Accuracy Evaluation Internet-Based GNSS for Kinematic Surveying the Case Study in Thailand

Transcription

1 Accuracy Evaluation Internet-Based GNSS for Kinematic Surveying the Case Study in Thailand Kritsada Anantakarn 1 1 Faculty of Engineering and Architectural : Uthenthawai campus. Rajamongala University of Technology Tawan-ok. ABSTRACT: This study evaluates the use of the Internet as the data transmission between the Internet Continuous Operation Reference Station (ICORS) and Global Navigation Satellite System (GNSS) rovers by using an Internet-based Real-Time Kinematic (RTK) system for GPS - GLONASS L1& L2 receivers in different vehicle speeds. An Internet-based RTK system based on the NTRIP provides a positioning coordinates that is essentially required for vehicle mobile mapping, automatic construction controller, aerial photogrammetry mapping and UAV land surveying. To evaluate accuracy of GNSS receiver positioning measurement, equipment and systems were integrated and tested in RTK survey mode with different velocities, and position accuracy will be compared with Total Station measurement technique by using measured polygon center coordinate, areas and perimeters. To maintain the same distance and route for comparing different positioning measurement of alternative vehicle velocities, a runway round of the sport stadium is selected for polygon measurement. The test results for all speed (10 55 km/h) show that horizontal error is less than 16 cm in comparing with polygon center, accuracy more than 98 % for area and 99% for polygon perimeter measurement. The system performance as the behavior of the GPRS wireless Internet connection to GNSS Virtual Reference Station (VRS) networks of Department of Land of Thailand in Bangkok. Keyword: GPS; GNSS; DGPS; Static; Kinematic; post-processing; NTRIP; Internet-based; Total Station; navigation; positioning; baseline; base-station; rover GNSS receiver.

2 1. INTRODUCTION Mobile mapping is the process of collecting geospatial data from a mobile vehicle, typically fitted with a range of photographic, radar, laser, LiDAR or any number of remote sensing systems. Such systems are composed of an integrated array of time synchronised navigation sensors and imaging sensors mounted on a mobile platform. The primary output from such systems include GIS data, digital maps, and georeferenced images and video. Navigation sensors or Global Positioning System (GPS) receivers have been used colleting accurate positions in real time at various travel speeds. This study is to evaluate the accuracy of typical Internet-based Real-Time Kinematic (RTK) GPS equipment by using GPRS internet connection in Bangkok. GNSS (Global Navigation Satellite System) is a satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' GPS and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). Europe's Galileo satellites have launched in 2011 and 2012 to reach full operational capacity. Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a network of ground stations. Satellite-based navigation systems use a version of triangulation to locate the user, through calculations involving information from a number of satellites. Each satellite transmits coded signals at precise intervals. The receiver converts signal information into position, velocity, and time estimates. Using this information, any receiver on or near the earth's surface can calculate the exact position of the transmitting satellite and the distance (from the transmission time delay) between it and the receiver. Coordinating current signal data from four or more satellites enables the receiver to determine its position. 1.1 Real Time Kinematics (RTK) 2

3 GPS Dual frequency system requires post-processing when operating in static or "fast static" mode. In Real Time Kinematics (RTK) GPS, the positional data are displayed and recorded immediately. Sub-centimeters to millimeter level accuracies (both horizontal and vertical) are obtainable with both single and dual frequency technologies (Rizos and Satirapod, 2011). To obtain latitude, longitude, and elevation for new point, both systems need to occupy a number of existing established points. Considerable skill, training, and expertise are required to operate either type of system effectively. RTK is now widely used for surveying and other precise positioning applications. The classical RTK technique requires that GPS data be transmitted from a single reference receiver to one or more roving units. Algorithms in the mobile unit combine the reference station data with measurements from the roving receiver to resolve the integer ambiguity required to calculate precise ranges from the GPS carrier phase measurements. The process of ambiguity resolution is often referred to as "initialization". RTK can provide centimeter position accuracy, but the accuracy and reliability of the standard RTK solution decreases with increasing distance from the reference station. This limitation on the distance between the roving GPS receiver and the RTK base station is due to the systematic effects of ephemeris, tropospheric and ionospheric errors (Wübbena, et al.,1996). These systematic errors result in reduced accuracy and increasing initialization time as the distance between base and rover increases. This phenomenon is becoming increasingly evident as we approach a maximum in the cycle of solar activity. 1.2 GNSS Network The concept of GNSS Network Reference Stations allows us to eliminate/reduce systematic errors in reference station data, i.e. allows to increase the distance to the reference station for RTK positioning while increasing the reliability of the system and reducing the initialization time (Raquet and Lachapelle, 2001). The GPS Network Reference Stations requires continuous modem line connections between the control center and all reference stations. Data is transmitted continuously to the center. The center will calculate and transmit optimized Radio Technical Commission for Maritime Services (RTCM) correction messages and transmits it to the users. Permanent GPS reference stations making it possible to receive RTK (Real Time Kinematics) correctional data everywhere. This means centimeter precise measurements in real time. By establishing an adequate 3

