Performance of Research-Based N-RTK Positioning … · Performance of Research-Based N-RTK...

2010 International Symposium on GPS/GNSS

Taipei, Taiwan.

October 26-28, 2010.

1

Performance of Research-Based N-RTK Positioning System in ISKANDAR Malaysia

Shariff, N. S. M., Musa, T. A., Omar, K., Ses, S. and Abdullah, K. A.

UTM-GNSS & Geodynamics (G&G) Research Group,

Infocomm Research Alliance (IcRA),

Faculty of Geoinformation Science and Engineering,

Universiti Teknologi Malaysia (UTM), 81310 UTM Skudai, Johor,

Malaysia.

Phone: +607-5530830; Fax: +607-5566163

[email protected]

Abstract A research-based N-RTK positioning system namely

ISKANDARnet which is situated in equatorial region has

been established. The network corrections are generated to

improve users’ position accuracy. In order to testify the

performance of the network corrections and positioning

quality, tests are conducted at various locations within the

network coverage. Real-time test results indicate that

positioning accuracy is achievable in centimetre-level.

Key words Distance-dependent errors, Network corrections, N-RTK.

1. Introduction

Over the past few years, GPS networks of Continuously

Operating Reference Stations (CORS) have been

extensively deployed for range of purposes. Particularly

for high accuracy positioning over large area, the CORS

network is beneficially being used by the Network-based

Real-time Kinematic (N-RTK) positioning technique. This

technique utilizes carrier phase-based measurements from

CORS network and models the distance-dependent errors

such as ionospheric, tropospheric and orbital errors that

respect to rover. A direct result of modeling the distance-

dependent errors is the ability to improve the resolution of

ambiguities, which are important to form high-precision

RTK survey [1]. However, in area of severe atmospheric

activities especially in equatorial region, there is a

challenge to reliably model the distance-dependent error

[2].

A research-based N-RTK system namely

ISKANDARnet, which is located in equatorial area has

been developed by Universiti Teknologi Malaysia (UTM)

and collaboration with University of New South Wales

(UNSW). The establishment of ISKANDARnet has

provided a good platform to various studies such as

atmosphere, meteorology and positioning in equatorial

area.

This paper generally describes the ISKANDARnet

system in term of architecture and operational as well as

evaluate its performance on N-RTK corrections and

positioning results. The assessment of network corrections

are tested against without apply network corrections. Tests

are also carried out in various locations of ISKANDARnet

area to perform N-RTK positioning results. Significant

experience in N-RTK have yield to discuss on extension of

ISKANDARnet service, include network integrity and

reverse-RTK.

2. Architecture of ISKANDARnet

Presently, ISKANDARnet consist of three CORS (see

Fig.1); ISKANDARnet1 (ISK1) is situated at Faculty of

Geoinformation Science and Engineering, UTM;

ISKANDARnet2 (ISK2) at the Port of Tanjung Pelepas

(PTP), Gelang Patah; and ISKANDARnet3 (ISK3) at

Kolej Komuniti, Pasir Gudang that covers metro-area of

Iskandar Malaysia (http://www.iskandarmalaysia.com.my/).

Fig. 1: Distribution of ISKANDARnet Reference Stations.

(http://www.fksg.utm.my/ISKANDARnet/about.html).

The ISKANDARnet system comprises of three main

components which are the CORS, a control center and user

applications (see Fig.2). Beside to aim for N-RTK service,

ISKANDARnet also provides multi-functional services

such as single-based RTK, Differential GPS (DGPS) and

Receiver Independent Exchange Format (RINEX) data

download.

mailto:[email protected]

http://www.iskandarmalaysia.com.my/

http://www.fksg.utm.my/ISKANDARnet/about.html


Taipei, Taiwan.

October 26-28, 2010.

2

Fig. 2: Architecture of ISKANDARnet.

For ISKANDARnet implementation, the N-RTK

processing software generally has four crucial steps which

are master-to-reference station processing, generating

network corrections, corrections distribution, and user

processing. In master-to-reference station processing stage,

one of reference station (usually the nearest) will be

selected as a master station to resolve the network

ambiguities and generate the network residuals. The use of

fixed network residuals will ensure that high-quality

network corrections can be generated through the

interpolation process [2]. Biases for any user in network

area are interpolated based on approximate user location.

Next, network corrections can be distributed via Virtual

Reference Station (VRS) method [3], [4] and [5] to

facilitate user-side processing.

3. ISKANDARnet N-RTK Performance A few observation campaigns have been carried out from

24th

- 27th

May, 5th

- 6th

Jun, 10th and 12

th August 2010.

There are seven proposed user locations (see Fig.3). Table

1 shows the distance of each test point from reference

station ISK1, ISK2 and ISK3.

Fig. 3: Observation Campaign Area.

