There are many kinds of surveillance equipment for monitoring visual, auditory and communications data. Some are widely available; others are legally or financially out of reach of anyone except government agencies. Technology is not limited simply to tools for gathering information, but extends to systems for interpreting data and evaluating patterns. Each kind has particular uses and limitations. Purposes of surveillance include detection of wrongdoing, crime prevention, safety, scientific research, personal security and public safety.
Cameras
Digital cameras continue to offer ever-increasing capability in smaller packages for lower cost. Coupled with the greater ease with which digital imagery may be analyzed and manipulated, this has led to the steady decline of analog cameras for most purposes. Microelectronics allows these devices to built very small for easy concealment. There is an inverse relationship between small size and resolution, but this kind of surveillance equipment usually offers adequate capability when used close to the target(s). Software can be used to collate and evaluate footage, but has only a limited ability to identify specific individuals or objects. Cameras which provide imagery outside the visible light spectrum are also available, but are more expensive. Infra-red imaging is the most common, but models which provide imaging using other kinds of electromagnetic emissions (for example, x-rays or gamma rays) can also be had.
Audio Surveillance Equipment
Most frequently, small and easily concealed microphones are used for intercepting conversations. Parabolic microphones are designed to receive sound from only one direction, and are valuable in crowds with a lot of background noise. Laser microphones bounce a laser off a hard surface; vibrations in the surface caused by sound waves change the way the laser light is reflected back to the transmitter, allowing sound recording. The mouthpiece within phones can also be remotely activated and used to record noise even when the phone is "off."
Signals Tracking
Tiny radio frequency ID tags, costing a few cents each, can be placed on objects or individuals and read from several meters away. Each tag can transmit a unique identifying signal, allowing scanners to identify the tagged subject. Larger transmitters, like GPS units, use satellite networks to achieve much greater range and are often employed for discreetly tracking vehicles. Scientists may also use GPS tagging to track wildlife migrations.
Signal Interception
Signal surveillance equipment can also be used to intercept electronic/radio communications. Electronic bugs can be physically attached to telephone lines and switchboxes, allowing calls to be rerouted or monitored as desired. Radio equipment can also be used to listen to conversations transmitted over the airwaves. However, this approach can have variable results depending on atmospheric conditions, whether the transmission is unidirectional or dispersed, and whether the conversation is conducted in code.
Computer Surveillance
Software embedded on a computer's hard drive (frequently as a virus) can record that system's activities including internet traffic and data storage. Specialized equipment can also detect electromagnetic emissions from a computer's monitor and keyboard, making inferences about what is being displayed on screen or being typed.
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