A. DasGupta

Bio: A. DasGupta is an academic researcher from University of Calcutta. The author has contributed to research in topics: TEC & Total electron content. The author has an hindex of 13, co-authored 24 publications receiving 438 citations.

Ionospheric perturbations observed by the GPS following the December 26th, 2004 Sumatra-Andaman earthquake

Abstract: Using the Total Electron Content (TEC) data recorded by the GPS receiver network, installed under the GPS and Geo Augmented Navigation (GAGAN) program, ionospheric electron content on the day of the great Sumatra-Andaman earthquake of December 26, 2004 was examined. A significant perturbation of 1.5 to 2 TEC units over a smooth variation of TEC in the morning hours was observed within 45 minutes of the quake at stations situated near the east coast of the Indian subcontinent. The disturbance was found to propagate northwestward with its origin situated about 2° northeast of the quake epicenter. Possible coupling mechanism of the crustal movement and the ionosphere are discussed.

Degradation of navigational accuracy with Global Positioning System during periods of scintillation at equatorial latitudes

Abstract: The effect of ionospheric scintillation on navigational accuracy with the GPS (global positioning system) in the equatorial region is presented. The accuracy of position fixing with the GPS as indicated by the PDOP (position dilution of precision) factor is degraded when the raypath from the satellite shows deep fading. It is understood that navigation, particularly using a moderately sophisticated GPS receiver, in the equatorial zone will be severely affected during maximum sunspot number years.

Long-term observations of VHF scintillation and total electron content near the crest of the equatorial anomaly in the Indian longitude zone

Abstract: Ionospheric VHF scintillation (SI ≥ 3 dB and saturated level) and total electron content (TEC) data obtained at Calcutta (subionospheric 21°N, 92.7°E geographic, 27°N dip) and ionosonde data at Kodaikanal ( 10.2°N, 77.5°E geographic, 3.5°N dip) for the period 1977-1990 have been analysed to show the importance of electrodynamic drift near the magnetic equator in controlling nighttime ionospheric F region ionization and irregularities in the equatorial region. Such long-term observations extending over a period of more than 13 years are possibly being reported for the first time from a location situated near the equatorial anomaly crest. Frequent and intense VHF scintillations near the equatorial anomaly crest during equinoctial and December solstice months around solar maximum years, have been identified with the equatorial F region irregularities. Simultaneous measurement of TEC at the same location shows that during solar maximum years the high F region ambient ionization is sustained for several hours in the postsunset period, often showing secondary enhancements during equinoctial months. Under solar minimum epoch, when scintillation is sparse, TEC in the above period shows a rapid decrease. At Kodaikanal, situated near the magnetic equator, during the equinoctial and December solstice months of solar maximum years, h/F values rise by more than 100 km in about an hour around sunset. These features are seldom observed during solar minimum epoch. A causative connection among h/F variation near the magnetic equator and the maintenance of high ambient ionization and occurrence of scintillation near the anomaly crest is established. Further, scintillation occurrence during the May-July months shows a remarkable hysteresis effect with solar activity level.

Errors in position‐fixing by GPS in an environment of strong equatorial scintillations in the Indian zone

Abstract: [1] The signal-to-noise ratio (SNR) of the L1 (1.6 GHz) transmission from the GPS and GLONASS satellites has been recorded at Calcutta (22.58°N, 88.38°E geographic; 32°N magnetic dip, 17.35°N dip latitude) since 1999 by a stand-alone coarse acquisition (C/A) code Ashtec receiver. The receiver usually tracks 10–15 satellites, sampling different sections of the ionosphere at different look angles from the station. Simultaneously, L-band (1.5 GHz) signals from geostationary INMARSAT (65°E) (350 km subionospheric point: 21.08°N, 86.59°E geographic; 28.74°N magnetic dip, 15.33°N dip latitude) and VHF (244 MHz) from FLEETSATCOM (73°E) (350 km subionospheric point: 21.10°N, 87.25°E geographic; 28.65°N magnetic dip, 15.28°N dip latitude) are also recorded. Calcutta is situated under the northern crest of the equatorial anomaly in the Indian longitude sector. The SNR of many GPS and GLONASS links, particularly in the southern sky and near overhead, has been found to scintillate frequently in between the local sunset and midnight hours. Scintillations of satellite signals near overhead are caused by irregularities in electron density distribution in an environment of high ambient ionization occurring near the crest of the equatorial anomaly. For the links at lower elevation angles in the southern sky, scintillations occur when satellites are viewed “end-on” through the field-aligned plasma bubbles. During periods of intense scintillations, in the high sunspot number years 1999–2002, it has frequently been observed that seven or eight GPS/GLONASS satellite links out of 15 may simultaneously show scintillations in excess of 10 dB. This paper presents an example of the above when the position determined with GPS shows fluctuations to the extent of 11 m in latitude and 8 m in longitude under such an environment.

