One of the more interesting modes is Meteor Scatter. That mode uses the ionized trails from meteors to bounce signals. The trails are normally good for a few, maybe tens of seconds at the most. The USFS used this to monitor snow level sensors out in Utah & the Rocky Mtn. states back in the '70's & '80's. A high power transmitter "polls" the nodes and when a node detects it's poll it transmits a signal back to the receiver. Since the information is low bandwidth, not time sensitive and can tolerate missed polls it was perfect.
Hams are starting to play with this method for email and positioning information with very narrow bandwidth DSP based data modems. They have used it with CW (morse code) for many years as has the US Military.
ShaneGibson responds, "Wow. So, there must be some pretty sophisticated tracking and aiming systems that go with the antennas for this? I'd imagine a meteor trail is an awfully small target to be trying to hit. Not to mention the issues of identifying one fast enough, aiming at it, and initiating your signal before it dissipated."
And Barry writes back, "No pointing. The transmitter antennas can be omni directional or wide beamwidth yagi's. Same with the receivers. I've heard meteor scatter signals, most sound like a "ping" and last no more than 1/4 -1 second, the longer trails start out as a sudden peak in a signal that decays over a few seconds. Hams working high speed CW (HSCW) normally coordinate on HF (15 meters / 21 mhz) via voice (SSB)."
Meteor scatter sites:
Sounds of meteor scatter: