Sometimes called beacon tags, location tags, proprietary VHF

radio devices, are attached to pallets or containers stored in a

large flat warehouse space. The tag generates an intermittent

signal with the tag's ID value that is broadcast over the warehouse

space. Usually, the tag is equipped with a motion sensor

so that the interval between broadcasts becomes shorter when

the tagged item is in motion. Readers are logistically located at

the corners of the warehouse space to receive the RF signals.

When a reader receives the beacon from the tag, it also receives

the strength of the signal, indicating the approximate distance

between the tag and the reader. In order to map the two-

dimensional location of the item in the warehouse space, the

beacon signal identification and signal strength must be simultaneously

read by a second reader. Both readers then send this

data to warehouse management software that maps the tagged

item location in two-dimensional space using triangulation.

Current commercial location tagging technology locates the

tagged item to an accuracy of about 3 meters using two readers.

Using more readers may increase the accuracy of location

but also extends the warehouse area.


Tags based on ultra wideband (UWB) communications are also

used in location service. In this system, all tags and readers

share a synchronized high-precision clock. The tag sends an

intermittent location signal with its ID and the time-stamp

from the tag's clock. The receiver adds its own time-stamp to

the message providing the system software with a differential

time delay between the tag and the reader. This time delay is

proportional to the distance between tag and reader. Readings

from multiple readers then allow the tagged item to be located

to an accuracy of less than one meter.


A third type of location system uses the time delay between the

time that a reader sends its signal, and the time it receives an

echo of that signal as an indication of the distance between the

tag and the reader's antenna to an accuracy of about 3 meters.

This system uses multiple antennas for each reader to reduce

the cost of the system.


It is theoretically possible to construct an RF tag with a Global

Positioning Satellite (GPS) receiver to transmit its location with

high accuracy. Here, the tag itself would compute location by

reception of signals from at least 3 GPS satellites. However,

such a tag would require an unobstructed view of the sky (outdoor

use only), and the GPS circuitry would probably use too

much energy to be powered from a battery. No commercial

GPS tags are presently available.