Tracking mobile assets with RTLS

Real-time location systems track critical assets to within a few feet of their actual location in the plant, yard and warehouse.

By Bob Trebilcock, Editor at Large -- Modern Materials Handling, 10/3/2007

Not having the right part for a manufacturing process or the right inventory for an order can bring a factory or warehouse to a screeching halt.

That’s where real-time location systems, or RTLS, come into play. These solutions use active and passive RFID technology to track the location of critical assets to within a few feet in real time.

That said, not all RFID solutions include real-time locating capabilities.

“In an RTLS solution, you have all the capabilities of an RFID system,” says Matt Armanino, executive vice president for worldwide operations for WhereNet, a provider of RTLS solutions using active RFID tags. “But you also get the addition of a layer of software that allows an RFID tag to be located on a real-time basis with x,y coordinates.”

The solution works because the RTLS tag is programmed to broadcast a signal identifying its location at regular intervals. The signal passes through that layer of software to calculate a location.

How accurate is an RTLS solution? Depending on the technology being deployed, RTLS systems can track an asset to within a few feet of its actual location. 

How real-time technology works
Not all RTLS systems work the same way. In fact, there are several approaches on the market today.

Time difference of arrival (TDOA): Also known as triangulation, TDOA solutions use three RFID readers to receive a signal coming from an active RFID tag. The readers record the time they receive the signals and forward that information to a central location. The central location measures the time difference between the three signals to triangulate a location. TDOA solutions require a direct line of sight between the tag and the reader. For that reason, they work best outside, or in facilities with high ceilings and open spaces.

Received signal strength indication (RSSI): RSSI solutions start with a map of the strength of the radio signals at various points in a facility. When a radio frequency (RF) signal is introduced into that environment, RF access points read the strength of the signal. The system then compares that to the archived RS fingerprint to determine a location. Due to the vagaries of a radio frequency, including interference, RSSI solutions work best in tight indoor environments.

RFID solutions: This approach relies on an active RFID tag programmed to send out a signal at specific time intervals to RFID readers. Those signals are used by the system to calculate location.

WiFi solutions: These solutions use specially designed WiFi-enabled tags that communicate their location over an existing 802.11 wireless infrastructure.

Sensor technology: In addition to providing information about the real-time location of an asset, the next step is to pair RTLS technology with sensors to monitor the physical condition of an asset. That might include attributes like the temperature or humidity of a storage area.  

Benefits of RTLS
Regardless of the technology, RTLS systems allow users to automate business processes that have been done manually for years and to collect more information than has been possible in the past.

“Users are deploying the technology because they need to have real-time connectivity to their mobile assets,” says Armanino. “You find many companies that have spent millions on decision support systems that are only as good as the timeliness of the information you’re feeding them. An RTLS system bridges the data divide between physical assets, a mobile workforce and a real time enterprise or execution system.”