Evaluation of the Trimble ProXR GPS Receiver Under a Hardwood Canopy Using CORS Broadcast Real-Time DGPS Corrections

Transcription

1 ... '" ' ' ": : J ::" /, 4// t,,) -].c United States Department of Agriculture.o - ////p//://:/4/a/ Forest Service = Technology & =,- =,- =, = HIE V" " I " i I 1! Development Program! October2000 O MTDC.,. 2200/2300/2400/2600/3400/5100/5400/, /7100 Evaluation of the Trimble ProXR GPS Receiver Under a Hardwood Canopy Using CORS Broadcast Real-Time DGPS Corrections Ross H. Taylor, Lands Program Manager; Tony Jasumback, GPS Coordinator; Dick Karsky, Program Leader; and Dale Weigel, North Central Forest Experiment Station. uring 1999, Ross Taylor, Lands Program Manager for the Hoosier =k National Forest, and Dale Weigel, Forester for the North o Central Forest Research Station, conducted a test for the Missoula " Technologyand DevelopmentCenter {'". (MTDC). They evaluated the real-.=,, time accuracy of the Trimble ProXa 8 GPS Receiver (Figure 1) using realtime broadcast signal corrections == acquired froma Continuously Opera-,," tingreferencestation(cors)station O 150 km (93.7 mi) away providing Dif- ferential GPS (DGPS) signals. The evaluation was conducted under a hardwood forestcanopy at the Forest Service GPS Hardwoods Test Site on the Hoosier National Forest 48 km (30mi)northof Bedford,IN.TheCORS. station was at Louisville, KY. This reportcompares the horizontal accu- racy of data obtained using the r differential broadcast corrections transmittedin realtimetothat obtained = e,.e by differential postprocessing. Figure 1 Trimble ProXR GPS receiver. All stations are second-order class or better (first-order class is the highest precision). The course consists of a A through G hasan area of ha The Trimble ProXR is a 12-channel, seven-stationclosedtraverse. Itstarts (3.323ac).These stations are located real-timedifferentialgpsreceiverthat atthetop of afingerridgeand proceeds underadense,uneven-ageoak,beech, includesanintegrateddgpsantenna. down the ridge into the bottom of a and hickory canopy, typical of the TheantennareceivestheGPSsignal gully,then back up an adjoining ridge eastern hardwood forests. The top of and real-time differential corrections to the starting point. The polygon the canopy is about 30.5 to 36.6 m broadcast by a radio beacon maindescribed by traversing from stations (100 to 120 ft) above the ground, tained by the U.S. Coast Guard or the "., ---j For additional Information contact: Dick Karsky, Program Leader; USDA Forest Service, MTDC; Bldg. 1, Fort Missoula; Missoula, MT Phone: ; Fax: ; Internet: rkarskv@fs, fed. us; Lotus Notes: Richard J Karsky/WO/USDAFS "d9 urejod uomdolo^oq pue/olouqooj,,oo!oslsojoq'omlno.l V jo

2 U.S. Army Corps of Engineers. This The Data Collector's The Data Collector's test used the U.S. Army Corps of Observations and Data-- Observations and Data-- Engineers DGPS real-time broadcast August 31 September 1 signal from Louisville, KY. Observations for file R083118a: The The Trimble software includes a The ProXR receiver used in the test first four stations (A through D) were Quickplan program that indicates contains Trimble firmware version collected quickly with 150 positions which satellites are visible and which The receiver can output the real- obtained in 20 minutes or less. At sta- signals are available at a given time differentially corrected position tion E the five satellites were located location foragiventime.thesoftware and the raw data for each position, almost directly above the position, indicated that six to seven satellites DatawerecollectedusingtheTrimble Satellites located loweron the horizon should have been available to the System Controller (TSC1) data logger were blocked out by tree canopy. The GPS receiver at thetest course. Howwith Asset Surveyor software version satellite geometry was poor, resulting ever, the alignment of those satellites The data collection interval was in a high PDOP reading (6.3). Be- did not allow the satellite signals to set at 1 second, the PDOP (position cause the PDOP switch was set at penetrate the tree canopy at stations dilutionofprecision)andsnr(signal- 6.0, no readings with a PDOP were A, B, and C. Data collection was abanto-noise ratio) masks were set at 6, recorded. The point was abandoned doned the morning of September 1. the elevation mask was set at 15 with only 16 positions recorded. At Unless the satellites were positioned degrees, and the antenna was set at stations F and G, satellites were not 35degreesorhigheroverthe horizon, 2m. PathfinderOfficesoftwareversion blocked out by the tree canopy and the dense canopy attenuated the 2.11 was used to differentially post- 150 positions were collected within 5 signal. Whenever signals from four process the raw-position data and minutes.thedgpsreal-timecorrec- satellites were received by the GPS display both the postprocessed and tion broadcast signal was always receiver, the radiolinkfromthe DGPS the real-time-corrected position data. present, real-time broadcast signal station was Base-stationdataforpostprocessing also present. On the afternoon of were obtained from the Bedford base September 1, stations that had been station PFCBS That station, just 18 km (11 mi) from the test site, records data at 5-second intervals. Table 1--Number of positions collected using real-time DGPS. Station A B C D E F G The test course (under canopy) was Total number of positions recorded traversedseventimeswiththeproxr Numberof positions real-time DGPScorrected receiver.at each station four different Number of positions not real-time DGPS corrected point files with about 150 position records were collected. Three different walk files were collected on the test course. Most of these records were real-time-corrected positions. At some locations, the canopy attenu- Hardwoodtest R083118A ated the DGPS correction signal Position files from such locations BAllPositionsPostDPGS contain both real-time-corrected and uncorrected positions. Table 1shows s [] SatelliteReal-TimeDPGS '-- that all the positions were real-time corrected at each station (A through. G) for the R083118a data collection o file. Figure 2 shows the horizontal position error for that file..o A B C D E F Stations G Figure 2--Horizontal position error (ft) for DGPS positions corrected in real time compared to postprocessed positions for file R083118a. 2 J

