Ionosphere scintillation Sample Clauses

Ionosphere scintillation. Scintillation is the rapid fluctuation of the amplitude and phase of radio waves caused by electron density irregularities in the atmosphere. The effects tend to be more severe for lower frequencies, e.g. VHF to L-band than higher frequencies. The effects also occur most strongly in special areas near the equator and to a lower degree in the aurora areas and near the poles. These effects can cause significant problems for L-band transmissions. Fluctuations of the amplitude of more than 20 dB have been measured for GPS signals /9/ and for Inmarsat near the equator/10/. This can be expected to be less problematic for higher frequencies such as Ku-band, but it may be noticeable for C- band. Iridium has its lowest satellite density near the equator. Simulations indicate that there will be in sum about 17 seconds each 24 hours where no satellites are visible at all /11/ at the equator. This should not be a problem during normal operation, but combined with the scintillation effects and decreased SNR, it may impair Iridium coverage. It is difficult to find actual measurements of performance in the literature, but one paper /12/ discusses the performance of Iridium in the USA. It quotes a frequency of not being able to connect or losing the connection before 3 minutes as 3 and 4 respectively out of 359 attempts in North California and correspondingly 4 and 15 out of 359 in Texas. Both places are at latitude above 30˚N and scintillation is probably not the cause here. However, it illustrates that there are significant disturbances in communication even at more optimal latitudes. L-band transmissions for Inmarsat C and Fleet 77 may also be impaired by scintillation, but the GEO orbit will in most cases give shorter transit through the atmosphere and no problems with Doppler shifts. It is also possible to use higher gain antennas to reduce or overcome this problem.
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