Page 120 - Intro to Space Sciences Spacecraft Applications

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Communications
the ionosphere at the time, some waves will propagate farther into the
ionosphere before being turned, appearing to be reflected at a higher alti-
tude.
Knowing the ionospheric makeup at any time, one can determine the
frequencies for which refraction of a signal will occur. The critical fre-
quency for this type of interaction is given by equation 5- 1.

where Ne represents the electron density (electrons/cm3) at a particular
region of the ionosphere. Equation 5-1 reveals the highest frequency that
will be reflected (with noml incidence) by a particular region of the
ionosphere. Lower frequencies will also be reflected, but higher frequen-
cies will not be refracted sufficiently to be turned completely back down
toward the surface.
As an example, from Figure 4-5 in Chapter 4 we can see that the high-
est electron density for the lowest region of the ionosphere (the “D’ region)
is about 103 electrons/cm3. From equation 5-1, the highest frequency that
this layer will reflect is about 280 kHz. We can determine the highest fre-
quency that will be reflected by the ionosphere as a whole by noticing that
the highest electron concentration indicated in Figure 4-5, associated with
the F2 region, is about 2 x lo6 electrons/cm3. Equation 5-1 indicates that
frequencies up to about 13 MHz will be reflected somewhere in the ionos-
phere, and frequencies higher than this may pass through these layers and
into space. Note that these results were achieved for the average daytime
electron concentrations as given in Figure 4-5. Actual concentrations may
be slightly different and, of course, conditions change significantly at
nighttime. Both these situations affect the critical frequencies and show
why continual monitoring of the ionosphere is important.
As noted above, equation 5-1 only addresses a signal propagating nor-
mal to the ionospheric layers. A signal that encounters the ionosphere at
some angle 9, as depicted in Figure 5-3, will encounter more charged par-
ticles as it passes through the regions and will have a better chance to be
refracted. As a result, higher frequencies than those obtained for normal
incidence will be turned depending on this angle, as can be determined
from equation 5-2.