Earthspace Telecommunications

Telecommunications have expanded enormously since the advent of the space age. In general, we are no longer limited to the coverage dictated by terrestrial systems, nor the data rate limitations and catastrophic ionospheric effects so long associated with lower frequencies. But this does not mean we are rid of the ionosphere and space weather effects. Table 4-7 contains a representative listing of effects for frequencies between 100 MHz and 1 GHz corresponding to a single traverse through the ionosphere at an elevation angle of 30 degrees. Also given is the frequency dependence. As may be seen, ionospheric effects are not entirely negligible at frequencies normally utilized on space platforms. For example, at 1 GHz, 250 nS corresponds to a group path delay error of ~ 250 feet, and this creates a problem for some single-frequency users of GPS systems requiring highly accurate radionavigation.

Table 4-7: Representative Maximum Effects on Earth-Space Systems (Note: The EC = 1018 electrons/m2, and the elevation = 30 degrees)

Effect

100 MHz

1 GHz

Frequency Dependence

Faraday Rotation

30 rotations

108 degrees

Ionospheric Delay

25 mS

250 nS

r1

Refraction

< 1 degree

<0.6 Minutes

r*

Absorption

5 dB

0.05 dB

r2

Time Dispersion

0.4 pS/Hz

0.0004 pS/Hz

r-*

For the time being we ignore Doppler effects and radiowave scintillation. Scintillation, arising from ionospheric inhomogeneities, has had a major influence on the selection of frequencies used for earth-space communication, and even with the upward drift in transmission frequencies, scintillation is still a factor. We shall soon see that for earth-space paths the estimated maxima given in Table 4-7 are directly related to the values of the Total Electron Content (TEC) of the ionosphere. As a result, measurements and predictions of the TEC take on a special significance for many applications.

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