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Comparative evaluation and validation of single and dual frequency GPS observations
 R. Ghosh P. Jayaprasad, B. Narender, Anjum Mahtab and Ajai Land use Planning and Photogrammetry Division Forestry, Land use and Photogrammetry Group Remote Sensing Application area Sunanda P Trivedi, Y. P. Rana and P. K. Srivastava Satellite Photogrammetry and Digital Cartography Division Signal Image Processing Group Space Applications Centre (ISRO) Ahmedabad-380 015. Gujarat. ranendu@hotmail.com
Introduction Global positioning system (GPS) technology today have a solution of position accuracy on earth from < 1m to 100 m depending upon the type of receiver and mode of observation one uses. “The basic observable of GPS – at least those that permit us to determine position, velocity and time – are the pseudorange and the carrier phase. Combining the basic observables in various ways can generate additional observable that have certain advantage. But the accuracy of the measured observable is not totally error free. The error sources are due to ionosphere, satellite and receiver clock, multipath, dilution of precision, selective availability and anti-spoofing. Dual frequency receiver takes care of the error due to ionosphere to a large extent. Rest of the errors significantly reduces in differential mode of observation. In order to answer some of these hypothesis objectively, an experiment was planned to measure the co-ordinates of ten points in and around Ahmedabad using both single and dual frequency receivers. The scope of the experiment were set are :
The purpose of this exercise was to obtain coordinates of few points distributed in and around Ahmedabad, using single and dual frequency GPS receivers. A comparative study of the performance of single and dual frequency was also carried out. Validation of the coordinates obtained was done, by comparing baseline distances computed using GPS with that from actual ground measurements. The accuracy of measurements was verified using triangle error of closure. Approach for GPS Observations Ten points in the vicinity of 20 Km were selected for the GPS observation. Out of these points, one point was considered as reference point and rest all as rover points. One dual and one single frequency GPS receivers were placed at the reference point. Since both the receivers cannot be placed at the same position simultaneously, the single frequency receiver was placed at known fix distance and direction away from the reference point. The dual frequency receiver was placed with the choke ring antenna in order to take care of the multipath conditions. At all the rover points one after another, the GPS receivers were set and GPS observations were taken for one hour at each point. For each of the point the minimum elevation angle 10 degrees, observation recording rate 10 seconds and set data collection parameters - compacted were selected. In the exercise two types of GPS receivers were used, three single frequency and two dual frequency Leica receivers. To process the GPS data SKI processing software version 2.3.1 was used. Analysis of the Exercises Performed The accuracy assessment of the experiment was done by the evaluation of the performance of various ionosheric models on the GPS measurements and the comparative performances of single and dual frequency observations. Performance evaluation of various models on the accuracy of the dual frequency observations The various types of ionoshperic models used in this study were ‘No model, Standard, Computed, Global/ Regional and Klobuchar. For each of the model, baselines were computed between reference and rover points. The processed data file gives the standard deviation in geodetic coordinates (Table-1). Table-1 shows that no model and the global model gives the same standard deviation values. As per the SKI software, the computed ionospheric model can be selected if there are at least 45 minutes of observations at the reference receiver for each baseline otherwise for baseline below 20 Km, the standard ionospheric model is used. It comes out from the table that for one hour GPS observation, there is not much variation in any of the model values but in case of point no. 3 where three hours GPS observation data is available, the variation in the effect of the model is appearing. Concluding that the computed model is suitable for one hour observation data, all further exercises were performed with computed ionospheric model.
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