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Provision of precise GPS control for the proposed international airport at Nagpur, India
 Prof. M. N. Kulkarni kulkarni@civil.iitb.ac.in  V. S. Tomar vstomar@civil.iitb.ac.in  Praveen Pillai praveen@civil.iitb.ac.in Department of Civil Engineering, IIT Bombay Powai, Mumbai
Introduction
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.
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