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A feature of the ionosphere is its ability to reflect radio waves. However, only radio
waves within a certain frequency range will be reflected and this range varies with a
number of factors.
The most widely used instrument for ionospheric measurement is the ionosonde. The
ionosonde is essentially a high frequency radar which sends short pulses of radio energy
into the ionosphere. If the radio frequency is not too high, the pulses are reflected
back to earth.
The ionosonde records the time delay between transmission and reception of the pulses.
By varying the frequency of the pulses (typically 1-22MHz), a record is obtained of
the time delay at different frequencies. This record is referred to as an ionogram.
The highest (see note) frequency which the ionosphere will reflect vertically is
called foF2. These foF2 measurements from various sites can be used to create a map of
foF2. The data used to produce the map of the USA region are from USAF observing sites
and obtained from Space
Environment Centre, Boulder Colorado.
The above map can be used as a guide to NVIS ionospheric frequency support.
Map is updated at approximately 40 minutes past the hour.
Note: The geomagnetic field splits a radio wave in the ionosphere into
two separate components, termed the ordinary(o) and extraordinary(x) waves. It is the
o-wave which is routinely scaled from ionograms.
The data presented in this page are experimental, and are derived from the automated
interpretation of regional ionograms.
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