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

Data Collection Strategy
In Fig. 4 a block diagram of the equipment required at each location is shown. The main equipment is the Ionospheric Scintillation and TEC Monitor. This is basically a dual frequency P code GPS receiver. It determines TEC by dual frequency psuedorange and carrier phase measurement. It determines amplitude and phase scintillation data as well. It runs on 230 V, 3 Amp power supply. The receiver can record either raw data every one second from all the visible GPS satellites (8-10 approximately) or a reduced data every minute on the minute. The raw data collection rate is approximately 400 bytes, per satellite, per second. Assuming a maximum visibility of 10 satellites at a time, the data recorded will be around 240 K Bytes per minute. This is quite large. However, for TEC modeling and ionospheric scintillation studies, measurements taken every minute are good enough. The receivers can also reduce the raw data every minute on the minute and record. The data recorded are :—” Lock time, Satellite PRN No., Azimuth, Elevation, TEC, ÄTEC, C/N₀ , Total S4 (Amplitude Scintillation Index) and 1,3,10,30,60 second Phase Sigma”. This data will be 136 bytes per satellite per minute. Assuming a maximum visibility of 10 satellites at a time, the total data will be about 2 M Bytes per day. This can easily be stored in a PC as a single file and downloaded remotely via Internet.

Fig 3. Locations selected for TEC receivers



Fig 4. Block Diagram of equipment configuration for TEC Data Collection Center



Fig 5. Scheme for Rack mounting of the equipment



Fig 6. Countrywide TEC Data Collection Network
The TEC Monitor is connected to an IBM PC-AT or equivalent . The TEC Monitor can automatically download the data and store it in a file in the PC. An Industrial type PC is required for continuous non-stop data collection. The PC is connected through a Modem and telephone/leased line to the Internet. For an uninterrupted data collection operation, an UPS will be used and all the equipment will get power from it as shown. All the equipment are to be mounted in a 19” rack as shown in Fig. 5 and kept in an Airconditioned Room .

Network Topology
In Fig. 6, TEC data collection network topology is shown. The main data acquisition centre will be at Space Applications Centre, Ahmedabad. The data will be downloaded via Internet at Ahmedabad periodically and archived and stored on CDROMs. Other centres engaged in the development of model and which require the data can also download the same from the Internet.

In addition to Internet, CD-Writers are also planned to be provided to take care of situations like malfunctioning in the Internet connection . The data will be recorded on CDs and these will be sent by post to SAC Ahmedabad periodically.

At places where Internet facility does not exist, VSAT connection will be provided. As the Data rate requirement is not high ,a single channel low speed VSAT terminal will give the required speed for both Data as well as voice communications.

Summary
To reduce Ionospheric errors, a grid based Ionospheric error correction model, as proposed by FAA, is planned to be implemented in India as part of satellite based air navigation (landing & take-off) system. 18 locations at the centres of 5°by 5°grids over India are chosen where Ionospheric TEC and Scintillation Monitors will be placed. An effective data collection and dissemination strategy is also outlined in this paper.

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