GAGAN - A SBAS TO SUPPORT CIVIL AVIATION OVER INDIAN AIR SPACE

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1 GAGAN - A SBAS TO SUPPORT CIVIL AVIATION OVER INDIAN AIR SPACE Shrey Agarwal a a Department of Civil Engineering, Indian Institute of Technology, Kanpur, India - ashrey@iitk.ac.in KEY WORDS: GAGAN, Satellite Based Augmentation System (SBAS), Indian Civil Aviation, Indian National Reference Systems (INRES), Airports Authority of India, Air Traffic Management System, Indian Regional Navigation Satellite System (IRNSS), Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS) ABSTRACT: This paper studies the implementation of regional satellite based augmentation system (SBAS) named GPS aided GEO augmented Navigation (GAGAN). by the Airports Authority of India and Indian Space Research Organization (ISRO). GAGAN is the augmentation system to increase the accuracy of navigation satellite constellations like GPS, GLONASS and GALILEO by providing correction terms to GPS signals. These basic constellations do not meet the accuracy requirements for safety critical applications like air traffic management. Also this system will help navigation during harsh weather conditions and also in landing in tough terrain like Leh and Mangalore airports. AAIs efforts towards implementation of operational SBAS can be viewed as the first step towards introduction of modern communication, navigation, surveillance/air Traffic Management system over Indian airspace. GAGAN will increase safety by using a three-dimensional approach operation with course guidance to the runway, which will reduce the risk of controlled flight into terrain. This paper deals with the basic concepts of SBAS, configuration and implementation of GAGAN and an overview of some of uses. 1. INTRODUCTION Satellite constellations like Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS) cannot provide the necessary accuracy and precision needed civil aviation navigational uses. Thus a need arose for one navigational system, available around the globe, at all times with extreme accuracy, trustworthy and easy to use. Such a system is Global Navigation Satellite System (GNSS). As the above mentioned satellite constellations fail to meet the necessary requirements of the civil aviation community, a need for augmenting these constellations arises to meet the required navigation performance (Rao, 2007). 1.1 Augmentation Augmentation of a navigation system is a method of improving its attributes, such as accuracy, reliability and availability, by the use of external information in the calculation process. Some of the aspects of navigation system that are taken care of by augmentation are: Integrity: Ability to protect user from incorrect information in a timely manner. As all the GPS satellites are not monitored at all times, integrity cannot be assured in satellite constellations alone. Time to Alert: The maximum allowable time elapsed from detection of problem until the equipment enunciates the alert. Continuity: Ability of the system to perform without any unpredicted interruptions during operation. Accuracy: Stands for the difference between the true and measured positions of a vehicle at a particular epoch. Availability: Ability of the system to be used by the user whenever necessary. Civil aviation requirements for satellite-based navigation as specified in ICAO SARPs which specify the Signal-in-Space performance requirements are as follows: 1. Integrity: Time to Alert: 6 seconds 3. Continuity: Accuracy (Horizontal): 16 meters 5. Accuracy (Vertical): 6 meters 6. Availability: The organization of this paper is as follows. A overview of Satellite based Augmentation System (SBAS) is given in section-2 followed by the theory of implementation of GAGAN in Indian airspace and on ground in section-3. IT is followed by conclusions and references in section-5 and section-6 respectively. Corresponding author. Tel. No ashrey@iitk.ac.in

2 2. SATELLITE BASED AUGMENTATION SYSTEM (SBAS) A satellite-based augmentation system (SBAS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. An SBAS consists of a network of precisely surveyed ground reference stations strategically positioned to monitor, collect and process satellite signals. The ground reference stations send satellite signal data to ground master stations, which then take measurements of signal delay and other errors (such as ionospheric and/or solar activity) that may impact the signal. Master stations develop corrections to the information obtained from the ground reference stations and send a corrected, or augmented, message to Geostationary Earth Orbit (GEO) communication satellites. These satellites broadcast the messages to the end users. Together, ground-based reference stations, satellite-based signals and aircraft sensor systems comprise the SBAS architecture for aircraft navigation. 2.1 Major SBAS segments Major SBAS segments are shown in Fig. 1. Ground segment: Consists of reference stations located st precisely surveyed locations for ranging adn integrity monitoring. Master Control Centre: Collects, estimates and processes the data to generate wide area correction messages and integrity information to the user. Navigation Land Earth Station: Up-links the data to geo-stationary satellites (GEO) for broadcasting to the end users. Space segment: Consists of satellite constellations like GPS, GLONASS and also GEO satellites for data transmission and ranging function. User segment: Consists of signal in space and receiver capable of receiving and decoding the GPS/GLONASS/GEO broadcast message. Figure 1: SBAS Architecture 2.2 SBAS operation SBAS operations can be described by the following steps: SBAS reference stations are deployed all over the region to be catered at accurately surveyed locations to calculate the ranges and carrier phases on L 1 & L 2 frequencies from the visible satellites. Above measurements sent to the Master station. Clock and ephemeris correction for each GPS satellite, ephemeris information for each GEO and ionospheric delays at fixed ionospheric grid points (IGPs) are calculated here.

