The signals intelligence field has many devices able to actively monitor areas of interest; however, the disadvantage of active monitoring is its intrusive and detectable nature. This led to the need for a passive, nondetectable method to monitor focal points within a specified area of interest.
SRC began experimenting with commonly found devices that emit RF signals to monitor the effect that human presence would have on the strength of the signals being transmitted. Using readily available technologies - a wireless router as a transmitter, and a computer laptop as a receiver - we discovered that you could accurately monitor human presence and even movement in a building. When humans were present, signal strength decreased and signal distortion increased. Monitoring the data over a period of time also allowed for the detection of the approximate number of people within the area.
The ability to use preexisting equipment and RF signals means that no extra equipment is required to actively emit RF signals, a key component to allowing for undetectable monitoring. This provides two unique advantages: the signals can originate from any number of sources ranging in frequency, including cordless phones and wireless routers; and the signals can pass through walls with minimal impact to the accuracy. This exciting leap in SIGINT data collection has many potential uses, such as monitoring buildings of interest and the movements of subjects under investigation, or gathering intelligence in urban warfare environments or volatile hostage situations.
This discovery, that radio frequency signals can be used to passively monitor human presence within an urban environment, is patented by SRC, Inc. (U.S. Patent No. 7,940,715 ).
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