05-10-2012, 10:31 AM
How Radar Works
[attachment=35350]
Radar is something that is in use all around us, although it is normally invisible. Air 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 guide weapons.
Meteorologists use radar to track storms, hurricanes and tornadoes. You even see a form of radar at
many grocery stores when the doors open automatically! Obviously, radar is an extremely useful
technology.
When people use radar, they are usually trying to accomplish one of three things:
l 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 has
detected.
l Detect the speed of an object - This is the reason why police use radar.
l 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.
Echo and Doppler Shift
Echo is 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.
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:
l Sound doesn't travel very far -- maybe a mile at the most.
l 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).
l Because the echo of the sound would be very faint, it is likely that it would be hard to detect.
[attachment=35350]
Radar is something that is in use all around us, although it is normally invisible. Air 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 guide weapons.
Meteorologists use radar to track storms, hurricanes and tornadoes. You even see a form of radar at
many grocery stores when the doors open automatically! Obviously, radar is an extremely useful
technology.
When people use radar, they are usually trying to accomplish one of three things:
l 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 has
detected.
l Detect the speed of an object - This is the reason why police use radar.
l 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.
Echo and Doppler Shift
Echo is 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.
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:
l Sound doesn't travel very far -- maybe a mile at the most.
l 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).
l Because the echo of the sound would be very faint, it is likely that it would be hard to detect.