Accuracy and signal reception of a hand-held Global Positioning System (GPS) receiver

by Stephen P. D'Eon

Accurate and precise repolting of forest survey locations is required La transmission fidkle et exacte de la localisation des points de to integrate forest survey data with Geographical Information Systems. sondage forestier est n&essaire h l'intkgration des d o n n k du sondage The accuracies of five Global Positioning System (GPS) survey aux systhmes d'information gCographique. La prCcision de cinq methods using a hand-held receiver were tested in a mixed for- mCthodes de sondage bas6 sur un systkme de positionnement GPS est of trembling aspen and spruce. Accuracy improved by elim- utilisant un rkcepteur portatif a CtC CvaluCe dans une for& inating positions obtained under poor satellite configurations mClangCe composCe de tremble et d'Cpinette. La prCcision a CtC and by using position averaging methods. Single fix positions, tak- amClior6e suite h I'Climination des positions obtenues en vertu d'une ing as little as two minutes to obtain, yielded better than accu- configuration inadequate des satellites et par l'utilisation de racy more than 80% of the time. Mowing the receiver to contjnu(~1~1y mCthodes permettant d'obtenir une position moyenne. Les posi- collect fixes for 15 to 30 minutes and then averaging the fixes yield- tionnements fixes individuels, ntcessitant pas plus de deux min- ed a median position error of 17 m. Sixty one stands represent- utes d'attente, se sont rCvC1Cs Ctre prdcis en deqa de loom dims ing a diversity of cover types, canopy heights, and crown closure plus de 80% des cas. En pernettant au receptew d'amasser des in the Petawawa Research Forest were tested during June and July donnCes sur une ptriode de temps de 15 h 30 minutes et en of 1992 for canopy interference with GPS sign&, A GPS posi- obtenant par la suite une moyenne de ces donnCes, il a CtC pos- tion was obtained under the canopy in 74% of the stands. sible d'obtenir une valeur mCdiane prkcise h 17m prks. Soixante Launches of additional GPS satellites since the summer of 1992 et un peuplements =@sentant divers Couverts forestiem, diffhcntes have further improved the probability of obtaining accurate gee- hauteurs de cime et plusieurs niveaux de fermet~re d~ COUVert, graphical positions under forest canopies. situCs dans la for& exphimentale de Petawawa, ont CtC meswCs

au cours de juin et juillet 1992 afin d'Cvaluer l'interfkrence du cou- K~~ words: positioning system, position accuracy, signal vert lors de 1'Cmission des signaux GPS. Un positionnement reception, canopy interference GPS a CtC obtenu sous couvert forestier clans 74% des peuplements.

La mise en orbite de satellites GPS additionnels qui a eu lieu au cours de 1'Ctk de 1992 a permis d'accroitre la probabilite d'obtenir des positionnements geographiques prCcis sous couvert forestier.

Mots cl6s: systkme de positionnement GPS, precision du posi- tionnement, rCception du signal, interference du couvert

Introduction Many forest managers use permanent sample plots and

temporary sample plots for monitoring forest conditions and forest inventory. Modem intensive forest management utiliz- ing Geographical Information Systems (GIs) requires accurate and precise plot locations. Forest ground surveys are often con- ducted in areas with few distinctive land features available for the surveyor to spatially reference the survey site. Conventional methods using maps usually allow field technicians to report the location of forest ground surveys to a precision no better than 1 km. Large scale National Topographic Series maps and up-to-date forest stand maps can help in reporting survey locations to a precision better than 1 km, but obtaining, trans- porting and using up-to-date forest stand maps are problems for forest technicians responsible for surveying large areas. Air photos do not usually have printed on them a geographical ref- erence grid (such as latitude and longitude). Global Positioning System (GPS) technology offers a potential solution to the prob- lem of reporting forest surveys accurately, precisely and easily.

Hand-held GPS receivers appear to be the appropriate choice for forest ground surveys, offering ease of use, porta- bility, and vendor-claimed accuracies in the 10 to 15 m range

Canadian Forest Service, Petawawa National Forestry Institute, Chalk River, Ontario, Canada KOJlJO.

when used without any additional equipment or differential cor- rections. Because GPS requires the receiver be placed where there is a clear line-of-sight to each of three satellites for a two- dimensional (2D) position, there is concern about the perfor- mance of GPS receivers under the forest canopy. This report describes results of a study to determine the suitability of hand-held GPS receivers in static forest ground surveys. Two aspects of hand-held GPS receivers were evaluated: accuracy of non-differentially corrected GPS positions in a forested area, and performance under various forest canopies.

