*Corresponding author (Kritsada Anantakarn) Tel +66 2 2527029 Fax +66 2 2527580,k.anantakarn@gmail.com 2015. American Transactions on Engineering & AppliedSciences. Volume 4 No.3 ISSN 2229-1652 eISSN 2229-1660 Online Available at http://TUENGR.COM/ATEAS/V04/129.pdf.

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American Transactions on Engineering & Applied Sciences

http://TuEngr.com/ATEAS

Accuracy Evaluation on NTRIP-Based GNSS RTK with Varied Speeds

Kritsada Anantakarn a*

a Department of Civil Engineering Technology, Faculty of Engineering and Architectures, Rajamongala University of Technology Tawan-ok, Uthenthawai Campus, THAILAND

A R T I C L E I N F O A B S T R A C T Article history: Received January 23, 2015 Received in revised form March 24, 2015 Accepted March 30, 2015 Available online April 01, 2015 Keywords: DGPS; Static; GPS Kinematic; post-processing; Total Station; Baseline.

This study evaluates NTRIP real-time kinematic to learn accuracy of a rover at various speeds. The rover is a dual-frequency (L1 and L2) GNSS (GPS and GLONASS) receiver. The rover travels along the running track around the football field at an outdoor stadium, in Pathumtani province, Thailand. Each test with the same route, the rover is moving at constant speed, ranging from 10-55 km/hr. Based stations are permanent stations of the Virtual Reference Station (VRS) from Department of Land, Thailand. On the test date, there are mostly 11-13 satellites available during the test, with PDOP 1.5-2.0. For combined code and phase data, NTRIP RTK positioning accuracy was compared with Total Station measurement technique. The result shows that NTRIP RTK can possibly provide centimeter accuracy at all speeds.

2015 Am. Trans. Eng. Appl. Sci.

1. IntroductionGNSS RTK has been advertised to have manufacturer’s horizontal accuracy at, say, 1cm +

1ppm (Pentax, 2012). However, this is for the condition of very low moving horizontal speeds at 2 cm/s (0.072 km/hr) of kinematic stop-and-go operations. This tradeoff between high accuracy and low moving speed is not practical for some applications where speeding is needed.

For GPS carrier phase signal, it is more precise compared to pseudoranges. The phase of the received carrier is compared to the oscillated phase generated inside GPS receiver. Transmitted

2015 American Transactions on Engineering & Applied Sciences.

carrier has a nominally constant frequency, while the received carrier keeps changing in frequency due to the Doppler shift (Langley, 1998). Doppler shift can also determine whether satellites trajectories are toward and away from the receiver (Hightower, 2008). In fact, the relative motion of the GPS satellite and the receiver induced this Doppler shift (Langley, 1998). Dynamic speeds of moving GPS receiver/antenna pay a roll such that the more quickly the receiver moves in space the greater the error would be (Hightower, 2008). This work explored accuracy of NTRIP-based RTK at various constant rover speeds (ranging from 10-55hm/hr) using GPS and GLONASS both code and phase signals.

2. Literature Review GPS has been used in many civilian applications especially after selective availability (SA)

was turned off since May, 2000. The GPS C/A-code sends a bit every 0.98 microsecond, causing a receiver to accurate about 0.01microseconds (equivalent to 3m). With this SA-unlock, civilian C/A-code users have immediate access to accuracy better than 20 meters (95%) (Witchayangkoon, 2000). For RTK system, the most concerned difficulty is properly aligning the signals. Using carrier signals, even though it is possible to obtain centimeter-level accuracy in differential mode (Langley, 1998), but losing lock can possibly cause positioning error (Wikipedia, 2015).

RTK-GPS positioning was experimented by virtual reference station (VRS) system with GPS-based control stations in japan (Namie, 2001). The test showed accuracy of a few centimeters, possible good result for outside area. Hu (2003) assessed RTK positioning via GPS VRS. The field test demonstrated accuracy better than 4cm and 1-6cm, for horizontal and vertical dimensions. Chen et al. (2004) reported a test of an Internet RTK system via NTRIP protocol, that during the study time it found GSM data connection had a better performance compared to GRPS.

