Earth and Space | Forces and Motion

Passive sensing with satellites

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Here you look at the performance of a satellite camera.

What to Prepare

  • a webcam connected to a computer, able to run both video capture and still image capture programs (low resolution is an advantage – you should easily be able to get as low as 640 by 480, but older cameras go as low as 160 by 160)
  • Duplo or other large children's building blocks
  • Lego or other small children's building blocks
  • A3 paper and felt tip
  • a high street catalogue detailing the current cost of digital cameras

What Happens During this Activity

Make up a landscape from both kinds of block. Fly the camera over them at a carefully chosen height, capturing an image or images as you go, so that you can easily distinguish each of the large blocks, but not the small. The images will show this because of their pixellation. You should be able to increase the magnification of the image, by zooming in, to see the pixels. Emphasise that the camera just cannot see things that are smaller than one pixel.

So why not make all satellite cameras very high resolution?

Two answers:

  1. Cost – you have to make 6 million picture elements all perfect on a chip to get a 6 megapixel camera. These are much more expensive than 2 megapixel cameras (a high street catalogue might come in useful here).
  2. The file sizes are also huge (each picture has to be transmitted as a series of 1s and 0s) and as you do not want to bring the satellite down again (why not?) all that information will need to be transmitted back to Earth. Have you ever had to wait for a very big picture to download from the Internet? Have one bookmarked here for demonstration if possible, but make sure it is not cached (so that it has to be downloaded).

If you can video with the camera, set it to a very low frame rate, then repeat the fly-by, but this time capturing a continuous series of frames. This allows you to develop a discussion about the need for a good choice of frame rate, to capture images often enough, particularly if you want to see movement. Get children to use these results to design landscapes where some features will be visible, and some not. Try out a selection of their landscapes, keeping the same camera settings and fly-by height, to see if they have understood the issues of resolution.

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