Prof. M. N. Kulkarni
V. S. Tomar
Department of Civil Engineering, IIT Bombay
The Global Positioning System (GPS) is the only navigation system, which has the capability to show the user his exact position on the Earth anytime, in any weather, anywhere. Due to its versatility, economy and reliability, and the rapid advancements in the field of GPS-aided aircraft navigation, most of the aircraft are now equipped with GPS navigations systems. As a follow-up, all the international airports in the world are now required to be surveyed with reference to the World Geodetic reference System: WGS84 international geodetic datum, that the GPS uses.
The GPS satellites, 24 in all, orbit at 11,000 nautical miles above the Earth. They are continuously monitored by the ground control stations, located worldwide. The satellites transmit signals that can be detected by any user equipped with a GPS receiver. Using the receiver, you can determine your location, and velocity, with great precision. GPS is one of history’s most exciting and revolutionary developments, and new applications of GPS are being discovered every day. The GPS system was originally developed to meet military needs of the Department of Defense of USA, but new ways to use its capabilities are continually being found. The system is being used for numerous applications, including surveying, geodesy, geodynamics, vehicle guidance systems, navigation in land-vehicles, aircraft and ships, etc.
In the case study presented here, GPS has been used to provide precise survey control for the proposed international airport at Nagpur. The work was taken up by the IIT Bombay GPS team for Geotech Consultants Pvt. Ltd.
Technical Details and Field Data Collection
The main task of this work consisted of providing the precise coordinates of the control points using GPS survey. The coordinates were required in WGS84 geodetic reference system, as well as UTM coordinate system. Mean SeaLevel (MSL) heights of all the control points were also required. The total area covered was approximately 5 km by 5 km, situated in the vicinity of the existing domestic airport near Nagpur city. The GPS control points were established in an approximate grid pattern. A total of 31 GPS control points have been provided, with one of them taken as a Base Station, continuously operating during the observation period. The field data was collected using three Trimble 4000SSI dual-frequency geodetic GPS receivers, with one kept fixed as a base station and other two used as roving stations. The base station was running continuously for approximately 48 hours. The minimum observation time on roving stations was kept 30 minutes. The logging interval was set as 15 seconds and elevation mask was kept 10 degrees, to avoid loss of data due to the presence of some tall structures in the area. In most of the cases, the GPS receiver could collect data simultaneously from more than 5 satellites, thus providing good quality data for processing. This triangular network methodology as shown in Fig. 2 was followed, till all the GPS points were covered.
Fig 1: Adjustment of the Traverse one
Data Processing and Results
The processing of the GPS data, thus collected, was done using Trimble GPS Data processing software GPSurvey, version 2.35. The small values of the standard deviation obtained as a result of this data processing, of the order of 5-7 cm, indicate the good precision of the results obtained. The UTM coordinates were obtained also using same GPSurvey software. The heights were converted from ellipsoidal heights, obtained from GPS, to MSL heights, using the latest World Gravity Model: EGM96, by National Imagery and Mapping Agency of USA. The base line lengths for the commonly observed points, as obtained from the software, were also provided. The Results are shown in Table 1 and the network map is shown in Fig. 2.
Checking of Integrated Adjustment of GPS & Total Station Traverses
The end-user carried out traverses using total station survey, based upon this GPS control. To check its accuracy, an integrated traverse adjustment was also done taking GPS points as a control points. The Geodetic as well as UTM coordinates were provided on WGS84 datum, with MSL heights.
Fig 2: Network Map of GPS control points
|GP01||25.619 N||32.494 E||306.4|
|GP02||54.491 N||57.315 E||306.2|
|GP03||47.976 N||36.464 E||302.1|
|GP04||48.177 N||16.022 E||303.9|
|GP05||15.504 N||48.136 E||298.5|
|GP06||41.882 N||32.281 E||305.1|
|GP07||06.772 N||19.306 E||299.5|
|GP08||01.786 N||51.994 E||301.0|
|GP09||32.912 N||08.135 E||308.3|
|GP10||34.934 N||43.631 E||312.2|
|GP11||10.594 N||56.705 E||309.8|
|GP12||57.596 N||15.354 E||310.2|
|GP13||22.025 N||58.987 E||312.0|
|GP14||34.816 N||28.369 E||311.6|
|GP15||36.941 N||00.794 E||305.6|
|GP16||49.165 N||27.363 E||305.4|
|GP17||21.783 N||33.454 E||315.5|
|GP18||07.670 N||03.180 E||308.5|
|GP19||14.279 N||22.434 E||315.3|
|GP20||40.974 N||00.366 E||317.9|
|GP21||51.648 N||50.650 E||313.9|
|GP22||07.074 N||23.305 E||313.4|
|GP23||14.775 N||23.304 E||309.7|
|GP24||39.471 N||03.389 E||310.7|
|GP25||08.174 N||45.242 E||314.6|
|GP26||28.572 N||11.396 E||322.1|
|GP27||18.057 N||04.498 E||319.8|
|GP28||46.517 N||53.340 E||316.1|
|GP29||33.825 N||31.281 E||303.0|
|GP30||57.219 N||44.194 E||304.6|
|BASE||44.354 N||19.829 E||311.7|
Methodology of Checking Work Carried Out
- The total airport area was surveyed by the end-user using three total station traverses.
- All the three traverses have been adjusted using standard software, which applies the Bowditch rule. For this, GPS points have been taken as fixed control points.
As an example, sample data from the adjustment of Traverse one is given in Table 2
|T1S1 – FIRST SEGMENT GP06 – GP28|
|STN. NAME||NORTHING (m)||EASTING (m)||MSL HEIGHT (m)|
Suggestions & Conclusions
The GPS control work described here is probably the first of its kind, for airport survey. Many valuable lessons were learnt from this work. Some of the conclusions and suggestions for future work are:
- The methodology adopted, of taking one base station and using the triangular network method, is found to be very much suited to this work.
- It would have been sufficient to use single frequency receivers for this type of work.
- The observation time adopted here, i.e. approximately 30 min. appears to be the optimum for such work.
- To carry out field checks to test accuracy of traverse stations from the GPS control points, about 4-5 consecutive traverse stations (not including GPS points) should be selected in each of the traverses, and adjustment of the total station traverse should be carried out. Using the given co-ordinates of the starting and closing stations as fixed points, co-ordinates of the 2 or 3 intermediate stations should be computed and compared with their co-ordinates provided. About 90% of the co-ordinates should agree within the specified accuracy limits, as per standard survey testing procedures.
- For checking the drawings prepared based on such control work, and the details in these drawings, co-ordinates/locations and heights of few well-defined points, as read from the drawing (softcopy or hardcopy) should be verified on ground. About 80% of the points thus checked should fall in the 1 meter accuracy level required for the 1:1000 scale drawing, as per standard survey & mapping testing procedures.