The Internet of Things: Real Time Location Services

The Internet of Things: Real Time Location Services

Written by Dave Harris, trueCABLE Technical Specialist, BICSI INST1 Certified

As you are probably already aware, the Internet isn’t just for people anymore. Nowadays there are actually more devices operating autonomously on the Internet than there are people. Telephones, cameras, security devices, wireless access points, smart doorbells, smart refrigerators and even smart toilet seats all use the Internet to do their jobs. Many more devices will feature Internet connectivity as more technologies are developed. Read The Internet of Things: PoE Lighting to learn more.

Real Time Location Services

Imagine that you are a biomedical engineer at a large health care facility. Among other things, biomedical engineers are responsible for the care and maintenance of medical equipment. A large hospital can have thousands of pieces of equipment in service at the same time. Many medical devices are portable, either hand-carried or rolled on wheels from location to location within the facility.

Most equipment will have an assigned “parking spot” so that they can be found when needed. But what if the device is in use? It might have been moved multiple times since it was first taken from its “spot.” How do you find it when you need it? You could send a relatively high-paid medical professional door to door to try to find it. That’s the old way. The new way uses a technology called “Real Time Location Services” (RTLS).

Asset Tags and Readers

To utilize RTLS, a small adhesive-backed “asset tag” is affixed to the equipment that needs to be locatable. That tag contains an RFID chip. Multiple network devices called “readers” are installed at various locations around the property. They're called “readers” even though they are radio devices that can both send and receive. The reader sends out a signal that the passive RFID chip receives. The tag has no power supply, but it can use the energy of the incoming signal to send out a small electronic “chirp” that is received back at the reader. If the chirp signal is received at more than two readers, software uses the elapsed times for the chirp to go from the tag to each reader to triangulate on the position of the asset tag.

There are some serious limitations to this method. Since the asset tag is unpowered, the chirp from the asset tag is actually a pretty weak signal. For that signal to be received at more than two different readers, the readers must be located within a relatively short distance from each other. In a facility as large as a hospital, the number of readers required for this is financially prohibitive.

In practice, the number of readers in the facility is limited to coverage of “choke points,” such as doorways, main hallways and elevator doors. In this way, the system logs the arrival of a passive tag at a particular reader, and its general location can be deduced. This is the least accurate method for RTLS and is normally used for smaller, less expensive, or less critical equipment.
An RTLS reader located above the ceiling near an elevator entrance
An RTLS reader located above the ceiling near an elevator entrance

Active Asset Tags

We can overcome some of these limitations by just beefing up the asset tag. Let’s make an asset tag that contains a battery, antenna, and radio that can both send and receive signals. We could even include an accelerometer to signal when the equipment is moving. This is called an “active tag” or a “transponder.” Because it’s powered, it can send a much stronger signal than the passive chirp of an RFID chip. This signal has the strength to reach multiple readers, so its location can be determined by triangulation. The system can even track equipment in motion in real time.

Needless to say, an active tag is larger than a passive tag. The passive tag is a sticker about the size of a large coin. The active tag can be about the size of a pillbox. The active tag is also more expensive, so fewer of them are used. This method is usually reserved for more expensive and critical equipment. Since the readers are still relatively far apart, this method can usually produce a resolution of a few meters. That’s still usually close enough to find the right room.

Bluetooth LE - Equipped Wireless Access Points

In a short amount of time, RTLS technology has progressed much further. In a commercial setting, WiFi access points are typically spaced fairly close together to ensure sufficient coverage and user density. In order to alleviate the distance problem in RTLS, readers are now being included in some commercial access points. Because these readers are close together, Bluetooth technology is used to communicate with the asset tags.

The Bluetooth reader within the access point contains multiple directional antennas. When a signal is shifted between elements of the antenna array the angle of the incoming signal can be calculated. The accuracy of this calculation can be further enhanced by “phase shifting” (not covered here) and by statistical methods.

This Bluetooth antenna array contains sixteen unique antennas. Eight antennas are reflectors to ensure that signals are directed away from the access point
This Bluetooth antenna array contains sixteen unique antennas. Eight antennas are reflectors to ensure that signals are directed away from the access point. The other eight are unique antennas used to determine the angle of arrival for a signal from an asset tag. Image by
When the signal from a tag is received by multiple locators, the tag location can be determined by calculating where the ‘rays’ from each locator intersect. This method is known as “Angle of Arrival (AoA) Technology.” This technology can produce resolutions of one to three meters.
Angle of Arrival (AoA) Position Determination
Angle of Arrival (AoA) Position Determination
As you might expect, this technology is not just about hospitals. You can stick an asset tag on just about anything. Some of the assets that it might be worthwhile to keep track of include:
  • Firearms
  • Lab equipment
  • Telecom equipment
  • Medical equipment
  • Vehicles
  • Artwork
  • Documents and files
  • Yes, even people

This represents a simple introduction to the technology behind Real Time Location Services. As this is an introduction, there are many methods and details that aren’t covered. (Phase shifting, anyone?) Keep an eye on these blogs for more examples from The Internet of Things.

Happy Networking!! 


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