Abstract
For reliable operation of WRS which is part of the SBAS (satellite Based Augmentation System) , GPS satellite signal observation should be available continuously at each WRS. Loss of signal availability, leading to degradation of the SBAS performance can occurs during periods of Ionospheric scintillation. Scintillation effects arise from small-scale irregularities in electron density, which is most common observed in the high latitude auroral region and the low latitude equatorial anomaly region. The auroral region expands equator ward during intense geomagnetic storms and scintillation effects may be observed in southern united state and Europe. The equatorial scintillation is expected to peak during the years of solar maximum, while high latitude storms activity is expected to peak in the year 2001-2003. SBAS are currently being implemented in United States Wide Area Augmentation System (WAAS), European Geo Satellite Navigation Overlay System ( EGNOS) , Japan, Multifunction Transportation Satellite Augmentation System ( MSAS) and India GPS / GLONASS and Geo-Satellite Augmented Navigation (GAGAN) regions which may be affected , to some extent, by the presence of scintillation. The impact of the scintillation activity is quantified by loss of phase and code observation, number of satellite observations lost simultaneously and duration and spatial extent of degraded receiver tracking performance.
The GAGAN will become fully operational in the year 2010 for providing Ionospheric correction to the GPS signal users flying over Indian Airspace. The GPS data is taken in fair weather condition at Delhi but GPS signal has experienced scintillation in both days for certain periods. It was also observed in the position error vs number of sample plot that the errors (Longitude, Latitude and Altitude) fluctuating during 2030 to 0030 time (IST) The Indian continent has diverse climate and it is also near to magnetic equator and also the Ionosphere variation over the Indian Airspace is not uniform. Therefore the scintillation intensity will vary at every two degrees latitude. For GAGAN project, it is suggested that more study in different WRS location is required before starting the modeling of the Ionosphere over Indian Airspace.
Introduction
The ionosphere is a diverse medium, in which radio frequency signals are refracted by an amount dependent upon signal frequency and Ionospheric electron density. In region of small-scale irregularities in electron density, rapid random phase variation can be produced by phase irregularities in the emerging wave front. Diffraction of the signal also leads to variations in signal amplitude-refers to as amplitude scintillation (or amplitude fading, for degradation in signal strengths). The WRS signal is also affected by the atmospheric condition i.e. medium of propagation, particularly the ionosphere. Trans-Ionospheric condition for GPS signal are perturbed due to group delay of the signal and fluctuations in the GPS signal characteristics (amplitude and phase) caused by irregularities in the electron density distribution of the ionosphere which causes the range error measurement.
To classify the Ionospheric conditions, the entire globe has been divided into three distinct regions i.e. equatorial, mid-latitude and high latitude zones. The equatorial region extends from the magnetic equator to about ±30-degree. The high latitude region covers locations above 65-degree around the magnetic poles. In between the above two boundaries, lies the mid-latitude zone. The Indian subcontinent extends from the magnetic equator, touching Trivandrum near the tip of the peninsula, to the mid-latitude zone in the north.
The equatorial region has two very prominent features: (1) the equatorial anomaly also called the Appleton Anomaly which is the latitudinal variation of high ambient F-region electron density at the magnetic equator and two crests around ±30 degree. During afternoon and early evening hours and (2) very severe irregularity structures in the electron density distribution. These irregularity structures also known as ‘bubbles’ are actually depleted ionization regions in the form of bananas or peeled orange sections over the magnetic equator and have vertical and spatial extents, extending from a few to hundreds of kilometers. The irregularities cause severe fluctuations known as scintillations in the signal strength. This phenomenon becomes a major issue for navigation applications because fluctuations provide additional stresses to the GPS / GLONASS receiver tracking loops and can induce cycle slips or even complete loss of lock.
