Mitigation of the effects of F layer regularities by using multiple paths
Abstract
In recent military operations trans-ionospheric satellite to ground communications have been utilized in the equatorial region. One problem for the predominantly 250 MHz systems has been fading due to irregularities in the region within plus and minus 20 degrees of the magnetic equator. A similar problem has been encountered at high latitudes. For minimizing the effect of the fading, one might expect that radio links sufficiently separated in frequency and polarization would be effective in combating scintillation. Unfortunately frequency separations would have to be of the order of 100 MHz to obtain an adequate diversity improvement. The use of right and left circular polarization or of orthogonal linear polarizations is not effective. However, time diversity is a demonstrated procedure for overcoming scintillation but there is a loss of using coding or redundancy and a reluctance to complicate the system with modems. If the paths from a single satellite are sufficiently well separated then fading on two links is uncorrelated and diversity gain may be achieved. Unfortunately at equatorial latitudes separations of the order of a kilometer are involved. Another use of 'space diversity' can be effective. If the paths from two satellites with spacing between them are available and there is recognition that scintillation is causing the fading on the path to one satellite, then operators can utilize the path to a second satellite. For equatorial paths the operator must be alert as to the time development of irregularities and the dynamics of plume development. The differences in activity on the paths are due to two aspects of the irregularity plume i.e. the start time as the sunset moves westward and the spacing between plumes on the very active days. Examples of the time difference between the start of equatorial scintillations on one path versus that of another have been recorded in many sectors of the world. The results of these studies as applied to the problem of multiple paths will be shown. At high latitudes during moderate magnetic activity, it is possible to find paths that are not dominated by irregularities. However at auroral latitudes when a magnetic storm commences, the irregularity development moves equatorward with great rapidity and encompasses large areas. It is then difficult to find a path free of irregularities.
- Publication:
-
In AGARD
- Pub Date:
- July 1994
- Bibcode:
- 1994mmpp.agar.....A
- Keywords:
-
- Equatorial Regions;
- F Region;
- Fading;
- Irregularities;
- Magnetic Equator;
- Military Operations;
- Polar Regions;
- Reluctance;
- Circular Polarization;
- Reception Diversity;
- Scintillation;
- Sunset;
- Time Response;
- Communications and Radar