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And Dual-Frequency GPS and GLONASS Observations on Point Accuracy Under Forest Canopies

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And Dual-Frequency GPS and GLONASS Observations on Point Accuracy Under Forest Canopies Effects of Differential Single- and Dual-Frequency GPS and GLONASS Observations on Point Accuracy under Forest Canopies Erlk Naesset Deckert and Bolstad (1996) and Sigrist et al. (1999) reported A 20-channel, dual-frequency receiver observing dual-fie- accuracies of approximately 3.1 to 4.4 m and 1.8 to 2.5 m for the quency pseudorange and carrier phase of both GPS and average position of 500 and 480 repeated measurements of indi- GLONASS was used to determine the positional accuracy of 29 vidual points under tree canopies, respectively, based on differ- points under tree canopies. The mean positional accuracy ential pseudorange. Nsesset (1999)found that, even under tree based on differential postprocessing of GPS+GLONASS single- canopies, carrier phase observations represent valuable addi- frequency observations ranged from 0.16 m to 1.I 6 m for 2.5 tional information as compared to traditional pseudorange min to 20 min of observation at points with basal area ranging acquisition. By using both single-frequency pseudorange and from <20 m2/ha to 230 m2/ha. The mean positional accuracy carrier phase observations in an adjustment with coordinates of differential postprocessing of dual-frequency GPS+GLONASS and carrier phase ambiguities as unknown parameters (float observations ranged from 0.08 m to 1.35 m. Using the dual- solution), an accuracy of 0.8 m was reported for two 12-chan- frequency carrier phase as main observable and fixing the nel receivers based on 30 min of observation. The correspond- initial integer phase ambiguities, i.e., a fixed solution, gave ing accuracy using pseudorange only was 1.2 to 1.9 m. the best accuracy. However, searching for fixed solutions Recently, Naesset et al. (2000)argued that, under less favor- increased the risk of large individual positional errors due to able conditions such as under forest canopies, the number of "false" fixed solutions. available satellites is a critical factor for a high positional accu- The accuracy increased with decreasing density of forest, racy. With additional satellites beyond those of the GPS pro- increasing length of observation period, and decreasing a gram, the probability of receiving signals from a required priori standard error as reported by the postprocessing soft- number of satellites with a good geometric distribution will ware. increase. This would be particulary useful in order to take full advantage of the carrier phase observations. By acquisition of Introduction the pseudorange and L1 carrier phase of both GPS and the Rus- Many forest survey applications need highly accurate spatial sian Global Navigation Satellite System (GLONASS),an accu- location of timber measurements made in the field. One such racy of approximately 0.4 m (float solution) was reported after example is the determination of tree heights and timber volume 30 min of observation. The corresponding accuracy using only from airborne laser scanner data (Naesset, 1997; Naesset and GPS observations was 0.7 m. Bjerknes, 2001). Global Positioning System (GPS)technology Furthermore, Naesset et al. (2000) showed that even under can provide few-millimeter accuracy with a surveying-grade tree canopies that are not too dense it might be possible to solve receiver under "clear sky." In forested landscapes, however, the initial carrier phase ambiguity, i.e., to obtain a so-called biologic and topographic obstacles tend to degrade the accu- fixed solution. Such solutions will often represent centimeter- racy obtained from the GPS observations and at times prevent level accuracy. In land surveyor applications with the highest the radio signals from reaching the GPS antenna on the ground. accuracy requirements, dual-frequency (LI+LZ) receivers are Differential positioning in forest environments, i.e., the often used. For ambiguity resolution, observation of both LI utilization of two GPS receivers-one base receiver sited at a and LZ is superior to L1 only (e.g., Hofrnan-Wellenhof et al., known position and a rover receiver used in the forest at 1997). The objective of this study was to compare the point unknown positions-has been applied both in traverse sur- accuracy of single-frequency (LI) carrier phase differential veys (Liu and Brantigan, 1995) and in the determination of the GPS+GLONASS under forest canopies with corresponding accu- geographical positions of individual points under forest cano- racies of dual-frequency (LI+LZ) observations. Fixed solutions pies (Deckert and Bolstad, 1996; Naesset, 1999; Sigrist et al., were also compared with float solutions, i.e., an adjustment 1999;Barrette et al., 2000; N~ssetet al., 2000). When differen- with both coordinates and carrier phase ambiguities as un- tial positioning