Radar is something
that is in use all around us, although it is normally
traffic control uses radar to track planes
both on the ground and in the air, and also to guide planes in
for smooth landings. Police use radar to detect the speed of
passing motorists. NASA uses radar to map the Earth and other
planets, to track satellites
and space debris and to help with things like docking and
maneuvering. The military uses it to detect the enemy and to
Photo courtesy Department of
Specialist 2nd Class Gilbert Lundgren operates radar
equipment in the combat information center of the USS
Meteorologists use radar to track storms, hurricanes
You even see a form of radar at many grocery stores when the
doors open automatically! Obviously, radar is an extremely
useful technology. In this edition of HowStuffWorks,
we'll uncover radar's secrets.
When people use radar, they are usually trying to
accomplish one of three things:
Detect the presence of an object at a distance -
Usually the "something" is moving, like an airplane, but
radar can also be used to detect stationary objects buried
underground. In some cases, radar can identify an object as
well; for example, it can identify the type of aircraft it
Detect the speed of an object - This is the
reason why police use radar.
Map something - The space
shuttle and orbiting satellites use something called
Synthetic Aperture Radar to create detailed
topographic maps of the surface of planets and moons.
All three of these activities can be accomplished
using two things you may be familiar with from everyday life:
echo and Doppler shift. These two concepts are
easy to understand in the realm of sound because your ears hear echo
and Doppler shift every day. Radar makes use of the same
techniques using radio
Let's look at the sound version first, since you are
already familiar with this concept.
Echo and Doppler Shift
When you shout into a well, the sound of your
shout travels down the well and is reflected (echoes)
off the surface of the water at the bottom of the well.
If you measure the time it takes for the echo to return
and if you know the speed of sound, you can calculate
the depth of the well fairly
something you experience all the time. If you shout into a
well or a canyon, the echo comes back a moment later. The echo
occurs because some of the sound waves in your shout reflect
off of a surface (either the water at the bottom of the well
or the canyon wall on the far side) and travel back to your
ears. The length of time between the moment you shout and the
moment that you hear the echo is determined by the distance
between you and the surface that creates the echo.
Doppler shift is also common. You probably
experience it daily (often without realizing it). Doppler
shift occurs when sound is generated by, or reflected off of,
a moving object. Doppler shift in the extreme creates sonic
booms (see below). Here's how to understand Doppler shift
(you may also want to try this experiment in an empty parking
lot). Let's say there is a car coming toward you at 60 miles
per hour (mph) and its horn is blaring. You will hear the horn
playing one "note" as the car approaches, but when the car
passes you the sound of the horn will suddenly shift to a
lower note. It's the same horn making the same sound the whole
time. The change you hear is caused by Doppler shift.
Here's what is happening. The speed of sound through
the air in the parking lot is fixed. For simplicity of
calculation, let's say it's 600 mph (the exact speed is
determined by the air's pressure, temperature and humidity).
Imagine that the car is standing still, it is exactly 1 mile
away from you and it toots its horn for exactly one minute.
The sound waves from the horn will propagate from the car
toward you at a rate of 600 mph. What you will hear is a
six-second delay (while the sound travels 1 mile at 600 mph)
followed by exactly one minute's worth of sound.
Doppler shift: The person behind the car
hears a lower tone than the driver because the car is
moving away. The person in front of the car hears a
higher tone than the driver because the car is
Now let's say that the car is moving toward you at 60 mph.
It starts from a mile away and toots it's horn for exactly one
minute. You will still hear the six-second delay. However, the
sound will only play for 54 seconds. That's because the car
will be right next to you after one minute, and the sound at
the end of the minute gets to you instantaneously. The car
(from the driver's perspective) is still blaring its horn for
one minute. Because the car is moving, however, the minute's
worth of sound gets packed into 54 seconds from your
perspective. The same number of sound waves are packed into a
smaller amount of time. Therefore, their frequency is
increased, and the horn's tone sounds higher to you. As the
car passes you and moves away, the process is reversed and the
sound expands to fill more time. Therefore, the tone is lower.
While we're here on
the topic of sound and motion, we can also understand
sonic booms. Say the car was moving toward you at
exactly the speed of sound -- 700 mph or so. The car is
blowing its horn. The sound waves generated by the horn
cannot go any faster than the speed of sound, so both
the car and the horn are coming at you at 700 mph, so
all of the sound coming from the car "stacks up." You
hear nothing, but you can see the car approaching. At
exactly the same moment the car arrives, so does all of
its sound and it is LOUD! That is a sonic boom.
The same phenomenon happens when a boat travels
through water faster than waves travel through the water
(waves in a lake move at a speed of perhaps 5 mph -- all
waves travel through their medium at a fixed speed). The
waves that the boat generates "stack up" and form the
V-shaped bow wave (wake) that you see behind the boat.
The bow wave is really a sonic boom of sorts. It is the
stacked-up combination of all of the waves the boat has
generated. The wake forms a V shape, and the angle of
the V is controlled by the speed of the boat.
You can combine echo and doppler shift in the following
way. Say you send out a loud sound toward a car moving toward
you. Some of the sound waves will bounce off the car (an
echo). Because the car is moving toward you, however, the
sound waves will be compressed. Therefore, the sound of
the echo will have a higher pitch than the original sound you
sent. If you measure the pitch of the echo, you can determine
how fast the car is going.
Understanding Radar We have seen that the
echo of a sound can be used to determine how far away
something is, and we have also seen that we can use the
Doppler shift of the echo to determine how fast something is
going. It is therefore possible to create a "sound radar," and
that is exactly what sonar is. Submarines
and boats use sonar all the time. You could use the same
principles with sound in the air, but sound in the air has a
couple of problems:
Sound doesn't travel very far -- maybe a mile at the
Almost everyone can hear sounds, so a "sound radar"
would definitely disturb the neighbors (you can eliminate
most of this problem by using ultrasound
instead of audible sound).
Because the echo of the sound would be very faint, it is
likely that it would be hard to detect.
therefore uses radio waves
instead of sound. Radio waves travel far, are invisible to
humans and are easy to detect even when they are faint.
Photo courtesy NASA
of Defense (right) Left:
Antennas at Goldstone Deep Space Communications Complex
(part of NASA's Deep Space Network) help provide radio
communications for NASA's interplanetary spacecraft.
Right: Surface search radar and air search radar are
mounted on the foremast of a guided missile
Let's take a typical radar set designed to detect airplanes
in flight. The radar set turns on its transmitter and shoots
out a short, high-intensity burst of high-frequency radio
waves. The burst might last a microsecond. The radar set then
turns off its transmitter, turns on its receiver and listens
for an echo. The radar set measures the time it takes for the
echo to arrive, as well as the Doppler shift of the echo.
Radio waves travel at the speed of light, roughly 1,000 feet
per microsecond; so if the radar set has a good high-speed
clock, it can measure the distance of the airplane very
accurately. Using special signal processing equipment, the
radar set can also measure the Doppler shift very accurately
and determine the speed of the airplane.
The radar antenna sends out a short, high-power
pulse of radio waves at a known frequency. When the waves hit
an object, they echo off of it and the speed of the object
Doppler-shifts the echo. The same antenna is used to receive
the much-weaker signals that return.
In ground-based radar, there's a lot more potential
interference than in air-based radar. When a police
radar shoots out a pulse, it echoes off of all sorts of
objects -- fences, bridges,
mountains, buildings. The easiest way to remove all of this
sort of clutter is to filter it out by recognizing that it is
not Doppler-shifted. A police radar looks only for
Doppler-shifted signals, and because the radar beam is tightly
focused it hits only one car.