The Effect of Very High Ionospheric Electron Content on Polarization Diversity Satellite Data Receiving Systems During the present very high level of general solar activi- ty, the Total Electron Content (TEC) of the ionosphere measured at Boulder has intermittently risen to unusually high levels. This is likely to continue for at least the next 18 months. On January 11th the vertical content rose to nearly 10^18 electrons per m^2 column. This leads to high values of critical frequency for propagation by ionospheric reflection. What may not be realized is that these high values of content can affect some kinds of transionospheric propagation from satellites even at GHz frequencies. Normally at mid latitudes these high contents have no effect on the strength of the received signals but at equato- rial and high latitudes instabilities in the ionosphere may give rise to a type of rapid fading called scintillation even at frequencies as high as 4GHz More likely to be encountered, however, is a more subtle effect which affects mainly mid and high latitudes. This is caused by the rotation of the plane of polarization of the radio wave propagating through the ionosphere in the presence of a parallel component of the magnetic field. This effect is known as Faraday Rotation because the mechanism is the same as the optical phenomenon discovered by Faraday. The following table shows the rotation in degrees caused to signals from 1-10 GHz by vertical propagation through a mid latitude ionosphere with a TEC of 10^18 electrons/m^2. Frequency Degrees Power Ratio Coupled GHz Rotation into Orthogonal Polarization 1 85.37 0.99 1.2 59.28 0.74 1.4 43.56 0.47 1.6 33.35 0.30 1.8 26.35 0.20 2 21.34 0.13 2.2 17.64 0.09 2.4 14.82 0.07 2.6 12.63 0.05 2.8 10.89 0.04 3 9.49 0.03 3.5 6.97 0.01 4 5.34 0.01 4.5 4.22 0.01 5 3.41 0.00 6 2.37 0.00 7 1.74 0.00 8 1.33 0.00 9 1.05 0.00 10 0.85 0.00 For practical paths, particularly for geostationary satellites observed at low elevation angles from high latitudes, these rotations may be increased by factors of 2 or more. The effect of the rotation on the receiving system will be zero if circular polarization is employed. However, for the more normal case of transmitted linear polarization where the receiv- ing antenna is linear and has been aligned for maximum signal, these rotations will cause a loss of signal. At 1GHz, for exam- ple, the ionosphere will cause the received signal to rotate nearly 90 degrees and become orthogonal to the receiver. This would lead to a drastic reduction in received power unless the receiving antenna is rotated to compensate. If polarization diversity is being employed, the effect of the rotation is to couple the unwanted signal into the orthogonal channel. At 4 GHz the coupling induced would be -40 dB for the case shown but this could possibly become much more if the path is at low angles or the coupling is already marginal for satisfactory system perform- ance. At least, one case has been reported where a 4GHz system has been adversely affected. The content of the ionosphere varies with the time of day and normally peaks at local noon or a little later unless the ionosphere is disturbed by geomagnetic activity. The rotation will be negligible at night and only reach significant values at 4GHz during the noon hours when the sun is particularly active. The polarization of the receiver antenna could be adjusted to track the Faraday rotation or it may be possible to achieve a compromise setting giving acceptable performance with equal errors at night and at noon. The magnitude of the effect on any given day can be corre- lated with the Solar 10cm Flux (F10) which had a value of 270 for the day used as an example above. To a rough approximation, when the 10cm flux increases by one unit, the TEC increases by 0.3 x 10^16 electrons/m^2. The values of 10cm flux can be obtained from the Solar Forecast or Data Listings on the SEL Bulletin Board ((303) 497-5000 300/1200 bps 8 bits no parity). Thus, if inter- mittent anomalous system behavior is observed which follows a daily pattern, it may be worth checking to see if the anomalies correspond to particularly high (>200) values of flux. If the effect is due to high electron contents, at least the mechanism is understood and it may be possible to take corrective measures. K. Davies R. Grubb NOAA/ERL/Space Environment Laboratory (303) 497-3284 2/9/89 This file recived via NOAA BBS at 303-497-5000  signal into the orthogonal channel. At 4 GHz the coupling induced