4 ms·

Math doesn't check out for me: plane flies at ~700mph or 0.2 miles per second. Satellite speed is on the order of 5 miles per second. Sensors separation is on t

• aspanu 13y ago
  I believe that what you are missing is the distance between the satellite and the plane. planes are at ~10,000 m altitude, GPS flies at 20,000,000 m altitude. A very small angle of distortion creates a very large distance at the plane.
  • zizzer 13y ago
    Just a note - It's not GPS satellites taking the pictures. It seems to be the RapidEye [1] satellites, which are much closer to earth at 630km. [1] http://en.wikipedia.org/wiki/RapidEye http://en.wikipedia.org/wiki/RapidEye
• aspanu 13y ago
  I think that you are missing relative height. Plane is at 10km, GPS satellite at 20Mm.
• InclinedPlane 13y ago
  I think you're missing optics. Each pixel maps to a perhaps meter wide spot on the Earth. Which means that each color line which is separated by maybe a few hundred pixels of distance on the sensor maps to hundreds of meters of distance different on the Earth. A plane travels at hundreds of meters per second, which is hundreds of pixels per second in sensor space.
  • tgb 13y ago
    My understanding is that these satellites are using push broom scanning [1]. So I don't see any meaningful sense in which the different scanners are separated by "a few hundred pixels". They're separated by inches, which corresponds to time. For this same reason, I don't see why the altitude of the satellite is important. But I've never really studied this so I'd be happy to be corrected. [1] http://en.wikipedia.org/wiki/Push_broom_scanner http://en.wikipedia.org/wiki/Push_broom_scanner
    • InclinedPlane 13y ago
      They're not mapping the Earth to a resolution of microns though. Does that help make sense of the fact that there isn't a 1-to-1 scale mapping between the Earth and the sensors? Each image sweeps each pixel across a length along the Earth of nearly a meter. If you try to imagine how this maps onto the sensor head it rapidly breaks down. Each pixel seemingly overlaps with other pixels, each line seemingly overlaps with other lines. But that's not what happens, instead there are optics which effect spreading the image plane over a large area on the Earth with a separation of several meters between each color line. Think about the optics in reverse. Imagine turning the satellite's sensor head into a hologram then sending it backwards through the optics until it reaches the Earth, where it would be magnified to an enormous size and each pixel would be the size of a human.
      • tgb 13y ago
        I do not understand your description in terms of push broom scanners. Scanning is done by a linear array of sensors that detect at varying angles that go off perpendicular to the angle of motion of the satellite. Imaging in the direction of motion of the satellite is done by physically moving the satellite - which of course is happening quite quickly. This is like looking through a slit in a door frame - you move the light sensor perpendicular to the slit to see the objects inside (except not a perfect analogy: it would be more like carrying the doorframe with you as you move so that you need to move x distance to see something x distance from what you were first seeing). The image is being taken orthographically along one direction. So for the red sensor to see what the green sensor is currently seeing, the satellite must move forward by a few inches so that the red sensor is above the same spot as the green sensor was earlier. This is not how traditional photography works. There is actually a one-to-one correspondence of motion of the sensor with displacement of the imaged object (in one direction). This is similar to how a "photo finish" is done [1] Since the satellite moves much faster than the plane and the displacement between the sensors is small, it seems that the plane should not have traveled very far. [1] http://en.wikipedia.org/wiki/Photo_finish http://en.wikipedia.org/wiki/Photo_finish