[50] Reliable Technology of Centimeter GPSGLONASS Surveying in Forest Environments

IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 53, NO. 2, FEBRUARY 2015 1029

Reliable Technology of Centimeter GPS/GLONASSSurveying in Forest Environments

Mieczysław Bakuła, Paweł Przestrzelski, and Rafał Kazmierczak

Abstract—This paper presents a reliable technology forGPS/Global’naya Navigatsionnaya Sputnikovaya Sistema(GLONASS) surveying under difficult observational conditions,i.e., in forest environments. The technology is based on therapid static and Real-Time Kinematic (RTK) methods. Testsurveys for the developed technology were carried out on a pointin a dense forest, with a high degree of difficulty for satelliteaccess. The coordinates of the point were determined by threeGPS/GLONASS receivers positioned in a line on a special base,which were separated by a fixed distance of 0.5 m. The testsurveys were conducted using Topcon HiPer Pro receivers with10-Hz RTK positioning and gathering data at a sampling rateof 10 Hz. Practical tests for the presented technology showedthat rapid static surveys based on three rover GPS/GLONASSreceivers were a quite efficient technology in forest environments(unlike the RTK) and allowed for reliable centimeter accuracy ofthe determined positions to be obtained in 5-min observationalsessions.

Index Terms—Data processing, error analysis, estimation,Global Positioning System, least squares methods, measurementerrors.

I. INTRODUCTION

P ERMANENT global navigation satellite system (GNSS)reference stations are currently widely used in many coun-

tries, enabling the modeling of errors related to distance decor-relation, i.e., ionosphere and troposphere errors [29]. In Poland,a reference station network system [the so-called Active Geode-tic Network European Position Determination System (ASG-EUPOS)] for the whole country was launched in mid-2008. TheASG-EUPOS system consists of around 100 GNSS stationsand offers real-time services for Real-Time Kinematic (RTK)surveys and for differential GPS (DGPS) code measurements[6]. The system also enables the automatic processing of datafrom static observations for any user, although the minimumobservation period is as much as 15 min and GNSS surveysmust be performed on points with good access to satellites.It is well known that a major limitation of satellite methodsis the need to perform measurements in an open area. If

Manuscript received November 20, 2013; revised April 23, 2014; acceptedJune 9, 2014. Date of publication July 11, 2014; date of current versionAugust 12, 2014.

M. Bakuła is with the Department of Satellite Geodesy and Navigation,University of Warmia and Mazury in Olsztyn, 10-724 Olsztyn, Poland, andalso with the Department of Air Navigation, Polish Air Force Academy,08-521 Deblin, Poland (e-mail: [email protected]).

P. Przestrzelski and R. Kazmierczak are with the Department of SatelliteGeodesy and Navigation, University of Warmia and Mazury in Olsztyn, 10-724Olsztyn, Poland (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TGRS.2014.2332372

obstructions are present, the survey becomes difficult or ofteneven impossible using traditional GNSS techniques because thepositions determined by the methods using satellite systemsoften have large gross errors [3]. Practice shows, however, thatthe performance of GNSS surveys under conditions of limitedaccess to satellites is sometimes necessary (investments inforested and tree-covered areas) [2]. Satellite technology is alsoused in digital photogrammetry methods in forest managementwhere practical forestry experience shows that the encoun-tered difficulties caused by variable or unclear forested areasnegatively influence using GNSS measurements [14]. GNSSmeasurements have been widely applied in aerial photogram-metry since the 1990s mainly to establish the coordinates ofthe ground control points for operations over vast and difficultareas, i.e., on small- and medium-mapping scales [33].