4 number of GPS stations and allocating data access to the reference stations via mobile phone using Global System for Mobile phones (GSM) data modems, they can offer correction data throughout the region of interest. All the GPS reference stations in a network send "on-line" raw GPS data via permanent connections to a super-computer housed in a secure Control Center. In this way all GPS observations can be gathered and weighted to the user's advantage. This solution gives the following advantages: - Uniform precision of the entire network, or in other words, no additional constants due to increased distance from the individual reference stations (a well-known problem in traditional GPS RTK surveying). - Single correction data from the entire network. - Safety and reliability to enhance the quality of GPS measurements. The Control Center of super computer takes care of the following numerous tasks: - Import of raw data and quality assurance routines - Storing of RINEX data - Correction of antenna phase center wandering - Modeling and estimation of systematic errors - Calculation of correction data in RTCM format for the users - Transmission of data to users in the field There exist many Networking approaches where GPS signals corrections can be send to mobile rover in the field for Real Time Positioning. 1.3 GPS base station in Thailand. The DPT GPS base stations consist of 11 stations distributed over Thailand as shown in Figure 1. The DPT GPS base stations project was established in 1996 with 4 stations implemented at the beginning. Presently a total of 11 stations are fully operational. The DPT GPS Base Stations were designed to serve as reference stations for both real-time and post-processed positioning applications. Each station is equipped with a dualfrequency GPS receiver (Leica GRX1200). 4

5 Figure 1: DPT GPS reference stations (source DPT website In 2005, the Royal Thai Survey Department (RTSD) also installed a new GNSS base station, called RTSD and designed for post-processed positioning applications. The Thai Meteorological Department (TMD) has established five CORS receivers as part of a tsunami and earthquake early warning system in Thailand and the installation was completed in late 2007 (Rizos and Satirapod, 2011) Subsequently, in early 2008, the Department of Lands (DOL) was the first organization in Thailand to provide a Real Time Kinematic (RTK) GNSS service using the Trimble VRS concept. Currently DOL s RTK network consists of 11 CORS receivers covering the Central Thailand as Figure 2. 5

6 Figure 2: DOL GPS reference stations (source DOL website Virtual Reference Station An RTK rover located near the center of several reference stations would be affected by systematic errors if using any one of these reference stations. If, however, measurements from all these reference stations are combined, a model of the geometric and atmospheric errors over the area can be determined and a VRS can be created adjacent to the rover's location, dramatically reducing the systematic errors. - Virtual Reference Station (VRS) Technique A pre-requisite of the virtual reference station (VRS) concept is the need of a duplex communication link between a node of the reference station network and the rover. The rover has to transmit its approximate coordinates to the network, which then interpolates from the state information a reference data stream VRS for the given position. The data relates to the observation space (Wübbena et. al, 1996). When the previous biases are estimated, the easiest and oldest way to broadcast differential corrections is the VRS. The idea is to create a synthetic correction generated as if the reference station is close to the rover as shown in Figure 3. For this reason, the rover communicates its approximate position (e.g. through an NMEA GGA message). Using this position, it is possible to interpolate data following different strategies, e.g. using: plane triangles (Vollath et al., 2000, Landau et al., 2002); 6

7 an Inverse Distance Weighting (IDW) estimation; least squares method for estimating polynomial coefficients; collocation (Raquet and Lachapelle, 2001). Figure 3: The VRS positioning concept This strategy can also be applied for post-processing positioning, by mean of a virtual data file (usually, in RINEX format). This file contains the observations that a virtual reference station may acquire in a wellknown position selected by the network user. 7

8 2. METHOD GNSS Static - Post Processing technique and GNSS RTK measurement in different speeds are the compare with measurement by Total Station as the general methodology as presented in Figure 4. GNSS Static Post Processing measurement GNSS RTK measurement in different speeds UTM Coordinate Transformation Total Station measurement Accuracy Assessment Polygon center coordinate Accuracy Assessment Polygon area Accuracy Assessment Polygon perimeter 2.1 Total station measurement Figure 4: General Method A total station is an electronic/optical instrument used in modern surveying. The total station is an electronic theodolite (transit) integrated with an electronic distance meter (EDM) to read slope distances from the instrument to a particular point as shown in Figure 5. Figure 5: Set-up for Total Station measurement 8

