3 ms·

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 pe

• mannykannot 13y ago
  You are assuming that the lines of sight of each of the arrays are parallel with one another, so that their separation on the ground is the same as the separation of the arrays on the satellite. If they are at an angle to one another, the separation of these lines at the surface will be different from (and possibly in a different order than) the separation of the arrays. It is the surface separation, divided by the speed of the satellite, that determines the time between images of the same point on the surface by different arrays. If the arrays all share the same lens, that would cause this effect.
  • tgb 13y ago
    I'm not sure what you're saying. I think you are either saying the same thing as InclinedPlane (see my response) or are suggesting that either intentionally or due to tolerances in machining or some such, the red and green sensor units are at some small angle from each other. Assuming you're saying the latter, since the distance is large, this small angle contributes a large displacement on the ground. Afterwards the image would be stitched together to compensate for that via some calibration routine which would work for only stationary objects. That is entirely plausible, I hadn't considered that before and had just been assuming that the satellite construction was 'ideal'. This sounds like the most likely reason for such an artifact to occur, though I still wonder whether this particular imperfection would be done given the high level of precision necessary for the entire project.
    • InclinedPlane 13y ago
      It's not due to imprecision, it's due to design and fundamental optical constraints. There's nothing special about the optics used for a push broom imager, it's just an ordinary lens. If you put a full frame sensor at the focal plane you could take an image of a several kilometer square patch of Earth. If instead you put several different lines of sensors in the focal plane and put each behind a color filter then you'd have a multi-spectral push broom sensor. But each line does not view the same part of the Earth at the same time. Because the sensors scan the Earth they end up imaging the same spot within a fairly short delay. But optically it's not possible to have each color line viewing the same horizontal slice of the image of Earth at the same time, that's not the way lenses work.
    • mannykannot 13y ago
      Each sensor will be on the focal plane of the lens it uses. If they are all are looking through the same lens, their lines of sight will all cross one another at the optical center of the lens, and so will be at an angle to one another. Therefore, each sensor has a different view of the surface - it is an example of parallax. (EDIT: I have changed 'lens system' to 'lens', because I think the former might be misleading. All I meant by 'lens system' is that to make a decent camera, you have to have several lens elements along the optical axis. Subsequently, I realized 'lens system' might be taken as implying an array of lenses. What I actually mean by 'lens' is simply as in 'telephoto lens'.) did a quick BOE calculation based on this assumption. The idea was to see if the implied focal length for the lens is plausible. I actually used the green-blue pair for the calculation, because I think it is slightly more likely that each pair share a lens, than that they all do. The green and blue images seem to be something more than a wingspan apart. We don't know what type of aircraft these are, so I picked a 737 as a mid-sized example. The latest models have a 34m wingspan, so let's say the aircraft has moved about 50m between images. Assuming the aircraft are moving relatively slowly, because they are searching, I used 100 m/s (200kts) for their speed, which means there is 0.5 sec between the images. The satellite is moving at "almost 5 miles per sec", so let's say 7 km/sec. That means the views of the two sensor arrays have a 3.5 km separation at the surface. Zizzer gives the satellite altitude as about 630 km. The ratio of the focal length of the lens to the separation of the sensor arrays is the same as the ratio of the altitude of the satellite to the distance between what part of the surface each array is imaging: f/a = 630/3.5, or f = 180a (a is the array separation). From the picture of the sensors, and principally using the connectors' pin-holes for a sense of scale, I guess the blue and green arrays are about 0.5 cm apart, implying f = 90cm, which seems plausible to me.
• InclinedPlane 13y ago
  Yes, there is a one to one correspondence in displacement but because the sensor elements view parts of the Earth that are farther apart the delay between lines is larger on the Earth than for the sensor elements in orbit. Just as the size of the pixels on the camera are larger than the size of the pixels on the sensor. That's because the view line of each sensor element is at an angle relative to the others. And that's what allows the sensor to see a multiple kilometer wide swathe of the Earth in each pass instead of just a few centimeters. Imagine a different satellite that instead of looking down looked out toward the Earth's horizon, as seen from orbit. And imagine it has two sensors, one which looks forward and one that looks backward, very nearly a 180 deg. difference. At the height of a satellite in low Earth orbit that could be a 1,000 km difference in what each sensor sees. Now, every millimeter the satellite moves the view moves as well, but because the views are separated by 180 deg. or 1,000 km of ground that means it takes much longer for one view to catch up to where the other was, even though the track of the sensor elements themselves in space do the same thing much quicker. Or, just imagine if the satellite rotated so the sensor was sideways, parallel to the direction of motion. It still views the same multiple kilometer wide slice of the Earth, but now it reimages the same thing line as it moves, shifting just one pixel forward each image. The sensor is only a few cm wide so it takes a tiny fraction of a second for the satellite to translate the position of the rearward most sensor pixel to where the forwardmost sensor pixel was, but it does not take the same amount of time for the viewed slice of the Earth to translate by an entire frame, because the frame is many kilometers wide.
  • tgb 13y ago
    No, I believe that you are still mis-interpretting the push broom scanners. A greyscale push broom scanner would have a single row of pixels that are lined up perpendicularly to the direction of travel and whose rays are in the plane perpendicular to the direction of travel. So yes, these rays would span many kilometers, but only in one direction. In order to image a 2D area of the planet the entire satellite would need to traverse down it linearly - which is something that satellites happen to be quite good at given how fast they are moving. This is almost like how a document scanner works - it translates a line of sensors across a document to construct a full 2D image. For a full color image, three linear sensors are used, each displaced some distance along the direction of motion. So the fact that the rays are spreading out in the perpendicular doesn't change the distance the satellite must travel to image the next part. For example, your second paragraph doesn't hold. The large separation of rays only occurs in the direction perpendicular to that of travelling. Note how your method would be imaging the same place multiple times on each pass whereas a push broom scanner would capture each one just one. For your third paragraph, I agree that this could be done but that's simply not how the sensors are aligned and that difference is important. In particular, it's the very difference that I'm trying to point out.