Placement of WAAS Reference Station (WRS) for Indian airspace


Arjun Singh & K. Ramalingam
Airports Authority of India

D. C. Reddy, P. Laxminarayna
Osmania University
Hyderabad
rcduaai@bol.net.in

Abstract
The standalone basic Global Positioning System (GPS) service fails to meet the required accuracy, availability, and integrity, which are critical to safety of Civil Aviation for precision approach. In the recent year, there has been wide spread growth in the development of satellite based augmentation system which is part of the Global Navigation Satellite System (GNSS), for enroute, precision approach and landing of the aircraft. This paper is presenting, briefly Wide Area Augmentation System (WAAS) principles, bowing effect for coverage analysis and identification of the WRS for Indian Air space based on FAA’s WAAS for GPS signal range correction error. The visibility of the satellite at each WRS with their PRN are observed at all the reference station and presented in the paper. In this paper, we developed some guidelines for optimal placement of the WRS for Indian WAAS (INWAAS). The method of placement of WRS has been discussed in length. The monitored area and bowing effects is calculated which demonstrate that if WRS number increased, the bowing effects will decrease and monitored area will increase.

Introduction
The GPS satellite signals are available worldwide; GPS represents a unique opportunity for the international aviation community to start converging toward the goal of a single, integrated and universalized GNSS for air navigation. This will eventually allow aviation users to reduce the number of different types of receivers required for navigation services in all phases of flight. The GPS will contribute to increased safety and efficiency of international Civil Aviation by supporting real-time surveillance of aircraft, thereby reducing separation requirements to increase the number of flights possible on the busy, most favorable transoceanic routes by overlying Geostationary Earth Orbit Satellite (GEOS).

The basic requirements for designing of WAAS reference station are discussed in shape of rectangular and Hexagonal configurations. It is verified that increase in number of WRS will increase the monitored area and reduction in bowing effect. The bowing effect is noticed only when distance between WRS stations are very high. When consider for regional coverage, the distance between WRS is decreased and monitored area is increased. Now in present discussion, it is very difficult to choose the site, which can provide the regular geometrical shape for easy way to calculate the coverage area. When Indian geographical condition is considered, the tedious task is to decide the place and getting coordinate of the place.

World wide WAAS
The major Airlines in the United States and around the world are flying to more international destinations; co-operation and joint alliances are being developed among them. Therefore the Airlines will implement WAAS only if it will lead to a seamless, worldwide system and allow them to reach their objective of flying and landing anywhere in the world with the single system. ICAO has already started the process of defining and implementing of worldwide GNSS Standard and Recommended Practices (SARPs). They have developed the Future Air Navigation Systems (FANS) concept and are in the process of defining the Satellite Based Augmentation Systems (SBAS) and its infrastructure, i.e. GNSS , will be part of FANS. The GNSS panel was convened in Montreal in October 1994 to start defining the process. Civil Aviation Authorities can follow to make the transition from the current, ground base navigation system to SBAS. The transitions involve the use of GPS and WAAS, as parts of an initial component of GNSS, to provide a smooth evolution to a worldwide, integrated GNSS with international participation and control.

WAAS is the first step toward an international GNSS, since countries or groups of countries can implement it, will have a voice over the control and management of GNSS and its use over their sovereign airspace. Since WAAS is more effective when it is an integrated system over a large area, its implementation will require regional coordination and cooperation. This requirement will encourage Civil Aviation Authorities to work together, and to initiate the process needed for an eventual worldwide and internationally based GNSS. Cooperation and coordination are essential. Since GNSS or any satellite-based system is too expensive for implementation by any one country, and by its nature is an inherently worldwide system, it requires standardization. WAAS implementation will require countries covering either large or small airspace, to work together towards a jointly operated and controlled navigation system. WAAS also represents a de-facto test bed to determine whether Civil Aviation Authorities around the world are ready to work together in a jointly operated system. The implementation of a seamless WAAS would be a first step toward a worldwide, internationally controlled GNSS.

WAAS Description
To provide enroute, non-precision approach and precision approach navigation service throughout Indian airspace, WAAS in its final form will use 10-12 stations placed throughout India to cover the entire Indian controlled Airspace. The GPS master station will collect the data and calculate the differential correction for GNSS core constellation. Ground Earth Station (GES) will transmit the corrections to Geo stationary Earth Orbit Satellites (GEOS), which will retransmit them to users. The GES transmission will employ a GPS wave form, including the coarse acquisition (C/A) code, which GEO will preserve, when it retransmit at the GPS L1 frequency (1575.42 Mhz); GEOS will thus appear to the user receiver to be additional GPS satellite that can be used for ranging as well as source of differential corrections to be applied to all visible satellites.

Fig: 1 Typical Wide Area Augmentation System (WAAS) Network
Corrections to be provided on a satellite-by-satellite basis are of two types. Long-term ephemeris and clock corrections are to be transmitted once a minute or longer. Fast corrections, transmitted every six seconds accounts for Selective Availability (SA) but also partially compensate for interim errors in the long-term corrections. The WAAS will also collect measurements of signal delay through the ionosphere at each station using dual frequency techniques taking advantages of GPS signals being broadcast on two widely separated frequencies, L1(1575.42 Mhz) and L2 (1227.60 Mhz) . This data will then be compiled at the master station to compute values for an imaginary grid over the service area, at 5 degrees spacing of the vertical Ionospheric delay (in meters) to be subtracted from the users range calculation to each satellite. The “grid” is broadcast to user via GEOS. To make use of this data, the user receiver must interpolate between the grid values to calculate the delay at the point at which its line-of-sight to each satellite in view pierces the Ionospheric shell. This vertical Ionospheric delay must then be rotated to the satellite line of sight angle. The WAAS station will therefore provide a continuous set of Ionospheric delay estimates that replace the generic, less accurate model of Ionospheric delay that is broadcasted by GPS satellite. Fig (1) shows the typical WAAS architecture.

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