Global Positioning System Reference
In-Depth Information
usually to identify itself for validation. The activity of tag reading creates a time
and position stamp which is then used in various enterprise software applications,
for example, to collect bills. In systems where every vehicle is tagged, the DSRC
can provide traffic flow information and also play an important part in crime
prevention and detection.
DSRC tags are transponders (similar in operation and principle to military
IFF systems discussed in Section 1.4.1) that handle a short two-way data
“conversation” and are therefore considered to be active tags. The range can be in
the order of tens of meters and therefore a power supply is needed. This is in
contrast to passive tags that are powered by the near-field communications radio
signals. Many cities globally have installed tolling systems (e.g., Singapore and
Toronto) and they are one of the most common ITS applications and a very useful
part of Whereness that could be exploited further.
6.2.13
Passive RFID
RFID technology is a rapidly growing business and a truly disruptive technology
that is changing the methods used in many logistics, access security, and fee
collection systems. As with the longer-range active tags discussed above, the
activity of tag reading provides an unambiguous time and position stamp. Passive
RFID tags are not passive when they are being used since they pick up power
from the local radio frequency field (usually using the near field), which energizes
the electronics in the microchip attached to their antennae. The anatomy of most
tags is similar. A loop antenna encompasses most of the tag (which is usually a
few cm square), and there is an associated capacitor that forms a tuned circuit
resonant at the tag frequency. There are many RFID frequencies allocated
worldwide but the only common dedicated band is the high frequency (HF) band
at 13.56MHz.
A typical large-scale system is the Oyster Card [18] used on the London
Underground rail network, where thousands of tag readers are installed at ticket
barriers and are used by millions of passengers daily. The cards are contactless but
must be placed in close proximity to the tag reader mounted on the barriers. The
tags can be reused since the tag can be recharged with cash via an Internet service.
Tag readers are actually transceivers and the tags contain a small amount of
data storage (1kb is typical) so that they can store information about available
credit and personal details. Normally, RFID tags contain pointers to data located
on databases elsewhere.
Whereas fee collection and building access systems use fixed tag readers and
mobile tags, it is also possible to use an RFID system the other way around. For
example, a mobile container crane may need to be accurately positioned along
side a dock or rail head or roadway in order to safely transfer shipping containers
between ships, trucks, and wagons. In this situation a mobile tag reader could be
fitted to the underside of the crane and the tags embedded under the concrete
roadway at very specific positions. When the crane moves, the tag reader would
