An impedance-modulated-reflector system

A radar signal reflected front a target (e.g., aircraft) has the same time variation as the incident signal. (This is aside from whatever Doppler shift is caused by aircraft motion.) But if the reflecting target has a time-varying reflectivity, the reflected signal will have that time variation encoded on it. This phenomenon permits communication between two (or more) points while having a radio-frequency (RF) power source at only one of them. Conventional radio communication requires a RF power source (transmitter) wherever messages are initiated in a network. The idea of using modulated reflectors for communicating is not new. An article Stockman (1948) considered this possibility, envisioning however that the modulation would need to be achieved by mechanical means. An advantage of impedance-modulated reflectors (IMRs) is that they are inherently covert and, therefore, hard to detect. The passive-node hardware can also be made small, light weight, ruggedly reliable and power parsimonious. (Only a small amount of power is needed to operate the ancillary electronics.) The possible ways the modulated-reflector communications might be used is only limited by the user's creativity. The system we describe was developed using an IMR as the passive node antenna (PNA). It is for a medium range (up to 10 s of km), low power (a few watts at the active node), slow-scan, remote video surveillance. This specific system also employs compression technology to permit utilization of narrow band satellite and telephone channels.