Equipment During field tests in the summer of 1992, the GPS satellite

constellation consisted of 18 active satellite slots of the 21 planned in the final wnstellation. Satellite launches since 1992 have brought the GPS constellation to near completion. The satellites are main- tained by the United States Department of Defence and broad- cast their signal free of charge. Each satellite broadcasts infor- mation about itself and its atomic clock time using low power radio signals. United States Department of Defence ground sta- tion precisely measure each satellite's altitude, position and speed twice a day. Minor deviations from the predicted orbit are cor- rected and relayed back to the satellite so it can broadcast the corrected information. GPS receivers contain an antenna, a clock and software to triangulate the receiver's position using its dis- tance from three or more satellites. Receivers can include options that improve their accuracy and precision, but these also increase price.

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A Magellan NAV 5000 PRO' hand-held GPS receiver was obtained in the spring of 1992. The NAV 5000 PRO is a five channel receiver measuring 21.5 cm X 9.0 cm X 5.0 cm and weighing 0.85 kg. It fits in the user's hand and is powered by six alkaline AA batteries, allowing complete portability. The five channels in the NAV 5000 PRO allow the receiver to track and obtain signals from up to five satellites at once. A multi- channel receiver was obtained in the expectation of difficul- ty in receiving signals and tracking satellites under the forest canopy. The vendor claimed accuracies of 15 m from a single GPS fix using the standard 10 cm antenna (Magellan 1992a). Riordan (1992) found the performance of the Magellan NAV 5000 PRO under forest canopies to be similar to the Trimble Pathfinder Basic Plus but slightly less than the Garmin GPS 100. Both the Trimble and the Gannin have vendor-claimed accu- racies similar to the Magellan (Trimble 1991; Gannin 1992).

Methods GPS Position Accuracy

An unordered benchmark located at 45" 59' 02"N, 77" 27' 14" W (NAD27) was used as a control point to test the accu- racy of GPS positions under operational conditions. The mon- ument was a bronze disc, set in a concrete cylinder at ground level. An unordered benchmark is not included in the current Geodetic Survey of Canada adjustment and evaluation but is considered to be within 1 to 2 m of its true position (National Defence 1984). The benchmark monument is located on the east side of a road in the Petawawa Research Forest and is surrounded to the north by a small clearing approximately 20 m in diam- eter. A mixed forest of 20 m tall trembling aspen and spruce enclose the monument to the south and to the west across the 10 m wide road. The monument was covered by 1 m tall ground vegetarian which was removed so the view of the sky from the monument was obstructed only by the surrounding forest. This surveyed benchmark was selected because it pre- sented a partially obstructed view of the sky expected under oper- ational field conditions.

The GPS receiver was initialized for the surveyed location and set to 2D mode. With its antenna raised, the receiver was powered on and placed on the benchmark monument. A "cold start" single fix position was attempted. The term "cold start" refers to the receiver having its power initiated while at the sur- vey location. A "warm start" involves powering the receiver on and obtaining some GPS fixes, then moving the receiver to the survey location while keeping it powered on. If a position was obtained, it was electronically recorded as a waypoint in the receiver's memory. Along with positional data, informa- tion about the satellites used, the signal strength, and the Position Dilution of Position (PDOP) are stored as part of a way- point. PDOP indicates the amount of positional error caused by the geometry of the satellites with respect to each other and with respect to the receiver.

If a single fix GPS position was obtained, the receiver was set to averaging mode and an average of 25 fixes was attempt- ed and recorded. The receiver was reset a third time and an aver- age of 100 fixes was attempted and recorded. The averaging

h he use of a particular product does not denote Canadian Forest Service sup port for that product.

of fixes requires the receiver to lock on to and maintain a sig- nal from each of three satellites. The NAV 5000 PRO obtains position fixes as fast as one per second once all three satellites are locked on. Should the signal from any of the three satellites be obstructed prior to the preset number of fixes being obtained, then the averaging software aborts. No post-processing of data was done. Visits to the monument were made randomly during normal work hours between 8 am and 5 pm.

A fourth method was tested where the receiver was initial- ized to the location and set to "auto select" mode. In this mode, the receiver searches for four satellites for a preset amount of time. If four satellites are acquired and their signals locked, the receiver sets itself to three dimensional (3D) mode and calculates a longitude, a latitude, and an elevation. Should the receiver not lock onto four satellites, it sets itself to 2D mode and attempts to acquire three satellites. The acquisition of a fourth satellite for 3D mode was tested to see if it gave more accurate latitude and longitude as compared to 2D mode. The receiv- er was left on the monument from 15 to 30 minutes to simu- late the amount of time it takes to do a field survey. Each fix the receiver obtained during this time was recorded in the receiv- er's buffer and was downloaded later in the day to an IBM/PC. The fixes were post-processed using Magellan software Version 2.4. The software averaged the fixes and calculated the error from the benchmark position. By post-processing the data for averaging, the receiver does not require a continuous lock on three or four satellites and may lose and acquire satellites any time during the session. Continuous collect sessions were subdivided during post-processing into 2D and 3D sessions.