Ionospheric total electron content (TEC) studies with GPS in the equatorial region

Abstract: This paper essentially deals with the effects of equatorial ionization anomaly gradient on space-based navigation systems like GPS. The equatorial region of the ionosphere, which extends about ±30 ο dip about the magnetic equator, is characterized by a steep latitudinal gradient, not only in the maximum ionization but also in the total electron content (TEC), through a major part of the day. This region also accounts for about one-third of the global electron content. The high ambient TEC results in large range errors for a major part of the day, affecting navigation and position-fixing using GPS. The gradient of the equatorial ionization anomaly between the trough and the crest is very sharp, which results in large temporal and spatial variation of the ionospheric electron content. A prediction of the range error introduced by the ionosphere in the equatorial zone is very difficult. Identification of a suitable ionospheric model for prediction of these errors in the geophysically sensitive equatorial region is necessary prior to the introduction of Indian SBAS network, GAGAN (GPS And Geo Augmented Navigation). For this purpose, ionospheric TEC measured from Calcutta, situated underneath the northern crest of the equatorial anomaly, has been compared with values generated by models like PIM1.6 and IRI-95 during 1977-1990. The equatorial anomaly gradient not only extends in the horizontal direction but with altitude also. Problems related to conversion of vertical to slant TEC and vice versa, as required for ionospheric range error corrections in satellite-based navigation with GPS, have been indicated and diagnostics suggested. It has been observed that sharp latitudinal gradient of TEC during the afternoon hours of equinoctial months of high sunspot number years is usually followed by generation of irregularities over the magnetic equator in the form of ‘bubbles’ or depletions. These depletions have sharp edges resulting in large range error rates on GPS links. Characteristics of bubbles, namely, amplitude and leading and trailing edge slopes, have been studied using GPS TEC data recorded at the Giant Meterwave Radio Telescope (GMRT) site during the vernal equinox of 2004. Use of GPS TEC measurements as a tool for studying ionospheric response to earthquakes has also been indicated.

Penetration of high latitude electric fields effects tolow latitude during Sundial 1984

Abstract: Electric-field-penetration events have been identified using F-region vertical-drift measurements obtained in the October 6-13, 1984 period by the Jicamarcan incoherent-backscatter radar and corresponding h-prime F measurements from ionosondes at Fortaleza, Cachoeira Paulista, and Dakar. Predictions made using the Rice Convection Model for the pattern, strength, and duration of the low-latitude electric field occurring in response to an increasing high-latitude convection agree with observations. The observed 1-2 h duration of the low-latitude response to decreased convection can be explained by the fossil-wind theory of Richmond (1983).

A survey of ionospheric effects on space-based radar

Abstract: In this survey, we fully review almost all potential ionospheric effects on the performance of space-based radar systems (SBRs), which operate in the ambient ionosphere environment; in particular, we review the use of space-based synthetic aperture radar systems (SARs) for imaging. There are two families of effects involved. One is the effects of the background ionosphere (non-turbulent ionosphere), such as dispersion, group delay, refraction, Faraday rotation, and phase shift. The other is the effects due to ionospheric irregularities, such as refractive index fluctuation, phase perturbation, angle-of-arrival fluctuation, pulse broadening, clutter, and amplitude scintillation. These effects adversely affect SAR imaging in several respects, such as by causing image shift in the range, and degradations of the range resolution, azimuthal resolution, and/or the resolution in height (elevation). We also review ionospheric irregularity characteristics and descriptions, propagation channel statistics, ...

From Sumatra 2004 to Tohoku-Oki 2011: The systematic GPS detection of the ionospheric signature induced by tsunamigenic earthquakes

Abstract: The recent tsunamigenic earthquake in Tohoku (11 March 2011) strongly affirms, one more time after the Sumatra event (26 December 2004), the necessity to open new paradigms in oceanic monitoring Detection of ionospheric anomalies following the Sumatra tsunami demonstrated that ionosphere is sensitive to the tsunami propagation Observations supported by modeling proved that tsunamigenic ionospheric anomalies are deterministic and reproducible by numerical modeling via the ocean/neutral-atmosphere/ionosphere coupling mechanism In essence, tsunami induces internal gravity waves propagating within the neutral atmosphere and detectable in the ionosphere Most of the ionospheric anomalies produced by tsunamis were observed in the far field where the tsunami signature in the ionosphere is clearly identifiable In this work, we highlight the early signature in the ionosphere produced by tsunamigenic earthquakes and observed by GPS, measuring the total electron content, close to the epicenter We focus on the first hour after the seismic rupture We demonstrate that acoustic-gravity waves generated at the epicenter by the direct vertical displacement of the source rupture and the gravity wave coupled with the tsunami can be discriminated with theoretical support We illustrate the systematic nature of those perturbations showing several observations: nominally the ionospheric perturbation following the tsunamigenic earthquakes in Sumatra on 26 December 2004 and 12 September 2007; in Chile on 14 November 2007; in Samoa on 29 September 2009; and the recent catastrophic Tohoku-Oki event on 11 March 2011 Based on the analytical description, we provide tracks for further modeling efforts and clues for the interpretation of complex--and thus often misleading--observations The routine detection of the early ionospheric anomalies following the rupture highlights the role of ionospheric sounding in the future ocean monitoring and tsunami detection

Three‐dimensional waveform modeling of ionospheric signature induced by the 2004 Sumatra tsunami

Abstract: [1] The Sumatra, December 26th, 2004, tsunami produced internal gravity waves in the neutral atmosphere and large disturbances in the overlying ionospheric plasma. To corroborate the tsunamigenic hypothesis of these perturbations, we reproduce, with a 3D numerical modeling of the ocean-atmosphere-ionosphere coupling, the tsunami signature in the Total Electron Content (TEC) data measured by the Jason-1 and Topex/Poseidon satellite altimeters. The agreement between the observed and synthetic TEC shows that ionospheric remote sensing can provide new tools for offshore tsunami detection and monitoring.