Weber et al. (2005a, 2005b) showed networked transport of RTCM via Internet Protocol (NTRIP). The study drew attention of IP-Streaming for Real-Time GNSS Applications via NTRIP and its potential for science and research as well as public and commercial services in many areas.

From studies done, it found no experiment show effect of varied speeds on accuracy on NTRIP-Based GNSS RTK.

3. Networked Transport of RTCM via Internet Protocol (NTRIP) Wireless Internet can be used for real time collection of continuous GNSS data streamed from

reference stations network. Then the differential GPS correction, as a derived product, is 130 Kritsada Anantakarn

broadcasted to authorized users. With a specific format of data dissemination, this technique is called Networked Transport RTCM via Internet Protocol (NTRIP) (Weber et al., 2005a). In fact, NTRIP uses the popular RTCM-104 streaming format.

The observed satellite data from each receiver is continuously sent to a NTRIP server, via the Internet. The data center will calculate and transmit optimized Radio Technical Commission for Maritime Services (RTCM) correction messages and transmits it to subscribed users by Networked Transport of RTCM via Internet Protocol (NTRIP).

4. VRS Network and NTRIP Virtual reference station (VRS) has been created with a concept of gaining high accuracy of

positioning, especially for real-time kinematic (RTK) mode. VRS of GNSS network architectures takes advantage of multiple reference stations. In VRS, Error from ionospheric and tropospheric refractions, and satellite orbit errors can be precisely modeled. The VRS network processing uses carrier phase data for computation. With known reference station coordinates, carrier phase ambiguities are resolved. Approximate coordinates of rover receiver are sent to VRS server (via GSM, GPRS, or 3G). In real time, the system then generates a virtual reference station at or near the initial rover coordinates. The generated VRS data is wireless transmitted to subscribe users via Internet Protocol (NTRIP). By this, users can get very accurate positioning coordinates. Conceptual detail of how VRS works is present by Leila (2011).

5. Experimental Equipment This study used permanent GNSS stations from Department of Land (DOL), Thailand (DOL,

2010), as base stations. There are total 11 stations in operation. These stations are dual-frequency GNSS receivers, continually observed GPS and GLONASS satellites in view.

The testing kinematic rover is a 136-channel dual-frequency PENTAX GNSS G3100-R1 receiver that can track both GPS (L1-C/A, L1-P(Y), L2-P(Y) and L2C) and GLONASS (C/A-code and carrier phase for both L1 and L2). Antenna is integrated with receiver, in one housing unit. The system has internal GSM modem for connection to (NTRIP) real time network.

A two-second-accuracy PENTAX Total Station (R-422VN model) is use for measurement to read slope distances from the instrument to a particular point. A reflector system is used containing one prism.

*Corresponding author (Kritsada Anantakarn) Tel +66 2 2527029 Fax +66 2 2527580, k.anantakarn@gmail.com 2015. American Transactions on Engineering & Applied Sciences. Volume 4 No.3 ISSN 2229-1652 eISSN 2229-1660 Online Available at http://TUENGR.COM/ATEAS/V04/129.pdf.

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6. Testing Procedure

6.1 Study area

The study area is located at the Queen Sirikit 60-year-old Memorial Stadium, No. 131 Moo 2 Tambon Thanyaburi, District Thanyaburi, Pathumthani Province, Bangkok, Thailand as illustrated in Figure 1 and Figure 3.

Figure 1: Football Field in District Thanyaburi, Pathumthani Province, Bangkok.

(Image courtesy of Google.com).

Figure 2: Evaluation Method

Field data collection and GPS – GLONASS data observation were conducted on 19 January 2013 (almost the end of tropical cool season), about 4pm local time. For the purpose of different driving speeds for the same route with unique distance and size, the standard stadium with 398.116-m oval athletics track (IAAF, 2008) was selected for the driving test. A Total Station was setup for measuring the size of the football field as well as perimeter and area. These measurements were in UTM coordinates with two reference points (Station 1 (STA1) and Station 2

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(STA2), see Figure 5). NTRIP-RTK measurements in different speeds were statistical analyzed and compared to the center coordinates, area and perimeter of the football field as illustrated in Figure 2.

