TEC Estimation and its Validation for Real Time Implementation of Satellite Navigation in GAGAN

摘要

GAGAN (GPS Aided Geo Augmented Navigation) is a joint project between Airports Authority India (AAI) and Indian Space Research Organization (ISRO) to demonstrate Space Based Augmentation System (SBAS) over Indian airspace. GAGAN aims to demonstrate Accuracy, Integrity, Availability and Continuity of SBAS signal over Indian airspace. rnAs the GPS signal travels from satellite to receiver on the ground, it experiences a delay due to ionosphere. The electron density in the ionospheric region varies widely with the time. India lies in the equatorial anomaly region where ionospheric conditions are very dynamic. The ionospheric delay in GPS signal is of the order of 15-20 meter in vertical direction and 55-60 meter in 15° slant direction. Ionospheric effects in GPS are required to be corrected to better than 0.5 meter for aircraft precision approach applications. Suitable Ionospheric model has to be developed for this purpose. rnTo study the ionospheric behaviour over Indian region, a network of 18 GPS stations has been setup at different locations all over India. They provide Pseudorange and Carrier phase measurements. A suitable technique is required to estimate the Total Electron Content (TEC), which is a measure of ionospheric delay, from these measurements. There are several factors which affect the accuracy of ionospheric delay estimation, such as GPS satellite and receiver L1/L2 Inter-Frequency Bias (IFB), cycle slips, slant to vertical mapping function, multipath, etc. rnWe have proposed a technique to estimate the ionospheric delays which give accuracy of 0.5 meter. Raw measurements-code and carrier phase from GPS dual frequency receivers can be used to determine the Slant Total Electron Content (STEC). Code measurement is noisy but absolute in nature while carrier is smooth but relative. Therefore carrier smoothing is done for absolute and noise free estimation of slant TEC. The Inter- Frequency Bias (IFB) of satellite is estimated using the Tgd values transmitted by GPS satellites while the Inter- Frequency Bias of receiver is estimated using the Kalman filter method. rnTo validate the technique and the accuracy of the results, we compared two co-located dual frequency GPS receivers over a longer period of time. The difference between STEC measured by receiver-1 and the receiver-2 after applying all the biases is calculated. It is observed that the difference is very little – of the order of less than 1 TEC unit, which corresponds to delay of 0.16 meter at L1