3 visited that morning were revisited. Table 2--Number of DGPS positions collected in real time (file R090118a). The point features were collected in a Station A B C D E F G short period of time. No more than 14 Total number of positions recorded minutes was required to collect 150 Number of positions real-time DGPS corrected positions. A walk file, R090120a, was Number of positions not real-time DGPS corrected collected around the course from station F back to station F. That file contained 250 positions. Table 2 shows that all the positions were real-time corrected at each station (A through HardwoodtestR090118A G) for the R090118a data collection []All Positions Post DPGS file. Figure 3 shows the horizontal position error for that file E] Satellite Real-Time DPGS Figure4 showsthewalkfilethat includes s positions that were not real-time corrected. Figure 5 shows the walk Q file with those 26 positions removed The area after the positions were removed was acres, for an error of percent. After that file was postprocessed, it had an area of acres, or an error of percent. A B C D E F Stations G The Data Collector's Observations and Data-- September 8 Figure 3--Position error (feet) for DPGS positions corrected in real time compared to all postprocessed positions for file R090118a. Observations for files R090818a and R090821a: On September 8, positions at stations A and (3 were easily collected. However, because of the topography and the position of the satellites, it took 11/2hours to collect 171 points at station B. At stations C, D, and E, positions were collected in 12 to 20 minutes. The DPGS signal was lost at station F for a short period of time. Table 3 shows that most positions at stations A through E were real-time corrected for the R090816a data collection file. However, at stations F and G many positions were i ] not real-timecorrected. Figure 6shows the horizontal position error for that Figure 4--Walk file R090120a, corrected in real time with 26 uncorrected positions (250 total). file. m 3

4 The Data Collector's Observations and Datam September 24 Conditions were dry when a walk file, R092418a, was made around the course and 210 positions were collected. Some were collected without the DGPS real-time beacon signal. A point file, R092419a, was started. Stations A, B, and Cwere collected in about13minutesper station.atstation D,locatedina smalldraw,only61positions were collected. The satellites were obscured by vegetation to the Figure 5--Walk File R090120a corrected in real time with the 26 uncorrected positions removed, west. At station E, on a ridge, 88 positions were recorded. The station was abandoned because of a poor Table 3---Number of positions corrected by DGPS in real time compared to those that were not satellite constellation (four satellites forfiler090818a, September 8, with a PDOP of 9.7). Because of the Station A B C D E F (3 poor satellite constellation and time Total numberof positions recorded constraints, positions were not col- No. of positions real-time DGPS corrected lected at stations F and G. No. of positions not real-time DGPS corrected Percent positions not real-time DGPS corrected Table 4 shows that most positions at stations A through D were real-time corrected for the R092419a data collection file. At station E, just 15 HardwoodtestR090818A percent of the points were real-time corrected.no positionswere collected at stations F and G. Figure 7 shows ; the horizontal position error for that " file Walk files were collected on September 1, 8, and 24. Table 5 shows Walk A B Stations C D E F G the results. Files Figure 6--Horizontal position errors from file R090818a corrected in real time and the file with postprocessed positions. 4