GNSS satellite technology is used in forest in situ biomassmeasurements for the analysis of Light Detection And Ranging(LiDAR) technology measurements, where the achieved accu-racy has been in the range of decimeters (or worse) due to diffi-cult environmental conditions [5]. Generally, GNSS surveyingis an efficient method for cadastral surveys, mapping, and otherrelated activities (e.g., establishing control points for terrestrialmeasurements). However, to date, GNSS measurements are be-ing avoided in very dense urban and forested environments. Theproblem of satellite receiver positioning under tree canopiesis not new. The research conducted to date and presented inliterature on the subject has used, e.g., the method of absolutemeasurement [9], [32], differential code measurement [22],[25], the modern Precise Point Positioning technology [18],static surveying [12], [17], [19], and the RTK method, whichis widely applied in surveying practice [20]. These papers con-cern accuracy analyses under conditions with limited access tosatellites, which clearly present the degradation of determinedmeter-level position accuracy with no possibility of obtainingreliable coordinates because of disruptions in satellite signalreception.

The new technology presented in this paper uses both theRTK survey and rapid static survey techniques. Only one base-line is always determined in traditional RTK surveys regardlessof the solution used [26], because of which such surveysare de facto devoid of sufficient control. For this reason, ourpaper uses a technology with three rover GPS/Global’nayaNavigatsionnaya Sputnikovaya Sistema (GLONASS) receivers(see Fig. 1). Three GNSS receivers are placed on an aluminumbeam [see Fig. 1(b)] at set distances from each other. Thebeam itself is mounted with a tribrach on the tripod, wherethe middle receiver is directly set over the measured point.Names were assigned to the receivers depending on the position

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1030 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 53, NO. 2, FEBRUARY 2015

Fig. 1. GNSS surveying based on the new method. (a) Technology concept. (b) Live view of the technology.

on the beam, i.e., N for the north receiver, M for the middlereceiver, and S for the south receiver. The compass attached tothe side of the beam allows the whole unit to be oriented in anorth–south direction. As many as six baselines for one pointdetermination were obtained in this way, i.e., three baselinesfrom the reference station (REF-N, REF-M, and REF-S) andthree baselines between the N, M, and S receivers (N–M,M–S, and N–S). This allowed additional redundant geometricconditions to be obtained, and it enabled the achievement ofreliable results in severe observational conditions [1], i.e., aconstant distance and height between the N–M–S receivers aswell as their orientation relative to each other.

To increase the number of measurements, it was additionallydecided to decrease the raw GPS/GLONASS data recordingand RTK positioning interval to 0.1 s. Positioning with adecreased measurement interval is mainly used in engineeringmeasurements for the monitoring of structure displacements[11], [21]. Although the postprocessing mode has been appliedso far, the application developed by the authors allows theresults to be directly recorded in less than 1-s sampling intervalto a portable computer. Additionally, in the RTK survey, theauthors’ application enables the analysis of the determined RTKpositions, which were obtained from the three receivers (N, M,and S), to find only one reliable position for the M receiverdirectly at a measurement site [4].

II. HARDWARE AND SOFTWARE

CONFIGURATION: TEST AREA

The measurements, which are both in the RTK and rapidstatic technologies, were performed based on the KROL stationof the ASG-EUPOS network located ca. 860 m from the placewhere the experiment was carried out using Topcon HiPer Proreceivers. Communication with the KROL station was pro-vided using the Networked Transport of Radio Technical Com-mission for Maritime Services (RTCM) via Internet Protocol(NTRIP) [28].

The data from the modem were simultaneously sent to thethree rover receivers with a suitably designed RS232 serialcable and RS232-LEMO cables, which transmitted the datato the selected ports of the receivers (see Fig. 2). The samecables were employed when sending the RTK survey results to

Fig. 2. Hardware setup of the GPS/GLONASS unit.

the portable computer using the GGA sentence of the NationalMarine Electronics Association standard. Efficient communica-tion between the GNSS receiver and the netbook was providedusing RS232-USB converters. Successively received sentenceswere processed with a specially prepared authors’ applicationfor positioning with a decreased interval. The program allowsthe recording and analyzing of positions from the RTK mea-surements from three receivers simultaneously.

Test surveys were carried out on October 23, 2012 from9:30 to 11:00 local time in a park belonging to the campusof the University of Warmia and Mazury in Olsztyn, Olsztyn,Poland. The observations on the test point were performedwith considerably difficult access to satellites [see Fig. 3(b) and(c)] simultaneously using GPS and GLONASS satellites [seeFig. 3(a)]. The use of two navigation systems in positioningresulted in a substantial increase in the number of satellitesevenly distributed in the sky. Despite the difficult observationalconditions, the number of satellites ranged between 17 and 23.Both the RTK survey and the raw GPS and GLONASS obser-vation recording, with a recording frequency of 0.1 s (10 Hz),were carried out during the whole measurement session. Broadobservational material was obtained this way for 54 000 posi-tions for each receiver. During the experiment, FIXED solutions(where the ambiguities of phase measurements are calculatedas integers) were just 8%–19% of all results, depending on the

BAKUŁA et al.: RELIABLE TECHNOLOGY OF CENTIMETER GPS/GLONASS SURVEYING IN FOREST ENVIRONMENTS 1031

Fig. 3. (a) Satellite configuration. (b) and (c) Obstructions at the measured point.

TABLE ISELECTED STATISTICS OF THE RTK SOLUTIONS

receiver. The vast majority, which was as much as 81–92%,were FLOAT solutions (i.e., the ambiguities are not integernumbers).

Despite ensuring almost identical survey conditions, i.e.,using the same satellites and three identical GPS/GLONASSreceivers, a quite significant difference appeared in the numberof fixed solutions for the rover receivers (north, middle, andsouth) placed on the survey unit.

III. RTK PERFORMANCE IN FOREST ENVIRONMENTS

Similar to the rapid static method, the RTK positioningresults from the RTK survey with three receivers were dividedinto 18 5-min sessions, which had parallel time equivalents insessions performed by the rapid static method. The optimumsolution in the form of a reliable position was searched in eachsession. The knowledge of the constant distance between in-dividual receivers, i.e., N−S = 1 m, N−M = M − S = 0.5 m,and the constant height of the whole survey unit were usedfor this purpose. Before starting to process the results, thepermissible difference (for horizontal positions) between theaforementioned values was arbitrarily adopted at the level of2 cm (dD). The height difference between the receivers (N, M,and S) could not exceed 5 cm (dH). The values of tolerance forhorizontal and vertical thresholds were empirically determined

and can be changed depending on the required accuracy. Tofind the optimum solution, we tried to obtain a reliable positionfor simultaneous measurement with three receivers (N–M–S).The FIXED RTK solutions, which were taken into account foreach solution for a given measurement epoch, had to fulfil theaforementioned geometric conditions. The horizontal (X andY ) and vertical coordinates (H) for the N–M–S solution werecomputed by

XN−M−S =XN+XM+XS

3YN−M−S =

YN+YM+YS

3

HN−M−S =HN +HM +HS

3(1)

and the final position (the averaged N–M–S solution) was thearithmetic mean from individual N–M–S solutions obtained ina given 5-min session. Additionally, because of a high numberof redundant observations in the proposed survey technology,an observation was regarded as reliable when only using tworover receivers (N–S, M–S, N–M,). Table I shows a quantitativecompilation of the considered solutions obtained throughoutthe experiment. The FIXED RTK positions occurred when two(N–S, M–S, N–M) or three (N–M–S) receivers simultaneouslyobtained FIXED and reliable solutions, e.g., for the N–S col-umn in a given epoch, N–S receivers calculated their positionsas FIXED, whereas the middle receiver had a FLOAT solution.


Fig. 4. Reliable RTK solutions obtained in the fourth session.

The FIXED RTK survey solutions for individual receiversmade up 8%–19% of all the results. Reliable solutions, ac-cording to the assumptions adopted by the authors, constitutedslightly over 2% of the whole survey. Gross errors did notoccur in any of the computed reliable solutions based on thepresented technology, which confirms the assumptions of thediscussed survey method. Despite difficult survey conditions,we managed to obtain a reliable N–M–S position in 206 mea-surement epochs, meeting both the geometric criteria (distanceand height) for three receivers simultaneously. This only rep-resents less than 21 s of measurement, which appeared during1.5 h of tests. Consequently, it is additionally possible to useN–S, M–S, and N–M solutions (see Table I) to increase thechance of obtaining reliable coordinates.

