Indian Journal of Radio & Space Physics Vol 42, June 2013, pp 175-181 Validation of GPS receiver instrumental bias results for precise navigation Dhiraj Sunehra JNTUH College of Engineering, Jagtial, Karimnagar 505 501, AP, India E-mail: dhirajsunehra@yahoo.co.in Received 6 November 2012; revised 14 February 2013; accepted 18 February 2013 The positional accuracy of Global Positioning System (GPS) is affected by several errors, the most predominant error being the ionospheric delay. This delay is proportional to the total electron content (TEC). The dual frequency GPS observables can be used to estimate the TEC. The line-of-sight TEC estimated from dual frequency GPS data is corrupted by the instrumental biases of the GPS satellites and the receiver. The instrumental biases exist as the signals at the two GPS frequencies (f 1 =1575.42 MHz; f 2 =1227.60 MHz) experience different delays within the GPS satellite and receiver hardware. The estimation of the receiver instrumental bias plays a significant role in achieving required navigation accuracy for civil aviation applications. In this paper, receiver instrumental bias results due to a modified fitted receiver bias method, Kalman filter and singular value decomposition (SVD) algorithms are compared. Keywords: Global Positioning System (GPS) accuracy, Ionospheric delay, GPS receiver instrumental bias, Fitted receiver bias, Kalman filter, Singular value decomposition (SVD) algorithm PACS Nos: 92.60.Ls; 84.40.Ua; 91.10.Fc 1 Introduction Global Navigation Satellite System (GNSS) is a collective term for those navigation systems that provide the user with a three dimensional positioning solution by passive ranging using radio signals transmitted by orbiting satellites. The Global Positioning System (GPS) is the most well known out of all the constellations. The positional accuracy of GPS is affected by several errors such as ionospheric delay, tropospheric delay, satellite and receiver clock offsets, instrumental biases of the satellite and receiver, receiver measurement noise and multipath. The stand alone GPS does not meet the positional accuracy required for Category I Precision Approach and landing phase of an aircraft. To use GPS for all phases of the flight, satellite based augmentation systems (SBAS) have been planned by various countries including USA, Europe, Japan and India. The Indian SBAS is under development stage and is named as GPS Aided Geo Augmented Navigation (GAGAN). The GAGAN network consists of several dual frequency GPS receivers of NovAtel make, located at various airports around the Indian subcontinent 1 . The ionospheric delay, which is a function of the total electron content (TEC), is one of the main sources of error in GPS precise positioning and navigation. The dual frequency GPS receiver can be used to estimate the TEC, taking advantage of the dispersive nature of ionosphere. However, line-of- sight TEC derived from dual frequency GPS data is corrupted by the instrumental biases of the GPS satellites and the receiver. The instrumental biases occur due to the frequency dependent delays of analog hardware within the GPS satellite and receiver. The estimation of the receiver instrumental biases is particularly important in achieving the Category-I Precision Approach (CAT-I PA) requirements of civil aviation. Several approaches based on the least squares fitting, Kalman filter, Self Calibration of pseudoRange Error (SCORE) algorithm and neural networks are reported in literature for estimation of TEC and instrumental biases 2-8 . Most of these methods have been applied to data from mid latitude regions. Literature survey suggests that no significant work has been reported on the estimation of TEC and instrumental biases using Kalman filter technique in low latitude regions especially in India. 2 GPS data processing The dual frequency GPS data provide both code and carrier phase measurements on the two GPS