ELECTRONIC monitoring equipment first became commercially viable for use in corrections in 1985. Since then, its use in home detention has distilled into two basic systems: programmed-contact systems and radio-frequency systems.Programmed-contact systems. The telephone linked to a computerized auto-dialer is the mainstay of this approach. A time-schedule for house arrest, worked out by corrections personnel, is established for each offender. A computer attempts periodically to contact an individual via telephone lines. Variables of age, offense, and so on, determine the intensity of contacts. When the telephone is answered, directions are given so as to verify the presence of the inmate. These can range from taking an electronic key, which is strapped to the offender, and connecting it to a modem in the phone, to placing a finger on a chin or other part of the face in front of a video-phone per command of a preprogrammed electronic voice. At the conclusion of a sequence of commands, a check is run by the central computer comparing the actual response to the expected response. A status report is generated and corrections personnel immediately check the outcome. The unpredictability of the contact combined with the threat of sanctions are what keep offenders from unauthorized absences. Radio-frequency systems. A transmitter, with a limited range (a radius not much larger than the offender's home) is strapped (usually to the ankle) to the offender serving a house-arrest sentence. A receiver/dialer is attached to the telephone and monitors the physical presence of the individual via the signals emitted by the transmitter. The unit in the offender's home uses telephone lines to call a host computer. A break in the signal caused by the offender stepping outside the range of the transmitter automatically triggers an alert call. Corrections personnel maintain the computer equipment, check that the offender's schedule of house arrest matches the stored schedule of his or her sentence, and generate daily status reports. Radio-frequency systems provide information to officials on or at a near real-time basis. Bracelets are tamper-proof and nonremovable.