All about Wi-Fi location tracking
Finding things is easy with Wi-Fi
By Joanie Wexler, Network World | Published: 10:00, 04 April 2006
There are a number of technical methods of wirelessly tracking down people, equipment, and goods. What applications require location capabilities, what are the various methods of wireless location tracking to support them, and how do they work?
Let's start with the applications. Here are a few things you might want to do with wireless location information:
- Find an emergency cellular or voice on Wi-Fi telephone caller.
- Find wireless devices that are unauthorised or otherwise might pose a security threat.
- Troubleshoot sources of wireless network interference.
- Conduct a WLAN site survey.
- Locate assets and equipment that have been stolen or misplaced.
- Speed and enhance a workflow process, such as the shipment of goods.
- Determine a colleague's "presence," or availability status, based on location.
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Depending on the application, there will be different requirements for the precision of the location measurement and its degree of accuracy. For example, it might be "close enough" to know that a person or object is on a certain floor of a building (degree of precision) and for that measurement to be successfully calculated 99 percent of the time (degree of accuracy).
The general whereabouts of a person might be sufficient for the presence application, for example. In other cases, greater precision may be needed with greater accuracy, as in the case of an emergency caller.
There are several algorithmic approaches to location tracking for use in these applications, and they have various degrees of precision and accuracy. Among them:
- Trilateration and its cousin, triangulation.
- Nearest sensor.
- Time distance of arrival (TDOA).
- Received signal strength (RSS).
- RF fingerprinting.
Let's take a look under the hood at the basics of these algorithms
This is the simplest method, though by itself, it is the least precise. This capability, supported by most wireless network vendors in their management systems, determines the 802.11 access point (AP) or cellular base station to which a client device is associated. It assumes that this sensor is the closest sensor to the device. It then computes how far the signal radiates.
The diameter of the 360-degree radiation "cell" surrounding the sensor (in three dimensions, mind you) is as precise as this method alone gets, even presuming that the client does indeed associate with the nearest sensor. If an 802.11b/g AP has
approximately a 100-by-100-foot coverage area, for example, the nearest-sensor method tracks the client to within a 10,000-square-foot area. Note, though, that a client might associate with a sensor a bit farther away if the nearest one is overloaded or its signal strength is otherwise not as strong.
The nearest-sensor measurement can be combined with others to pinpoint location more precisely. "Triangulation" measures the angles between three or more nearby sensors (or other reference points). Where they intersect is calculated as the client location. Precision within 50 metres is generally accepted for triangulation, according to Diana Kelley, senior analyst at Burton Group. Trilateration measures the distance between sensors or other reference points, rather than the angles between them.
GPS, TDOA and RSSI
On the cellular side, GPS systems combine triangulation with a measurement called time difference of arrival (TDOA) over a network of satellites. TDOA measures the relative time delay of signals arriving at different sensors and can be used with triangulation in 802.11 networks, too. Because time is proportional to the distance traveled, the distance to each sensor within range can be estimated and, consequently, the location of the client. In addition to TDOA measurements, received signal strength indication (RSSI) can be used to measure the RF power loss between transmitter and receiver to calculate distance.
To date, GPS isn't used much in 802.11 WLANs because GPS chipsets are expensive, compared to using information radiating from a Wi-Fi client, and satellite reception within buildings can be iffy.
A more sophisticated category of location tracking used in 802.11-based WLANs is called RF fingerprinting. This technique uses intelligent algorithms to improve location-tracking precision by accounting for the environmental effects - such as an object, human, mirrors, windows, attenuation and multi-path - on the wireless signal. A "fingerprint" of the wireless environment is calculated by a physical walk-around using a handheld spectrum analysis device. These measurements are later compared to deviations in the real-time environment to locate the client device.
Several Wi-Fi system vendors support a form of RF fingerprinting in their management systems to enable security and management of their own WLANs. So do a number of third-party location specialists.
Let's take a look at those companies.
Some location-tracking technology suppliers are makers of wireless LAN communications systems that layer location services onto their systems. Others are third-party location specialists that provide overlay wireless tracking systems.
Several fall into the "RF fingerprinting" category, using intelligent algorithms to account for environmental effects on wireless signals. As described above, a wireless "fingerprint" of the radio environment is created by a physical walk-around using a handheld spectrum analyzer (or are auto-calibrated in some systems). They are then compared to deviations in the real-time environment to locate the client.
Below is a sampling of vendors that fall into the RF fingerprinting category (companies are Wi-Fi communications systems vendors unless otherwise noted):
Auto-calibrates the fingerprint. Claims precision of less than 12 feet depending on access point (AP) vendor. Primarily focused on security applications.
Auto-calibrates the fingerprint. Claims precision of "within a few metres." Requires Cisco lightweight APs, controllers, and
Cisco 2700 Wireless Location Appliance. Uses technology for many applications.
Performs manual fingerprinting using handheld spectrum analyzer. Focused on troubleshooting and security applications, locates any device sharing the 2.4GHz or 5GHz unlicensed WLAN spectrum that could cause interference. Among them: 802.11 devices, microwave ovens, cordless phones and Bluetooth devices.
Newbury Networks (overlay)
Holds a patent for its particular RF fingerprinting algorithm (U.S. Patent No. 6,674,403 B2). Has expanded its 802.11-centric Newbury Location Platform, adding asset tracking and content delivery applications to its existing WLAN security application. It also says it intends to open up the platform to third-party partners.
Uses fingerprinting technology of partner Ekahau. Also uses internally developed Received Signal Strength Indicator (RSSI),
triangulation, nearest AP technology. Focused on site-survey applications.
Among the companies that don't use fingerprinting:
Aruba Wireless Networks
Advocates deploying a very dense grid of its 802.11 sensors, which continually communicate with one another, so that changes in the environment are always accounted for.
Network Chemistry (overlay)Creates a "signal propagation model" for each sensor showing what signal would be expected to be received at the sensor from every possible floor plan location. Then compares model with actual sensor readings, computes error between actual and model, and determines location based on where error is lowest.