6.2 GNSS Observation

GNSS observation was conducted in two steps: (1) static post-processing for identifying the UTM coordinates of two reference points and (2) NTRIP-RTK measurements by moving in different speeds by attaching receiver to the motorcycle traveling along the gutter grate of the football field.

Figure 3: Map of Bangkok, Thailand and DOL-GNSS Reference Stations located in Bangkok,

Thailand (Source of image on the right: DOL Thailand website).

6.3 GNSS Static Post-Processing GNSS receiver (PENTAX G3100-R1) uses the L1/L2 carrier phase that is applied for Static –

Post Process data collection. The measurement is conducted simultaneously for two referenced points – Station 1 (STA1) and Station 2 (STA2) – with 5-second epoch interval having observation longer than one hour. Observations in RINEX format were downloaded from DOL VRS webserver for the same observational time for GNSS base stations BLAN, PKKT and BPLE as illustrated in Figure 3. Unfortunately, the data from nearer stations (AYYA and OKRK stations) were not available due to server maintenance. The GNSS static processing results of STA1 and STA2 were then used for coordinates transform from Total Station coordination into UTM

*Corresponding author (Kritsada Anantakarn) Tel +66 2 2527029 Fax +66 2 2527580, k.anantakarn@gmail.com 2015. American Transactions on Engineering & Applied Sciences. Volume 4 No.3 ISSN 2229-1652 eISSN 2229-1660 Online Available at http://TUENGR.COM/ATEAS/V04/129.pdf.

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coordination for comparing with NTRIP-RTK measurements. Thus, adjusted coordinates of STA1 and STA2 are readily known, with horizontal accuracy 4 mm.

6.4 NTRIP-RTK measurements

GNSS receiver PENTAX model G3100-R1 with GPS and GLONASS L1/L2 carrier phase (Pentax, 2012) has been connected to Department of Land NTRIP server VRS system by using GPRS internet connection (GSM SIM card) as shown in Figure 4. The controller links with the GPS-GLONASS receiver for configuring RTK 1-second epoch interval automatically as shown in Figure 4. The system was integrated to a YAMAHA (FINO) motorcycle because it could run on markers of the stadium. In order to attain good results, RTK measurements have been collected during RTK fixed status. After VRS system connection and initial time, fixed RTK status was achieved for the driving test at fixed oval track with different speeds ranging from 10- 55 km/hr. In the same track, the stable speed was controlled by maintaining the vehicle gauging meter to controlled speed. The driving test experiment repeats twice at each constant speed.

Figure 4: GNSS RTK measurement in different speeds.

6.5 Total Station measurement

Total Station was set up at station 1 (STA1) located at the football field center. The reflector was set up at station 2 (STA2) located outside the football field as illustrated in Figure 5. The football field has dimensions met the guideline in Track and Field Facilities Manual of the International Association of Athletics Federations (IAAF, 2008).

Total 72 points were measured by the Total Station with 9 m interval for the straight lines and 4.5 m distance for the curve lines of the football field, as shown in Figure 5.

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Figure 5: Setup for Total Station measurement.

7. Results and Discussions Measurement by GNSS RTK in different speeds was compared with center coordinates, shape

area and perimeter. In comparing with Total station measurement, center coordinates average accuracy of all speeds in North and East are 16.0 cm and 9.0 cm respectively as illustrated in Figure 6.

Figure 6: RTK averaged horizontal error (compared with Total Station).

At the speed of 10 km/hr, the highest accuracy we can get in area measurement as 99.21% and

perimeter measurement as 99.57% as shown in Table 1. For measurement mode RTK 10 km/hr, the huge differences between numbers of epochs for round 1 and 2 come from unstable of VRS server happened during round2 experiment. However, the lowest accuracy for area measurement is 98.13% with perimeter 99.06% for the speed 55 km/hr.

*Corresponding author (Kritsada Anantakarn) Tel +66 2 2527029 Fax +66 2 2527580, k.anantakarn@gmail.com 2015. American Transactions on Engineering & Applied Sciences. Volume 4 No.3 ISSN 2229-1652 eISSN 2229-1660 Online Available at http://TUENGR.COM/ATEAS/V04/129.pdf.

