Background
Post– sunset in the equatorial ionosphere routinely cause rapid phase and amplitude fluctuations of radio waves propagating through the disturbed regions. The intensity of scintillation is positively correlated with the solar cycle and associated signal fades will often exceeds 20dB at L-band frequencies during solar maximum. The effects of such an environment on the performance on GPS/ GLONASS navigation systems is poorly understood [6]. Ionospheric scintillations in the equatorial region have been studies extensively over the last two decades on VHF,UHF L-band etc. In Fig. 1 three zones have been identified i.e. most intense, intense and moderately intense scintillations in India. The Indian subcontinent covers the geographic latitude range of 10 to 45 degree N and the magnetic equator touches the Southern tip of the Peninsula. The well-known equatorial anomaly occurs with the northern crest along the line joining Kolkata in the east and Ahmedabad in the west. In other works, this region covers the northern half of the equatorial belt in the Asian Zone and extends into the transitional latitudes from the equatorial to middle latitudes. Ionospheric observations in Indian subcontinent have been extensive and the contributions made from India in terms of the global aspects of equatorial scintillation are significant. However, most of the observations have been oriented towards understanding the physical plasma processes [2]. No serious attempt has, however, been made to derive models of Ionospheric scintillation, which can be used for aviation application in SBAS. It will be very critical while implementing the GAGAN for Indian Airspace.
Fig-1 Identify the different zone where the scintillations most intense, intense and moderate
Characteristics of scintillation
It has been well-established fact that the signal strength and frequency of scintillation is highly correlated with 11 years cycle of sunspot number. The percentage occurrence of scintillation is also a function of the local time of a day, season, and magnetic activity. Equatorial scintillation has been found to increase significantly with an increase in the solar activity level,. The control is more pronounced at all-equatorial locations around the anomaly crest than at the equator [1]. It has also been observed that there is the dramatic increase in the scintillation activity during the equinoxes (February to April and August to November) and December solstice months with the increase of sunspot number. It has also been observed that scintillation is very prominent during post sunset hour of these periods. This related to the equatorial Ionospheric plasma processes. It is also interesting to observe that scintillation occurrence in the June solstice is spares and almost independent of solar activity.
It has been established that the equatorial irregularities can frequently produce signal fading of L- band transmissions from Geo Earth Orbiting Satellite (GEOS) in excess of 20 to 25dB at locations like Ascension Island (7.9 degree S, 14.4 degree W geographic 31 degree S dip), Kolkata 22.58 degree N, 88.38 degree E geographic, 32 degree N dip) and Hong Kong (22.12 degree N, 114.12 degree E geographic, 30 degree N dip) situated near the crest of the equatorial anomaly [2].