Table 1: Distance Test Points from Reference Stations. Points ISK1 ISK2 ISK3

PT1 10.9 km 24 km 21.8 km

PT2 5.8 km 21.5 km 27.3 km

PT3 14.8 km 29.6 km 16.8 km

PT4 25.1 km 35.8 km 7.0 km

PT5 17.7 km 24.9 km 17.9 km

PT6 19.2 km 5.3 km 37.6 km

PT7 12.4 km 11.5 km 34.6 km

In order to evaluate performance of ISKANDARnet

N-RTK system, three tests were carried out as in the

following sub-sections:

3.1. N-RTK vs Single-based RTK This test evaluates the performance of N-RTK (i.e. with

network corrections) relative to the single-based RTK (i.e.

without network corrections) approach. The RTKLIB-rtk

post processing software [6] has been used to process all

test points with the nearest reference station (see table 1).

Static observation was also carried out during the

campaign to obtain ‘known’ coordinates of all points.

Table 2: Position Errors - with (w) and without (w/o)

Network Corrections.

(a) PT1 dNorth

(cm)

dEast

(cm)

dUp

(cm)

Ave Std RMS Ave Std RMS Ave Std RMS

w/o 2.47 0.75 2.58 -2.28 0.44 2.32 -2.91 1.49 3.28

w 1.04 0.58 1.19 -2.09 0.61 2.18 1.52 1.56 2.17

% Improved 53.9 % Improved 6.0 % Improved 33.8

(b) PT2 dNorth

(cm)

dEast

(cm)

dUp

(cm)


w/o 1.63 0.77 1.81 -1.18 0.70 1.37 -2.54 1.67 3.04

w 0.72 1.24 1.44 -1.04 1.01 1.45 -1.01 4.17 4.29

% Improved 20.4 % Improved -5.8 % Improved -41.1

(c) PT3 dNorth

(cm)

dEast

(cm)

dUp

(cm)


w/o -0.60 1.33 1.46 0.69 0.77 1.64 -11.5 6.31 13.19

w -0.48 1.20 1.29 1.77 0.62 1.88 -2.63 3.72 4.55

% Improved 11.6 % Improved -14.6 % Improved 65.5

(d) PT4 dNorth

(cm)

dEast

(cm)

dUp

(cm)


w/o 2.32 1.22 2.62 1.68 1.08 1.99 -4.31 2.76 5.12

w 1.99 1.22 2.29 -1.02 1.08 1.38 -2.57 2.76 4.69


(e) PT5 dNorth

(cm)

dEast

(cm)

dUp

(cm)


w/o 7.32 0.75 7.36 8.02 0.28 8.03 12.1 0.07 12.1

w -2.01 0.39 2.05 -0.83 0.59 1.01 2.90 0.86 3.02



Taipei, Taiwan.

October 26-28, 2010.

3

(f) PT6 dNorth

(cm)

dEast

(cm)

dUp

(cm)


w/o 8.31 0.63 8.33 -4.08 0.09 4.08 2.49 1.50 2.91

w -1.61 0.93 1.85 -0.55 0.63 0.84 4.36 1.25 4.54

% Improved 77.8 % Improved 79.4 % Improved -56.0

(g) PT7 dNorth

(cm)

dEast

(cm)

dUp

(cm)


w/o -7.46 1.02 7.53 -2.01 0.91 2.21 -5.88 1.57 6.09

w 1.64 0.47 1.70 -0.53 0.83 0.98 -0.54 3.13 3.17


Tables 2(a) - 2(g) show numerical results of position

errors at each test point over its ‘known’ coordinates. It can

be noticed that the position errors of N-RTK approach

constantly provide centimetre level - in terms of average,

standard deviation and RMS values. These tables also

indicate percentage in RMS of N-RTK is much improved

over the single-based approach, except for dEast

component of PT2 and PT3, and dUp component of PT2

and PT6. The test point of PT5 shows significant

improvement up to 72.1%, 87.4% and 75% in dNorth,

dEast and dUp components, respectively. Since the

distance of PT5 is over than 17 km from ISK1 (see Table

1), this result obviously explains that the network

correction effectively reduces the effect of distance-

dependent errors compared to the single-based approach.

Figures 4(a) - 4(g) provide the time series of position

errors for both the N-RTK (green line) and single-based

RTK (blue line). All figures show discrepancies in position

errors of N-RTK in the dNorth, dEast and dUp components

are fairly consistent at the zero value. However, there are a

few drop-off in the position errors of the N-RTK approach

at certain points as shown in Figures 4(b), 4(d) and 4(f).

This could be the results of poor quality of the network

corrections. Further inspection has found that these points

(PT2, PT4 and PT6 – see Fig. 3) were located at the corner

of the ISKANDARnet triangle. This result support the

finding by [7] indicates that the performance of the

network corrections decrease as the N-RTK user is moved

far away from the center of the network.

