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International Research Journal of Management Science & Technology ISSN 2250 – 1959(0nline) 2348 – 9367 (Print)

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IRJMST Vol 7 Issue 1 [Year 2016] ISSN 2250 – 1959 (0nline) 2348 – 9367 (Print)

International Research Journal of Management Science & Technology http://www.irjmst.com 9

Comparative Study of Online GPS Post Processing Services and Effects on DGPS Data

Processing

*Manas Kumar Jha, **Saurabh Singh, **Nisha Upadhyay, *** Nishant Khare *Manager (GIS), IL&FS Environmental Infrastructure & Services Ltd.

**M.Tech. (Geomatics), Indian Institute of Surveying and Mapping, Survey of India, Hyderabad, India

***M.Tech. (Spatial Information Technology), Devi Ahilya Vishwavidyalaya Indore, India

Abstract:

Increasing dependency on Differential Global

Positioning System (DGPS)

survey for

accurate surveying and mapping project is

common across the globe. Whether someone

require a point data or want to align the

LiDAR scans or point cloud data the precise

DGPS value is very much required. The

precision in the accuracy of the DGPS value

determines the overall accuracy of the project

output. To achieve a considerable accuracy

DGPS survey methodology and its post

processing technique must be defined

beforehand. DGPS data acquisition in RTK

(Real time kinematic) and PPK (post

processing kinematic) is common in survey

projects. The objective of processing of DGPS

data in both RTK mode and PPK mode is to

transform the absolute positioning of a point

(X, Y, Z) in a relative positioning with

reference to the other point of a known

coordinates in a similar reference system. This

paper will discuss in details the comparison of

DGPS processing output using the software

based post processing where control points get

validated using various online internet based

GPS processing services.

Keywords: Global Positioning System,

DGPS, RTK, Post Processing, GNSS,

Accuracy Assessment, Surveying, Post

Processing.

Introduction:

Traditionally in geodesy and surveying, the

measurements are collected over temporarily

or permanently monumental points and stored

for data-mission processing. In many cases,

the position of the user’s antenna has to be

determined in real time such as navigation or

mobile mapping. Real time determination of

position involves wireless transmission system

over radio frequencies, and L-band or C-band

geo-stationery satellites for coverage for entire

continents. Depending on the accuracy

requirements, two modes of real-time

operation can be used: DGPS or RTK.

DGPS [1] requires a reference station receiver

transmitting pseudo range corrections to the

users, whose receiver use this information

together with their measured pseudo-

ranges for positioning at the meter to few

centimeter accuracy levels. RTK [2] on the

other hand, is based on transmitting reference

station carrier phase data to the user’s

receivers.

In its simplest form, a DGPS reference

receiver is set up at a site with known

coordinates. After it has been configured to

operate as the `base station’, the reference

receiver tracks continuously all the visible

satellites and determines the corrections

necessary for its pseudo range data to be able

to compute the Single Point Positioning (SPP)

[4] result that is identical to the known

coordinates of the site.

There are two methods of GPS (Global

Positioning System) data processing. These

are Post-processing and Real-time Processing.

The Post-processing [3] method of

involves downloading of complete survey data

and processing in office using various

software such as Trimble Business Center,


International Research Journal of Management Science & Technology http://www.irjmst.com 0

GrafNet, etc.. In real-time processing the

correction data from base antenna is then

transmitted to the user (via some form of

wireless data link), receiver then apply the

corrections to their pseudo range data before

computing these SPP solutions.

The method of Post-processing of data is not

vulnerable to many limitations of real time

processing such as poor satellite visibility,

multi path and unreliable data link from the

reference station. Post processing even

provides a much better positional accuracy.

There are six popular post processing free

services are available. These services take

advantage of both the IGS Stations

Network and the IGS product range, and

works with data collected anywhere on Earth.

a) Online Positioning User Service

(OPUS) operated by United States

National Geodetic Survey (NGS)

b) Scripps Coordinate Update Tool

(SCOUT) operated by Scripps Orbit

and Permanent Array Center (SOPAC)

c) Australian Online GPS Processing

Service (AUSPOS) operated by

National Mapping Division of

GeoScience Australia

d) Auto Gipsy operated by NASA JPL

(Jet Propulsion Laboratory)

e) CSRS-Precise Point Positioning

(CSRS-PPP) operated by Canada

Geodetic Survey Division of Natural

Resource Canada (NRCan).

f) CenterPoint RTX post Processing by

Trimble

The free online GPS processing services are

solutions to obtain control quality coordinate.

