Morphological and spectral characteristics of L-band and VHF scintillations and their impact on trans-ionospheric communications

Abstract

Amplitude scintillations recorded at 1.5 GHz frequency during the high (1998)(1999) and low (2004)(2005) sunspot activity periods over a low latitude station, Waltair (17.7 • N, 83.3• E) revealed that the L-band scintillations mostly occur during the post-sunset to midnight hours peaking around 21:00 hr local time with maximum occurrence during equinoxes, moderate during winter and minimum during the summer months. The occurrence, as well as the intensity of scintillations, is found to be strongly dependant on both the season of the year and the sunspot number. Strong (S4-index >0.45) and fast fading scintillations (fading rates >40 fads/min) observed during the post-sunset hours of equinoxes and winter months manifest as several short duration patches at both VHF (244 MHz) and L-band (1.5 GHz) frequencies and are found to be always associated with the range or total Spread-F on ionograms and bubbles/depletions in the Total Electron Content (TEC) measured from a colocated dual frequency GPS receiver, suggesting that these scintillations are of the Plasma Bubble Induced (PBI) type. On the other hand, relatively weak and slow fading scintillations (fading rates <8 fads/min) observed around the post-midnight hours of the summer months which appear as long-duration patches (>3 hr) at 244 MHz signal (with practically no scintillation activity at the L-band frequencies) are often found to be associated with frequency Spread-F on ionograms with no depletions in TEC. Further, the presence of Fresnel oscillations observed in the spectrum of 244 MHz suggests that the long-duration scintillations observed are due to the presence of a thin layer of irregularities in the bottom side F-region which are generally known as Bottom Side Sinusoidal (BSS) irregularities. Further, the PBI-type scintillations at L-band frequencies are often found to exceed 10 dB power levels (S4 > 0.45) even during the low sunspot activity period of [2004][2005], and cause Loss of Lock in the GPS receivers resulting in a total interruption in the received signals.

Study of spatial and temporal characteristics of L-band scintillations over the Indian low-latitude region and their possible effects on GPS navigation

“…Also, the occurrence of scintillations is modulated by the seasonal effect, with maximum occurrences during the equinox months followed by winter and a minimum occurrence during summer months. During the moderate to high sunspot activity periods, the seasonal modulation in the occurrence pattern of scintillations is significant (DasGupta et al, 1983;Rama Rao et al, 2006). However, during relatively low sunspot number periods, such as [2004][2005], and during the descending phase of the sunspot number, the seasonal control on the scintillation activity is predominantly perceptible over the sunspot number dependence, as may be seen from Fig.…”

Variability of ionospheric scintillation near the equatorial anomaly crest of the Indian zone

“…Steep power spectral slope as high as ∼ 5-6 for early evening scintillation at 257 MHz frequency near equatorial region of solar maximum period is reported by Basu et al (1983), and in the range of 2.5-5.6 from equatorial anomaly region by Iyer et al (2006). The high (low) values of spectral indices in the pre-midnight (post-midnight) hours may be attributed to a thick layer (comparatively thinner) of irregularities, common during equinoctial months of the solar maximum epoch (Rama Rao et al, 2006). In the developing phase, although irregularities of various scale sizes may generate simultaneously, lower scale size irregularities decay first.…”

Long-term morphological and power spectral studies of VHF amplitude scintillations recorded over Waltair (17.7°N, 83.3°E), India

“…3) are showing moderate fade rates that vary between ~7 and ~23 fad min -1 (~0.11 and 0.38 cycles s -1 ), and the fading rates of scintillations recorded on 22 March 2013 (Fig. 4) As the moderate intensity and relatively fast fading rates of VHF scintillations are due to multiple scattered radio waves with a wide range of irregularities embedded in plasma bubbles, which are moving across the FLEETSAT satellite propagation path, the observed scintillations in the present study would have originated from a wide range of irregularities that were initiated initially over the equatorial station (Iyer et al 2006;Rama Rao et al 2006;Basu et al 2009;Prasad et al 2012). …”

“…The severe scintillations, for which the scintillation index is usually greater than 0.45, which is ≈10 dB, are able to interrupt the GPS receiver operation by causing in-phase and quadrature signal fading, which makes the proper phase determination using a tracking loop nearly impossible. Further, it was reported that a typical GPS receiver loses its lock for a short duration from 1 -4 min during severe scintillation events, thereby affecting even the integrity of satellite based augmentation systems (SBAS) (Rama Rao et al 2006).…”