You timed your question well - in exactly a week we will have the 20th anniversary of the Omega system permanent shutdown.

Kritchlow said: ↑

   What exactly is "Omega"? ...Is the signal transmitted from the ground?

Not is but was. We are taking past tense here. Yes, a number of countries participated to have this almost global aviation/oceanic global navigation system through system of ground antennas/transmitters. The best positional accuracy you could achieve was about 3-4 miles. VLF don't 'bounce', they hug the terrain, can go around mountain ranges, all that by the principle of diffraction. My atomic watch today is also getting 'synced' up using (similar) LF signal (antenna in Fort Collins, Co). Omega receiver was a small suitcase -like box and I bet you needed a trained operator. I think (I could be wrong) Omega was ultimately much more popular on water than in the air.