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Table 1: Average accuracies of GNSS RTK testing with different speeds. Measurement mode #Epochs Accuracy for Area

(%) Accuracy for perimeter

(%) RTK 10 km/hr Round 1 115 99.21 99.57 RTK 10 km/hr Round 2 77 99.09 99.52 RTK 15 km/hr Round 1 59 98.92 99.41 RTK 15 km/hr Round 2 59 98.82 99.39 RTK 20 km/hr Round 1 53 98.73 99.35 RTK 20 km/hr Round 2 50 98.63 99.29 RTK 25 km/hr Round 1 45 98.56 99.23 RTK 25 km/hr Round 2 42 98.38 99.41 RTK 30 km/hr Round 1 39 98.32 99.12 RTK 30 km/hr Round 2 41 98.48 99.21 RTK 35 km/hr Round 1 39 98.42 99.16 RTK 35 km/hr Round 2 38 98.46 99.21 RTK 40 km/hr Round 1 36 98.38 99.17 RTK 40 km/hr Round 2 34 98.45 99.22 RTK 45 km/hr Round 1 34 98.46 99.18 RTK 45 km/hr Round 2 53 98.29 99.13 RTK 50 km/hr Round 1 31 98.32 99.14 RTK 50 km/hr Round 2 32 98.25 99.09 RTK 55 km/hr Round 1 29 98.13 99.06 RTK 55 km/hr Round 2 30 98.23 99.07

Figure 7: Number of satellites and Positional Dilution of precision (PDOP).

During the experiment, most of numbers of available satellites are 11 – 13, see Figure 7.

Positional Dilution of Precision (PDOP) values are about 1.4 -1.8 which is considered very good. Table 2 shows result of statistics analysis on correlation of satellite Dilution of precision (DOP), accuracy and travel speed. It is found that the travel speed has strong negative relationship with position accuracy Horizontal (HSDV) and Vertical Standard Deviation (VSDV) as the Pearson correlation values as -0.321 and -0.323 respectively.

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Table 2: Correlations HSDV (m) VSDV (m) Sat PDOP HDOP VDOP

Speed (km/hr)

HSDV (m)

Pearson Cor. 1.000 .999** .007 .166** .070* .190** -.321** Sig. (2-tailed) .000 .837 .000 .026 .000 .000 N 1000.000 1000 999 999 999 999 999

VSDV (m)

Pearson Cor. .999** 1.000 .012 .168** .067* .195** -.323** Sig. (2-tailed) .000 .715 .000 .034 .000 .000 N 1000 1000.000 999 999 999 999 999

Sat Pearson Cor. .007 .012 1.000 -.823** -.882** -.752** -.075* Sig. (2-tailed) .837 .715 .000 .000 .000 .018 N 999 999 999.000 999 999 999 999

PDOP Pearson Cor. .166** .168** -.823** 1.000 .875** .985** -.162** Sig. (2-tailed) .000 .000 .000 .000 .000 .000 N 999 999 999 999.000 999 999 999

HDOP Pearson Cor. .070* .067* -.882** .875** 1.000 .777** -.042 Sig. (2-tailed) .026 .034 .000 .000 .000 .188 N 999 999 999 999 999.000 999 999

VDOP Pearson Cor. .190** .195** -.752** .985** .777** 1.000 -.196** Sig. (2-tailed) .000 .000 .000 .000 .000 .000 N 999 999 999 999 999 999.000 999

Speed (km/h)

Pearson Cor. -.321** -.323** -.075* -.162** -.042 -.196** 1.000 Sig. (2-tailed) .000 .000 .018 .000 .188 .000 N 999 999 999 999 999 999 999.000

**. Correlation is significant at the 0.01 level (2-tailed). *. Correlation is significant at the 0.05 level (2-tailed).

8. Conclusion This study evaluate accuracy of NTRIP real-time kinematic at various specific speeds of

dual-frequency (L1 and L2) GNSS (GPS and GLONASS) rover. The rover was attached to the motorbike which was set to travels along the running track around the football field. The motorbike (rover) was moving at constant speed for each test, ranging from 10-55 km/hr. Based stations are permanent stations of the Virtual Reference Station (VRS) from Department of Land, Thailand. With combined code and phase data, NTRIP RTK positioning accuracy was compared with Total Station measurement technique. The result shows that NTRIP RTK can possibly provide accuracy at 10cm or better. The NTRIP-Based GNSS RTK is thus very suitable for applications such as vehicle mobile mapping, machine control and automatic construction controller.

