CODE coefficients based Single frequency Ionospheric Error Correction Model to improve GPS Positional Accuracy of the Low-latitude regions

Abstract

GPS (Global Positioning System) is a satellite based navigation system which provides reliable positioning information to its users (receivers) based on the concept of Time of Arrival (TOA) of the signal at the receiver. The positional accuracy of GPS is affected by several sources such as atmosphere, multipath and satellite-receiver geometry etc., of which ionosphere is one of the major sources of error for single frequency GPS user. When GPS signal propagates through the ionosphere, due to the change in refractive index in atmospheric layers, the signal bends which causes a delay in the arrival of the signal at the receiver. This delay of the signal introduces an error in ranging measurements at the receiver which in turn leads to an error in positioning information. Hence, to improve the positional accuracy, it is necessary estimate the delay of the signal in ionosphere during its transit from satellite to receiver. As this delay is caused by the electron content of the ionosphere, it is also necessary to estimate the Total Electron Content (TEC) in the signal path. The only way of estimating the ionospheric delay for single frequency receivers is by using Klobuchar algorithm which is based on the GPS broadcast ephemeris data. But this algorithm is optimized for mid latitudes and is very poor in high and low latitude regions. Hence it is required to have a model that is suitable for low latitude regions and near-equatorial regions such as Indian subcontinent. In this paper, low latitude suitable CODE coefficients based Klobuchar algorithm is implemented and is validated. The paper aims to investigate the performance of the CODE based Klobuchar algorithm by estimating the ionospheric delay using the CODE coefficients for data due to a low-latitude receiver data i.e. IGS station (HYDE: Lat/Long: 17°24'39N/78°33'4E) located in Hyderabad, Andhra Pradesh, India. This CODE based ionospheric delay is compared with delay due to Klobuchar model using the broadcast ephemeris data of the IGS receiver (HYDE). The model performance is validated with the ionospheric delay computed using the IONO-Lab data for the IGS station (HYDE). The CODE based algorithm is implemented, compared and validated for several days data of the IGS receiver (HYDE) and in this paper, it is presented for a typical day i.e. 11 th September 2014. The work yields the conclusion that Klobuchar model is simple due to short