Abstract
Abstract. The scintillation data (S4-index) at the L-band frequency of 1.575GHz, recorded from a total of 18 GPS receivers installed at different locations in India under the GAGAN project, have provided us with a 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 18-month, low sunspot activity (LSSA) period from January 2004 to July 2005. During this period, the occurrence of scintillations is found to be maximum around the pre-midnight hours of equinox months, with very little activity during the post-midnight hours. No significant scintillation activity is observed during the summer and winter months of the period of observation. The intensity (S4 index) of the scintillation activity is stronger around the equatorial ionization anomaly (EIA) region in the geographic latitude range of 15° to 25° N in the Indian region. These scintillations are often accompanied by the TEC depletions with durations ranging from 5 to 25 min and magnitudes from 5 to 15 TEC units which affect the positional accuracy of the GPS by 1 to 3 m. Further, during the intense scintillation events (S4>0.45≈10 dB), the GPS receiver is found to lose its lock for a short duration of 1 to 4 min, increasing the error bounds effecting the integrity of the SBAS operation. During the present period of study, a total of 395 loss of lock events are observed in the Indian EIA region; this number is likely to increase during the high sunspot activity (HSSA) period, creating more adverse conditions for the trans-ionospheric communications and the GPS-based navigation systems.
Highlights
The Global Positioning System (GPS) is a satellite-based navigation system, which provides good positional accuracy of the user at any point of the globe, and at any given time using the L-band frequencies of L1 (1575.42 MHz) and L2 (1227.60 MHz)
We present here the results on the spatio-temporal and intensity characteristics of the L-band scintillations (S4 index), simultaneously measured by these receivers in the Indian region and the possible effects on GPS navigation
In the present study the amplitude scintillation (S4 index) data at 1.575 Hz, recorded by the dual frequency GPS receivers installed at the 18 different locations in the Indian region under the Indian Geo Augmented Navigation (GAGAN) programme during the eighteen-month period from January 2004 to July 2005, are used
Summary
The Global Positioning System (GPS) is a satellite-based navigation system, which provides good positional accuracy of the user at any point of the globe, and at any given time using the L-band frequencies of L1 (1575.42 MHz) and L2 (1227.60 MHz). The eastward post-sunset electric fields enhance the R-T instability, while westward fields quench it These irregularities can grow to become large ionospheric depletions, often called equatorial plasma bubbles, which are elongated along the magnetic flux tubes. We present here the results on the spatio-temporal and intensity characteristics of the L-band scintillations (S4 index), simultaneously measured by these receivers in the Indian region and the possible effects on GPS navigation. At low elevation angles high S4 index values are observed (even during daytime hours), because the amplitude scintillation depends on the electron density deviations and on the thickness of the irregularity layer, both of which increase apparently at low elevation angles, causing stronger scintillations, and high S4 index values due to multipath effects. The 40◦ mask angle may reduce the number of satellites available for the actual Satellite Based Augmentation System (SBAS) operation, but allows one to study the effects of ionospheric irregularities alone on the GPS navigation, limiting the tropospheric and multipath effects at the low elevation angles
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