A few comments to The Custodian's excellent node:
When you direction find in aeronautical navigation, it's true you use NDBs and VOR and VORTAC transmitters, but the aircraft pilots also have to know where they are - so all aviation maps show NDBs and VOR stations - their locations on maps. Then the pilots can triangulate where they are. Aircraft do not so much know the positions of the NDBs as they know the angles to the NDBs when they are flying - and this helps them compute a solution for their location in the air.
The solution, by the way, gives only latitude and longitude. It does not tell them their altitude. For that they have to use the aircraft altimeter (a fancy barometer) and, when close to the ground for landing, radar altimeters if the aircraft are so equipped.
NDB: Non-directional beacon
VOR: VHF Omnidirectional Range
VORTAC: VOR-TACAN transmitting station - a joint FAA/military station that can be used by both commercial aircraft (that use VHF aeronautical direction-finding frequencies), and military, that use UHF frequencies and special military signals not accessible by commercial or private aircraft.
A comment on the use of the term sideband. Single sideband (SSB) is a narrowband transmission technology. It's really easy to both detect the signals and to compute a direction to them. On a spectrum analyzer, a SSB signal is very visible - it stands above the noise floor by 10 dB or more - like a mountain over a flat valley.
By contrast military spread spectrum signals have their energy spread over an incredibly wide bandwidth relative to the carrier frequency. The power spectral density is so low that some systems (like one I helped design for Harris Corp) have PSD levels BELOW the kTB noise floor. That can't be detected by RDF principles. (The only times you can are when a signal is unencrypted and momentarily not spread as widely, as is the case during initial signal acquision.)
Honeywell (and the company before it, I forget...) sold an ADF bit of equipment that used two Adcock antennas to triangulate right from one setup. Another RDF manufacturer is Rohde and Schwartz. They sell to Euro governments, who drive around in vans and detect unauthorized television receivers by sensing their intermediate frequencies. R&S equipment is kickass - and expensive. Another company is, I believe, Frequentis, an Austrian outfit.
I wrote this in the form of a list of errata when The Custodian asked me to proof his work. So I did, and he recommended that I just repackage the comments and post them as an addendum to his piece. I have worked in the general field of communications engineering (and in satellite design, communications systems architecture, navigation, radars, wireless, aeronautics, standards committees, etc.) for about thirty years.