As an example, Fig. 4 shows the measurement results fromthe fourth session, during which the best results of the RTKmeasurement were obtained, including the only N–M–S solu-tion acquired in the whole experiment. The other reliable solu-tions (N–S and M–S) are also characterized by high precisionand centimeter accuracy. Other results of the RTK measure-ments from the 5-min sessions are no longer characterized bysuch precision of determinations, and gross errors also occurredin many cases of individual rover receivers. However, due tothe application of three GPS/GLONASS receivers, all grosserrors were easily eliminated. As a result, we managed to obtaincoordinates with high reliability and centimeter accuracy withN–S, M–S, and N–M solutions. They were only obtained in themeasurement sessions numbered 5, 10, and 18.

The standard deviation of the final RTK results was 0.017 mfor the northern component, 0.013 m for the eastern component,and 0.015 m for the height. The RMS error values, in relationto the true coordinates [16], using the RTK method were 0.018,0.024, and 0.037 m for the northern and eastern coordinatesand the height, respectively. The position error (RMS) of thefinal averaged N–M–S solution was only 0.002 m horizontallyand 0.037 m for the height measurement. The quite rigorousgeometric conditions imposed a priori produced very goodresults.

IV. RAPID STATIC PERFORMANCE IN

FOREST ENVIRONMENTS

Eighteen 5-min sessions were obtained for each receiverduring the rapid static survey, i.e., for the north, middle, andsouth receivers. All coordinates of these receivers were radiallycomputed relative to the KROL station using the GPS andGLONASS systems. The software Topcon Tools v. 6.11 [24]was used for baseline computations in postprocessing.

Fig. 5 shows a sketch of the FLOAT or FIXED baselinesobtained in individual sessions with marked points whose coor-dinates were determined. The local time of the performed mea-surements with an ordinal number was additionally providedfor each session. The final result is the coordinate (similar tothe RTK survey) obtained by (1). In most cases, it was possibleto obtain the coordinates of at least two receivers (M–S, N–M,N–S), which also allowed reliable coordinates of the middlepoint (M) to be determined.

The results considered reliable by the authors included atleast two baselines each from the reference station determinedas FIXED and two baselines between the N–M–S receivers.Therefore, for the measurement with just three rover GNSSreceivers to one reference station, the user can obtain as manyas 16 combinations of such solutions (see Fig. 6).

The GPS/GLONASS baselines between the reference stationand the rover receivers (N–M–S) were adjusted by the least-squares method in the following two variants.

1) Minimally constrained network adjustment (MCNA),which uses the minimum number of constraints to definethe coordinate system (i.e., by holding the referencestation KROL fixed in 3-D), with results presented inFigs. 7(a) and 8.

2) Constrained network adjustment (CNA) with fixed ref-erence station (KROL) and distance conditions, i.e., theconstant distances between the rover receivers and theconstant height of the whole survey rover unit, ob-tained by the following conditions: N−S = 1 m, N−M =M−S = 0.5 m, and HN−HS = HN−HM = HM−HS =0. The horizontal and vertical results of this adjustmentare presented in Figs. 7(b) and 8.

The new types of observations (the constant distances andheights of the rover receivers) directly refer to the parameters inthe least-squares adjustment due to the infinitely large weights[15]. Note that, in the second variant of the adjustment, we donot need the calculated GPS/GLONASS baselines between therover (N–M–S) receivers.

The computed coordinates and baselines, as well as theconstant distances between them and the heights of the re-ceivers, allow the obtained results to be verified. This provideseven more possibilities for accurate analyses than for the RTKsurvey. Out of all the sessions, only in one case (session 14)was it not possible to obtain any result because no baselinefrom the reference station to the rover receivers had beencorrectly computed. It should be noted here that the KROLstation recorded GPS/GLONASS observations with only a 1-sinterval, which is similar to all stations of the ASG-EUPOSsystem, although the RTK survey was possible with a 0.1-s


Fig. 5. Sketch of the baseline solutions in the rapid static mode during the tests.