0 200 400 600 800 1000 1200 1400 1600 1800-5

0

5Position Error at PT1

dN

ort

h [cm

]

0 200 400 600 800 1000 1200 1400 1600 1800-4

-2

0

dE

ast [c

m]

0 200 400 600 800 1000 1200 1400 1600 1800-10

0

10

Epochs

dU

p [cm

]

(a)

0 500 1000 1500 2000 2500 3000-20

0


dN

ort

h [cm

]

0 500 1000 1500 2000 2500 3000-10

0

10

dE

ast [c

m]

0 500 1000 1500 2000 2500 3000-20

0

20

Epochs

dU

p [cm

]

(b)

0 100 200 300 400 500-5

0


dN

ort

h [cm

]

0 100 200 300 400 500-5

0

5

dE

ast [c

m]

0 100 200 300 400 500

-20

0

20

Epochs

dU

p [cm

]

(c)

0 500 1000 1500-5

0


dN

ort

h [cm

]

0 500 1000 1500-5

0

5

dE

ast [c

m]

0 500 1000 1500

-10

0

10

Epochs

dU

p [cm

]

(d)


Taipei, Taiwan.

October 26-28, 2010.

4

0 20 40 60 80 100 120-10

0


dN

ort

h [cm

]

0 20 40 60 80 100 120-10

0

10

dE

ast [c

m]

0 20 40 60 80 100 1200

10

20

Epochs

dU

p [cm

]

(e)

0 500 1000 1500 2000

-10

0

10

Position Error at PT6

dN

ort

h [cm

]

0 500 1000 1500 2000-5

0

5

dE

ast [c

m]

0 500 1000 1500 2000

-5

0

5

Epochs

dU

p [cm

]

(f)

0 50 100 150 200 250 300 350 400-10

0


dN

ort

h [cm

]

0 50 100 150 200 250 300 350 400-5

0

5

dE

ast [c

m]

0 50 100 150 200 250 300 350 400-10

0

10

Epochs

dU

p [cm

]

(g)

Fig. 4: Position Errors in dNorth, dEast and dUp

Component of the N-RTK (green) and single-based RTK

(blue) at All Test Points.

3.2. Real-Time Test of ISKANDARnet The ISKANDARnet was also evaluated in real-time mode

by collecting 1000 epochs of N-RTK positioning at each test

point (see Fig.3). The N-RTK coordinates are compared

with the ‘known’ coordinate (as obtained from static

observation in section 3.1).

Fig. 5 – Fig.11 show scatter plot of position errors in

dNorth (dN), dEast (dE) and dUp. The blue dots indicate the

position errors that relative to the ‘known’ position (denoted

as red dot). From these figures, it can be seen that small

variations of position errors in horizontal component with

standard deviation achievable is less than 1.3 cm and up to

4.1 cm in vertical component. These figures also show that

the position error distributions are not exactly close to the

‘known’ point. Perhaps, it is due to ‘known’ position is

offset from the ‘true’ value. However, the accuracy achieved

is reasonable, according to average values are in the range of

centimeter level.

Fig. 5: Scatter Plot of Position Error for PT1.


-6

-4

-2

0

2

-2

0

2

4-2

0

2

4

6

8

10

d East (cm)

Position Error of PT1

d North (cm)

d U

p (

cm

)

-6

-4

-2

0

2

-2

0

2

4-10

-5

0

5

10

d East (cm)


d North (cm)

d U

p (

cm

)


Taipei, Taiwan.

October 26-28, 2010.

5






3.3 ISKANDARnet vs MyRTKnet Test of ISKANDARnet N-RTK performance was also

carried out by comparing with the commercially available

national N-RTK system, Malaysia Real-Time Kinematic

GNSS Network, known as MyRTKnet [8]. A total of 1000

epochs of fixed positioning solutions were recorded for both

N-RTK systems, thus average it to be compared. It is

assumed that there is no rapid atmospheric change since

there is only short delay of about 15 minutes between

ISKANDARnet and MyRTKnet observations.

Table 3 shows the coordinates average and standard

deviation from ISKANDARnet as compared to MyRTKnet

at PT1, PT2 and PT7. Overall, the average coordinates

from both N-RTK systems differ only in centimeter level

with maximum of 2.2 cm for the dUp component. The

results also shows small coordinates deviation between the

two network systems as indicated in standard deviation

values in Table 3. Hence, the ISKANDARnet positioning

results is reasonably compatible with the MyRTKnet.

Table 3: Position Average and Standard Deviation for

NRTK of ISKANDARnet Compared to MyRTKnet.

