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

Abstract

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.

Climatology and modeling of ionospheric scintillations and irregularity zonal drifts at the equatorial anomaly crest region

“…Thus, we have chosen cases with no occurrence of loss of receiver lock, which was considered a difficult task and reduced the set of data to be analyzed, since loss of lock is a very common process during the occurrence of strong scintillation. Loss of receiver lock during very strong scintillation events tends to occur more frequently when there is a close alignment between the magnetic field lines and the satellite-to-receiver signal paths (DasGupta et al, 2004;Moraes et al, 2017). However, this geometrical factor should not be considered a general rule.…”

“…They presented evidence of a latitudinal gradient in the irregularity zonal velocities associated with the vertical shear of the zonal drift in the topside equatorial ionosphere. Also using a VHF system, de Paula et al (2010) estimated the zonal drift of irregularities at two magnetic conjugate sites in the Brazilian sector and showed that the magnitude of the eastward zonal velocities in the SAMA region may be ∼ 12 % larger than at the conjugate point owing to the weakening of the total magnetic field intensity. Valladares et al (2002) showed strong correlation in the nighttime variations in UHF scintillation zonal drifts and thermospheric zonal winds, indicating the importance of neutrals in controlling the plasma motion during nighttime.…”

“…In the South American sector, the zonal drift of scintillationproducing irregularities has been studied in the last few decades through the detection of VHF geostationary satellite signals (Abdu et al, 1985;Basu et al, 1996;de Paula et al, 2010), from UHF geosynchronous satellite measurements Sheehan and Valladares, 2004), and through an L-band GPS-spaced antenna system (Kil et al, 2000(Kil et al, , 2002Ledvina et al, 2004;Kintner et al, 2004;Muella et al, 2013Muella et al, , 2014. Basu et al (1996) estimated the zonal drift of the kilometer-scale irregularities causing VHF scintillations above the equatorial site of Ancón (MLAT 1.46 • N) in Peru and at the low-latitude station of Agua Verde (MLAT 11.3 • S) in Chile.…”

Morphological and spectral characteristics of L-band and VHF scintillations and their impact on trans-ionospheric communications

“…Further, it is to be expected that the scintillation activity increases substantially during high sunspot activity periods and will adversely effect the trans-ionospheric communications and navigation. Das Gupta et al (2004) have also shown that, seven or eight GPS/GLONASS satellite links out of 15 satellites simultaneously showed scintillations in excess of 10 dB, which resulted in a position error of 11 m in latitude and 8 m in longitude during the high sunspot activity years of 1999-2002.…”

Characteristics of L-band (1.5GHz) and VHF (244MHz) amplitude scintillations recorded at Kolkata during 1996–2006 and development of models for the occurrence probability of scintillations using neural network

“…When the depth of fading exceeds the fade margin of a receiver, the signal becomes buried in noise and signal loss and cycle slips (Kelly et al, 1996) are encountered. In Global Positioning System (GPS), amplitude scintillation may cause degradation of position fixing by standalone GPS receivers (Bandyopadhyay et al, 1997;DasGupta et al, 2004). Phase scintillations induce a frequency shift and when this shift exceeds the phase locked loop bandwidth, the loop is stretched.…”