Fig. 6. Diagram of possible baselines in the reliable rapid static approach.

interval. Hence, the determinations of the KROL-N, KROL-M,and KROL-S baselines were computable in a 1-s interval,whereas the N–M, M–S, and N–S baselines were computedusing observations with a 0.1-s interval. Nevertheless, due to

the assumptions described in the presented technology andredundant observations, we managed to obtain high-accuracycoordinates. The decrease in the positioning interval (to 0.1 s)resulted in a substantial number of FIXED baselines (as muchas 96% of the baselines between the N, M, and S receivers)for receivers located under tree canopies. The reduction of theobservation interval to 1 s (for receivers N, M, and S) onlycaused 43% of them to be obtained as FIXED. The observationscollected within the entire 1.5-h session did not allow allpossible baselines to be computed (see Fig. 5). The cases ofsessions 4 and 7 show that better results can be obtained with ashort observation period than with a long measurement. This isconnected with the problems in the ambiguity estimation withextensive and qualitatively heterogeneous observation materialrecorded under difficult survey conditions. Recently, a methodof precise positioning without explicit ambiguity resolutionhas been developed [8], although it also does not resolvethe validation problem. The ambiguity validation process isstill a challenge, particularly in GNSS positioning performed


Fig. 7. Horizontal distribution of reliable positions in the rapid static method performed in a forest area. (a) MCNA. (b) CNA.

Fig. 8. Vertical differences between the results obtained in the rapid static method and the true position. (a) MCNA solution. (b) CNA solution.

in severe observational conditions. Therefore, the method ofGNSS surveying and postprocessing is extremely important inorder to achieve reliable accuracy.

Table II compiles the lengths of all baselines computedas FIXED between the N–M–S receivers and the absolutevalues of deviation from the constant height (|dH|) betweenthe determined pairs of receivers. However, in analyzing thecontents of Table II, the appearance of many gross errors canbe observed, with decimeter and even meter values, despite aFIXED solution. The baselines with gross errors make up 33%of the baselines computed as FIXED (17 incorrect baselines),whereas for the 1-s observation interval, they only made up9% (2 incorrect baselines). However, with the increase in thepositioning frequency, the number of baselines with gross errorsrose while a substantial increase (by 14) in the number ofbaselines computed as FIXED and not biased with gross errorswas also recorded. Fig. 7(a) and (b) shows the horizontalscattering of reliable rapid static survey results based on theMCNA and CNA solutions. The final results are spread acrossan area of about 7.5 cm2; a little better solution was obtainedin the CNA approach. The height accuracy obtained by therapid static method in the two variants of the least-squaresadjustments is presented in Fig. 8. The reference value isthe coordinate obtained from repeated earlier measurements

using classical methods. The point on which the tests wereperformed is a test point, which is particularly designed forresearch on GNSS positioning under conditions with limitedsatellite availability. The results obtained in sessions 10 and 12were a radial measurement, which were only obtained basedon a single baseline from the reference station (see Fig. 5) andwere eliminated from the group of reliable solutions becauseof no sufficient control. For session 13, gross errors wereobserved for the FIXED baseline solution, and in session 14,no baselines from the reference station with a FIXED solutionwere obtained. It should be noted here again that the ASG-EUPOS system reference stations provide observations with a1-s interval at most, whereas the measurements were performedwith a 0.1-s interval. This is why the baselines between the M–Sand N–S receivers in session 14 were determined correctly, withthe number of GPS/GLONASS observations increased tenfold(see Table II).

The precision of reliable rapid static results in the MCNA/CNA approaches, which was expressed by standard deviation,was estimated at 0.007/0.006 m for the northern component,0.006/0.006 m for the eastern component, and 0.013/0.017m for the height. The RMS error values of the MCNA/CNAwere 0.008/0.007 m, 0.017/0.012 m, and 0.024/0.029 m for thenorthern component, the eastern component, and the height,


TABLE IIPRECISION OF THE FIXED BASELINES COMPUTED BETWEEN THE N, M, AND S RECEIVERS (ERRORS ARE UNDERLINED)

respectively. The consistency of the RMS errors with the valuesof standard deviations, for both the RTK and rapid static meth-ods, proves the full reliability of the presented GPS/GLONASSobservation measurement and processing technology.

