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How does radar find the position of an object?

  1. Oct 29, 2007 #1
    How does radar find the position of an object? And how does radar make an image? Isn't the reicever a dish so wouldn't only be able to receive magnitude related information like a microphone does with sound?
  2. jcsd
  3. Oct 29, 2007 #2
    Two-way time, send out a beam and see how long it takes for it to come back. Radars, as far as I am aware do not make images in any physically meaningful sense, they simply indicate position by assuming that signal velocity in air is known.

    A more interesting question is whether the radar is Euclidean or Lagrangian.
  4. Oct 29, 2007 #3
    Radar transponders carried in craft can add information that shows on the scope.
  5. Oct 29, 2007 #4
    GiTS, never seen those old radar screens in sci-fi, where it sweeps around at the rate that the dish on the roof is rotating? (Although these days, we'd probably be more likely to digitally calculate the position using several receivers to triangulate the reflected pulse.)

    Billiards, what do you mean by "whether the radar is Euclidean or Lagrangian"?
  6. Oct 31, 2007 #5
    im still confused.

    I can see how you could get the distance x,y position from the rotating radar. But what about the altitude? And what about the size of an object?
  7. Oct 31, 2007 #6


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    Staff Emeritus
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    Gold Member

    There is no size, shape or altitude information in basic radar. Modern computerized radar systems are able to get altitude, I am not sure how it is done.

    In the old days (I worked on naval radar systems in the early '70s) we had a separate height finding radar which we would point at a target identified on a long range search radar. The height finder then scanned vertically in the specified direction to obtain altitude data.
  8. Nov 3, 2007 #7

    So if I wanted to take a picture with radar or lidar, how would i do it?
  9. Nov 4, 2007 #8
    If you have to ask (how you would do it), then I'm inclined to answer "with difficulty".

    One may be interested in this demonstration of how radio antenna arrays produce pictures.
    Last edited: Nov 4, 2007
  10. Nov 4, 2007 #9
    What happens is the antenna sends a short burst of Electro Magnetic energy out. It is reflected by the object and then one receives a shifted version of the energy that was sent out. The mathematical process of correlation picks the return signal out of the background noise and the amount of shift determines the distance.

    One can use radar or lidar to build up a image of an object by sending out bursts of energy to different locations on the object, measuring the distance of each. Keep in mind that one is limited by the wave length of the electromagnetic energy. You can't resolve features smaller than the wavelength one is using. So long wave radar is useful for something like the surface of Venus which is much larger than even low frequency waves while it is useless for resolving small features like a person's face.
  11. Nov 4, 2007 #10


    Staff: Mentor

    You need to be a little more specific. What are you taking a picture of? What information do you want in that picture? Is your radar mobile or stationary?

    This page has some cool radar pictures produced with synthetic aperture radar, which is probably the most commonly used technique for mobile radar platforms.

  12. Nov 4, 2007 #11
    Wildman's answer seems the most correct, with a few details omitted. First, the position and orientation of the transmitting antenna is known, and the same goes for the receiving antenna. These may be the same but they don't have to be. Let's assume they're the same, and that the antennas are parabolic, so they only transmit and receive energy in the direction it's pointing.

    The transmitter sends out a short pulse at a known frequency and then switches to receiver mode. The return signal is in line with the direction the antenna is pointing to, and the round trip time gives you the distance. There. You've just fixed the distance to an object in a single direction, so you've located it in 3D space. If you can also measure the doppler shift of the returned signal, you also know the velocity of the object. To do this, the pulsewidth has to be less than the round trip time to the object.

    To scan the whole sky you have to rotate the antennae, as well as sweep it up and down in azimuth. For a big dish (the bigger the more sensitivity and directional discrimination) this can be hard to do. With phase array radars, the antenna can be flat and stationary but contains thousands of smaller emmitter/receivers, and by phase shifting the signals from each emmitter independently, interference can sweep the beam over about 180 degrees much faster.

    That's the simple answer to how radar works in principle. In practice, with multiple reflections, scattering, ground clutter, etc, let alone active jamming and stealth, things are a lot more complicated.
  13. Nov 4, 2007 #12
    lol, i have to wonder when i see things like this weather the poster is more interested in furthering the understanding of others, or just plain showing off!!

    i like wildmans and bob314...'s answer.

    i like [rightly or wrongly] to think of radar as having a flashlight in the dark
    the flashlight sends out "waves" of light, objects then reflect theese, and the reflections are detected by my eyes! now replace my flashlight and eyes with a radar dish and light with microwaves!! (however i cant really range what i'm seing with the flashlight in the same way radar does!)

    and i have a further question,
    does the radar dish send out pulses of em waves or a continuous one?
    if its continuous then how is the time between emmiting and recieving measured.
    eg, if i have a radar dish pointed only in one direction and its emmitting radar waves continuously, suddenly it dectects a reflected one? hows it to know the time at which the reflected wave was emmited?? (unless it continuously changes frequency?, is the reflected frequency the same as the emmitted??) lol thanks :D
  14. Nov 4, 2007 #13


    Staff: Mentor

    Both pulsed and continuous wave radar schemes exist. The basic continuous wave radar scheme only detects moving targets where the doppler shift moves the "echo" sufficiently far away from the transmitter. You can also do frequency-modulated continuous wave radar in order to get the ranging information. So, you send out a chirp, and you listen for a delayed and doppler shifted version of the chirp. The delay gives you the range and the doppler shift gives you the speed of the target.
  15. Nov 5, 2007 #14
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