G. Sasibhushana Rao

GPS receiver SPS accuracy assessment using LS and LQ estimators for precise navigation

Global Positioning System (GPS) is the emerging technology extending its applications in the field of surveying, navigation, military etc., and these applications demands multiple levels of accuracy ranging from tens of meters to sub-millimetre level. Hence there is a need to develop systems, methods and a technique to meet the demand of high accuracy.GPS provides its services in two different standards, Standard Positioning Services (SPS) and Precise Positioning Services (PPS).The accuracy of these services is influenced by various measurement biases and random errors. Many of the biases and errors can be minimized by proper modelling of the biases, GPS technique employed and position estimate solution. This paper focuses in improving the accuracy of SPS GPS receiver by correcting relativistic and satellite clock errors in the C/A (Coarse/Acquisition) measurements. It also proposes an accurate position estimate method among Least Squares Estimator (LSE) and Linear Quadratic Estimator (LQE).The performance of these estimators are analysed with Position Accuracy Measures (PAM) and Statistical Error Measures (SAM). The practical data used for processing and analysis is collected on L1 (1575.42MHz) frequency over a period of 24 hrs from the SPS Dual Frequency GPS (DFGPS) receivers located at IISC, Bangalore (Lat/Lon: 13.01° N / 77.56° E) and Andhra University (AU), Visakhapatnam (Lat/Lon: 17.72°N / 83.32°E).

Convergence issues of taylor series method in determining unknown target location using hyperbolic multilateration

Modern Electronic Warfare systems need to provide real time location information in a consistent, unambiguous and timely manner. The time difference of arrival (TDOA) techniques used for location determination make use of the method of hyperbolic multilateration. One technique which is widely used for solving non linear hyperbolic equations is the Taylor Series method. This paper discusses the convergence issues the method faces and analyses the possible effect of variations in receiver distribution on coPnvergence of the solution.

Performance evaluation of standard positioning service GPS receiver with LS and LQ estimators over southern region of Indian sub continent

With the discontinuity of Selective Availability (SA) by U.S government, obtaining accuracies upto tens of meters by GPS system became simpler. Though the performance level reached to millimeter level accuracy, it is only restricted to military domains and is provided under Precise Positioning Service (PPS) standards. The two different GPS services include PPS and Standard Positioning Service (SPS). All the Civil applications of GPS fall under this SPS Standard that can provide upto 40 meters of accuracy under ideal sky conditions. This paper mainly focuses in improving the SPS GPS receiver performance in terms of accuracy and precision by modeling the C/A (Coarse/Acquisition) measurements for Satellite Clock biases and Relativistic error. It also concentrates in implementing two different Position estimators, Least Squares (LS) and Linear Quadratic (LQ) Estimators and there by analyzing their performance in terms of Position Accuracy Measures. The data collected on LI (1575.42MHz) frequency of the SPS Dual Frequency GPS (DFGPS) receiver located at IISC, Bangalore (Lat/Lon: 13.01o N / 77.56o E) and Andhra University (AU), Visakhapatnam (Lat/Lon: 17.72oN / 83.32oE) are used for performance evaluation.

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.

A Modified Variance Kalman Filter for GPS Applications

The advancements in technology have made global positioning system (GPS) part and parcel of human daily life. Apart from its domestic applications, GPS is used as a position determination system in the field of defence for guiding missiles, navigation of ships, landing aircrafts, etc. These systems require precise position estimate and is only possible with the reduced measurement uncertainty and efficient navigation solution. Due to its robustness to noisy measurements and exceptional performance in wide range of real-time applications, Kalman filter (KF) is used often in defence applications. In order to meet the increase in demands of defence systems for high precise estimates, the KF needs upgradation, and this paper proposes a new covariance update method for conventional Kalman filter that improves its performance accuracy. To evaluate the performance of this developed algorithm called modified variance Kalman filter (MVKF), real-time data collected from GPS receiver located at Andhra University College of Engineering (AUCE), Visakhapatnam (Lat/Lon: 17.72°N/83.32°E) is used. GPS statistical accuracy measures (SAM) such as distance root mean square (DRMS), circular error probability (CEP), and spherical error probability (SEP) are used for performance evaluation.

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.

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.