V. QUALITY CONTROL OF GPS/GLONASS DATA

OBTAINED IN THE FOREST ENVIRONMENT

There are a number of different error sources affecting thecode and phase measurements used in the RTK and rapidstatic positioning. Most of these errors can be eliminated usingdifferential techniques because of their spatial correlation, e.g.,the effect of the ionosphere or troposphere layer. For high-accuracy field surveys, the main source of error (and difficultto eliminate) is multipath [30], [31]. The notion of multipathconsists of the recording of reflected signals by the GNSSreceiver besides the direct signal from the satellite. There aretwo possibilities of signal reflection. It can be reflected by anobstacle standing near the receiver (e.g., buildings and trees)or reflected off the ground and reach the receiver’s antenna[16]. Because of the movement of satellites along the orbitand the continually changing geometry of satellites, the levelof multipath impact varies in time and depends on the altitudeof a given satellite. The signals of low satellites are at morerisk of the multipath effect than those high in the zenith. Inmeasurements at the reference stations, the multipath effect isrepeated in 15–30-min cycles [23]. Hence, it can be minimizedfor long measurement sessions. In rapid static and kinematicGNSS surveys, the multipath effect is considered one of themain sources of errors [27] and poses the greatest challenge forsessions of several minutes [7]. Hoffmann-Wellenhof et al. [13]dealt in their research with the determination of the multipatheffect using several receivers.

The values of the multipath error for code measurementsat the L1 and L2 frequencies can be computed using thefollowing [10]:

MP1 ≡P1 −(1 +

2

α− 1

)L1 +

(2

α− 1

)L2

=M1 +B1 −(1 +

2

α− 1

)m1 +

(2

α− 1

)m2 (2)

MP2 ≡P2 −(

2α

α− 1

)L1 +

(2α

α− 1− 1

)L2

=M2 +B2 −(

2α

α− 1

)m1 +

(2α

α− 1− 1

)m2 (3)

where

B1 ≡ −(1 +

2

α− 1

)n1λ1 +

(2

α− 1

)n2λ2 (4)

B2 ≡ −(

2α

α− 1

)n1λ1 +

(2α

α− 1− 1

)n2λ2 (5)

α ≡ f21

f22

(6)

f1GPS: 1575.42 MHz;f2GPS: 1227.60 MHz;f1GLONASS: 1602.00 + k × 9/16 MHz, where k is the inte-

ger frequency number for GLONASS;f2GLONASS: 1246.00 + k × 7/16 MHz, where k is the inte-

ger frequency number for GLONASS;niλi: the integer wavelength phase ambiguity for fre-

quency i;Pi: pseudorange observable for frequency i;Li: phase observable for frequency i;Mi: the pseudorange multipath for frequency i;mi: the phase multipath for frequency i.


Fig. 9. Multipath values for the GPS satellites for the north, middle, and south receivers.

The multipath values for the GPS and GLONASS satellitesobserved during the experiment performed in the forest arepresented in Figs. 9 and 10. Analyzing the graphs (see Figs. 9and 10), it can be observed that despite the small distancesof the receivers from each other (0.5 m), the recorded mul-tipath impact is different at each of the north, middle, andsouth receivers. For the GPS system satellites, the multipathvalues change in short periods of time (under 1 min) andassume the values of ±10 m. Comparing the signals for indi-vidual satellites, it can be observed that the multipath effectvalues for the three rover receivers differ for the G02 andG31 satellites for the MP1 combination [see (2)] and for theG29 and G31 satellites for the MP2 combination [see (3)].The maximum amplitudes of the multipath values recordedat the three rover receivers for the same measurement epochexceed 20 m.

