Morphological Characteristics of L-Band Scintillations and Their Impact on GPS Signals – A Quantitative Study on the Precursors for the Occurrence of Scintillations

The scintillation data (S4-index) at the L-band frequency of 1.575 GHz recorded from 18 GPS receivers installed at different locations in India under the GAGAN project has given an unique opportunity, for the first time in the Indian region, to make a simultaneous study of spatio-temporal and intensity characteristics of the trans-ionospheric scintillations during the low sunspot activity (LSSA) period from January 2004 to July 2005. During this period the occurrence of intense (S4 > 0.45 ≈ 10 dB) scintillations are found to be mostly confine to the pre-midnight hours of equinoctial months and around the equatorial ionization anomaly (EIA) region of geographic latitudes from 15 to 25 N in the Indian sector. These scintillations are often accompanied by the TEC depletions with durations ranging from 5 to 25 minutes and magnitudes from 5 to 15 TEC units that are found to affect the positional accuracy of GPS by about 1 to 3 meters. Further, during the times of intense scintillations, the GPS receivers are found to loose their lock for a short duration of 1 to 4 min increasing the error bounds and thus effecting the integrity of the SBAS operation, which may substantially increase during the high sunspot activity periods. The pre-reversal enhancement (PRE) in ExB drift is the most important parameter in controlling the occurrence of scintillations. Also, these drifts are found to decrease with increasing geomagnetic activity (Kp), more significantly during equinoctial months. On a day-to-day basis, it is found that an upward drift velocity ≥ 30 m/s at the equator, is the necessary condition for strong scintillations to occur over Waltair (20N dip) on magnetically quiet days with average Kp ≤ 2 (6 hrs prior to the local sunset) during the high sunspot year of 2001. This threshold value of the upward drift reduces to 20 m/s during the low sunspot activity year of 2004. Further, it is found that the post sunset vertical drifts at the equator are found to increase linearly with the increase in the anomaly gradients in TEC during afternoon hours. Also, strong (≥ 10 dB) L-band scintillations are observed during the days on which this afternoon anomaly gradient exceeds 1.25 and the PRE ExB drift exceeds 20 m/s, suggesting that a well developed anomaly around the afternoon to early evening hours is a precursor for large post sunset vertical drifts at the equator and a subsequent occurrence of scintillations. Rama Rao, P.V.S.; Tulasi Ram, S.; Gopi Krishna, S.; Niranjan, K.; Prasad, D.S.V.V.D. (2006) Morphological Characteristics of L-Band Scintillations and Their Impact on GPS Signals – A Quantitative Study on the Precursors for the Occurrence of Scintillations. In Characterising the Ionosphere (pp. 6-1 – 6-22). Meeting Proceedings RTO-MP-IST-056, Paper 6. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.