An ionogram is a graphical representation of HF propagation, gathered from ionosondes. An ionosonde is a specialized HF transmitter and receiver that is capable of bouncing a radio signal off of the ionosphere, and listening for the bounce. Ionosondes are also known as chirpsounders. Using this information, it is possible to determine the critical frequency of the F2 layer. The critical frequency of the F2 layer is important in NVIS communications, because it tells you whether or not your signal will propagate at your chosen frequency. The maximum usable frequency is three times the critical frequency.

As promised, here is a link to the Region 6 Army Mars Solar Weather site:
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