School of Surveying and Spatial Information Systems

The University of New South Wales


GPS-RTK on the UNSW Campus

by T. Farrow

Supervised by Assoc. Prof. C. Rizos

Edited by J. M. Rüeger

October 2002


Introduction

The School of Surveying and Spatial Information Systems has recently acquired four state-of-the-art Leica System 500 GPS receivers.  These are predominately used for undergraduate education, via a series of survey exercises on the UNSW campus.  The Leica System 500 receivers can also be used in RTK (“real-time kinematic”) mode, if the instruments are suitably configured and a radio link between the reference receiver and the 'rover' receiver is provided.  However, the radios are of comparatively low power (to conform to regulation), and their signals are prone to fade. The radios are of the UHF variety, which requires line-of-sight between the two receivers.  Maintaining the radio link from a 'reference' receiver to a 'rover' receiver is therefore a challenge, especially in environments with many buildings, as on the UNSW campus and in most cities.  Below is a figure illustrating the RTK technique.

 

Figure 1: Real Time Kinematic GPS (http://www.paccrst.com).

 

Aim  

This project was concerned with an investigation of the use of RTK for survey applications.  First, a literature search was conducted to identify articles and reports that describe guidelines for the use of GPS-RTK, any standards/specifications regarding its use, and critical issues that might need to be addressed when using this technique. Second, the radio reception on the UNSW campus was tested to determine the best locations for GPS-RTK field exercises.  An additional task was to prepare a set of field instructions to be used by the students when operating GPS-RTK.

 

Experimentation

The experimental design for the project was a step-by-step process. This involved getting 'hands-on' experience with the equipment, familiarisation with the UNSW campus and a working knowledge of the GPS-RTK technique.  An insight into how well certain areas of the campus receive the radio signal was also obtained.

The hands-on experience involved configuring the reference and rover receivers for real-time GPs surveying.  The possible field work locations on the campus were visited. These are: Gym Lawn, Village Green, Science Square, Physics Lawn, Naked Lady Lawn, Barker Street Parking Station, Main Walkway, International Square, Quadrangle Lawn, Civil Engineering Lawn, Commerce Courtyard, Library Lawn, Michael Birt Gardens, and Botany Street Parking Station. 

Figure 2: Field work sites on the UNSW Campus (base map courtesy of J. Cosentino 2002).

 

Once the different types of environments on the campus were evaluated, the sites that deemed to be satisfactory for fieldwork where used for the final experiments.  As sites were tested, the following information was recorded: date, time, name of site, number of satellites locked on to, number of satellites available, availability of radio link, and notes on maters of interest.  On sites, where GPS-RTK was possible, a feature (eg. a path on the Physics Lawn, as shown in Figure 3) was surveyed to simulate a typical survey task. 

Figure 3:  Detail survey using the RTK technique  (Farrow, 2002). The precision of the survey could be improved by using struts on the antenna rod (ed.).

 

Results

Surprisingly the field experiments showed that the main issue with RTK on the UNSW campus is not the radio link, but rather GPs issues such as satellite availability, high PDOP and GDOP, and loss-of-lock due to obstructions such as trees and buildings. A reference location on the Electrical Engineering Building (on top of the SE liftwell) provides radio coverage to all sites tested on campus.  RTK GPs surveys were, however, only possible on the International Square, Physics Lawn, Village Green and the Gym Lawn.

The campus control marks  Pavilion (B425) and Botany Street Car Park (B410), were also found to be suitable locations for a reference receiver. However, they  did not provide radio coverage to all sites tested on campus. The radio link from the Pavilion (B425) reference receiver was available at all sites except for the Michael Birt Gardens.  RTK was only possible at Village Green, Barker Street Car Park, International Square. The radio link from a reference receiver on the Botany Street Car Park (B410) was only operational on the Physics Lawn, Barker Street Car Park, Civil Engineering Lawn, Library Lawn, Commerce Courtyard, and the middle of the Main Walkway.  The only sites where RTK was possible were the Physics Lawn, Barker Street Car Park and the middle of the Main Walkway.

 

Further Information

For more information contact
Prof. C. Rizos (supervisor)

Email:     

C. Rizos@unsw.edu.au

 

Mail:     

School of Surveying and Spatial Information Systems

University of New South Wales

UNSW SYDNEY NSW 2052

Australia

 

Phone: +61-2-9385-4205

Fax: +61-2- 9313-7493

WWW: http://www.gmat.unsw.edu.au