9 2.2 GNSS measurement Setting up RTK Base-station and Rover position Since the RTK base station's position is assumed to be a known point, its accuracy is very important. Having a clear view of the sky, from the top of a building allows the base-station to collect data from a maximum of visible satellites. This approach is highly recommended to perform a successful, accurate and fast survey. A static Internet-based RS and RTK experiment was conducted using Department of Land (DOL) CORS as shown in Figure 6. Figure 6: DOL - GNSS Reference Station (Source: DOL website) A data link was established from the base-station to the rover using wireless Internet. GPRS was used over the wireless link Post-processing Post-processing is used in Differential GNSS to obtain precise positions of unknown points by relating them to known points such as reference station. The GNSS measurements are usually stored in computer memory in the GNSS receivers, and are subsequently transferred to a computer running the GNSS post-processing software. The software computes baselines using simultaneous measurement data from two or more GNSS receivers. The baselines represent a three-dimensional line drawn between the two points occupied by each pair of GPS antennas. The post-processed measurements allow more precise positioning, because most GPS errors are corrected. 9

10 The dual frequency GNSS receiver uses the L1/L2 carrier phase is applied for Static Post Process data collection. The measurement is conducted in 1 second rate of recoding with duration more than one hour. GNSS Rover data GNSS Base data Septentrio Software Convert to RINEX Internet RINEX format files GNSS Solution Software for Post Processing Coordinate report Lat/Long - UTM converter Figure 7: static surveying and post-processing work flow. 10

11 The Post-Procession result is then used for coordinate transform from Total Station into UTM coordination for comparing with GNSS RTK coordinates. Figure 7 and Figure 8 presents the static surveying and postprocessing technique. Figure 8: static surveying and post-processing technique Real-time Kinematic (RTK) rover Dual frequency RTK receiver with GPS and GLONASS L1/L2 carrier phase was used for rover GPS receiver using GPRS connection to the Internet Server for receiving corrected and computing data from the base-station as illustrated in Figure 9. The RTK position is measure at the rate of 1 second automatically. 2.3 Study area Figure 9: GNSS RTK measurement in different speeds 11

12 The study area is located at Ratchmangla University Thanyaburi Campus, Bangkok Thailand as illustrated in Figure 10. Figure 10: Ratchmangla University Thanyaburi Campus, Bangkok Thailand. (GoogleEarth image) 3. RESULTS AND DISCUSSION 3.1 Measurement by Total Station There are 74 points are measured by Total Station as shown in Blue points in Figure 11. Figure 11: Measurement results from Total Station The polygon center coordinate of the polygon in UTM map projection is extracted as in Table 1. 12

13 Table1: Measurement results from Total Station Polygon Center coordinate North (m) East (m) Area (sqm) Perimeter (m) Measurement mode Total station R425VN , Measurement by GNSS Static for Post-Processing There are two points in the Stadium for static measurement in more than one hour of data collection. These two log points named TIA1 and TIA2 were corrected and adjusted by using DOL CORS data as BLAN, PKKT and BPLE as shown in Figure 12. Due to atmospheric problems, AYYA station has no data (Appendix 4) that makes the network not in a good triangle shape. Figure 12: Measurement by GNSS Static for Post-Processing The coordinate and horizontal accuracy at 1 cm of these two log points are shown in Table 2. 13

14 Table 2: The coordinate and accuracy of these two log points Name Coordinate Components Error Status TIA1 East Adjusted North Adjusted Ellips height Adjusted TIA2 East Adjusted North Adjusted Ellips height Adjusted Coordinates of these two points then used for coordinate transformation from 74 points measured by Total Station to UTM coordinate. 3.3 Measurement by GNSS RTK in different speeds In comparing with Total station measurement, center coordinates average accuracy of all speeds in North and East are 5.5 cm and 3.5 cm respectively as illustrate in Figure 13 and Table 3. Figure 13: Horizontal Error for center coordinates in compare with Total Station measurement 14