For the GLONASS satellites, practically all signals were alsobiased by the multipath impact. It is worth noting that, forindividual satellites in the studied case, the multipath valueschange with different frequencies. For the rising satellite R24,which was at the elevation angle of 23◦ at the start of themeasurement session and moved toward the zenith, the mul-tipath values changed by around a 1-min interval and werein the range of ±10 m. For the descending of satellites R13(elevation of 85◦ → 15◦) and R22 (elevation of 45◦ → 10◦), themultipath effect persisted for a long term (around 15 min) andexceeded the value of +10 m. Because of numerous obstruc-tions surrounding the studied point, observational conditionsare different at each of the receivers. This fact translates into themultipath values and the observation moment at which a mul-

tipath is recorded at an individual GNSS receiver. This is veryclear for the R13 and R22 satellites (see Fig. 10). The multipathimpact has direct influence on the baseline solution accuracyand reliability. Consequently, with such large obstructions, thepresented survey technology, because of redundant observa-tions and the geometric conditions of the N–M–S receivers,still enables the determination of coordinates with centimeteraccuracy.

VI. SUMMARY AND CONCLUSION

Broad observational material, i.e., over 160 000 measurementepochs, was collected from the GPS/GLONASS (10 Hz) mea-surements carried out in Kortowo Park. Raw GPS/GLONASSobservation data and coordinates obtained by the RTKmethod with a 0.1-s measurement interval were simultaneouslyrecorded during the experiment for three GNSS receivers. Theywere set on a special measurement beam designed for thispurpose. The GPS/GLONASS data were divided into 18 5-minrapid static sessions and corresponding RTK measurement(10 Hz) sessions and were then analyzed. The use of twonavigation systems for the RTK positioning resulted in theincreased reliability of the obtained solutions. Increasing thepositioning frequency caused an enhancement in the FIXEDdeterminations. The measurement under dense tree canopiescaused four (out of 18) reliable solutions to be obtained inthe RTK positioning during the experiment. This made upjust 22% of all measurement sessions. The RMS error for theRTK coordinate determination was 0.018 m for the northerncomponent, 0.024 m for the eastern component, and 0.037 m


Fig. 10. Multipath values for the GLONASS satellites for the north, middle, and south receivers.

for the height. The rapid static measurement performed muchbetter, with 78% of the sessions being reliable according to theadopted assumptions. The RMS error of the MCNA/CNA vari-ants for the determination of the northern component, the east-ern component, and the height were 0.008/0.007, 0.017/0.012,and 0.024/0.029 m, respectively. The presented measurementtechnology allowed all gross errors to be eliminated, and itconsiderably reduced systematic errors. The combination oftwo measuring techniques resulted in obtaining reliable coor-dinates with centimeter accuracy. Coordinates with centimeteraccuracy were obtained in 15 out of the 18 conducted 5-minsessions on a point located in a dense forest with completelyobstructed access to satellites. The presented technology canbe primarily used to measure geodetic control networks ordetermine the coordinates of control points for the calibrationof aerial photographs or satellite images in a forest or denselydeveloped areas, i.e., for measurements in places that are diffi-cult for satellites to access.

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Mieczysław Bakuła received the Ph.D. degree insatellite geodesy from the University of Warmia andMazury in Olsztyn, Olsztyn, Poland, in 1999.

He is an Associate Professor with the Depart-ment of Satellite Geodesy and Navigation, Univer-sity of Warmia and Mazury in Olsztyn. Since 1993,his research has been focused on global navigationsatellite system (GNSS) positioning and their ap-plications in surveying and navigation, GNSS dataprocessing, and network adjustments. Since 2011,he has been involved in precise navigation with the

Department of Air Navigation, Polish Air Force Academy, Deblin, Poland.

Paweł Przestrzelski is currently working toward thePh.D. degree in the Department of Satellite Geodesyand Navigation, University of Warmia and Mazuryin Olsztyn, Olsztyn, Poland.

His research interests include software develop-ment, global navigation satellite system (GNSS) po-sitioning in obstructed areas, and differential GNSSpositioning.

Rafał Kazmierczak is currently working toward thePh.D. degree in the Department of Satellite Geodesyand Navigation, University of Warmia and Mazuryin Olsztyn, Olsztyn, Poland.

His research interests include rapid static GlobalPositioning System/Global’naya NavigatsionnayaSputnikovaya Sistema (GPS/GLONASS) position-ing and the study of multipath influence in globalnavigation satellite system positioning.

Documents

[50] Reliable Technology of Centimeter GPSGLONASS Surveying in Forest Environments