Hardware Reference
In-Depth Information
Determining Distance
Electronic locating systems—like GPS, mobile phone location, and sonar—seem
magical at first because there's no visible evidence as to how they work. However, when
you break the job down into its components, it becomes relatively straightforward. Most
physical location systems are based on one of two methods: measuring the time of a
signal's travel from a known location, or measuring its strength at the point of reception.
Both methods combine measurements from multiple sources to determine a position in
two or three dimensions using trilateration.
For example, a GPS receiver determines its position on
the surface of the planet by measuring the time delay
of received radio signals from several geosynchronous
satellites. Mobile phone location systems function
similarly, using the signal from nearby cell towers to
determine the phone's position. Systems like Skyhook
(
www.skyhookwireless.com
) use several different systems
(WiFi, GPS, and cell tower location) to refine their posi-
tional accuracy. Sonar and infrared-ranging sensors work
by sending out an acoustic signal (sonar) or an infrared
signal (IR rangers), and then measuring the strength of
that signal when it's reflected off the target.
Sometimes distance ranging is used for acquiring a
position; other times, it's used for refining it. In the
following examples, the passive distance rangers deliver a
measurement of physical distance.
Passive Distance Ranging
Ultrasonic rangers like the MaxBotix LV-EZ1, and infrared
rangers like the Sharp GP2Y0A21YK, shown in Figure 8-2,
are examples of
distance rangers.
The MaxBotix sensor
sends out an ultrasonic signal and listens for an echo.
The Sharp sensor sends out an infrared light beam, and
senses the reflection of that beam. These sensors work in
a short range only. The Sharp sensor can read about 10cm
to 80cm, and the MaxBotix sensor reads from about 0 to
7.5m. Passive sensors like these are handy when you want
to measure the distance of a person in a limited space, and
you don't want to put any hardware on the person. They're
also useful when you're building moving objects that need
to know their proximity to other objects in the same space
as they move.
X
Distance ranging techniques can be classified as
active
or
passive
. In active systems, the target has a radio, light, or
acoustic source on it, and the receiver listens for the signal
generated directly by the target. In passive systems, the
target doesn't need to have any technology on board. The
receiver emits a signal, then listens for the signal reflected
back from the target. Mobile phone location is active
because it relies on a two-way transmission between
the phone and the cell tower. GPS is also active, even
though the transmission is one-way, because the signal
used to determine location is direct, not reflected. The
target is a radio receiver. Sonar and infrared ranging are
passive because the signal is reflected off the target, not
generated by it.
