Snapshot or movie? Exploring the differences between RFID and RTLS

The terms Radio Frequency Identification (RFID) and Real-Time Location System (RTLS) can cause confusion since they are often used interchangeably. In fact, they mean something very different and RFID and RTLS solutions are used in very different ways.

RFID is the term used to describe the wireless retrieval of an identification code from an electronic tag using a radio signal emitted from the tag. These systems were developed to solve some of the shortcomings of barcodes, which can only be read at very short range, are easily damaged and become unreadable - if you can’t see the barcode you can’t read it. RFID systems solve this problem by using a radio signal rather than an optical scanner, and that radio signal can be read over longer ranges, even when no line-of sight exists between the tag and the reader. RFID tags come in two varieties: passive and active. Passive tags use no battery, and instead derive power from the radio signal emitted by a reader. These tags can truly be thought of as “next generation barcodes.” By contrast, active tags are powered by batteries, and as a result can transmit signals much further (10s and even more than hundred meters). This type of tag has advantages in certain situations but the battery makes them more expensive both from a manufacturing and maintenance standpoint.

Degrees of location

Real-Time Location Systems (RTLS) not only retrieve the ID of an electronic tag but also pinpoint its location in real-time. In order to do this, the signal from the tag is received at multiple sensor locations which use methods such as triangulation or multilateration to calculate the tag position. This seems to be very different from the definition of an RFID system. So why is there confusion? The confusion comes from defining exactly what is meant by “location information” and the following examples helps to illustrate the point.

Area location

RFID systems only provide
 limited location information

A single RFID reader is placed at the center of a warehouse,and items are identified using long range RFID tags. Since the reader cannot physically hear tags outside the warehouse, it makes a list of all the tags that it can hear, and labels them as “in the warehouse.” In this case “in the warehouse” is location information derived from an RFID system comprising a single reader and active tags.

Proximity location

Two RFID readers are placed at either end of a warehouse populated as above with active tags. Both readers hear all tags, but they are connected via a network to a software application which compares the signal strength received. For a given tag, this application determines that the northern reader is receiving far more signal than the southern reader, and determines that the tag is therefore in the northern part of the warehouse. Again “in the northern part of the warehouse” is location information derived from a RFID system comprising multiple readers and active tags.

Choke point location

A single RFID reader is placed over a doorway, under which items pass which are identified using passive tags. The reader can only detect tags that are within 1,5 meters (due to their short range), confirming that any tag that it does hear must be in the vicinity of the doorway. Each tag ID is recorded as “at the doorway” along with the time at which it was detected. In this case “at the doorway” is location information derived from a RFID system comprising a single reader and passive tags.

Real-time location

A network of RTLS sensors is distributed throughout a building, and items in the building are identified using active tags. Anywhere in the building, multiple sensors can receive the signal from a given tag and measure either its signal strength, its time of arrival or its angle of arrival (or sometimes more than one of these). This information is passed via a network to an application that calculates the tags coordinates in the building. In this case “coordinates in the building” is location information derived from a network of RTLS sensors and active tags.

RTLS allows users to identify products at any stage
 of the assembly line and to control processes
based on the relative location of
products, tools, and mobile devices.

A better definition

These examples make both the reason for the confusion and the solution clear. Any system can be thought of as providing “location information” - even a barcode scanner. But a better definition of the term “RTLS” can immediately distinguish these systems from RFID systems: A Real-Time Location System is any system using a network of sensors to determine the coordinates of a tag in real-time, anywhere within an instrumented area. Using this definition we immediately understand that “in the warehouse” and “at the door just a minute ago” do not fall under the definition of RTLS. Rather “on shelf B7 on aisle 9 of the warehouse” and “entering through the door and turning towards aisle 3” are the hallmarks of location information delivered by an RTLS solution.This definition also makes no mention of radio frequency on purpose. RFID very clearly means radio frequency ID, without exception.

One further differentiator is that RTLS tags and sensors, whereas predominantly RF, can also use other technologies such as infrared and ultrasound. A good way to think about the difference is to think of RFID as a fixed still camera, and RTLS as a panning video camera. The fixed still camera can provide information about the scene within its field of view at the time a photo is taken. By contrast, a video camera that can pan and zoom provides continuous, real-time scene information over the entire area. This is the fundamental difference between RFID and RTLS: snapshot vs. movie. A series of snapshots at strategic times and locations can help build a picture of what was going on in the past, but only a video camera with a live feed can give real-time information about what’s going on, all the time.  That’s the power of RTLS.