15 Table 3: Summary for RTK Dynamic Measurement Polygon Center coordinate North (m) East (m) Area (sqm) Perimeter (m) Error for North (m) Error for East (m) Error for Area (sqm) Error for Perimeter (m) Average , Standard deviation The highest accuracy for North at 1 mm and East at 6 mm for center coordinates of the polygon can be achieved at the speed of 50 km/h and 45 km /h respectively while the lowest accuracy falls to the speed of 20 km/h for North and 40 km/h for East coordinate as illustrated in Table 4. Table 4: Accuracy assessment for GNSS RTK in different speeds Polygon Center coordinate Error for North (m) Error for East (m) Error for Area (Sqm) Error for Perimeter (m) Measurement mode North (m) East (m) Area (sqm) Perimeter (m) Total station R425VN , RTK 10 km/h r , RTK 10 km/h r , RTK 15 km/h r , RTK 15 km/h r , RTK 20 km/h r , RTK 20 km/h r , RTK 25 km/h r , RTK 25 km/h r , RTK 30 km/h r , RTK 30 km/h r , RTK 35 km/h r , RTK 35 km/h r , RTK 40 km/h r , RTK 40 km/h r , RTK 45 km/h r , RTK 45 km/h r , RTK 50 km/h r , RTK 50 km/h r , RTK 55 km/h r , RTK 55 km/h r ,

16 At the speed of 10 km / h, the highest accuracy we can get in area measurement as 99.2% and perimeter measurement as 99.6%. However, the lowest accuracy for area measurement is 98.1% and perimeter measurement is 99.1% for the speed of 55 km/h. 4. CONCLUSION The travel speeds do not affect so much to the horizontal accuracy of GNSS RTK measurements as 5.5 cm when using VRS (DOL CORS) in Bangkok and its vicinity. Since we already get the Fixed RTK measurement status for the setting of every one second data recording, the faster the speed, the fewer measurements we can get for the same polygon. These sources of accuracy measurement for area measurements is 98.1 % and perimeter measurements 99.1% when the speed is increased up to 55 km/h. The results show that an Internet-based RTK system based on the NTRIP provides a positioning coordinates that is essentially required for vehicle mobile mapping, automatic construction controller, aerial photogrammetry mapping and UAV land surveying. 5. REFERENCES Landau H, Vollath U, Chen X (2002) Virtual reference station systems, Journal of Global Positioning Systems 1(2): Raquet J. and Lachapelle G. (2001) RTK positioning with multiple reference stations. GPS World, 12(4), 48-53,2001. Rizos C. and Satirapod S. (2011). Contribution of GNSS CORS Infrastructure to the mission of Modern Geodesy and Status of GNSS CORS in Thailand. Engineering Journal: Volume 15 Issue 1 ISSN Vollath U, Landau H, Chen X, Doucet K, Pagels C (2002) Network RTK versus single base RTK Understanding the error characteristics, Proceedings of ION GPS 2002, Portland, OR,

17 Wübbena, G., A. Bagge, G. Seeber, V. Böder, P. Hankemeier (1996). Reducing Distance Dependant Errors for Realtime Precise DGPS Applications by Establishing Reference Station Networks. Proceedings International Technical Meeting, ION GPS 96, Kansas City, Missouri,

18 APPENDIX 1: Totoal Station measurement

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20 APPENDIX 2: Static Post-Processing Report Land Survey Overview GNSS Solutions, Copyright (C) 2011 Ashtech. 1/22/2013 3:59:29 PM Project Name : Thanyaburi2556Jan13 Spatial Reference System : UTM/WGS 84/UTM zone 47N Time Zone : (GMT+07:00) Bangkok, Hanoi, Jakarta Linear Units : Meters Coordinate System Summary Coordinate system Name : UTM/WGS 84/UTM zone 47N Type : Projected Unit name : Meters Meters per unit : 1 Vertical datum : Ellipsoid Vertical unit : Meters Meters per unit : 1 Datum Name : WGS 84 Ellipsoid Name : WGS 84 Semi-major Axis : m Inverse Flattening : DX to WGS84 : m DY to WGS84 : m DY to WGS84 : m RX to WGS84 : " RY to WGS84 : " RZ to WGS84 : " ppm to WGS84 : Projection Projection Class : Transverse_Mercator latitude_of_origin 0ฐ 00' "N central_meridian 99ฐ 00' "E scale_factor false_easting m false_northing m Control Points 95% Name Components Error Status Control Error BLAN East FIXED North FIXED Ellips height FIXED BPLE East FIXED North FIXED Ellips height FIXED PKKT East FIXED North FIXED Ellips height FIXED 20

