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Study of space weather effects using GPS

Effects of ionosphere
Irregularly structured ionosphere (i.e. inhomogenities in refractive index) can cause fluctuations (due to refraction effects) on the radio signal that is passing through it. These fluctuations are called ionospheric scintillations. Low latitude region is suitable for studying these scintillations; we analyzed the GPS data collected on Nov 24, 2001 at Tirunelveli (Lat. 8^o 37’ N, Long. 77^o 49’ E) located in south India. The data was collected at 30 sec sampling interval using Trimble 4000 SSI duel frequency receiver. The x-axis in the Fig.3 shows number of epochs and the entire axis represents 24 hours duration with starting time 00 UT. On this day, an intense geomagnetic storm with Ap index 104 occurred. Rate of TEC (ROT) is often used to describe the ionospheric scintillations (Bhattacharyya et al, 1999).

In Fig 3 ROT is plotted for each satellite in view (shown in different colors). The ROT values are relative with its level fixing at zero (at 0000 UT). From the Fig 3, it is seen that the scintillation activity started around 1400 UT and lasted for about 8 hours. The pattern of scintillations is characterized by a central minimum and oscillatory amplitude fluctuations on both the side of the minimum. This type of scintillations is caused by a mechanism intrinsically deferent with that of random scintillations (Hajkowica, 1994). It is clear from the Fig.3 that the maximum number of cycle slips occurred during the time of intense scintillation activity. Cycle slip is indication of loss of lock to the satellite and is represented as vertical line extended upwards in the Fig 3.


Fig. 3 Rate of TEC variations on Nov 24, 2001 at Tirunelveli.
GPS cycle slips are seen as the vertical lines extended to the top of the figure.

Conclusions
The radio signals are affected by space weather. The ionosphere imposes the most detrimental effects on the radio signal passing through it. Dispersive nature of the ionosphere and use of two frequencies in GPS facilitate estimation of ionospheric TEC which is an important parameter in the study of L-Band communication though the ionosphere. Rate of TEC (ROT) from GPS can illustrate the features of the ionospheric scintillations. Highly disturbed ionsosphere can cause cycle slips in GPS data. Very good correlation is found between the ionospheric scintillations and the number of cycle slips in GPS data.

The US Dept. of Defense planning to introduce L3 frequency in GPS, which further enhances the use of GPS in space weather monitoring. Like the weather forecasting from meteorological departments, the space weather forecasting is gaining popularity and many agencies already started space weather forecasting.

References

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