By this one can get free of cost unlimited

access for online processing of GPS data. This

process is recognized and adopted widely in

GPS community.

The minimum number of hours for

observation to get the precise value from

online GPS processing system differs with

different agencies; however AUSPOS –

Australia suggest/recommends minimum 6

hours of observation to get precise

coordinates.

In this study a comparison between three

online processing facilities AUSPOS, OPUS

and Center Point RTX by Trimble is made

The major objective of the study is to know

the deviation in output of the rover data

processed using differentially corrected base

coordinate of various online GPS processing

Services.

Methodology:

In this study, DGPS system is used to acquire

the data. A GNSS antenna (Trimble TRMR6-

3) is placed at an unknown point as a base

receiver for more than 7 hours with a DOP

(all) of less than 4. The rover GNSS antenna

(Trimble TRMR6-3) is used to collect 15

minutes of observation. The baseline distance

from Base to rover was limited up to 500

meters.

Altogether 21 points using GNSS rover was

acquired in the open sky area.

The base data collected at an unknown point

cannot be used as a control quality in post-

processing of GPS data, so there is a need to

obtain appropriate control quality coordinate

values. Online GPS processing services are

freely available solutions to obtain a control

quality coordinate.


International Research Journal of Management Science & Technology http://www.irjmst.com Page 31

Figure 1. DOP value for GPS Satellite

Figure 2. DOP value for GLONASS Satellite

Base Data Processing:

Output base data were changed into the

RINEX (Receiver Independent Exchange

Format) [5] format and uploaded on the

following online GPS processing services for

post-processing;

a) CenterPoint RTX post Processing by

Trimble

b) Australian Online GPS Processing

Service (AUSPOS) operated by

National Mapping Division of

GeoScience Australia

c) Online Positioning User Service

(OPUS) operated by United States

National Geodetic Survey (NGS)

Trimble RTX gives processing result error in

terms of standard deviation (SD), the average

SD was 0.006 (m).

AUSPOS indicates a reliable solution in terms

of ambiguity resolution; in this study average

ambiguity resolution obtained for base data is

76.1%.



OPUS defines processing results in terms of

overall RMS which is 0.015 (m) with the base

data.

Received precise control quality coordinate

from above three online GPS processing

services were used with rover data in post

processing process.

Post Processing;

The Rover data was post processed with

reference to the raw base data using Trimble

Business Center (TBC) software. The post

processing process was also repeated three

times with the precise base coordinate

received by three different online GPS

processing methodology (AUSPOS, OPUS

and Trimble RTX).

Analysis:

An analysis was made for accuracy

assessment for different base coordinates

obtained from three different online

processing services with same rover

coordinates.

Observation with Base Coordinates:

1. Raw – unprocessed Base coordinate Vs

Precise Base Coordinate received from

Online GPS processing services.

The coordinate of the base receiver after data

acquisition was observed and noted using

Trimble Business center software. Again the

precise coordinate received from online GPS

services was then compared with the

coordinate of base receiver data. There was

variation in precise coordinate received from

online GPS processing services was observed

as shown below in Table 1.

Table 1

S.NO Precise Value for Base Station DX DY DZ

1 Raw Data - Trimble Trex 0.336 4.431 1.481

2 Raw Data - AUSPOS 0.3361 4.429 1.534

3 Raw Data - OPUS 0.3167 4.426 1.537

Figure 3. Deviation in X, Y, Z between raw base coordinate and Precise coordinate received

from Online Processing services.

2. Precise Base Coordinate received from Online GPS processing services, Trimble RTX

Vs AUSPOS Vs OPUS:

Precise coordinate received from three online GPS processing services was then compared,

Difference in position up to 2 cm was observed between AUSPOS and OPUS.



Table 2

S.NO Precise Value for Base Station DX DY DZ

1 Trimble Trex - AUSPOS 0.0000 -0.0028 0.0530

2 Trimble Trex - OPUS -0.019 -0.005524 0.056

3 AUSPOS - OPUS -0.0200 -0.0020 0.0030

Figure 4. Deviation in X, Y, Z between raw base coordinate and Precise coordinate received

from Online Processing services.

3. Observation with Rover Coordinates:

To compare and analyze the result of the rover points, all rover data were processed using

Trimble business center with three different precise coordinates received from online GPS

processing services. Results were then plotted as a deviation in X, Y, Z as mentioned below.

