0
\$\begingroup\$

I understand how (one of the) principle(s) of RADAR works: send a signal, record the received scattering copy off the target, and measure the elapsing time. You might need to consider Doppler effects, but let's not worry about this. But this signal only gives you a topography of a single point. How can an analog RADAR obtain a topography map (per pulse) that has finite 2D area?

I know the pulse spreads spherically as it propagates, and so is the scattering copy, but still the antenna will receive one signal -- how can it tell which is which?

EDIT: To be more specific: Suppose your radar is in an airplane and directed downwards. It sends a pulse. On ground, there are two objects -cars, for instance- separated by distance x, and each of diameter d. When the pulse reflects off the two cars and reach the RADAR, how is the x and y axis differentiated? I'd assume the antenna lump sums everything into one energy pulse, thus losing all spatial details. This should not be an issue, however, in case of one object only.

\$\endgroup\$
  • 5
    \$\begingroup\$ The RADAR antenna is directional, so the signal travels in a narrow beam, instead of propagating omnidirectionally. As the antenna rotates, it emits multiple pulses. See here. I have specifically chosen an old video, so the antenna is not a phased array. \$\endgroup\$ – Nick Alexeev Mar 30 '15 at 4:26
  • \$\begingroup\$ Any large part of the signal that spreads out (away from the path of travel) is not likely to return above the threshold anyway. Also, once you do start using a phased array, the beam can be made quite a lot narrower, and it can scan back and forth very quickly, yielding useful 3d information with relative precision. \$\endgroup\$ – Sean Boddy Mar 30 '15 at 6:36
  • \$\begingroup\$ Required reading: Most Secret War, RV Jones and Glide Path, Arthur C Clarke \$\endgroup\$ – tomnexus Mar 30 '15 at 17:38
4
\$\begingroup\$

The radar signal encodes only one thing: The amount of energy received from objects at various distances from the antenna. To get information about the placement of those objects in dimensions perpendicular to the radar beam, you need to scan it, either mechanically or electrically (phased array).

\$\endgroup\$
2
\$\begingroup\$

An early rotating-antenna RADAR unit would have worked by having a rotating unidirectional antenna whose orientation was synchronized with a rotating coil around the display tube. The antenna would output pulses at some rate, and the field in the display tube's coil would be changed in sync with those pulses such that the amount of beam deflection would be proportional to the amount of time since the last pulse. Provided that the time between pulses was sufficient that no pulse would be sent while a reflection from an earlier one might still be received, the distance of the beam from the center would correspond to the time since the last pulse, and the direction of the beam from the center would correspond to the direction the antenna was pointing when the pulse was sent and received.

Modern RADAR systems don't use rotating coils in display tubes, of course, but many of the overall principles remain the same.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.