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



[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.