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# A GPS system for indoor environments

## Indoor positioning systems calculate your location within 12 inches

When I was a cop, one could interrupt any conversation with an announcement of “I’ve got IPS” and get everyone’s immediate attention. “IPS” was our local shorthand for “Important Police Stuff” (okay, we didn’t say “stuff”), and covered genuine operational details that, if not properly disseminated, might make someone look foolish or get them hurt.

A technology development around the corner is also called IPS, but refers to something entirely different: Indoor Positioning System.

We’ve gotten used to having Global Positioning Satellite (GPS) information available, and it’s easy to forget how cool it is. A constellation of satellites maintained by the U.S. Air Force orbits the earth at an altitude of about 12,600 miles, distributed so that at least six are visible from any point on the planet’s surface at any given moment (there is an excellent animation of this arrangement here).

Each carries an extremely accurate clock and a radio transmitter. The satellites broadcast a time signal to the GPS receiver in your car, phone or other device. With the time and speed of light as constants, the distance between the satellite and the receiver is calculated by noting the time difference between the received signal and the actual time. If you know the distance between the receiver and at least four satellites, you can calculate your position, including altitude. The GPS receiver does this on a continuous basis, updating the location data several times a second and translating it to latitude and longitude coordinates or a point on a map display.

This isn’t to say that GPS doesn’t have its limitations. If you don’t have a clear view of the sky, the signals (which are only 50 watts at the origin) may not penetrate to your receiver. Inside a building, you’re lucky to get a fix at all, much less determine where you are or on what floor.

IPS calculates position not from overhead satellites, but from proximity to earthbound Wi-Fi, Bluetooth and Near Field Communications (NFC) transmitters that have been mapped and identified. The cellular industry has for many years been using a similar method for calculating the position of mobile phones not equipped with GPS. By measuring signal strength and time codes transmitted between a cellular handset and multiple transmission towers, a not-especially-precise location is calculated. IPS uses this method between local, low-power transmitters usually installed in buildings, rather than outdoors.

IPS can yield extraordinarily precise location data, within about a foot. The more transmitters that are in range of the receiver, the more precise the determination. In places like malls and office buildings, Wi-Fi access points are ubiquitous. You may not see all those available when you power up your laptop or smartphone if the owner has the service set identifier (SSID) transmission turned off to discourage freeloaders or people attempting to break into the network, but they are still detectable and their signals measurable for IPS purposes. Bluetooth beacons established for just this purpose would help to bridge dead areas where Wi-Fi signals don’t penetrate, and NFC transmitters are becoming more commonplace at points of sale where people can use smartphones to make purchases.

A company called Broadcom recently introduced a chip for smartphones that allows IPS location data to be generated from these Wi-Fi, Bluetooth and NFC radios. They will be commonplace within a few years, as smartphones replace “dumb” cell phones.

Why should you care? It’s fairly easy to locate someone in an open area if they have a GPS device, but not so easy if they’re in a building—especially a multi-story building. Cops and firefighters search buildings all the time, and when someone is in trouble and needs help, locating them inside that structure is often problematic. Some law enforcement agencies function inside buildings most of the time, and police in larger cities conduct “vertical patrols” where they walk the floors of high-rise buildings.

With the combination of GPS and IPS and the other sensors inside your smartphone, your path and present location are easily determined. The GPS receiver tracks you until you enter the building, and IPS takes over from there. If you wander away from the necessary transmitters, the phone’s gyroscope, accelerometer and compass can estimate your travel from the last known data point by dead reckoning.

This could be bad news if you’re trying to hide from your supervisor, but the added safety of being able to find officers calling for assistance more than makes up for it.

Tim Dees is a writer, editor, trainer, and former law enforcement officer. After 15 years as a police officer with the Reno Police Department and elsewhere in Northern Nevada, Tim taught criminal justice as a full-time professor and instructor at colleges in Wisconsin, West Virginia, Georgia, and Oregon.

He was also a regional training coordinator for the Oregon Dept. of Public Safety Standards & Training, providing in-service training to 65 criminal justice agencies in central and eastern Oregon.

Tim has written more than 300 articles for nearly every national law enforcement publication in the United States, and is the author of The Truth About Cops, published by Hyperink Press. In 2005, Tim became the first editor-in-chief for Officer.com, moving to the same position for LawOfficer.com at the beginning of 2008. He now writes on applications of technology in law enforcement from his home in SE Washington state.

Tim holds a bachelor’s degree in biological science from San José State University, a master’s degree in criminal justice from The University of Alabama, and the Certified Protection Professional credential from ASIS International. He serves on the executive board of the Public Safety Writers Association.

Dees can be reached at tim.dees@policeone.com.

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