Nocturnal winter increases in total electron content are observed at Bozeman, Boulder, and Dallas with the ATS 6 radio beacon. Results of this investigation suggest that the increases take place mainly in the ionosphere. Changes in plasmaspheric contents are much smaller. The downward motion of the F2 layer and the east‐to‐west movement of the nocturnal maximum (NM) suggest the presence of an electric field. A westward electric field will move plasma from higher L shells to lower L shells with smaller volumes and thus increase the plasma pressure, the result being an enhanced flow of plasma from plasmasphere to ionosphere. The total flux required to produce the NM is of the order of 1×1013 m−2 s−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.

Abstract. The irregularities in the electron density distribution of the ionosphere over the equatorial region frequently disrupt space-based communication and navigation links by causing severe amplitude and phase scintillations of signals. Development of a specification and forecast system for scintillations is needed in view of the increased reliance on spacebased communication and navigation systems, which are vulnerable to ionospheric scintillations. It has been suggested

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] Ionospheric total electron content (TEC) and scintillations have been recorded continuously since April 2003 using a dual-frequency GPS receiver at Pune, India (geographic latitude 19.1°N, longitude 74.05°E; 24°N dip), situated in between the magnetic equator and the northern crest of the equatorial anomaly. The TEC often shows bite-outs when severe amplitude scintillations are observed on the GPS L1 carrier level. The apparent duration of the bite-outs may be different from the true east-west duration, as observed with geostationary links, because of the presence of a relative velocity between the irregularity cloud and the satellite. The trajectory of a GPS satellite plays an important role in observing the bubble characteristics. The distributions of amplitude and duration of the bubbles have been obtained during the equinoctial months February through April of 2004. The median values are found to be 9 TEC units (1 TECU = 10 16 el/m 2 ) and 3.3 min, respectively. The range error at GPS L1 frequency corresponding to the median TEC depletion is 1.4 m, while that corresponding to the 95th percentile value is 4.5 m. An asymmetry in the east-west walls of the bubble and sharp edges of the depletions resulting in high range error rates $30 cm/min has been noted.