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 LI 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. Copyright 2006 by the American Geophysical Union.

ASHIK PAUL

Radio Physics and Electronics

A DASGUPTA

S RAY

A DAS

S ANANTHAKRISHNAN

Fulltext

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

&nbsp;

The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

&nbsp;

It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

Equatorial bubbles as observed with GPS measurements over Pune, India

Radio Science

Introduction of new navigation signals L2C (1227.60 MHz) and L5 (1176.45 MHz) to the existing GPS (Global Positioning System) spectrum, under the modernization program of GPS offers the improvement of position accuracy. The present study aims to understand the relative robustness of the L2C and L5 signals compared to legacy L1 C/A signal during periods of scintillations in terms of durations of cycle slips encountered from an anomaly crest location, Calcutta (22.58°N, 88.38°E geographic; magnetic dip 32°N). The data analyzed in this study were recorded during the vernal equinox of 2014 (February–April), a period of high solar activity of cycle 24. Results obtained from the comparative analyses, which are perhaps one of the first from the Indian longitude sector, indicate GPS L5 to be more robust than L1 C/A and L2C in terms of occurrence and duration of cycle slips under adverse ionospheric conditions. Furthermore, loss-of-lock events of duration greater than 6 s are found to be more frequent for S4 ≥ 0.6. It is found that frequency sensitivity of the GPS spectrum, in terms of occurrence of cycle slips and loss of locks are in conformity with earlier results from the equatorial region but are different from the high latitudes with respect to local time of occurrence and geomagnetic activity. ©2019. American Geophysical Union. All Rights Reserved.

Space Weather

Interfrequency Performance Characterizations of GPS During Signal Outages From an Anomaly Crest Location

Detrimental effects of satellite signal outages during periods of intense equatorial ionospheric scintillations could be mitigated using multiconstellation satellites if provisions for interoperability of these satellite signals exist. In view of the sharp spatial gradient of ionization occurring in the equatorial region, comparison of satellite signal fluctuations from different constellations should be performed over limited spatial volume. This could be effective for maintaining and improving the performance of satellite-based communication and navigation without compromising the accuracy and integrity. This paper presents a comparative study of robustness of GPS, GLONASS, and GALILEO satellites over a common ionospheric volume during periods of ionospheric scintillations for the equinoctial months of 2014 and March 2016 from Calcutta (22.58°N 88.38°E geographic; 32°N magnetic dip) and for September 2016 from Siliguri (26.72°N, 88.39°E; 39.49°N magnetic dip), located near the northern crest of equatorial ionization anomaly in the Indian longitude sector. It is found that for all the cases, carrier-to-noise ratio fluctuations over limited ionospheric volume from satellites of different constellations are comparable, thereby rendering them interoperable. ©2018. American Geophysical Union. All Rights Reserved.

Study of Relative Performance of Different Navigational Satellite Constellations Under Adverse Ionospheric Conditions

Application of multiconstellation satellites to address the issue of satellite signal outages during periods of equatorial ionospheric scintillations could prove to be an effective tool for maintaining the performance of satellite-based communication and navigation without compromise in accuracy and integrity. A receiver capable of tracking GPS, Global Navigation Satellite System (GLONASS), and Galileo satellites is operational at the Institute of Radio Physics and Electronics, University of Calcutta, Calcutta, India, located near the northern crest of the equatorial ionization anomaly in the Indian longitude sector. The present paper shows increased availability of satellites combining GPS, GLONASS, and Galileo constellations from Calcutta compared to GPS-only scenario and estimates intense scintillation-free (S4 &lt; 0.6) satellite vehicle look angles at different hours of the postsunset period 19:00–01:00 LT during March 2014. A representative case of 1 March 2014 is highlighted in the paper and overall statistics for March 2014 presented to indicate quantitative advantages in terms of scintillation-free satellite vehicle look angles that may be utilized for planning communication and navigation channel spatial distribution under adverse ionospheric conditions. The number of satellites tracked and receiver position deviations has been found to show a good correspondence with the occurrence of intense scintillations and poor user receiver-satellite link geometry. The ground projection of the 350 km subionospheric points corresponding to multiconstellation shows extended spatial coverage during periods of scintillations (0.2 &lt; S4 &lt; 0.6) compared to GPS. ©2017. American Geophysical Union. All Rights Reserved.

Impact of multiconstellation satellite signal reception on performance of satellite-based navigation under adverse ionospheric conditions

Transionospheric satellite navigation links operate primarily at L band and are frequently subject to severe degradation of performances arising out of ionospheric irregularities. Various characteristic features of equatorial ionospheric irregularity bubbles like the drift velocity, characteristic velocity, decorrelation time, and decorrelation distance can be determined using spaced aerial measurements at VHF. These parameters measured at VHF from a station Calcutta situated near the northern crest of the Equatorial Ionization Anomaly (EIA) in the geophysically sensitive Indian longitude sector have been correlated with L band scintillation indices and GPS position accuracy parameters for identifying possible proxies to L band scintillations. Good correspondences have been observed between decorrelation times and distances at VHF with GPS S<inf>4</inf> and Position Dilution of Precision during periods of GPS scintillations (S<inf>4</inf>>0.3) for February-April 2011, August-October 2011, and February-April 2012. A functional relation has been developed between irregularity drift velocity measured at VHF and S<inf>4</inf> at L band during February-April 2011, and validation of measured S<inf>4</inf> and predicted values performed during August-October 2011 and February-April 2012. Significant improvement in L band scintillation prediction and consequent navigational accuracy will result using such relations derived from VHF irregularity measurements which are much simpler and inexpensive. ©2014. American Geophysical Union. All Rights Reserved.

Effects of transionospheric signal decorrelation on Global Navigation Satellite Systems (GNSS) performance studied from irregularity dynamics around the northern crest of the EIA

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. ©2013. American Geophysical Union. All Rights Reserved.

Impact of space weather events on satellite-based navigation

The errors in position-fixing by GPS in an environment of strong equatorial scintillations in the Indian zone are discussed. The signal-to-noise ratio (SNR) of the L1 transmission from the GPS and GLONASS satellites has been recorded at Calcutta since 1999 by a stand-alone coarse acquisition code Ashtec receiver. The receiver usually tracks 10-15 satellites, sampling different sections of the ionosphere at different look angles from the station. The SNR of many GPS and GLONASS links has been found to scintillate frequently in between the local sunset and midnight hours.

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

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.

Journal	Radio Science
Publisher	AMER GEOPHYSICAL UNION
ISSN	0048-6604
Open Access	No