Figure 4. Deviation Plotted for X, Y, Z against Raw rover coordinates as Processed Coordinates

Figure 5. Deviation Plotted for X, Y, Z against Raw rover coordinates as Processed Coordinates



Table 3

S.NO Analysis criteria DX DY DZ

1

Raw Rover Data Vs Processed Rover data using Raw

Base Data 0.9359 1.3616 -1.6011

2

Raw Data Vs Processed rover data using Base

Coordinate from Trimble RTX 0.336048 4.431333 1.480762

3


Coordinate from AUSPOS 0.336095 4.428524 1.533762

4


Coordinate from OPUS 0.316667 4.426 1.536762

Results:

Received precise coordinate from the three

service provider Trimble RTX, AUSPOS and

OPUS was used as an input for base

coordinate and with reference to the same all

rover data was processed. During the analysis,

it was observed that precise coordinate

received from all three online GPS processing

services has resemblance in processing

parameter.

Deviation in output observed between Trimble

RTX, OPUS, and AUSPOS. There is a

constant shift of approx. 1cm was observed.

Between Trimble RTX and OPUS however,

deviation was up to 2 cm between AUSPOS

and OPUS.

Table 4

S.NO Mean Deviation DX DY DZ

1 Processed rover data using Base Coordinate from

Trimble Vs AUSPOS 0.0000 -0.0028 0.0530


Trimble Vs OPUS -0.019 -0.00552 0.056


AUSPOS Vs OPUS -0.0200 -0.0020 0.0030

Figure 6. Deviation Plotted for X, Y, Z against Processed data using precise coordinate from

Trimble RTX and AUSPOS and OPUS.

Conclusion:

To fasten the GPS survey with desired

accuracy online GPS processing services can

be considered as a boon for surveying

industries. To get the accurate coordinate user

dependency on the establishment of a base



station can be minimized if we process the

data using online services. It can be done

without having detailed knowledge of data

processing. The minimum observation time

with a receiver also gets reduced up to 15

minutes that can be post-processed using these

services. With this study it is evident of the

fact that any Online GPS processing services

can be used for fastening data processing at

the user end. However to know the difference

in observed value it is very much required for

use of services as per project and accuracy

requirements.

Reference:

1. Morgan-Owen, G.J.; Johnston, G.T.,

"Differential GPS positioning,"

in Electronics & Communication

Engineering Journal , vol.7, no.1, pp.11-

21, Feb 1995 doi: 10.1049/ecej:19950104

2. Manabu Omae, Naohisa Hashimoto,

Takehiko Fujioka, Hiroshi Shimizu, “The

Application Of Rtk-Gps And Steer-Bywire

Technology To The Automatic Driving Of

Vehicles And An Evaluation Of Driver

Behavior,” IATSS Research Vol.30 No.2,

2006

3. Francois Koenig and David Wong,

“Differential Global Positioning System

(DGPS) Operation and Post-Processing

Method for the Synchronous Impulse

Reconstruction (SIRE) Radar,” Army

Research Laboratory, ARL-TN-0281,

June 2007

4. Rock Santerre, Lin Pan, Changsheng

Cai, Jianjun Zhu, “Single Point Positioning

Using GPS, GLONASS and BeiDou

Satellites,” Published Online November 2014

in Scientific Research.,

http://www.scirp.org/journal/pos,

http://dx.doi.org/10.4236/pos.2014.54013

5. RINEX: The Receiver Independent

Exchange Format, Werner Gurtner,

Astronomical Institute University of Bern,

https://igscb.jpl.nasa.gov/igscb/data/format/r

inex300.pdf

Other Internet Sites:

IGS Stations Network: http://igs.org/network

OPUS: http://www.ngs.noaa.gov/OPUS/

SOPAC: http://sopac.ucsd.edu/scout.shtml

Auto Gipsy: http://apps.gdgps.net/

Trimble RTX: http://www.trimblertx.com/

AUSPOS: http://www.ga.gov.au/scientific-

topics/positioning-avigation/geodesy/auspos

CSRS-PPP: http://goo.gl/RQfwg5

http://www.scirp.org/journal/pos

http://dx.doi.org/10.4236/pos.2014.54013

http://igs.org/network

http://www.ngs.noaa.gov/OPUS/

http://sopac.ucsd.edu/scout.shtml

http://www.trimblertx.com/

http://www.ga.gov.au/scientific-topics/positioning-avigation/geodesy/auspos

http://www.ga.gov.au/scientific-topics/positioning-avigation/geodesy/auspos

http://goo.gl/RQfwg5

..

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