Mapping the Underworld – GPS Based Positioning System

he research proposed here is part of a larger research project; investigating the development of techniques and systems able to locate and position underground pipes and cables. The research here will investigate various areas that will augment kinematic GPS, able to position with a precision of approximately 1cm. The augmentation will take place in two means, firstly physical augmentation using INS, HSGPS and pseudolites, and secondly simulating the possibilities of the near future (~ 5 years) when Galileo and future GPS is available.

Aim.
The overall objective of the sub-project is the research and development of a prototype positioning system to conduct precise 3D positioning at a centimetre level, using pseudolite transmitters, currently available commercial GNSS technology and GNSS simulators developed and available at the IESSG. A suite of algorithms and a software package for the data processing and simulation of GPS, Galileo and pseudolite measurements will be researched and developed. System validation will be conducted at Nottingham, as well as field trials with the remainder of the project teams.

Introduction.
The accuracy, availability and reliability of satellite based positioning are very dependent on the number of tracked satellites and their spatial geometry [Santerre, 1991]. One of the limiting factors in using GPS is the requirement of having direct line of sight with the satellites themselves and the GPS receiver. Most of the mapping of the “underworld” will be based in built up areas where line of sight to a sufficient number of satellites is not always possible. In addition, the presence of trees, as well as buildings, can cause masking issues, as well as introducing multipath errors.

This part of the project will research various means of improving the position availability, integrity and precision through a GPS based system, augments with other systems such as Galileo, INS and pseudolites. The reliable and accurate positioning of the overall system is an underpinning issue.

B.Scientific/Technological Relevance
Relevance to Beneficiaries
The relevance of this project feeds into the larger project, whereby the relevance to the beneficiaries are all based on the fact that this work will allow the locations of buried pipes and cables to be known. The beneficiaries include the general public, who will suffer less congestion and inconvenience, the companies involved in digging up streets who will be able to avoid the buried pipes and cables as well as being able to accurately locate the pipes or cables they need to dig for.

C.Dissemination and Exploitation
The partners through normal meetings and technical publications will disseminate the results of the research. In addition, the work will feed into the larger project, allow dissemination through this. It is also planned to disseminate the positioning aspect of the work through FIG, ICE and ICES meetings and publications, both nationally and internationally. In addition, it is planned to disseminate the results through civil engineering and surveying international journals and magazines.

D.The Proposed Work
Project Objective

There are two aspects to the mapping, that of the surface mounted system and that of the in-pipe based system. Both these will be based on precise RTK-GPS (Real Time Kinematic Global Positioning System) and INS (Inertial Navigation Systems) as well as introducing novel approaches such as utilising pseudolites and simulators to predict the advantages of using near-future GNSS (Global Navigation Satellite Systems) constellations.

Recently, real life applications of pseudolite transmitters have been researched in Australia, UK and Canada [Barnes et al, 2002, 2003; Dai et al., 2000; Dodson et al., 2003; Meng et al., 2002a, 2002b, 2003; Morley, 1997; Raquet et al., 1996; Roberts et al., 2003; Wang, 2002]. This technology is very much in its infancy, but potentially presents many answers to the issues concerning the use of GPS in built up areas.

The research into mapping of a surface based unit will focus on improving the availability of GNSS data through a number of ways. Firstly investigation into the use of High Sensitivity GPS (HSGPS) receives will see whether this is a feasible means of gathering data from satellites whose data is weak due to masking issues. Currently, these GPS receivers are used for pseudorange observables only, the carrier phase will be required for precise positioning. A second area of research is that of integrating a pseudolite into the observations. These are pseudo-satellites, or ground based satellites [Cobb, 1997; Integrinautics, 2002; Astrium, 2003]. They comprise of a data transmitter located upon a tripod over a known coordinate. Research into this field is very limited, with Nottingham being one of only a handful actively looking into this area [Meng et al, 2003]. The issue of multipath in such a built up area and in a kinematic application will also be addressed. Multipath occurs when the signal from the satellite reaches the receiver via more than one pathway through reflecting off of buildings etc [Dodson et al, 2001]. The integration of GPS with INS will be investigated as a means of providing 3D data even when the unit can not see any satellites. Finally, the in-house GNSS data simulator developed at Nottingham as well as a GNSS hardware simulator will be used to assess the potential use of future GNSS constellations [Roberts et al, 2003]. This includes Galileo as well as future GPS satellites which are planned to have a stronger signal than currently available.

The research into mapping of an in-pipe unit will focus on using an INS, and being able to provide 3D coordinates from this as accurately as required. INS drift will be the main limit in this, and various techniques will be used to address this. Such techniques will involve filtering the data, taking into account zero velocities and calibrating the INS with these, as well as using surface mounted locations, coordinated using GPS, and using transponder technology to co locate the underground INS. The Network RTK, to be installed around Nottingham through the SRIF money, will be used to validate the field trials, providing a “truth”.

No resources are required as the equipment recently purchased through a SRIF fund will be used. The only resource required is travelling to and from meetings and sites for field trials, as well as a PhD studentship and the employment of a post doctoral researcher.

Overall Deliverables; accurate, reliable 3D positions for the surface or underground based units.

The project was funded by the UK's Engineering and Physical Sciences Research Council. The research grant for the sub-project was worth 120,000 UK pounds.