Jimmie D wrote:

SNIPPED


I know how a VOR works its the concept of space modulation I dont
understand. How can radiated enery from multiple sources combine in space
and modulate each other. This is as described by the FAA manual. I am
thinking this can nit happen but the modulation takes place only in the
receiver, not out in space as described by the manual.



I don't know if the term Space Modulation is applicable to this effect, but the
ionosphere does cause AM modulation.

The atmosphere from sea level up to approximately 300K feet altitude is an
ionized thickness. This ionization peaks at the Paschen altitude of around 200
Kft. The ionization source is the sun. We see this with xray flares and proton
effects [northern lights] and the loss of HF propagation. The lowest levels have
little ionization [C layer, D layer, etc]. At xray peak fluxes we experience
total loss of HF propagation and high absorption of 80/40 meter ground wave signals.

This ionization is not uniform nor is it static. It is quite dynamic and varies
with air density [wind, rain, snow, etc]. The variation in ionization causes
amplitude modulation of signals being propagated through the ionosphere. We
normally experience it as QSB on HF and as flutter on some over the horizon VHF
AM signals [not aurora related].

I had the bad experience of the modulation causing a false trigger in a flight
test vehicle. We had about 5% AM on a near earth telemetry signal. We had to
redesign the detector circuits to accommodate the effect.

Aircraft communications is AM and VHF. I assume, if this effect is Space
Modulation, that it becomes important on weak signal paths.