is used, the two receivers collect data simulta- known parameters. neously, and common errors in the two receivers are elimi- In practical field surveys of remote forest areas, it is not nated. However, site-dependent errors are not reduced by dif- economically feasible to revisit a site. It is therefore important ferencing between receivers. to ensure that a proper quality of all the collected GPS+GLONASS Two observables are available for positioning with GPS, i.e., the pseudorange and the carrier phase. The carrier phase is the basis of the techniques used for high-precision GPS surveys. Photogrammetric Engineering & Remote Sensing Vol. 67, No. 9, September 2001, pp. 1021-1026. Department of Forest Sciences, qgricultural University of 0099-1112/01/6709-1021$3.00/0 Norway, P.O. Box 5044, N-1432 As, Norway (erik.naesset@ O 2001 American Society for Photogrammetry isf.nlh.no). and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING September 2001 1021 observations be obtained on the first attempt. To achieve a TABLE1. SUMMARYOF FOREST STAND CHARACTERISTICS FOR 29 SUB required accuracy at sites with high tree densities, it may be CANOPYPOINTS~ necessary that a position in an adjacent opening be measured Range Mean and that the position of the object of interest be computed from measurements of distance and bearing. However, it would be Age (years) 18-115 68 useful if the surveyor could decide in the field whether it is h~ [m) 5.1-28.2 17.8 likely that an accurate position can be obtained. Forest density, G [m2/ha) 5.0-42.0 24.7 V (m3/ha) 15.0-508.5 226.8 tree height, length of the observation period, and geometric sat- % spruce 0.0-100.0 55.2 ellite distribution are factors that affect accuracy (Deckert and % pine 0.0-100.0 28.6 Bolstad, 1996; Naesset, 1999; Naesset et al., 2000) and that may % birch 0.0-100.0 16.2 be considered by planning in advance or by tree measurements No. of sites dominated by in situ. How these factors affect accuracy was, therefore, spruce 16 evaluated. pine 9 Furthermore, when the field measurements are completed deciduous 4 and the positions finally are computed by differential post- "hL= Lorey's mean height, G = basal area, V = total timber volume. processing, it is useful to assess the reliability of the computed positions. Because no ground truth will exist, the expected accuracy has to be computed from indicators that are known to sample trees and expressed by the so-called Lorey's mean be correlated with the accuracy and that are made available height (ht),i.e., mean height weighted by basal area. during the field work or after the postprocessing. These indica- tors comprise the factors mentioned above and the a priori stan- GPS and GLONASS Data Collection dard errors of the computed coordinates reported by the Two identical Javad Legacy receivers (Javad Positioning Sys- postprocessing software. I evaluated how these factors and tems, San Jose, California) were used to collect the GPS and indicators affect the observed accuracy. GLONASS observations, one receiver serving as a base station for differential correction sited at the university campus ("Base Material Methods Station 1") and near the Oslo International Airport ("Base Sta- and tion 2") and one used as a rover receiver in the forest at the sub- Field Reference Data canopy sites. The base stations were located 0.8 to 5.5 km from The study was accomplished in a forest in the municipality of the respective sub-canopy sites. The Javad Legacy are 20-chan- As (N 5g040' E 10°45', 40 to 120 m a.s.l.1, near the Agricultural nel dual-frequency receivers observing pseudorange and car- University of Norway, and in the municipality of Ullensaker rier phase (LI+L~)of both GPS and GLONASS. (N 60°11' E 11°13', 200 m a.s.l.), southeast Norway, near the The GPS and GLONASS obs~rvationsfor the 23 sub-canopy Oslo International Airport. points in the municipality of As were acquired on 20 and 21 Ten sites were selected forethetrial. Eight of them were July 1999. For the six sub-canopy sites in the municipality of located in the municipality of As within a radius of 5 km and Ullensaker, the data were collected on 30 October 1999. On two of them were located in the municipality of Ullensaker both occasions 27 operating GPS satellites were available within a radius of 0.8 km. Each site was comprised of a mixture (Anon., 1999a),whereas the number of operating GLONASS sat- of open areas and closed forest stands. The closed stands repre- ellites in July and October were 16 and 10, respectively (Anon., sented different combinations of tree heights, stand densities, 1999b).No mission planning was done to survey under opti- and tree species. In an open area at each of the ten sites, the posi- mal satellite configurations. The rover receiver was positioned tions of two subjectively selected points were accurately deter- accurately with a tripod over each sub-canopy point. The an- mined using differential GPS.
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