Piggyback secure encrypted link

Satellites broadcasting television programmes intended to be picked up by relatively small antennas on the rooftops of the intended audience have high power transmitters. They effectively act as "amplifying mirrors", receiving the feed from earth, translating it to a different frequency, and then re-transmitting it earthwards.

A transmitter can inject intelligence, disguised as random noise, into the uplink. This can then be picked up by any receiver in the service area of that satellite transponder. Normal receivers are designed to reject such random interference, but a custom modified receiver can reject the legitimate programme, tune into the seemingly random interference, and extract the coded information. This link can be used to exchange keys used for secure communication by other means, or short text messages.

The transmitter is equipped with a generator of pseudorandom noise and the intelligence is mixed with it to obtain a noise-like signal. These are shaped into pulses, of a sufficiently low rate in order not to attract attention (this limits the speed of the link) and then used to generate noise pulses having a bandwidth equal to the passband of the transponder. This can be extremely simple, a tuned antenna and a spark gap could possibly suffice.

The receiver is equipped with the identical PRN generator and extracts the pulses using digital processing of the decoded baseband signal. A normal satellite receiver, if equipped with a DSP receiver, could be reprogrammed to do this.

Since the transmitter and receiver could be located anywhere in sight of the DTH satellite, this technique has the potential to span large distances.

It is secure, because there is no record of the conversation as would be the case if, for example, telephones were used, either wired or wireless.