Plain-language answer: An ionosonde is a swept-frequency radar that measures the overhead ionosphere and produces an ionogram.
Why operators care
It can reveal critical frequencies and virtual heights near the instrument. Those local vertical measurements help explain conditions but do not directly measure every oblique path.
How to use it well
Use a nearby current ionogram, check its timestamp and quality flags, then combine it with path geometry and on-air evidence.
The common mistake
Applying one distant ionosonde reading unchanged to an entire continent.
Operating boundary
Predictions, reports, schedules, and mode descriptions guide a decision; they do not override current regulations, equipment limits, exact station conditions, or what the operator verifies on air.