21 Logged Points 95% Name Components Error Status TIA1 East Adjusted North Adjusted Ellips height Adjusted TIA2 East Adjusted North Adjusted Ellips height Adjusted Files Name Start Time Sampling Epochs Size (Kb) Type PKKT019F.13o 13/01/19 12:00: L1/L2 GPS/GLONASS BPLE019F.13o 13/01/19 12:00: L1/L2 GPS/GLONASS BLAN019F.13o 13/01/19 12:00: L1/L2 GPS/GLONASS TIA1_ O 13/01/19 14:14: L1/L2 GPS/GLONASS TIA2_ O 13/01/19 12:51: L1/L2 GPS/GLONASS Occupations Site Start Time Time span Type File PKKT January :00: :59:55.00 Static PKKT019F.13o BPLE January :00: :59:55.00 Static BPLE019F.13o BLAN January :00: :59:55.00 Static BLAN019F.13o TIA1 January :14: :22:45.00 Static TIA1_ O TIA2 January :51: :25:15.00 Static TIA2_ O Processes Reference Reference File Rover Rover File Mode Num BPLE BPLE019F.13o BLAN BLAN019F.13o Static 1 BPLE BPLE019F.13o PKKT PKKT019F.13o Static 2 BPLE BPLE019F.13o TIA1 TIA1_ O Static 3 BPLE BPLE019F.13o TIA2 TIA2_ O Static 4 BLAN BLAN019F.13o PKKT PKKT019F.13o Static 5 BLAN BLAN019F.13o TIA1 TIA1_ O Static 6 BLAN BLAN019F.13o TIA2 TIA2_ O Static 7 PKKT PKKT019F.13o TIA1 TIA1_ O Static 8 PKKT PKKT019F.13o TIA2 TIA2_ O Static 9 TIA2 TIA2_ O TIA1 TIA1_ O Static 10 Processed vectors Vector 95% Vector 95% Vector Identifier Length Error Components Error SV PDOP QA Solution PKKT - TIA X No Fixed 13/01/19 12:51:50.00 Y :25:15.00 Z PKKT - TIA X No Fixed 13/01/19 14:14:10.00 Y :22:45.00 Z BPLE - TIA X No Fixed 13/01/19 12:51:50.00 Y :25:15.00 Z BPLE - TIA X No Fixed 21

22 13/01/19 14:14:10.00 Y :22:45.00 Z BPLE - PKKT X No Fixed 13/01/19 12:00:00.00 Y :59:55.00 Z BPLE - BLAN X No Fixed 13/01/19 12:00:00.00 Y :59:55.00 Z BLAN - TIA X No Fixed 13/01/19 12:51:50.00 Y :25:15.00 Z BLAN - TIA X No Fixed 13/01/19 14:14:10.00 Y :22:45.00 Z BLAN - PKKT X No Fixed 13/01/19 12:00:00.00 Y :59:55.00 Z TIA2 - TIA X Fixed 13/01/19 14:14:10.00 Y :02:55.00 Z Adjusted vectors Vector Length Vector Tau Vector Identifier Length Resid. Components Resid. Test QA PKKT - TIA X /01/19 12:51:50.00 Y Z PKKT - TIA X /01/19 14:14:10.00 Y Z BPLE - TIA X /01/19 12:51:50.00 Y Z BPLE - TIA X /01/19 14:14:10.00 Y Z BPLE - PKKT X /01/19 12:00:00.00 Y Z BPLE - BLAN X /01/19 12:00:00.00 Y Z BLAN - TIA X /01/19 12:51:50.00 Y Z BLAN - TIA X /01/19 14:14:10.00 Y Z BLAN - PKKT X /01/19 12:00:00.00 Y Z TIA2 - TIA X /01/19 14:14:10.00 Y Z

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24 APPENDIX 3: GNSS RTK measurement report Summary for RTK Dynamic Measurement Measurement mode Polygon Center coordinate North (m) East (m) Area (sqm) Perimeter (m) Error for North (m) Error for East (m) Error for Area (Sqm) Error for Perimeter (m) Total station R425VN , RTK 10 km/h round , RTK 10 km/h round , RTK 15 km/h round , RTK 15 km/h round , RTK 20 km/h round , RTK 20 km/h round , RTK 25 km/h round , RTK 25 km/h round , RTK 30 km/h round , RTK 30 km/h round , RTK 35 km/h round , RTK 35 km/h round , RTK 40 km/h round , RTK 40 km/h round , RTK 45 km/h round , RTK 45 km/h round , RTK 50 km/h round , RTK 50 km/h round , RTK 55 km/h round , RTK 55 km/h round ,

25 APPENDIX 4: DOL CORS lack of Satellites for Post Processing 25