5 Table 4--Number of DGPS positions corrected in real time compared to those that were not file: stations D, E, and F were abanforfiler092419a. doned because of weather. The 35- Station A B C D E F G percent error reflects the incomplete- Total numberof positionsrecorded Bess of the file. No. of positionsreal-timedgpscorrected No.of positionsnot real-timedgpscorrected A heavy canopy can attenuate the % of positionsnot real-timedgpscorrected broadcast DPGS signal. The radio link between the DGPS broadcast station in Louisville, KY, was lost only occasionally when collecting the HardwoodtestR090924A individual files. Table 2 shows percentacquisitionofthe COBSDGPS real-time signal. Ifyou are planning a [] AllPositionsPostDPGS DGPS real-time operation under the F, canopy, allow extra time to get the [] SatelliteReal-TimeDPGS data.you shoulduse satellitevisibility software to predict the times when enough satellites are at higher elevations with good PDOP This test indicated that when the TrimbleProXRGPSreceiverwas used under the hardwood canopy and the A B C D E F positionswere real-timecorrectedwith Stations G the COBS DGPS broadcast signal, the receiverprovidednearly 1-m (1.02 Figure time compared 7--Horizontal with postprocessed position error from positions. file R090924a for DPGS positions collected in real to5.29 ft) average horizontal-position accuracy. If the data are postprocessed,the ProXRwill producebetter Table 5--Number of DGPS positions that were corrected in real time compared to those that (1.204 to 3.194ft) averagehorizontal were not for walk files R090120a, RO90821a, and R092418a. Acreage errors are included, accuracy. Walk file number R090120a R090821a R092418a Total number of positions recorded Number of positions real-time DGPS corrected Number of positions not real-time DGPS corrected Percent positions not real-time DGPS corrected Summary Acreageerror Acreage of file, real-time and DGPS corrected " Percent error from actual The Trimble ProXR will work under a hardwood canopy in conditions when Acreage of file real-time, and not DGPS corrected Percent error from actual a GPS constellation is high above the Actual acreage of walk course horizon.therewere manytimes when four to five satellites were tracked but Incomplete course. Some points were not included in the acreage, with a PDOP of 7 or higher (particularly, in the morning sessions). The satellites neededto provide an acceptable Discussion for Stations F in Table 3 and Stations PDOP typically are low in the horizon F and G in Table 4. In some cases, and notaccessiblethroughtheheavy This test indicates that a heavy hard- only a portion of the positions were canopy and treetrunks. The radiolink wood canopy can attenuate satellite real-time corrected: stations B, C, E, with the COBS station in Louisville GPS and real-time COBS DGPS F, and G in Table 3, stations B, C, E, 150 km (90 mi) away did not present correction signals, preventing them F, and G in Table 4, and a small much of a problem. Although the link from being picked up by the receiver, number of positions in Table 5. File was lost on occasion, the loss was No real-timecorrectionswere received R090821a was an incomplete walk infrequent. 5

6 About the Authors Ross Taylor is a Lands Program Manager for the Hoosier National Forest. Tony Jasumback retired in the spring of 2000 as the GPS Coordinator at MTDC. He has been involved in the development and evaluation of GPS equipment for Forest Service use since Dick Karsky is Program Leader in Forest Health Protection at MTDC. He has been involved in most of the center's resource programs throughout the years and has worked with GPS since Dale Weigel is a Forester for the North Central Research Station. Additional single copies of this document may be ordered from: USDA Forest Service, MTDC Building 1, Fort Missoula Missoula, MT Phone: Fax: Internet: For further technical information, contact Dick Karsky at the address above. Phone: Fax: Internet: rkarsky@fs.fed.us Lotus Notes: Richard J Karsky/WO/ USDAFS An electronic copy of this document is available on the Forest Service's FSWeb intranet at: mtdc.wo.fs.fed, us = The Forest Service, United States Department of Agriculture tute an endorsement by the Department of any product or service means for communication of program information (Braille, large (USDA), has developed this information for the guidance of its to the exclusion of others that may be suitable. The U.S. Depart- print, audiotape, etc.) should contact USDA's TARGET Center at employees, itscontractors, and itscooperating Federal and State ment of Agriculture (USDA) prohibits discrimination in all its (202) (voice and TDD). To file a complaint of discrimiagencies, and is not responsible for the interpretation or use of programs and activities onthe basis of race, color, nationalorigin, nation, write USDA, Director, Office of CivilRights, Room 326-W, this information by anyone except its own employees. The use of sex, religion, age, disability, politicalbeliefs, sexual orientation, or Whitten Building, 1400 Independence Avenue, SW, Wash ngton, trade, firm, or corporation names in this document is for the marital or family status. (Not all prohibited bases apply to all D.C orcall(202) (voiceandTDD). USDA information and convenience of the reader, and does not consti- programs.) Persons with disabilities who require alternative is an equal opportunity provider and employer. 6