Average (cm) Standard Deviation (cm)

dNorth dEast dUp dNorth dEast dUp

PT1 -1.6 -0.2 2.0 0.2 0.0 0.7

PT2 0.3 -1.8 2.2 0.2 0.3 0.3

PT7 -1.3 -0.6 -0.5 1.2 1.8 3.8

4. Overview of Future Works

Uncertainty of N-RTK environment possibly provides

unreliable network corrections. It is becomes critical when

user is not noticeable of any malfunction that could affect

their position accuracy. For instance, section 3.1 has shown

that N-RTK has accuracy drop-off even though in fixed

positioning solution. Thus, to ameliorate potential data

quality concerns, the integrity monitoring is crucial to be

undertaken by giving a timely warning to users when large

position errors occur [9]. In future, ISKANDARnet is

inspired to adopt integrity monitoring system which detects

the status of reference stations, atmosphere conditions

within network coverage, and availability of real-time data

streams thus to inform users about the quality and fitness-

for-purpose of N-RTK positioning results.

-10

-5

0

5

-4

-2

0

20

2

4

6

8

10

d East (cm)


d North (cm)

d U

p (

cm

)

0

2

4

6

-2

0

2

4-10

-5

0

5

10

d East (cm)


d North (cm)

d U

p (

cm

)

-2-1

01

23

-1

0

1

2

3-10

-5

0

5

10

d East (cm)


d North (cm)

d U

p (

cm

)

-4

-2

0

2

4

-2

0

2

4

6-10

-5

0

5

10

d East (cm)


d North (cm)

d U

p (

cm

)

-20

24

68

-4

-2

0

2

4-10

-5

0

5

10

d East (cm)


d North (cm)

d U

p (

cm

)


Taipei, Taiwan.

October 26-28, 2010.

6

Such of quality indicator is not only potential for N-

RTK system, but become increasingly attractive for

advanced positioning method called reverse-RTK. The

reverse-RTK is a server-based approach, which service

provider can exercises control over the generated products

and place a true commercial value [10] by providing a

value-added product to the user. However, it can be

expected that the preparation towards implementing the

reverse RTK will involves with great data processing and

management as well as communication links.

5. Concluding Remarks

The performance of ISKANDARnet has been

demonstrated in this study. The observation campaigns

indicate that the network correction yields significant

improvements in position errors relative to single-based

approach due to the capability of N-RTK to model the

effects of distance-dependent errors. Furthermore,

reasonable accuracy of centimeter-level is achieved in real-

time test of the system. The positioning result of

ISKANDARnet is also found compatible within centimeter

level with the commercial MyRTKnet system. Currently,

implementation of network integrity monitoring for

ISKANDARnet is in progress. Further work will also

include a development of reverse-RTK system for

ISKANDARnet.

References

[1] Hu, G. R., Khoo, H. S., Goh, P. C. and Law, C. L.

(2003). Development and Assesment of GPS Virtual

Refernce Stations for RTK Positioning. Journal of

Geodesy. 77, 292-302.

[2] Musa, T. A., Lim, S., Yan, T. and Rizos, C. (2006).

Mitigation of Distance-Dependent Error for GPS

Network Positioning. International Global Navigation

Satellite Systems Society IGNSS Symposium, 17-21

July, Queensland, Australia.

[3] Landau, H., Vollath, U. and Chen, X., 2002. Virtual

Reference Station Systems, Journal of Global

Positioning Syste. Vol. 1(2),137-143.

[4] Wanninger L (2002) Virtual Reference Stations for

Centimeter-Level Kinematic Positioning. ION GPS,

September 24– 27, 2002, Portland, Oregon, pp 1400–

1407.

[5] Rizos, C. and Han, S. (2003). Reference Station

Network-based RTK Systems-Concepts and Progress.

Wuhan University Journal of Natural Sciences. 8 (2),

566-574.

[6] Takasu, T. (2010). RTKLIB ver. 2.4.0. Manual.

[7] Dao, D., Alves, P. and Lachapelle, G. (2004).

Performance Evaluation of Multiple Reference Station

GPS RTK for a Medium Scale Network. Journal of

Global Positioning Systems. 3 (1), 173-182.

[8] Jamil, H., Mohamed, A. and Chang, D. (2010). The

Malaysia Real-Time Kinematic GNSS Network

(MyRTKnet) in 2010 and Beyond. FIG Congress 2010,

11-16 April, Sydney, Australia.

[9] Chen, W., Hu, C., Ding, X., Chen, Y. and Kwok, S.

(2002). Critical Issues on GPS RTK Operation using

Hong Kong GPS Active Network. Journal of

Geospatial Engineering. 4(1), 31-40.

[10] Rizos, C. (2007). Alternatives to current GPS-RTK

Services and Some Implications for CORS

Infrastructure and Operations. GPS Solution 11: 151-

157.

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