RFID: Old technology, new possibilities

For a technology only now achieving mainstream recognition, RFID has a long history. The first practical application was developed in World War II, when a pressing need to remotely identify planes led to IFF -- Identification Friend or Foe. This embodied all the basic features of RFID: an interrogatory radio signal was picked up by the object to be identified, which then automatically replied by transmitting an identifying code.

Although civilian aviation has continually developed this technology so that aircraft can be automatically identified on radar screens together with height, speed and direction, it took a long time for it to find other uses. RFID technology is limited by the cost of the components, their ability to operate at various frequencies and the amount of power they draw; it took the development of high speed, low cost and low power silicon circuits to widen the market. High value items -- notably nuclear components -- were among the first to get their own tags, but throughout the 60s, 70s and 80s the idea spread to transportation for road tolls, manufacturing for component management through production lines, animal husbandry and security.

Tagged nuclear components
The first and still the most widespread use of RFID in the retail channel was EAS, which stands for Electronic Article Surveillance. This uses one-bit tags -- they merely respond with a simple signal saying they're present -- to trigger readers on the exit of shops, libraries and so on. However, inventory and supply chain management demands full ID for tagged items, so multi-bit RFID systems are now receiving most attention. Such tags can be read-only -- they keep the identity code given to them at manufacture -- or be read/write, which is far more flexible but has a more limited lifetime and more security concerns.

Typically, a reader sends a request transmission to a tag which replies with its identity; the reader has to distinguish between potential multiple responses when more than one tag is within range, and the system has to be set up so that the tags can be read during normal warehousing or transport. RFID systems that require major changes to existing procedures have proved difficult to implement and unreliable.

Active and passive
There are two main classes of RFID tag -- active and passive. Passive tags either have no battery -- a class called pure passive or RF powered -- or a small one that runs the electronics but doesn't power the transmitter, known as active/passive. All classes of passive tags have a short range, because they rely on the energy in the radio signal from the reader to power the transmitted reply: this is normally in the order of microwatts. However, pure passive tags have no maintenance requirement and can be made very robust, capable of surviving very high temperatures and aggressive environments -- and can also be made far more cheaply than any other sort. Even active/passive types can be made so that the battery life is the same as the natural working life of the tag.

Active tags have a battery or other power source that runs the transmitter. This lets the tag radiate much higher power radio signals, and thus increases the range significantly. Even small active tags with low capacity batteries can achieve a range of several hundred metres, and provided that the power requirements can be met, an active RFID system can have the same range as any other radio system. The record here is held by the Pioneer 10 space probe, which recently went silent after 30 years of operation. It had a nuclear-powered transponder on board that located the spacecraft at a distance of 12 billion miles from Earth. Given motivation and engineering, there is no practical limit to RFID range.