Localization using LEO satellites

• Main Author: Xia, Yingyu
• Other Authors: School of Electrical and Electronic Engineering
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Wireless communication systems
• Online Access: http://hdl.handle.net/10356/61342
• Institution: Nanyang Technological University

Localization have been commonly performed utilizing signals from navigational satellites such as the Global Positioning System (GPS). However, in an urban environment this proves to be a challenge where GPS signals were known to deteriorate significantly [1] [2]. In an attempt to improve the localization reliability, this project explores the feasibility of using Low Earth Orbit (LEO) satellites for positioning services. In this dissertation, after giving an introduction to the project in chapter I. The different techniques of localization were discussed. In our project, passive localization was performed, as such no information regarding the exact time of signal transmission was available. In addition, the avoidance of complex array design narrows our choice down to using Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA) as our localization technique. Extraction of these variables of interest simultaneously was made possible by the computation of Complex Ambiguity Function (CAF), as such it was studied and a mathematical intuition behind it was also examined. In addition, with simulated signals it was discovered that addition of noise, shorter integration length as well as a smaller bandwidth deteriorates the performance of the CAF. Therefore, it is important to keep these variables in mind during the computation. Investigation of the existing literature in the computation of CAF was carried out. In our localization problem, the accuracy is highly dependent on the TDOA and FDOA estimates. It is therefore important to improve their precision. However, if one were to follow the traditional techniques in the calculation then it will only lead to high computation loads as well as out of memory scenarios. Indeed, it is important to find an efficient method and this formulates the problem for the project. Three main methods were tried out, their merits, their shortcomings discussed. The final method proved to be able to attain the resolution required as well as a shorter computation time in using Iridium satellite data. After the attainment of the required TDOA and FDOA measurements, one will be able to do positioning. In this section, the localization scenario was illustrated using Systems Tool Kit (STK). Mathematical formulation of the underlying relationship between the measurements as well as the receiver and satellite positions were also shown. The problem presents itself here as the difficulty in solving the set of nonlinear equations. In this project, the Taylor Series was then used to linearize the problem. The Root Mean Square Error (RMSE) was looked into as well as the effect of the initial estimate on the final localization result. In addition the CRLB was also derived. On top of that the comparison between only using TDOA estimate and when including the differential Doppler information was also made. Lastly, the conclusion was presented, where the findings were summarized and further improvements that could be made to this project identified.