Pradeep V. Khekale
M. R. Sivaraman
Satcom & Information Technology Application Area
Space Applications Centre
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.
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.
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 8×18 = 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
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 .
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.
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.