How Do Ankle Monitors Work? GPS, RF, Range and Accuracy (2026)

Almost every ankle monitor is built around one of two technologies, and knowing which one you're wearing tells you what it can and cannot do.

Radio frequency (RF) is the older, simpler, cheaper technology. It answers one question: are you home right now? A transmitter strapped to your ankle broadcasts a continuous low-power radio signal, and a base station (a small box plugged in at your residence) listens for it. RF does not know your coordinates and cannot follow you around town — it only knows whether your ankle unit is within radio range of the box in your living room. It's used for house arrest and curfew enforcement.

GPS is the location-tracking technology. Instead of a home base station, the ankle unit itself reads signals from satellites to calculate your position, then reports that position over a cellular network. GPS monitors are used for higher-risk supervision, pretrial release, parole, and cases with exclusion zones (for example, staying away from a victim's home or a school). Many modern GPS units also include an RF/home-beacon mode so they can confirm curfew compliance at night and track location during the day.

A third category, continuous alcohol monitoring (SCRAM CAM), uses the ankle strap to sample the sweat on your skin every 30 minutes for alcohol rather than to track location — some devices combine GPS and alcohol sensing in one unit.

An RF setup has two parts: the ankle transmitter and a base receiver unit that gets installed in your home and plugged into power (and often a phone line or cellular link). The transmitter pulses a coded radio signal every few seconds; the base station listens for that specific code so it can't be spoofed by a neighbor's device.

Range is the whole game with RF. The base unit is typically programmed to detect the ankle transmitter within roughly 50 to 150 feet, though published ranges run as wide as 100 to 300 feet depending on the equipment and the building. Manufacturer specs for a common 433 MHz link cite on the order of 50 meters (about 165 feet) indoors and up to 200 meters (about 650 feet) in open air. Because walls, concrete, metal, and even large appliances weaken the signal, the installer performs a range test at your specific address to set the real boundary — which is why your usable range can be smaller than the spec sheet suggests.

When you step outside the range during a curfew window, the base station logs the departure time and sends an alert to the monitoring center; it logs your return the same way. During approved hours (work, treatment, court), your officer sets a schedule so those exits don't trigger a violation. If your monitor requires a landline or the base loses power, it can generate false or missed reports, so keep the base plugged in and never move it without telling your officer.

A GPS ankle unit contains a satellite receiver, a cellular modem, tamper sensors, and a battery. It calculates location using global navigation satellites — modern devices combine several constellations (GPS, GLONASS, Galileo, BeiDou) and often add Wi-Fi and cell-tower positioning as backup for when the sky view is poor. The position fix is computed on the device, then uploaded over a commercial cellular network to the monitoring platform. If there's no cell signal, the unit stores the points and forwards them once it reconnects, so a gap in coverage isn't the same as a gap in tracking.

There are two reporting modes. Active GPS transmits your location at frequent intervals (often every 1 to 5 minutes) for near-real-time supervision — this is what people picture when they think of GPS monitoring. Passive GPS records your locations on the device and uploads them in a batch to be reviewed later, which is cheaper and used for lower-risk cases. Some programs switch a device between modes.

The supervising agency draws geofences on a map: inclusion zones are places you must stay inside (such as home during curfew), and exclusion zones are places you're barred from (a victim's address, a school, a park, a co-defendant's neighborhood). Officers can add a buffer around an exclusion zone so an alert fires as you approach, not only once you cross the line. Enter or leave a zone outside your approved schedule and the system flags it automatically.

GPS is accurate but not perfect, and the gap matters because a bad location fix can look like a violation. In the open with a clear view of the sky, a modern unit typically resolves your position to within a few meters — some 2026 devices advertise under 2 meters under ideal conditions.

Accuracy falls apart in what's called the urban canyon: tall buildings, parking garages, dense tree cover, and structures with thick concrete or metal can block or bounce the satellite signals. In those conditions the reported location can drift from your true position by tens of meters. Devices that fall back to Wi-Fi or cell-tower positioning indoors usually hold accuracy in the 15 to 30 meter range — good enough to know your neighborhood, not good enough to prove which side of a property line you're on.

This is why drift is worth documenting. If a monitor briefly shows you inside an exclusion zone you never entered, or 'jumping' across a map, that can be a signal artifact rather than a violation — being in a parking garage, a basement, or a downtown core is a common cause. Keep a simple log of where you actually were (with times) and any receipts or witnesses; if an alert is disputed, that record and the raw tracking data help your officer or attorney separate a real breach from GPS error.

Battery life depends on the device and how often it reports. Traditional GPS units run roughly 24 to 72 hours per charge (BI's LOC8 and SCRAM's GPS units cite around 50 to 60 hours), while some 2026 low-power models stretch to about a week on infrequent reporting. Charging is done while you wear it — a magnetic or clip-on cradle plugs into a wall outlet, and most units reach a full or near-full charge in one to two hours. A dead battery is treated as a violation, so daily charging (many people charge while showering or sleeping) is part of the routine, and a low battery triggers its own alert to the monitoring center.

Most modern monitors carry a water-resistance rating (some as high as IP68) and are fine for showering and getting caught in the rain. They are not designed for swimming, baths, hot tubs, or long submersion — sitting underwater can damage the unit or block its signal, and either can read as tampering. When in doubt, keep it out of standing water and never remove it to get around the water rule.

Who actually watches the data: the technology is usually operated by a private company running a 24/7 monitoring center — the largest are BI Incorporated (GEO Group), Attenti (Allied Universal), Sentinel, and Satellite Tracking of People (Securus); a handful of firms hold most U.S. contracts. That center collects the alerts; your probation or parole officer sets the rules and decides consequences. And the question people ask most: no, a standard GPS or RF ankle monitor has no microphone and cannot hear or record your conversations — it transmits location and status data only. A small number of newer models add two-way voice so the monitoring center can call the device to reach you, but covert audio surveillance is not a feature of ordinary court-ordered monitors and would require separate legal authorization.