The position error (PE) indicates the accuracy of a GPS posi- tion because it represents the nearness of the GPS position to the known position of a benchmark. The PE is calculated as the horizontal error of the averaged position for the session. In this study, sessions averaged from one to 1034 fixes.

Forest Canopy Interference A total of 61 stands in the Petawawa Research Forest were

sampled during June and July of 1992 for interference with GPS signals. The stands ranged from young softwood plantations to mature hardwood stands. Stands were selected for sampling by species composition, height, and density class (crown clo- sure) as listed on the Petawawa Research Forest Cover Map (Canadian Forestry Servicel986). Plantations that were not clas- sified on the Forest Cover Map for height and crown closure were evaluated for these parameters from the ground. If the cur- rent stand characteristics did not match the Forest Cover Map, the stand was excluded from sampling. Occasionally the intended stand could not be correctly located and was replaced with a substitute of similar characteristics. The height class of dominant trees in stands sampled ranged from class 2 (6-10 m) to class 6 (26-30 m). Crown closure in stands sampled ranged from class 1 (0-25% closure) to class 4 (76-100% closure). The species composition of stands sampled included various com- binations of nine coniferous species and seven deciduous species. The terrain varied from flat to gently rolling with rock outcrops. Understory characteristics varied greatly in terms of size, density and species composition.

Operational conditions were simulated using a standard antenna with the receiver, no mission planning was done to sur- vey under optunal satellite configurations, and no post-processing of data was done. Each stand was tested by entering the stand

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Table 1. Effect of PDOP on position error for five GPS survey methods

Median GPS survey PDOP Number of position error Greatest error Least error method value positions (m) (m) (m)

Single fix > 6.0 6 229.2 515.7 96.9 >4and<6 7 50.2 185.9 31.0

< 4 27 42.4 100.2 10.2 < 3 19 31.4 92.4 12.0 < 2 11 24.3 52.4 12.0

Average 25 fixes >4and<6 6 109.8 147.7 19.0 1 4 21 41.1 75.7 4.1 < 3 17 41.1 75.7 4.1 < 2 9 32.0 75.7 4.1

Average 100 fixes >4and<6 4 55.6 93.3 10.4 t 4 13 29.4 173.5 8.1 1 3 11 28.2 173.5 8.1 < 2 7 24.7 61.5 8.1

Continuous all 16 17.0 132.2 5.7 collect 2D < 4 12 17.0 88.3 5.7

Continuous all 17 16.8 52.1 3.9 collect 3D < 6 12 14.0 52.1 3.9

and selecting a spot representative of the stand. The GPS receiver was then powered on and held 1.8 m above the ground ("cold start"). The operator slowly moved around until a position was obtained. If a position was not obtained with- in five minutes, the operator returned to the road or a nearby clearing, powered on the GPS receiver and attempted to obtain a fix. With the receiver still powered on and tracking satellites, the operator re-entered the stand and returned to the representative spot and tried to obtain a position ("warm start"). If a position was not obtained, the stand was revisited at the opposite time of day (morning vs afternoon) and the method repeated. If a posi- tion was not obtained on the second visit, the stand was revis- ited a third time during the winter season when all the hardwood foliage had fallen. The third visit was conducted six months later under a different satellite constellation. For each stand the for- est cover information was supplemented with the type of fix obtained (cold, warm, or none), the waypoint information, and a brief description of the understory.

Results GPS Position Accuracy

Single fix GPS positions on the benchmark were attempt- ed 48 times (Table 1). A position was obtained 40 times (83%) with the signal being blocked the other eight times. To obtain a single fix from a "cold start" required as little as 2 min- utes. GPS positions for sessions using the two averaging methods required a continuous lock on each of three satellites. The mixed forest surrounding the benchmark tended to block or interrupt the signal from satellites low on the horizon, causing the averaging software to abort prior to calculating a position. Sessions averaging 25 and 100 fixes yielded GPS posi- tion 27 and 17 times, respectively. These averaging sessions took from 3 to 5 minutes.

