Unconventional applications with single frequency DGPS

RAJNEESH GUPTA, VIVEK BANSAL, BRIG M C DHAMIJA (RETD)

CURRENTLY the trend is that more and more project authorities are mandating use of GPS for control work and private surveyors have to comply with this requirement. Dual frequency DGPS have been beyond the financial reach of these surveyors as such most prefer to invest in single frequency DGPS, which cost almost one third the cost of dual frequency DGPS.

Conventionally DGPS have been used so far for horizontal control. Recently however there have been a number of articles on using GPS for leveling. All the authors have used a dual frequency DGPS. These authors belong to national mapping agencies or academic institutions as such had access to leveling Bench Marks (BMs) of their countries. However, India being a very price sensitive market compounded with the fact, that private surveyors have no access to spirit level height BMs. Even if they work for govt. agencies, obtaining height data involves lot of hassles, security implications, takes unviable time frames and is quite expensive. Finally, when height data is made available, there is no certainty that the BM’s would be found intact on ground. Therefore, the general practice by private surveyors is to make do with alternatives such as commencing leveling from an arbitrary point with arbitrary height value assigned to it. To be competitive in pricing, hardly anybody does a closed loop, nevertheless the work is by and large acceptable by the consultants and project contractors for survey requirements relating to planning, design and checking as built of highways, pipelines, SEZ and so on.

As is well known, DGPS come in two modes i.e. Dual Frequency and Single Frequency. The generally specified accuracies for both are 5mm +1ppm for Horizontal and 10mm +2ppm for vertical in static post processing solutions albeit with a range limit of approx 15 km for single frequency DGPS and upward of 50 km for a dual frequency DGPS. The versatility of both considerably differs for various applications.

Single frequency DGPS can also be used in Static PP, Stop & GO Kinematic Post Processed (PP) and continuous Kinematic PP. Besides single frequency DGPS can also be used for GIS data acquisition with PP accuracy of submeter for Base lines up to 50 km.

Background

Many of our colleagues in the Institution of Surveyors (India) and private surveyors had been querying us to suggest methodology for utilizing single frequency DGPS for extended range to cover their projects involving large distances. This article is an endeavor to present some experiments with single frequency DGPS both for distance as well
elevation accuracies over large distances.

Pan India has been an accredited in house R&D agency by the Dept. of Scientific & Industrial Research, Ministry of Science & Technology, Govt. of India. Pan India in its ambit of R&D activity has undertaken a detailed study to examine some of the unconventional solutions for surveys using a single frequency DGPS in project spanning distances over 100-150 km.

However, before undertaking investigations we had a wide ranging discussion with various experts, it has been our experience that there are mainly 3 classes of users, the first is an organization such as Survey of India (SOI), who are very well versed with equipment as well as technology and surveying practices.

The second category of surveyors are those such as M/s Punj Lloyd / M/s Jaypee etc. who’s main job is construction related and surveying is a essential component mainly for acquiring Base data for design, subsequently for ground layout and finally for checking as built. The projects are related to Highways, Pipelines and layouts for townships or SEZ etc.

The third category of users are private surveying companies who generally work as sub-contractors for survey works for infrastructure companies and consultants undertaking preparation of DPRS designs etc.

It is for benefit of last 2 categories of surveyors that we had undertaken different

projects to evaluate the utility of single frequency DGPS beyond the range of12-15 km. These survey companies need establishment of control over a distance of 100 to 200 km for highways, L Section and Cross section, layout surveys for pipelines extending over distances in excess of 100 km, DEM/DTM for town planning & SEZ’s and soon.

The project

Four different types of projects were selected for the unconventional procedures undertaken for evaluation of utility of single frequency DGPS as follows:
1) Control work for highway alignment covering a distance of over 118 km in UP.
2) L section & Cross section survey for about 2 km.
3 ) Data acquisition for DEM for 20 Acres Part of SEZ.
4 ) As built survey of 130 km part of Oil pipe line.

The procedure

As already mentioned the Single Frequency DGPS has optimum range of to 12-15 km to achieve specified accuracy in post processing. In order to increase this range the obvious solution is to use Base and Rover in leap-frog mode i.e. first set up DGPS on Base and Rover on stations about 8-10 km apart. Next whilekeeping the Rover intact, remove Base GPS and put it on to the next station as Rover 8-10 km further away, in this way the DGPS unit acting as Rover earlier will now act as Base. The control points were thus extended in similar way by making leap frog point between 6 to 11 km apart for the complete route of about 120 km. During post processing, it was made mandatory that data for 1st Base & 1st Rover position was processed. In the second Base-Rover processing the Base (which was Rover in first observation) was assigned the same coordinates derived from earlier processing & so on. For automatic processing in similar way for all position the utility existing in the software though never used earlier, was utilized. With this utility, the processing is ordered in the desired sequencing of Base-Rover-Base with automatic assigning of processed co-ordinates which were obtained for the position as earlier Rover to that of Base in subsequent processing to follow the procedure as mentioned above. In a route spanning 120 km a total of 12 stations were established in this manner (see diag 1).

Results

Extension of Control Points

To check the over all accuracy of results, a single quadrilateral was observed over entire distance using Dual Frequency DGPS and processed by trilateration. The initial azimuth was taken as the one between Base & 1st Rover. The closing azimuth was checked with the last Base & Rover station. The averaged value of coordinates of 1st Base observed over a period of 2 hours was taken as starting co-ordinate in UTM zone 43 and WGS-84 Datum.

The results on comparison showed that the distance vector between end points, which were 107 km direct devised by single frequency GPS in Leap frog mode are in agreement with in 4 ppm. The azimuth is in agreement in less than one second. The ellipsoidal elevation by both single frequency DGPS at 1st Base and last Rover (approx 107 km direct) & (118 kmby leap-frog) were in agreement to with in single tertiary Class II specification as compared with Dual Frequency.

Since absolute MSL height of starting & end point was not available, EGM 96 was applied to obtain Orthometric Heights. Comparison between Δh and ΔH was done to study the variation in ΔN with respect to EGM-96 which showed that the value of N in this project area was varying between -0.269 to + 0.257 over a distance of 118 km in generally fl at terrain (table 1).

Subsequently a BM found near point 401 which is a canal BM and second near point 413 which is a railway BM. These are in different series and by different departments. However when the heights were observed at these points by DGPS and substituted for point 401 instead of value obtained in EGM 96 and with this height fixed, all other points ware datumed on to it.. The value of height of railway BM at point 413 as compared with the height inscribed on the railway BM was found to be in agreement in better than 47 cms in the leap-frog series (table-2).