Detrimental effects of the equatorial ionospheric irregularities on satellite‐based communication and navigation systems have been studied over the past few decades as space weather events have the potential to seriously disturb the technological infrastructure of modern society. The present paper tries to understand operational compliance of Global Positioning System (GPS) receivers to International Civil Aviation Organization (ICAO) standards under scintillation conditions by recording the received phase of the L1(1575.42 MHz) signal from two stations, namely Calcutta situated near the northern crest of the Equatorial Ionization Anomaly and Siliguri, situated beyond the northern crest, at a subionospheric latitude separation of 4° along the same meridian. A causative approach is adopted whereby GPS phase scintillations have been monitored and receiver performance prior to loss of lock and cycle slips have been analyzed during August–October 2011 at Calcutta and September 2011 at Siliguri. The received phase at GPS‐L1 frequency has often been found to fluctuate at kilohertz, often megahertz rates, thereby causing carrier‐tracking loop malfunctions. It should be borne in mind that normal GPS receivers' carrier‐tracking loops have a typical dynamic range of 14–18 Hz. Cycle slips have been observed with durations far exceeding ICAO specified levels for high dynamic platforms like aircrafts. Differences in cycle slips between Calcutta and Siliguri indicate possible evolution of irregularity structures even across small subionospheric swath. Significant improvement in present understanding of GPS phase scintillations should be developed and implemented in receiver designs prior to application of Satellite Based Augmentation System services for civil aviation, particularly in the geophysically sensitive equatorial region.

[1] Using a dual-frequency high-resolution software-based GPS receiver, TEC and phase have been monitored from Calcutta, India situated near the northern crest of the Equatorial Ionization Anomaly for studying some Space Weather events during 2008-2010. Data from a dual-frequency Ionospheric TEC and Scintillation Monitor operational at this station under the international SCINDA program of the U.S. Air Force have also been used. This paper presents two cases of intense Space Weather events occurring in the equatorial latitudes under magnetically quiet conditions during the abnormally prolonged minimum of solar cycle 24. High values of S 4 with maximum ∼0.8 were noted on GPS links located almost due south of Calcutta (22.58°N, 88.38°E geographic; magnetic dip: 32°N) when the look angles of the satellites are more-or-less aligned with the axis of the anisotropic field-aligned irregularities over the magnetic equator. Associated bite-outs in TEC of amplitude 40 units were recorded in the local post-sunset hours. Well-defined patches of phase scintillations and associated cycle slips were identified. On these days, higher values of ambient ionization were noted and the diurnal maximum of the electrojet strength was found to be delayed followed by a significant rise of the F region with a high upward drift velocity over the magnetic equator around sunset indicated by ionosonde. Measurements of in situ ion density using LEO DMSP corroborate the F region height rise. Presence of irregularities in ionization density distributions around 450km was found from C/NOFS measurements.Citation: Paul, A., B. Roy, S. Ray, A. Das, and A. DasGupta (2011), Characteristics of intense space weather events as observed from a low latitude station during solar minimum,

The largest geomagnetic storm in solar cycle 24 occurred during 17–18 March 2015 where the main phase of the storm commenced from 07:00 UT of 17 March 2015 and reached the Dst negative minimum at 22:00 UT. The present paper reports observations of total electron content (TEC), amplitude, and phase scintillations from different GPS stations of India during the storm of 17 March and highlights its effects on GPS. It also presents the global equatorial spread F (ESF) occurrence during the storm using total ion density drift measurements from Communication and Navigation Outage Forecast System (C/NOFS) satellite. TEC enhancements were noted from stations along 77°E meridian around 10:00 UT on 17 March compared to 16 and 18 March indicating positive storm effects arising out of equatorward neutral wind in the local morning to noon sector of the main phase. Intense scintillation observations from Calcutta were most extensive during 15:00–16:00 UT, 17 March, and the receiver recorded a longitude deviation of 5.2 m during this time. Cycle slips of the order of 8 s could be observed during periods of intense phase scintillations on the same night. Intense scintillation observation from Palampur is an exceptional phenomenon attributed to the dramatic enhancement of the electric field due to prompt penetrating (undershielded) electric leading to a very high upward ion velocity over the magnetic equator as recorded by C/NOFS. The total ion density measured globally by C/NOFS reveals two distinct longitude regions of ESF occurrence during the storm: (i) East Pacific sector and (ii) Indian longitude during the storm. The time and longitude of ESF occurrence could be predicted using the time of southward turning of interplanetary magnetic field Bz.

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