Abstract: This paper focuses on modeling the errors which normally degrade the accuracy of Global Poisoning System (GPS). The performance of the GPS is mainly affected by ionospheric errors. SiRF Star III single frequency receiver is used for collecting and projecting datum in World Geodetic System 1984 (WGS-84) co-ordinate form. To project the ellipsoidal model onto a map model, datum conversion from WGS-84 to Universal Transverse Mercator (UTM) form is needed. The conversion introduces errors in the datum. The variation in the errors can be observed from day to night, area to area and also due to the seasonal changes. The datum is collected from Ameerpet region of Hyderabad, which is a heavily populated area with heavy traffic and tall buildings. The variation in the datum has been observed from afternoon to evening.
Keywords: GPS, WGS-84, UTM, Ionospheric errors, Northings and Eastings
The space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth with sub-meter accuracy is called Global Positioning System (GPS) . It consists of a constellation of 28 satellites in six different orbits which give the information of the position of the user with sub meter accuracy . If there are four or more GPS satellites in unobstructed line of sight with the receiver, accurate spatial co-ordinates can be obtained . The datum obtained from the satellites, contain the information about the position and timing by calculating the the Keplerian orbit elements. The location information from GPS is based on the choice of coordinate system and datum . The coordinate systems that are mostly used in GPS are World Geodetic System 1984 (WGS-84) and Universal Transverse Mercator (UTM). The datum provides the location and should be less erroneous .
The different types of errors that affect the performance of GPS are external errors like Ionospheric errors, Atmospheric errors, Satellite and Clocking errors, Multipath errors, Selective Availability and Anti Spoofing, of which the major error is the Ionospheric error in the radio propagation signal . The conversion from the global coordinate system WGS-84 to the local UTM coordinates also introduces errors. The positioning accuracy tends to degrade as the baseline increases.
II. Ionospheric Errors
Ionosphere is the top most layer of the atmosphere which is composed of ionized particles and its characteristics indicate diurnal as well as seasonal variations. The change in characteristics of the layer may be the cause for the variation of the errors from day to night.
The Ionospheric errors in the GPS observations can significantly bias the computed user position. The ionosphere affects the electromagnetic waves that pass through it by inducing an additional transmission time delay, under worst case conditions, Ionospheric scintillations can rapidly vary amplitude and phase...