Keeping Privacy While Tracking

What does “privacy” mean in EM Location Tracking context

One of the most troubling aspects (mostly in Europe) while tracking an offender by a “GPS” device is the privacy. My interpretation of it, in this sense, is that the authorities respect the offender’s right to keep his location information to himself, unless this offender performs any breach. In most of the cases, the usage of GPS tracking is to enforce exclusion and inclusion zones (special zones) regime. So if this is the case, why do we need to collect location information while not in the special zones?

What are we trying to achieve with GPS tracking

From the supervision agency perspective, the “GPS” tracking intends to supervise the offender out of his residential place. Trying to understand what it means “to supervise” brings me to the following options:

  1. Knowing the location of the supervised subject continuously (very high risk, privacy is not an issue, etc.)
  2. Enforcement of special zones

The location tracking solutions, currently, are serving both options without distinction between the required option. Even if the agency is not interested with the location points of the subject while not in a special zone, the devices are reporting their lat/long points (the location information) which are saved in the memory and data-base of the EM system.  I wonder why it is needed for the second point, why do we need to store all location points even if not in the neighbourhood of the special zones? I guess the reason is that from the technology perspective it covers all options. On the other hand, when the agency wants to keep the offender’s privacy, these registered points are prohibited.

 How to combine privacy and GPS tracking objectives

The answer is quite simple: let the device report its location only while offender is in violation, such as crossing exclusion zone, or missing the inclusion zone.

There are two main approaches the EM vendors are taking with regards to the “GPS” device capabilities:

  1. Using a relatively simple device without significant calculations capabilities. The backend EM system is taking care of all calculations. In this approach, most of the devices are not aware of the special zones. They report all the location points and the EM backend system calculates whether there is a zone violation. In such architecture, it is obvious that locations points should be uploaded into the system. This is the first breach of privacy.
  2. The other approach brings almost complete independency of the location tracking device. These devices are aware of the special zones, they can calculate whether there is a zone violation or not and they can keep many location points in the internal device memory. In addition, these devices report the location point.

Using the second approach devices and disabling the upload of the location points can serve the enforcement of the special zones while maintaining the offender privacy. In order to support high reliability of such an approach, it is highly recommended to have “secondary evidence”, by another location mean, such as LBS.

Another advantage of such an approach may be increased battery life. If there is no need to upload routinely the location points, the device can decide on the enable/disable location mechanism. If the offender is far away from a special zone and movements are low, the location mechanism can be disabled and will extend the battery life.

We will be happy to get your thoughts and feedbacks in order to better understand if this approach can mitigate the conflict of privacy and offender tracking. You can send your feeback here, or by email to

GPS Location Tracking For Electronic Monitoring – Part 2

In this part I’ll describe the most common approaches for location tracking, the one and two piece solutions.


  1. 1Piece

The 1Piece solution is a single unit attached to a subject’s ankle. It can compute location and communicate with backend (EM) servers via a cellular network. In recent years, most 1Piece products have also been able to communicate with HUs via RF, so the 1Piece can be used both for location tracking and for home detention. In theory, this device covers everything; location services while outside the house, and RF proximity in house (or work)  where GPS is unavailable.

Vendors provide different types of home units. Some use RF receivers, while others use special “Beacon” equipment, which usually has a subset of RF features, like a Beacon without independent communication ability. In this case, the 1P device is still responsible for communication with backend servers. Beacons may be very small and battery-operated, rendering charging unnecessary.

Traditionally, the 1Piece is charged by connecting it to an outlet via a wired charger – in effect, connecting the subject’s leg to the wall. This is a severe limitation, but should vanish in the future with the advent of “wireless” chargers, where a device is connected to the 1Piece and charges it without needing an electrical outlet. Even today most of the vendors propose wireless charging.


  1. 2Piece

The 2Piece solution contains a GPS receiver carried by the subject on their bag or belt, along with a bracelet attached to the subject’s ankle with which it communicates by RF. The solution  can identify the GPS receiver unit’s location and communicate that through the cellular network (usually GSM).

One major difference between the 1Piece and 2Piece solutions is that the location of the unit is the same as the location of the subject in the former, whereas in 2Piece, it is only the location of the unit (the GPS receiver). With 2Piece, the subject is responsible for taking the GPS unit with them. This is not the case with 1Piece.

For 2Piece, communication must be maintained between the GPS and the RF unit at all times, as this indicates that the GPS unit is actually on the subject. 2Piece allows more flexibility in the design of the GPS unit as it’s not actually on the subject’s body. The GPS unit can operate similarly to a cell phone, with text or voice messages to communicate with the subject. There are new solutions today, which are using Cell phones as the GPS receiver for EM, since this solution contains all needed capabilities, like GSM communication, text messages and voice calls.

There is a slight difference in the installation process between installing the two solutions; the 1Piece only requires activating one device, whereas the 2Piece obviously needs two.

