4 ms·

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

by 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.
- tgb 13y ago
  To be clear: you're saying that each of the the sensor lines are at slightly different angles and that this is due to sharing (at least for some of them) the same lens? If so, that shared lens would be exactly the part of this that I was missing. Honestly, it doesn't look like there's a shared lens between the red and the other two, as you point out, but I guess it could be sharing a lens with the "unused lines" marked in the diagram that puts it at a distinct angle. As a test of my understanding - would I be correct in saying that when the article says "The red and infrared rows are paired on one CCD array, while the green and blue sensors are on a second array, which is why the red plane often seems much further ahead than the green and blue planes." it's going a little too far: there's no reason that the red line would have to be inherently at a greater angle than the other three? I used to interpret this line as just meaning "the green and blue are closer together than the red is, so the red image will be further apart" but that should be insignificant. I'm curious - does anyone know how they calibrate this kind of equipment or are the tolerances good enough that they don't really need to? Do they take passes over known objects to calibrate or do some algorithmic test for how well the three colors match up (in a non-moving example).
  - mannykannot 13y ago
    To the first question: yes, and after a bit more thought, I realize that this assembly must be designed to use a single lens, as the sensor arrays are close together, which would make it difficult to have any other optical arrangement. The pictured assembly would form the back of the camera, where the rectangular CCD array (or film) goes in a conventional camera. The picture shows that the red sensor is further from the green one than the blue is, simply because it is on a separate sub-assembly. If the infra-red image showed on the pictures, it would be as far from the red one as the green one is from the blue. Given the order of the arrays on the camera, the IR image of the plane would be ahead of the red image, meaning that the IR is the last of the four images taken. Incidentally, the pictures show the aircraft are experiencing considerable wind drift, especially in the case of the top-left picture, where you can see the contrail. This indicates strong winds, and probably also a relatively low airspeed for the aircraft. The orientation of the waves suggests a head- or tail-wind, but to get that drift, it must be different at the airplane's altitude (which probably is not high if it is searching.)

by tgb 13y ago

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.

tgb 13y ago
To be clear: you're saying that each of the the sensor lines are at slightly different angles and that this is due to sharing (at least for some of them) the same lens? If so, that shared lens would be exactly the part of this that I was missing. Honestly, it doesn't look like there's a shared lens between the red and the other two, as you point out, but I guess it could be sharing a lens with the "unused lines" marked in the diagram that puts it at a distinct angle. As a test of my understanding - would I be correct in saying that when the article says "The red and infrared rows are paired on one CCD array, while the green and blue sensors are on a second array, which is why the red plane often seems much further ahead than the green and blue planes." it's going a little too far: there's no reason that the red line would have to be inherently at a greater angle than the other three? I used to interpret this line as just meaning "the green and blue are closer together than the red is, so the red image will be further apart" but that should be insignificant. I'm curious - does anyone know how they calibrate this kind of equipment or are the tolerances good enough that they don't really need to? Do they take passes over known objects to calibrate or do some algorithmic test for how well the three colors match up (in a non-moving example).
- mannykannot 13y ago
  To the first question: yes, and after a bit more thought, I realize that this assembly must be designed to use a single lens, as the sensor arrays are close together, which would make it difficult to have any other optical arrangement. The pictured assembly would form the back of the camera, where the rectangular CCD array (or film) goes in a conventional camera. The picture shows that the red sensor is further from the green one than the blue is, simply because it is on a separate sub-assembly. If the infra-red image showed on the pictures, it would be as far from the red one as the green one is from the blue. Given the order of the arrays on the camera, the IR image of the plane would be ahead of the red image, meaning that the IR is the last of the four images taken. Incidentally, the pictures show the aircraft are experiencing considerable wind drift, especially in the case of the top-left picture, where you can see the contrail. This indicates strong winds, and probably also a relatively low airspeed for the aircraft. The orientation of the waves suggests a head- or tail-wind, but to get that drift, it must be different at the airplane's altitude (which probably is not high if it is searching.)