The exclusion of positions with high PDOP values and the use of averaging yielded more accurate positions for the three GPS survey methods not requiring post-processing. The medi- an PE for all single fix positions was 5 1.3 m (Table 1). For this method under near ideal satellite conligurations (PDOP values

less than 2) the median PE was 24.3 m. The most accurate sin- gle fix position was obtained with a PDOP value of 3.5. Averaging 25 fixes derived a similar trend, with the more accurate positions being obtained with lower PDOP values. Averaging 100 fixes yielded more accurate positions although one position with a low PDOP value of 2.9 gave a position emr of 173.5 m.

A further 22 sessions were made using continuous sampling where the receiver collected fixes in its buffer for post-processing. Using this method, interruptions in the satellite signal caused the receiver to suspend rather than abort the calculation of its position. Once the satellite signal was re-established, the receiver resumed calculating positions. These continuous col- lect sessions obtained from 54 to 1034 fixes taking from 15 to 30 minutes.

The continuous collect methods yielded a median PE of 17.0 m for 2D positions and a median PE of 16.8 m for 3D positions. The 3D method yielded positions with a narrower range of val- ues than the 2D positions (Table 2). With the exception of two 2D sessions and one 3D session, all continuous collect PEs were less than 50 m. In one 2D session (session 7), the PDOP for indi- vidual fixes ranged h m 13.7 to 49.5 indicating a very poor satel- lite configuration and yielding a very poor position with a PE of 132.2. Once a fourth satellite was obtained during ses- sion 7, the receiver switched to 3D mode yielding a PE of 11.3 and reasonable PDOPs ranging from 5.2 to 7.5.

Canopy Interference A GPS position under the forest canopy was obtained on the

first visit in 35 of the 61 (57%) stands sampled (Table 3). In 10 of the 26 stands where a GPS position was not obtained on the first visit, a position was obtained on the second visit at the opposite time of day (morning vs afternoon). Thus, in 74% of the stands, a GPS position was obtained on the first or second visit. Good positions, as indicated by PDOP values less than 4, were obtained for 73% of positions during first or second vis- its. In this study, hand-held GPS in a mixed wood forest yielded positions under the canopy in approximately two out

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Table 2. Number of 6xes and position errors for two continuous sampling methods

Number of PE all 2D Number of PE all 3D Session 2D 6xes fixes(m) 3 D h e s fixes (m)

Table 3. Frequency of GPS positions and PDOP value for 61 stands sam- pled in the Petawawa Research Forest

Number of stands Position PDOP value lSt visit 2nd visit1 31d visit2

Good ( ~ 4 . 0 ) 27 6 15 Medium (4.0-10.0) 5 2 0 Poor (>10.0) 3 2 1 Total positions obtained 35 10 16 Position not obtained 26 16 0 Total stands sampled 61 26 16

'Second visits were made at the opposite time of day (morning vs afternoon) to stands where a position was not obtained during the visit. =~hird visits were made six months later during the winter months when hard- wood foliage had fallen. Only stands where a position was not obtained dur- ing the second visit were sampled a third time.

of three stand types. Further study is underway to character- ize canopy and understory interference with GPS signals.

Discussion Errors in GPS positions come from satellites not being in their

pmhcted orbital position, enrns in the satellite's clocks, the receiv- er's accuracy, the satellite's signal bouncing off objects prior to being acquired by the receiver, atmospheric and ionos- pheric induced error, and the United States military purpose- ly randomizing the satellite's signal (known as Selective Availability) (Hum 1989). Under ideal conditions without Selective Availability, manufacturers of hand-held GPS receivers claim accuracies for single fix positions of about 15 m (Gamin 1992, Magellan 1992a; Trimble 1991). Selective Availability introduces the largest error with the United States military having the right to scramble GPS signals by up to 100 m None of the above error components are recorded by the receiver, making it is impossible to determine the accuracy of a GPS position without referencing to a know position.

One error component that is recorded by the receiver is the PDOP.GPS receivers determine their distance from each vis- ible satellite by comparing the satellite's clock time with the

receiver's clock time. The distance from each satellite is deter- mined with some error. Once three or more satellites are con- tacted, the receiver triangulates its position. The size of the tri- angulation solution set is indcated by the PDOP (Fig. 1). The receiver will attempt to triangulate its position from satel- lites that yield the best solution set (lowest PDOP) value). If a satellite's signal is blocked, the receiver will choose anoth- er satellite, leading to a less than ideal satellite configuration.

The forest canopy can block the signal from satellites, causing the receiver to use a less than ideal satellite configu- ration yielding a position with a high PDOP value and ques- tionable accuracy. Values of PDOP of four or less are considered good for 2D positions and PDOP values of six or less are con- sidered good for 3D positions (Magellan 1992b). GPS field meth- ods that increase the number of satellites the receiver can track should be employed to improve the opportmities for obtain- ing a GPS position from satellites with a good configuration. Satellite launches since this study was conducted in the sum- mer of 1992 have improved satellite availability.

