A. DasGupta

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

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.

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

Abstract VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earths horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst

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

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.

The Usefulness of GLONASS for Positioning in the Presence of GPS in the Indian Subcontinent

The effort to integrate the use of GPS and GLONASS constellations resulted in the production of a special receiver, which can use both constellations in combination. These receivers may be used in GPS only mode, GLONASS mode and combined (both GPS and GLONASS) mode. Utilising this type of receiver, GPS and GLONASS signals were monitored for one calendar year simultaneously in different places in India to study the status of visibility of satellites and the positioning accuracy. The number of satellites in GLONASS constellation gradually depleted from 16 to 7 during the course of this study. So a 3-D solution was rarely possible using only GLONASS satellites. However, appreciable improvement in PDOP was observed in the combined mode. Before the withdrawal of GPS Selective Availability (SA), significant improvement of position accuracy could be observed in the combined mode. After the removal of GPS-SA, the accuracies of the combined mode and that of GPS-only mode have been found to be of the same order. While this does not apparently reflect any advantage, it indirectly confirms that both the GPS and the GLONASS systems have similar limits of accuracy and also confirms that optimal interoperability of two systems has been achieved. These studies reveal that the combined use of GLONASS and GPS will always be beneficial to a varying degree depending on different applications and circumstances.

Effect of Scintillation on Timing Applications of GPS in Indian Subcontinent

On the Indian subcontinent, scintillations are quite frequent and intense and have a longer duration during a peak period of an 11-year solar cycle. This phenomenon quite significantly informs us about the performance of the Global Positioning System (GPS). A special experiment has particularly been conducted to study the effect of scintillations on GPS time. This paper discusses the findings of this experiment. A degradation of timing accuracy of the order of 60 ns has been observed.