  1. Comparison between 1P and 2P
  • Co-operation of subject – 1P only requires subject co-operation for charging, while the 2P also requires the subject to carry the device with them. Disorganised subjects tend to have difficulty with 2P devices.
  • Communication with subject is more powerful using the 2P solution, which allows text messages and voice calls, whereas the 1P allows only more primitive communication like LEDs, vibrations and sounds. Implementation of the 2P solution in future will probably have the GPS unit in a cell phone.
  • Battery charging – until recently, the 2P had a significant subject’s mobility advantage here. However, with the new wireless charging, this advantage almost disappears.
  • GPS and cellular reception are better with the 2P because the unit is at waist rather than ankle level.

The 1P is usually cheaper and simpler than the 2P solution. Great consideration should be taken before neglecting 1P in favour of the more complex 2P.


EMiS-CON staff will be happy to assist and answer questions.

GPS Location Tracking for Electronic Monitoring – Part 1


This article is the first part out of a two, describing technical and operational aspects of using Location Tracking (GPS) solution for electronic monitoring of offenders. The second part will provide insights about the common location tracking approaches, one and two piece solutions. The working definition for Electronic Monitoring is a system that monitors people of interest to the law or under probation or supervision.

Because most of the experts, users and decision-makers in this industry are not technologically-focused, we aim to give the technical and operational background that will increase your knowledge of the area without technical explanations and jargon.

Location Tracking (GPS)

Location tracking enables community supervision agencies to track subjects anywhere, not just when they’re close to a predefined point (the RF receiver). This enables them to know where subjects are at any point in time.

Unlike RF technology, location tracking for subject supervision began about     a decade ago, in the US. A few years ago, the only EM systems in Europe were based on RF technology. However, use of location tracking technology in Europe has grown in the past four years, although it remains low in comparison to North and Latin America. Most new EM product development focuses on location tracking rather than RF technology.

The basis of location tracking is a device worn on the subject’s ankle. This device contains a GPS receiver module, a cellular communication module (usually GSM in Europe), control, indication and memory elements. The GPS receiver manages the device’s satellite signal reception and location calculation. This location is reported to back-end servers by the communication module.

Location tracking technology should meet two criteria. Firstly, it should accurately identify the device location at all times and anywhere, including underground, underwater, indoors, outdoors, etc. Secondly, privacy considerations must be kept to the fore. As location tracking devices are usually used to enforce subjects staying inside an inclusion zone or outside an exclusion zone, law enforcement agencies should not need to know where subjects are most of the time, only when they are crossing a prohibited boundary. For this reason, data collected by location tracking devices must be carefully handled.

Problems arise because to calculate location, a GPS needs to acquire information from four satellites. This means that the probability of receiving the required signals while the subject is at home is low. For this reason, there is usually no functional GPS when a subject is at home. Even outside the house, in urban and underground areas for example, reception can be dodgy, so there is a significant amount of time when GPS is non-functional.

To mitigate this problem, most equipment vendors include additional RF transmitter/receivers to locate the offender while they are in a predefined location e.g. school, work, home. This additional module, which is part of the GPS ankle bracelet, communicates with a home unit (HU), or beacon, put in set locations. When the RF and HU units can communicate, there is no need for GPS as the location is assumed known.

The problem becomes more complex in other locations. LBS (Location-Based Services) are often used instead of GPS. LBS derive location from the proximity of tracking devices and cell towers. However, as this location is usually based on a single tower, information can be inaccurate, ambiguous or misleading.

A complementary approach is assisted GPS (A-GPS). One method is assistance with time to first fix (TTFF), i.e. getting GPS location faster when the satellite signal reception is renewed. Another uses LBS as described earlier. However, the accuracy of LBS can be improved by using a minimum of three towers. Location is computed using triangulation of information about cell tower IDs and the signal strength the tracking device receives from each tower.

Other location tracking options include using other satellite systems like Glonass, which may improve reception depending on the  location over the globe; dead reckoning based on inertial measurements; WiFi-based tracking; computation of the propagation of RF waves.

A device that could track location anywhere would probably use a smart algorithm to combine and integrate some or all of those technologies.

Battery Life in Location Tracking Devices

 Battery life is a particularly important factor in location tracking devices. This includes the time it lasts after charging, and how many charge cycles the battery can withstand before replacement. Operation time after charging depends on the following aspects:

  • Battery condition – The electrical capacity of the battery degrades after a certain number of charging cycles. A battery’s ability to withstand many charge cycles depends on its quality, but in EM equipment this period is usually between 12 and 18 months (taking one charging cycle per day). Battery performance is also affected by the environment (e.g. temperature, humidity), by usage profiles and more.
  • Operational working profile – The GPS (location) and GSM (communication) modules are heavy electrical power consumers. As requirements for GPS samples and communication to the backend increase, device operation time between charging cycles decreases. Hybrid or passive modes can come in useful here. EM vendors declare a battery life between 36 and 96 hours, but without information about the usage profile this information is not useful and cannot be comparable.
  • Initial electric capacity of battery – Obviously, a fully-charged battery will last longer between charge cycles.


Pros and Cons of Location Tracking Technology vs. RF technology


  1. Tracking (almost) everywhere, not just at home.
  2. Boundary (inclusion and exclusion zones) enforcement.
  3. Potential for expansion tools (e.g. crime correlation).


  1. Subject co-operation required, especially for charging.
  2. Poor indoor performance as a standalone solution.
  3. More staff needed than for home detention.