Sunday, September 26, 2021

Introduction GPS and LAAS

1.GPS and LAAS Introduction

The Global position System (GPS) is used for wide variety of navigation system. The aircraft navigation, which is part of navigation system, uses GPS for wide variety of operations.

These include flight over oceanic routes, enroute and terminal areas. The final approach of an aircraft requires greater safety and reliability. The final approach includes precision and non precision approach. To serve precision approach, the GPS must be augmented by LAAS or WAAS (Enge, 1999). In WAAS/LAAS, the WAAS is in operation since 2003 (Rife, et al., 2008). The WAAS/LAAS collects measurement across reference stations. WAAS transmits differential vector correction to the user, where as LAAS transmits differential scalar correction. The LAAS is developed to be primary radio navigation system for Category I, II and III precision approach (Pervan and Gratton, 2005). The most important and challenging job when using LAAS is guaranteeing the integrity while user is in operation (Walter et. al., 1997). In LAAS integrity parameter is broadcast by LGF. One of the important integrity parameter send by LAAS is sigma ground also known as ground reference receiver’s pseudo-range error correction parameter 

1.2 Scope of the LAAS

It is aimed to monitor the integrity in LAAS. The monitoring of integrity is done by estimating vertical protection levels (VPL) and horizontal protection levels (HPL). In this thesis VPL is considered because vertical errors are more sensitive than horizontal errors.

1.3 LAAS Technical approach

In order to simulate LAAS there should be a minimum of two reference receiver. In this thesis Dual Station 1 and Dual Station 2 stations are considered as reference receivers and its RINEX file is extracted for processing the data. These stations are responsible for monitoring of integrity by sending integrity parameters to the user. The user estimates the protection levels by processing the integrity parameters sent by LGF.

1.4 LAAS Literature survey

It is essential to understand the architecture of GPS, its augmentation system and LAAS. Standard books (Misra, 2001; Parkinson, 1996; Kaplan, 1996; Grewal, 2002) and the journals (IEEE Transaction on Aerospace and Electronic System, IEEE Position Location and Navigation Symposium, Proceeding of IEEE, Journal of Institute of Navigation, Royal Institute of Navigation Cambridge University press and ION GPS/GNSS) are referred to understand the basic and advanced topics related to this thesis. To know the GPS and its augmentation systems, various books (Grewal, 2002; Hoffman, 2001; Misra, 2001; Ramjee, 2002) and journal papers are studied. To understand the concept of LAAS, various books (Grewal, 2002), journal papers (Braff, 1996; Enge, 1999; Misra, 1999;) are referred. To understand the concept of Protection level, journal papers (Rowson. 1998; Enge, 1996; Vassileva, 2006; Walter, 1997, Rife and Phelt, 2008) are referred. To know about overbounding concept related to VPL, journals (Decleene, 2000; Rife and Pervan, 2004; Rife and etc, 2006; Blanch and etc, 2008; Braff and Shively, 2005, Pervan and Sayim, 2001).  Estimation of B- value and ground error variance (Pervan, 2005, Lee, 2006; Matteo, 2004) papers are referred. 

Local Area Augmentation System (LAAS) is an augmentation system to Global Positioning System (GPS) that has been designed to meet RNP parameters to achieve Cat I, II, and III precision approach and landing for civil aviation applications. Integrity which is one of the important RNP parameter is directly linked with safety. The Horizontal Protection Level (HPL) and Vertical Protection Level (VPL) are two parameters used to monitor   integrity in position domain. In this thesis VPL is considered.  For Civil aviation applications India is looking to have LAAS at each airport. In this context the simulation of LAAS ground facility (LGF) is done by considering the Station 1 and Station 2 dual frequency receivers as its Reference Receivers. The receiver is considered as the user to estimate VPL for 24 hours, known as VPL time series. The ground error () is modelled with respect to elevation angle of GPS satellites, using   VPL is analysed over one day data.  It is observed that the computed VPL is greater if mean ground error is considered compared to zero mean ground error. It is further observed that the VPL density is following Gaussian distribution

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