Turn radio echoes into an ionogram

A topside sounder sends radio pulses at a sequence of frequencies and records returning signals. An ionogram organizes the response by frequency and delay, often with delay expressed as apparent range. It is a record of radio propagation, not an optical photograph of a solid atmospheric surface.

NASA’s Alouette 2 dataset describes swept-frequency operation, rather than the additional fixed-frequency operation available on the later ISIS instruments. One sweep samples frequencies sequentially. Treating every point as a simultaneous snapshot can hide changes during acquisition, especially when comparing rapidly varying conditions.

For a simple range coordinate, multiply the round-trip delay by the speed of light and divide by two: apparent range = c × delay ÷ 2. With c = 300,000 km/s, an invented 20-microsecond delay gives 3 km. Convert microseconds to seconds first: 20 microseconds is 0.000020 seconds. The division by two accounts for outward and returning travel.

The real restored Alouette 2 recordings were digitized at 40,000 samples per second: 25 microseconds between samples, corresponding to 3.75 km in apparent range. Sample spacing is not a promise of equally accurate true altitude. Propagation through plasma and the geometry of the path require further interpretation.

One echo has an outward and a return journey

The Alouette 1 figure compares a scanned ionogram with extracted points. The plot labels its vertical coordinate depth; those mapped coordinates do not automatically become an electron-density profile. Grid marks, noise, metadata and instrument signatures can also create dark features that an extraction algorithm must distinguish from useful traces.

Before comparing two ionograms, identify the spacecraft, acquisition time, station, frequency scale and processing version. Alouette 1 and Alouette 2 are related instruments with separate records. A recent dataset update date tells you when a catalogue changed, not when the spacecraft acquired the observation.