www.fs.fed.us/database/gps/mtdc/gps2000/gps_comparison.htm
This degradation feature Selective Availability (SA) is the intentional degradation of the Global Positioning System (GPS) signals. This will mean that civilian users of GPS will be able to pinpoint locations up to ten times more accurately than before. With SA off position accuracy in the autonomous mode increased from +/- 100 meters to less than +/- 10 meters (see Figure 1). The decision to discontinue SA is the latest measure in an on-going effort to make GPS more responsive to civil and commercial users worldwide. Last year, Vice President Gore announced plans to modernize GPS by adding two new civilian signals to enhance the civil and commercial service. The change in position error when SA turned off Test courses Powell, Idaho GPS test course This GPS test site is located on the Powell RD on the Clearwater NF. It is located on Highway US 12 near the Idaho Highway Maintenance area about 12 miles west of Lolo Pass and about 55 miles SW of Missoula, MT. The test site is located near the Lochsa River, and has mountains on the north and south sides of the course, with a obstructed angle of 10 degrees on those two sides. The conventional survey was accomplished with a Topcon total station. The GPS control survey was accomplished with four Trimble 4000SSE/SSI geodetic GPS receivers with L1/L2 compact dome geodetic antennas operating in the static mode. Three data sets were observed from HARN (High Accuracy Reference Network) stations "LOLO GPS" and "W522". Final results were calculated from a least squares adjustment utilizing fixed integer baselines from the geodetic observations. The error estimates for the network observations are in the 2-centimeter range. Coordinates are NAD83 (1992) Montana HARN Latitude, Longitude. Site Diagram for Powell GPS test course Lubrecht Test Course The Lubrecht Test Course is located at the Lubrecht Experimental Forest about 30 miles NE of Missoula, MT. This course is a polygon with seven turning points (stations) and is located on gentle terrain, under a mixed Lodgepole and Ponderosa Pine canopy. The trees are about 19 meters tall with a minimal understory and would probably be classified as a light to medium canopy. Station B-31 is located in the open, with a clear view of the sky down to an angle of 15 degrees (See Figure 3). Site Diagram for Lubrecht GPS Test Course Clackmas test course The Pacific Northwest Regional Surveyor established a test course at the request of the Forest Service's GPS Steering Group in a typical dense West Coast Douglas-fir stand. The Test Network geographic positions were established by GPS and conventional geodetic survey. Geographic positions for the network points are accurate to 5 cm. Each point is monumented with 5/8" rebar with plastic caps and a nearby orange carsonite fence post, with "survey monument" sticker attached. All points are intervisible and the lines between the survey points are brushed and flagged. Site Plan for Clackmas GPS Test Course Test procedure The receivers were turned on and allowed to collect data for approximately 20 minutes to insure that a current almanac was stored in the receiver before the tests were run. External antennas were used on all receivers for most of the tests. Some tests were run with the built-in internal antennas to obtain a comparison between the 2 types of antennas. Different numbers of positions (1, 60, and 120) were averaged at the different stations to determine what affect that had on the accuracy. When possible the PDOP (Precision Dilution of Precision) and EPE (Estimated Position Error) values were monitored to determine what type of constellation was being accessed by the receivers. The position errors obtained at each station were then averaged over the complete course to determine the average error of the individual receiver. Receivers tested The receivers tested on the Lubrecht and Powell courses were: 1. Rockwell PLGR-96, PPS, S/N 165268, with sw:613-9868-015 Software. Trimble Pro XR, S/N 0220174140 with TSC1, and Asset Surveyor Ver. It has been evaluated in other MTDC reports and the accuracy of the receiver hasn't changed significantly with S/A turned off. The receiver contains a security module that can eliminate the positional error intentionally introduced when selective availability (SA) was enabled and it decodes the encrypted signal, which prevents its unauthorized use, called anti-spoof (AS). The receiver is not classified, but it is an accountable property item and should remain in control of the authorized user. It can store 999 waypoints and has 15 user definable reversible routes that can have up to 25 legs each. It can store 500 waypoints with symbols and 20 reversible routes, and a track log of 1900 points. It has a built-in detailed basemap that covers lakes, rivers, interstates, national/state highways, secondary roads in metro areas, cities, railroads, airports and a detailed exit database for the Federal Highway system). It has a NMEA 0183 and RTCM 104 DGPS corrections interface. The operator can select if DGPS is to be used and which real-time broadcast station to use. It is capable of outputting both the real-time differentially corrected position as well as the raw data for each position. Base-station data for post processing were obtained from the Forest Service base station in Missoula for the Powell and Lubrecht Site. That station, was a About 30 miles from the Lubrecht Site and about 55 miles from the Powell test site, and records data at 5-second intervals. It has an integrated, high-performance 12-channel GPS receiver and antenna. The receiver firmware provides for easy GIS data collection, easy data maintenance of existing GIS databases, cable-free real-time differential GPS with Beacon-on-a-Belt receiver and it provides ARC/INFO, AutoCAD, Intergraph, MGE, ERDAS and GRASS support. GPS Pathfinder Office software is included for data processing and GIS export. It has a built-in worldwide background map and detailed basemap for the Americas includes cities, lakes, rivers, railroads, coastlines, interstate, national and state highways. It automatically averages your position when the receiver is stationary. It has 9 graphic navigation screens, a re-settable trip odometer, and "EZstart" initialization and built-in simulator for training and education. It has Upload/Download Capability, has NMEA output and is DGPS ready. Results Most error results shown in the following charts may be expressed in 2DRMS values. The 2DRMS value approximates the 95% probability of error for a position. This means that 95% of the time or 19 out of 20 position readings will have an error less that what is shown on the charts. Chart shows the position error in 2DRMS, (95% of the time the error will be less than shown), for the different receivers on the Powell GPS test course. The chart in figure 5 shows the position errors for the different receivers on the Powell test course. The Trimble Pro XR and Geo 3 receivers collected files that could be post-processed and those files were post-processed in figure 5. The error over the course with the Pro XR was often less than 2 meters under the forest canopy. Figure 6 shows position errors for the different receivers on the Lubrecht test course and the Trimble Pro XR and Geo 3 were not corrected or post-processed is this graph. The position errors for the Trimble Pro XR and Geo 3 receivers are more similar to the other receivers compared to figure 5 because of the uncorrected or non-post processed data. Most position errors were less than 7 meters for all receivers except for the Magellan receiver. This graph shows the position errors at the different stations for the Lubrecht GPS test Course. In this graph the Pro XR and Geo 3 were not post-processed. Station 31 is in the open and not under a forest canopy. As can be seen in figure 6, all of the receivers have position errors of less than 5 meters. If all the position records are averaged for all the stations on the course, an overall position error can be determined for each receiver over the complete GPS test course. This graph shows the position error for each of the receivers on the Lubrecht GPS test Course. I...
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