Hardware Reference
In-Depth Information
Active Distance Ranging
The ultrasonic and infrared rangers in the preceding
sections are passive distance-sensing systems. Mobile
phones and the Global Positioning System (GPS) measure
longer distances by using ranging as well. These systems
include a radio beacon (the cell tower or GPS satellite)
and a radio receiver (the phone or GPS receiver). The
receiver determines its distance from the beacon based
on the received signal from the beacon. These systems
can measure much greater distances on an urban or
global scale. The disadvantage of active distance ranging
is that you must have a powered device at both ends. You
can't measure a person's distance from somewhere using
active distance ranging unless you attach a receiver to the
person.
ZigBee, and WiFi radios all provide data about signal
strength as well. In order to relate this to distance, you
need to be able to calculate that distance as a function
of signal strength. The main function of a GPS receiver
is to calculate distances to the GPS satellites based on
signal strength, and then determine a position using those
distances. The other radio systems mentioned here don't
do those calculations for you.
In many applications, though, you don't need to know the
distance—you just need to know how relatively near or far
one person or object is to another. For example, if you're
making a pet door lock that opens in response to the pet,
you could imagine a Bluetooth beacon on the pet's collar
and a receiver on the door lock. When the signal strength
from the pet's collar is strong enough, the door lock opens.
In this case, and in others like it, there's no need to know
the actual distance.
X
GPS and cellular location systems don't actually give you
the distance from their radio beacons, just the relative
signal strength of the radio signal. Bluetooth, 802.15.4,
Figure 8-6
Active vs. passive
distance ranging
Base unit (sensor) sends out
signal, reads reflection from
mobile object or person
Initial signal generated by
base unit (e.g., cell tower)
Response signal generated by
mobile unit (e.g., cell phone).
Passive distance ranging
Active distance ranging