Vendor-claimed accuracies of 15 musing the described equip- ment were not achieved in this study. Selecting the appropri- ate GPS survey method will efficiently utilize field time and yield results with the desired accuracy (Fig. 2). For less restric- tive surveys demanding 100 m accuracy, single fbi surveys under good satellite configurations (PDOP less than 4) are the appro- priate choice. For surveys demanding 20 m accuracy, the two continuous collect methods with post-processing are the appre priate choice.

A more complicated method for obtaining GPS positions is differential processing; it compensates for Selective Availability, but was not used in this study. Differential processing requires a second receiver placed at a known position and synchronized to collect data from the same satellites as the roving receiver. The second receiver can be a GPS base station that tracks all satellites in view and logs data directly to a computer. A GPS base station can provide differential corrections for any num- ber of rovers within several hundred km, although distances greater than 50 km are not recommended (Magellan 1992b). Differential processing can yield vendor-claimed accuracies of 5 to 10 m for single fixes and 3 to 5 m for positions that average numer- ous fixes (Magellan 1992a). Averaging autonomous fixes as done in this study is not a substitute for differential process- ing.

GPS-produced elevations can provide useful data in moun- tainous terrain where contour lines on topographic maps are close together. Elevation data can be obtained using GPS if the receiver is set to 3D mode and four satellites are tracked. GPS-produced elevations will contain approximately double the error as horizontal positions and will not be obtained as often as 2D positions due to blockage of the extra satellite (Magellan 1992b).

In a forest typical of the Great Lakes-St. Lawrence region, about two thirds of all stands yielded a GPS position in less than five minutes using a hand-held receiver without extending the antenna above 1.8 m. Approximately three quarter of the positions obtained were from satellites configured to yield accu- rate positions. Under operational conditions, a GPS position within 45 m of its true value should be obtained under the canopy in more than half the stands.

Survey location reporting can be recorded precisely and quick- ly using GPS. In static forest ground surveys the crew remains

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solutibn set I

solution set

a) Good geometry (low PDOP) b) Poor geometry (high PDOP)

Figure 1. The influence of satellite geometry on GPS position accuracy.

0.0 1 1 ~ 1 1 1 1 ~ 1 1 1 1 , 1 1 1 1 ~ 1 1 / I , 1 / 1 1 ~ 1 1 l 1 ~ 1 1 1 1 ~ 1 1 1 ~ ~ ! 1 1 1 ,

0 10 20 30 40 50 60 70 80 90 100

Position Error (m)

Method - Average 100 - Average 25 I"-* Continuous 2D Continuous 3D * Single fix

Figure 2. Probability of obtaining positions within a specified posi- tion error for five non-differentially corrected GPS methods.

for a period of t h e in one location making forest measurements and observations. The GPS receiver can be initiated at the start of the survey and left powered on for the duration of the sur- vey. The GPS location data can be appended to the survey data once the survey is completed. The receiver can also be used to

store the location in the receiver's buffer, eliminating the pos- sibility of transcription errors. GPS data can be electronical- ly transferred to a database or GIs for survey location mapping and analysis with respect to other GIs features. The Global Positioning System provides quick, accurate, and precise for- est survey location reporting.

Acknowledgements The author wishes to acknowledge with thanks C. Mach for

assistance in collecting the field data. Dr. L. Marshall and an anonymous reviewer provided very useful review comments that improved this paper.

References Canadian Forestry Service. 1986. Forest Cover. Petawawa National Forestsy Institute. Map. Garmin International Inc. 1992. The GPS 100 Personal Surveyor. Garmin International Inc., Lenexa, Kansas. Hum, J. 1989. GPS -A guide to the next utility. Trimble Navigation Ltd., Sunnyvale, CA. Magellan Systems Corp. 1992a. Magellan GPS NAV 5000 PRO. Magellan Systems Corp. San Dimas, CA. Magellan Systems Corp. 1992b. Magellan GPS NAV 5000 PRO user guide. Magellan Systems Corp., San Dimas, CA. National Defence. 1984. Trig List CFB Petawawa MCE 35 and 132. Department of National Defence, Mapping and Charting Establishment, Ottawa, Ontario. Riordan, D. 1992. Application of Global Positioning Systems for per- manent sample plots. Ontario Ministry of Natural Resources, Northeast Science and Technology, Timmins, Ontario. Trimble Navigation Ltd. 1991. GPS Pathfinder Basic. Trimble Navigation Ltd., Sunnyvale, CA.

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