The use of high-sensitivity GPS for mapping sub-surface utilities

Urban areas with its tall buildings and narrow corridors are extremely challenging environments to use a positioning system such as GPS which depends on an open sky view in order to have a line of sight to each satellite. However, there are many urban applications for example the positioning of buried assets such as pipes and cables which require the advantages that GPS positioninng brings.
There are over 4 million kilimetres of buried cables and pipes currently in the UK [1]. Many of these have not been accurately positioned thereby leading to increased disruptions through street works as trial pits have to be dug in order to locate the required utility. As a result, projects such as the DBERR-TSB (Dept for Business, Enterprise and Regulatory Reform - Technology Strategy Board, formerly the Department of Trade and Industry) funded VISTA and the EPSRC funded Mapping the Underworld projects are being undertaken to deal with the challenge of locating and visualizing both past, present and future buried utilities. The University of Nottingham as part of the VISTA and Mapping the Underworld projects has carried out some trials to investigate the use of High Sensitivity GPS (HSGPS) receivers to overcome the challenge posed by the urban environment on GPS surveying.
Several high sensitivity GPS tests were conducted around the University of Nottingham urban canyon test site, as well as around the IESSG building. The tests were performed in order to assess the quality of positioning obtainable from High Sensitivity GPS in different environments especially in areas with limited sky view such as urban canyons or close to buildings. Tests were also carried out to investigate the advantages if any it may offer when integrated with total station measurements using the Continuous Updating Technique (CUPT) designed as part of this project.
A Ublox Antaris 4 L1 receiver was used in these tests. It is a high sensitivity receiver able to track low power GPS signals. GPS signals in general are very weak by the time they reach the earth, in the order of -130dBm (-160dBW). High sensitivity GPS receivers are able to integrate the received signal for much longer than conventional receivers, thus enabling them to track signals at levels as low as 160dBm [2].
Short static (6 minutes) tests were carried out with the Ublox HSGPS on a pre-surveyed point connected to a Thales antenna mounted on a tripod. Tests were conducted in a fairly open environment and then a partially obstructed environment right next to a building. The data was post processed using Leica GeoOffice. The results from the fairly open environment showed a difference from the truth in east and north directions of 0.014m and 0.065m respectively. However, another similar static test in another fairly open environment occupied for 8 minutes gave deviations from the truth of 0.004m, 0.027m, -0.037m respectively. A float-only solution was obtained for the point located in an obstructed environment, close to the IESSG building. The difference from the truth was 0.051m, -0.079m and 0.156m in eastings, northings and heights respectively. Further tests were also carried out indoors with the Ublox.
Static data was collected inside the IESSG building for about an hour and 10 minutes, and then post-processed. The deviation from the truth was 25.749m in eastings, 20.398m in the northings and -6.981m in height. In kinematic tests conducted, the Ublox was able to recompute the integer ambiguity very quickly compared to a Thales L1 receiver after loss of phase solution. The various tests conducted showed that HSGPS was able to provide L1 fixed phase solutions in areas where standard GPS was only able to provide a code solution and was able to provide either a float or a code solutions where standard GPS was unable to function. However there were issues of reliability raised which was addressed by integration with Total Station data using CUPT.