.us/search

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