3 Figure 2: SBAS Data Flow (Rao, 2007) Master station also calculates the error bounds for ionospheric corrections at each IGP, and also combined error bounds for clock and ephemeris corrections. These corrections and error bounds are sent to the users through GEO satellites. Corrections are used for improving the accuracy of their positions by user avionics. The error bounds are used to calculate error bounds on the position error called the Vertical Protection Level (VPL) and Horizontal Protection Level (HPL). 2.3 Ionospheric Corrections The ionospheric errors are the most dominant type of error in GPS positioning. Some major effects on GPS signal are: (i) group delay of signal modulation (absolute range error) (ii) carrier phase advance (relative range error) (iii) Doppler Shift (iv) refraction or bending of radio wave (v) signal amplitude scintillation etc... The ionosphere is a region of plasma that extends from roughly 50km to 2000km above the surface of the earth. Divided into several layers in altitude depending upon the electron density. The ionosphere introduces frequency dependent delays in the signal (function of total electron content (TOC)). The master station collects all the ionosphere data collected from all the reference stations and calculates vertical delays and error bounds of corrections which are then sent to users for accurate positioning. 2.4 Other Errors The ephemeris error in the data downlinked through the GPS L1 frequency to ordinary GPS receivers is in the vicinity of about 7 8 meters. This translates to an equivalent position error. In SBAS a better ephemeris model is transmitted to the Geo-Stationary satellite which enables a modified GPS receiver to improve the ionospheric and ephemeris related errors resulting in better position accuracies. Clock errors are offset through accurate measurements made at ground stations since this is a one dimensional error Ephemeris Ephemeris stands for the 6 orbital elements of a Keplerian orbit in which the navigation satellites operate. With global ground networks, GPS is achieving sub centimeter level accuracies in orbit determination. 2.5 Some Implementations of SBAS WAAS The Wide Area Augmentation System (WAAS) was the first SBAS approved for aviation use. It was developed by the US Federal Aviation Administration (FAA) to augment the GPS, with the goal of improving its accuracy, integrity and availability. WAAS serves North America, with benefits extending into Central America, South America and over the Atlantic and Pacific Oceans. It was commissioned in EGNOS The European Geostationary Navigation Overlay Service (EGNOS) is an SBAS developed by the European Space Agency, the European Commission and EUROCONTROL. Commissioned in 2011, EGNOS serves Europe and the northern portion of Africa MSAS A Japanese SBAS, the Multi-functional Satellite Augmentation System (MSAS), supports differential GPS designed to supplement the GPS system by reporting and improving on the reliability and accuracy of those signals. Commissioned in 2007, MSAS serves Japan and the surrounding area.

