Strategy for development of grid based ionospheric model over India using GPS for WAAS implementation


Pradeep V. Khekale


M. R. Sivaraman
Satcom & Information Technology Application Area
Space Applications Centre
Ahmedabad-380 015
khekale@yahoo.com

Abstract
One of the major error sources in conventional GPS is the Ionospheric propogation delay. Using suitable Ionospheric models, this error can be corrected and position accuracy using GPS can be improved. For Category I precision landing of aircrafts over Indian airspace using GPS, a Grid based Ionospheric model is to be developed. This paper describes the plan for development and implementation of Grid based Ionospheric model to be used in the Indian WAAS implementation.

Introduction
Indian Space Research Organisation (ISRO), in collaboration with Airport Authority of India (AAI) is planning to implement Satellite Navigation in India to meet the requirements of Category I precision landing for aircrafts. The technique to be used is the well known Wide Area Augmentation system (WAAS) (See Fig 1) . In this technique, the existing GPS (Global Positioning System) satellite constellation will be augmented with a Regional Geostationary Satellite (GSO) and a suitable ground segment. The ground segment will collect necessary ranging data from all the visible GPS satellites, calculate the GPS broadcast ephemeris and satellite clock corrections as well as ionospheric correction parameters (WAAS Correction data) and transmit them through the GSO, carrying a Cx L transponder, to the user.The user aircraft carrying a WAAS receiver, can receive GPS as well as the GSO Satellite signals in L band, correct its pseudorange measurements and determine its position more accurately. Because of the corrections, the aircraft position accuracy improves from 100m to lt; 5 m.

One of the major errors in ranging in GPS, is the ionospheric effect at 1.57542 GHz. This is of the order of 15-20 m, when the satellite is overhead and 55-60 m, when the satellite is at an elevation angle of 15 0 , during solar maximum period, over Indian region. For WAAS, suitable ionospheric models are required to be developed to correct this ionospheric error to better than 0.5 m. This note describes the plan for the development of the model and its validation.

Basic Approach
As of today, many Ionospheric models are available. The four models viz. Klobuchar, Bent, IRI (International Reference Ionosphere) and PIM (Parameterized Ionospheric Model) models are the most popular ones. But none of them can meet the accuracy requirements for WAAS planned for the Indian aviation requirements.

Federal Aviation Administration (FAA) of US, responsible for implementation of WAAS over US, have recommended a Grid-based Ionospheric Model, using near real time TEC measurements from GPS dual frequency TEC receivers, over a number of stations in the region of service. Accordingly, the message format of the WAAS correction data, transmitted from the ground segment via GSO to the user aircrafts, has been standardized. According to this scheme, the ionospheric delay corrections are broadcast via GSO as vertical delay estimates at specified 5 0 by 5 0 Ionospheric Grid Points (IGP’s). (See Fig. 2). The ground segment estimates the vertical ionospheric delay at the grid points, using the slant TEC measurements from a network of TEC receivers. A user aircraft, depending on the grid in which it is located, uses the vertical delay estimates at the corners of that grid to estimate the vertical delay at its location and converts it to slant direction depending on the satellite direction he uses for range measurements. All WAAS receivers are designed to accept only this grid based Ionospheric model vertical delay estimates for ionospheric correction of range measurements.

Plan for WAAS
In Fig. 3, we have shown the 5 0 by 5 0 grid drawn over Indian region. For providing WAAS service covering all the airports in India, we have to determine the vertical ionospheric delay at the corners of the 18 grids as shown. In Fig.3, we have shown the selected locations, where the GPS based TEC measurement receivers are planned to be kept for TEC data collection purpose.

The locations chosen are approximately at the centre of the grids. From one location, a TEC receiver can see atleast 8 GPS satellites on an average, and measure slant TECs in 8 different directions. These slant TEC values can be converted to vertical TEC values using proper mapping functions.These vertical TEC values will be the values corresponding to the latitude and longitude at the ionospheric pierce points. Thus at any instant, there will be atleast 8x18 = 144 vertical TEC values over the Indian region, which can be used to estimate, through interpolation, the vertical TEC values at the corners of the required 18 grids.



Fig 2. FAA Predefined Global IGP Grid used in Waas

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