Bharati Bidikar

Ionospheric Time Delay Estimation Algorithm for GPS Applications

The global positioning system (GPS) signal transit time delay in ionosphere comprised of ionized plasma is a major error source in GPS range measurements. As the density of the ionized plasma varies, the velocity of the radio waves differs from the velocity of light. Due to this, the GPS signals experience group delay or phase advance. Hence, the GPS signal transit time measurement is affected, and this time delay directly propagates into pseudorange measurements when scaled by the velocity of light. The delay depends on elevation angle of the satellite since the signal takes the longer propagation path when transmitted by the satellites tracked at lower elevation angle. The delay also depends on the solar activity conditions since the ionized plasma is a result of solar radiation. To achieve the precise navigation solution, the delay in ionosphere is estimated using conventional method where the total electron content (TEC) is modeled and pseudorange measurements of Link1 (L 1) and Link2 (L 2) frequencies are used. In this method, the TEC is an additional parameter to be calculated and the accurate range measurements determine the accuracy of the TEC. To overcome this, an eigenvector algorithm is proposed in this paper. The algorithm decomposes the pseudorange and carrier phase measurement coefficient matrix. The ionospheric time delay estimates of the proposed algorithm and conventional method are presented in this paper. The delays are estimated for the typical data collected on April 7, 2015, from dual frequency (DF) GPS receiver located in a typical geographic location over Bay of Bengal (Lat: 17.73° N/Long: 83.319° E). The proposed algorithm can be implemented for military and civil aircraft navigation and also in precise surveying applications.

GPS C/A Code Multipath Error Estimation for Surveying Applications in Urban Canyon

Global Positioning System (GPS) is satellite based navigation system implemented on the principle of trilateration, provides instantaneous 3D PVT (position, velocity and time) in the common reference system anywhere on or above the earth surface. But the positional accuracy of the GPS receiver is impaired by various errors which may be originating at the satellite, receiver or in the propagation path. These errors have assumed importance due to the high accuracy and precision requirements in number of applications like the static and kinematic surveying, altitude determination, CAT I aircrafts landing and missile guidance. In this paper, the error originating at the receiver due to multiple paths of the satellite transmitted radio frequency (RF) signal is estimated. Multipath phenomenon is prevalent particularly in urban canyons, which is the major error among other GPS error sources originating at the receiver. The algorithm proposed in this paper estimates the error using coarse/acquisition (C/A) code range, carrier phase range and Link1 (L1) and Link2 (L2) carrier frequencies. This algorithm avoids the complexity of the error estimation using conventional methods where sensitive parameters such as the geometry or the reflection coefficient of the nearby reflectors are considered. The error impact analysis presented in this paper will be useful in selecting the site for GPS receiving antenna where the reflection coefficients are hard to measure up to the required accuracy. Analysis of the change in intensity of this error with respect to elevation angle of the satellite will facilitate in selecting pseudoranges with least error. Error estimation and range modeling proposed in this paper will be a valuable aid in precise navigation, surveying and ground based geodetic studies.

A Novel Multipath Mitigation Technique for SPSGPS Receivers in Indian Urban Canyons

3D Position, Velocity, and Time (PVT) provided by GPS receiver is biased by various errors and the incorrect measurement of the GPS observables. Among all the possible errors multipath phenomenon is of major concern, particularly in urban canyons, as it depends on the environment around GPS receiver antenna. Considering its importance, a number of studies were conducted to analyze the multipath effects on GPS signal, but most of the research concluded in the requirement of new architecture of GPS receiving antenna to reject the multipath signals or change in reflecting environment geometry around the GPS receiving antenna. But these conventional methods may not mitigate the multipath error of SPS (Standard Positioning Service) GPS applications in urban canyon, where the geometry or the reflection coefficient of nearby reflectors cannot be determined accurately. In this paper, the multipath error mitigation technique is proposed which is based on the linear combinations of pseudorange and carrier phase observations. The analysis of the error given in this paper will facilitate in processing the observables of satellites with high elevation angle and least signal multipath. The pseudorange correction and the analysis of the residual range error presented in this paper will be useful in selecting the low-multipath location for GPS receiving antenna placement, navigation, surveying, and ground-based geodetic studies.

Signal emission time effect on orbital and navigation solutions for precise applications

Time is a fundamental part of position estimation and Global Positioning System (GPS) works on time measurements between known position and unknown position. So far most of the research was on modelling the propagation path delays, pseudorange correction, instrument bias errors and integer ambiguity resolution. But not much research is dedicated to model onboard clock correction and the impact of relativistic error (due to eccentricity) on signal emission time. These errors not only degrade the satellite position accuracy but also manifest as large pseudorange error when it is scaled by speed of light, which in turn degrade the receiver position accuracy. In this paper the signal emission time from satellite antenna phase center is modelled by considering the clock correction parameters, signal reception time at the receiver and relativistic error. The impact of satellite clock error and relativistic error on satellite position and receiver position are estimated and analysed. This precise timekeeping will improve the accuracy of the satellite position, which minimise the error in pseudorange and in turn receiver position accuracy is enhanced. This research work will be useful for GPS augmented systems like GAGAN(GPS Aided GEO Augmented Navigation) and WAAS(Wide Area Augmentation System).

A novel SET estimation algorithm for precise geosynchronous orbital solutions

Global Positioning System (GPS) accuracy, availability, reliability and integrity depends on the synchronisation of onboard atomic clock with GPS system time. The lack of syncronisation (satellite clock offset) impacts Signal Emission Time (SET) which not only degrade the orbital solutions but also manifest as large pseudorange error when it is scaled by speed of light, which results in inaccurate assessment of navigation solution. Another important parameter that impacts Signal Emission Time (SET) is the change in one of the Keplerian orbit elements i.e. eccentricity. Geosynchronous satellite orbits are elliptical in shape which causes the change in eccentricity. This results in change in satellite altitude which adds an error to SET. In this paper, the signal emission time from satellite antenna phase centre is modeled by considering the clock correction parameters, signal reception time at the receiver and eccentricity. The SET is estimated for a typical day (24 Hrs ephemerides data of year 2011), which is solar peak activity year. Though many researches were done on signal propagation path delays, pseudorange correction, instrument bias errors and integer ambiguity resolution, not much attention is paid on the satellite clock error and eccentric corrections impact on geosynchronous orbital and navigation solutions. In critical navigation applications like CAT I/II aircrafts landing and missile navigation the proposed SET algorithm can be implemented to achieve the required accuracy. In all these applications, higher positional accuracy is required compared to the existing 16.5 meters horizontal and 4.5 meters vertical accuracy. Hence for precise navigation applications accurate timekeeping is inevitable.