4 2.5.4 GAGAN The GPS-Aided Geo-Augmentation Navigation (GAGAN) is a planned implementation of a regional SBAS by the Indian government to improve the accuracy of a Global Navigation Satellite System (GNSS) receiver by providing reference signals. Currently under development, GAGAN will serve India and the surrounding area SDCM The GLONASS System for Differential Correction and Monitoring (SDCM), proposed by Russia. Figure 3: SBAS Service Areas 3. GPS AIDED GEO AUGMENTED NAVIGATION Indian Space Research Organization (ISRO) along with Airports Authority of India (AAI) worked out a joint programme to implement the SBAS using GPS/GLONASS over Indian airspace. Major segments of GAGAN are: Indian Reference Stations (INRESs): They collect measurement data and broadcast messages from all the GPS and GEO satellites in view and forward it to the Indian Mission Control Centre. The project plans to establish 15 INRESs in mainland India. Indian Mission Control Centre (INMCC): The GAGAN INMCC is established at Bangalore. The measurement data collected every second from each of the INRES receiver chains are transmitted in real-time to the INMCC for correction and integrity processing and generation of SBAS messages. Indian Navigation Land Up-link Station (INLUS): INLUS communicates with the Indian Nav. Payload. This earth station receives messages from the INRESs through the INMCC, format these messages and transmit them to the GEO satellite navigation payload for broadcast to users. The INLUS also provides GEO Ranging information and corrections to the GEO satellite clocks. Message formats and timing are as per the ICAO SARPs. Navigation Payload: Proposed Navigation payload compatible with GPS L1 and L5 frequency on an Indian satellite to be positioned in the Indian Ocean Region. Functions of the Navigation Payload are: (i) to relay Geostationary overlay signal compatible with GPS L1 frequency for use by modified GPS receivers (ii) to provide a CxC path for ranging by INRESs with an up-link from the INLUS. The L1/L5 frequency payload footprint is shown in Fig. 4. Three phases planned for reaching Full Operational Capability of GAGAN are: 1. Phase 1: Technology Demonstration System (TDS): The objective of the Technology Demonstration System (TDS) is to develop indigenous capability in SBAS implementation. TDS was successfully completed during 2007 by installing eight Indian Reference Stations (INRESs) at eight Indian airports and linked to the Master Control Center (INMCC) located near Bangalore. A successful demonstration was carried out in August The achieved position accuracies were computed to be better than 7.6 meters on a test AAI aircraft. 2. Phase 2: Initial Experimentation Phase (IEP): Redundancies will be provided to the space segment, INMCC, INLUS and the systme validation to be carried out over entire Indian airspace. 3. Phase 3: Final Operational Phase (FOP): During the Final Operational Phase (FOP), additional INRESs will be established as required and the communication systems will be established with all redundancies. INMCC and NLES will be augmented with operational hardware and adequate redundancy. INRESs will be augmented with operational hardware. The Preliminary System Acceptance Test (PSAT) was recently conducted and has reconfirmed that the achievable accuracies are much better than the 7.6 meter achieved during TDS.

5 Figure 4: GAGAN Navigation Payload Footprint (Kibe, 2011) 3.1 Study of Ionosphere One essential component of the GAGAN project is the study of the ionospheric behavior over the Indian region. This has been specially taken up in view of the rather uncertain nature of the behavior of the ionosphere in the region. The study will lead to the optimization of the algorithms for the ionospheric corrections in the region. Iono-Tropo modelling and scintillation studies in the L-band will be carried out over the entire Indian Airspace. About 20 total electron content (TEC) receivers shall be located at the Centre of the 25 deg x 5 deg. ionospheric grid points (IGP) over the Indian region. Data from the TEC stations is being collected for almost 8 years now. This is a vast amount of data for better iono tropo models. ISRO and AAI engineers have developed accurate iono tropo models over the Indian Flight Information Region (FIR). 3.2 Indian Regional Navigational Satellite System The Indian government has stated that it intends to use the experience of creating the GAGAN system to enable the creation of an autonomous regional navigation system called the Indian Regional Navigational Satellite System (IRNSS). The first satellite of IRNSS constellation was launched onboard PSLV (C22) on 1 July 2013 while the full constellation is planned to be realized during 2014 time frame. 4. CONCLUSIONS GAGAN after its final operational phase completion will be compatible with other SBAS systems such as the Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS) and the Multi-functional Satellite Augmentation System (MSAS) and will provide seamless air navigation service across regional boundaries. GAGAN will increase safety by using a three-dimensional approach operation with course guidance to the runway, which will reduce the risk of controlled flight into terrain i.e., an accident whereby an airworthy aircraft, under pilot control, inadvertently flies into terrain, an obstacle, or water. GAGAN will also offer high position accuracies over a wide geographical area like the Indian airspace. These positions accuracies will be simultaneously available to 80 civilian and more than 200 non-civilian airports and airfields and will facilitate an increase in the number of airports to 500 as planned. These position accuracies can be further enhanced with ground based augmentation system. 5. REFERENCES GAGAN. Available: notices/aaisite test/faq gagan.jsp, Last Accessed 10th Nov GAGAN REDEFINING NAVIGATION - ENABLER FOR SEAMLESS AIR TRAFFIC MANAGEMENT, International Civil Aviation Organisation, AN-Conf/12-WP/90

6 Kibe, S. V. (2011) Indian plan for Satellite-Based Navigation Systems for Civil Aviation. Available: Last Accessed 10th Nov Rao, K. N. S., (2007) GAGAN - The Indian satellite based augmentation system Indian Journal of Radio & Space Physics, 36,pp White Paper (2013) Operating in Satellite-Based Augmentation System (SBAS) Airspace Universal Avionics Systems Corporation Doc: WHTP , pp. 2-3