9. References Chen, R., Li, X., & Weber, G. (2004). Test results of an Internet RTK system based on the NTRIP

protocol. The European GNSS. ftp://77.234.201.131/Support/Navis/RTCM SC-104/Chen_GNSS2004.pdf Assessed November 2013.

*Corresponding author (Kritsada Anantakarn) Tel +66 2 2527029 Fax +66 2 2527580, k.anantakarn@gmail.com 2015. American Transactions on Engineering & Applied Sciences. Volume 4 No.3 ISSN 2229-1652 eISSN 2229-1660 Online Available at http://TUENGR.COM/ATEAS/V04/129.pdf.

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DOL. (2010). Department of Land referenced stations. DOL Publishing Web. http://110.164.49.162. Assessed November 2013.

Hightower, P. (2008). Motion Effects on GPS Receiver Time Accuracy. Instrumentation Technology Systems. http://www.itsamerica.com/Technical_Descriptions/Dynamics Modes and Motion Effects on the GPS recei.pdf

Hu, G. R., Khoo, H. S., Goh, P. C., & Law, C. L. (2003). Development and assessment of GPS virtual reference stations for RTK positioning. Journal of Geodesy, 77(5-6), 292-302.

IAAF. (2008). Track and Field Facilities Manual. International Association of Athletics Federations. Editions EGC

Langley, R. B. (1998). RTK GPS. GPS World, 9(9), 70-76. http://www2.unb.ca/gge/Resources/gpsworld.september98.pdf Accessed October 2014.

Leila, K. (2011). What is a virtual reference station and how does it work. Inside GNSS magazine 6:28-31.

Namie, H., Hagiwara, N., Kim, H., Nitta, N., Shibahara, Y., Imakiire, T., & Yasuda, A. (2001, January). RTK-GPS positioning in Japan by virtual reference station (VRS) system with GPS-based control stations. In Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) (pp. 353-361).

Pentax. (2012). PENTAX G3100-R1. Pentax Publishing Web. http://pdf.directindustry.com/pdf/pentax-precision/g3100r1/22660-337935.html. Accessed August 2014

Weber G, Dettmering D and Gebhard H. (2005a) Networked transport of RTCM via internet protocol (NTRIP). In: A Window on the Future of Geodesy. Springer-Verlag, Berlin Heidelberg, pp 60-64.

Weber, G., Dettmering, D., Gebhard, H., & Kalafus, R. (2005b). Networked transport of RTCM via Internet Protocol (Ntrip)-IP-streaming for real-time GNSS applications. In ION GNSS 18th International Technical Meeting of the Satellite Division, September (pp. 13-16). http://www.beidoudb.com:88/document/uploads/6db61471-e394-4573-9208-14d26593c1ba.pdf Accessed November 2014.

Wikipedia. (2015). Real Time Kinematic. http://en.wikipedia.org/wiki/Real_Time_Kinematic Accessed January 2015.

Witchayangkoon, B. (2000). Elements of GPS Precise Point Positioning. PhD Thesis, University of Maine, USA. DOI: 10.13140/RG.2.1.3282.6402

Kritsada Anantakarn is Lecturer in the Department of Civil Engineering Technology, Faculty of Engineering and Architectures, Rajamongala University of Technology Tawan-ok, Uthenthawai Campus, THAILAND. He earned his Bachelor of Engineering (Civil Engineering) from Faculty of Engineering Rajamangala Institute of Engineering, and a Master’s degree in of Urban and Environmental Planning from King Mongkut's Institute of Technology Ladkrabang. He is interested in positioning with GPS/GNSS technology.

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