5 ms·

Why ghostly false positives cropped up in MH370 satellite hunt

21 points by adenner 13y ago

tgb 13y ago
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 the order of a few inches. It would take about 15 microseconds between first and last sensor passing over the same spot. The plane can only move around 5 millimeters in this amount of time. It looks like it's moved several hundred feet. What am I missing?
- 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
yaur 13y ago
Since there shouldn't be too much moving around the open ocean besides planes, couldn't you use this artifact to build a semidecent search algorithim rather than going through all the data by hand?
- afreak 13y ago
  In addition to planes and boats, "islands" of garbage also exist and they range from the size of small cars to that of small cities.
  - yaur 13y ago
    This particular type artifact is created because the plane is bright and moving FAST enough that it isn't in the same position relative to the sensor from the beginning of the image to the end. My best guess is that you need something moving at least 100MPH to see this effect at all and the position of the double image (assuming the same sensor) should be a function of the speed of the object. Islands of garbage moving fast enough to be mistaken for a plane are probably interesting in their own right.
    - yaur 13y ago
      As it turns out that the real problem here is that the all of the data from the 8th is in a pretty useless spot given what we know now. So what is useful to find is wreckage not planes. After looking at the data I'm also unconvinced that speed has anything to do with it and that the more likely cause is altitude.
- pixl97 13y ago
  You do realize that the surface of the ocean is moving around rather drastically? That part of the ocean, because of how wide open it is, has some of the largest swells of any ocean. I feel that this would make things rather difficult.
throwwit 13y ago
Out of all the news coverage showing the search crews manning the planes, never once did I see anyone wearing sunglasses. Wouldn't polarized glasses actually help with reflection/glare?
- mannykannot 13y ago
  Polarized glasses help with reflection at a shallow angle, but the observers will be looking down, not across the surface, and when the sun is low on the horizon, all observers will be looking away from the sun, as it would be a waste of time to try to see anything up-sun, even with polarized glasses. In addition, using polarized glasses might lead to seeing the colored bands of stress birefringence in the window, depending on what material it is made of.
malandrew 13y ago
Thus far, have we found even a single piece of debris that we know came from the MH370? I just don't buy the idea that the plane ended up as far south as it did and I haven't really read of any solid evidence to support this other than one blurry satellite image with a large but unidentifiable object. If that object was in fact part of the plane, I have yet to hear of any explanation as to why the plane was that far south of it's last known position instead of continuing north west.
- gamblor956 13y ago
  It's a multitude of satellite images, combined with a Doppler analysis of satellite pings of the aircraft, plus a fuel-range analysis, plus a radar analysis of the northern half of the potential travel routes... I have yet to hear of any explanation as to why the plane was that far south of it's last known position instead of continuing north west. No one knows that, yet (possibly ever). But they do know that the plane did not travel north west, as it didn't show up on the civilian or military radars of multiple countries.
  - malandrew 13y ago
    Do we have any good theories explaining why the plane would have even traveled in that direction? AFAICT there is nothing in that direction on the map. What were they flying towards?

21 points by adenner 13y ago

tgb 13y ago

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 the order of a few inches. It would take about 15 microseconds between first and last sensor passing over the same spot. The plane can only move around 5 millimeters in this amount of time. It looks like it's moved several hundred feet. What am I missing?

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

yaur 13y ago

Since there shouldn't be too much moving around the open ocean besides planes, couldn't you use this artifact to build a semidecent search algorithim rather than going through all the data by hand?

afreak 13y ago
In addition to planes and boats, "islands" of garbage also exist and they range from the size of small cars to that of small cities.
- yaur 13y ago
  This particular type artifact is created because the plane is bright and moving FAST enough that it isn't in the same position relative to the sensor from the beginning of the image to the end. My best guess is that you need something moving at least 100MPH to see this effect at all and the position of the double image (assuming the same sensor) should be a function of the speed of the object. Islands of garbage moving fast enough to be mistaken for a plane are probably interesting in their own right.
  - yaur 13y ago
    As it turns out that the real problem here is that the all of the data from the 8th is in a pretty useless spot given what we know now. So what is useful to find is wreckage not planes. After looking at the data I'm also unconvinced that speed has anything to do with it and that the more likely cause is altitude.
pixl97 13y ago
You do realize that the surface of the ocean is moving around rather drastically? That part of the ocean, because of how wide open it is, has some of the largest swells of any ocean. I feel that this would make things rather difficult.

throwwit 13y ago

Out of all the news coverage showing the search crews manning the planes, never once did I see anyone wearing sunglasses. Wouldn't polarized glasses actually help with reflection/glare?

mannykannot 13y ago
Polarized glasses help with reflection at a shallow angle, but the observers will be looking down, not across the surface, and when the sun is low on the horizon, all observers will be looking away from the sun, as it would be a waste of time to try to see anything up-sun, even with polarized glasses. In addition, using polarized glasses might lead to seeing the colored bands of stress birefringence in the window, depending on what material it is made of.

malandrew 13y ago

Thus far, have we found even a single piece of debris that we know came from the MH370? I just don't buy the idea that the plane ended up as far south as it did and I haven't really read of any solid evidence to support this other than one blurry satellite image with a large but unidentifiable object. If that object was in fact part of the plane, I have yet to hear of any explanation as to why the plane was that far south of it's last known position instead of continuing north west.

gamblor956 13y ago
It's a multitude of satellite images, combined with a Doppler analysis of satellite pings of the aircraft, plus a fuel-range analysis, plus a radar analysis of the northern half of the potential travel routes... I have yet to hear of any explanation as to why the plane was that far south of it's last known position instead of continuing north west. No one knows that, yet (possibly ever). But they do know that the plane did not travel north west, as it didn't show up on the civilian or military radars of multiple countries.
- malandrew 13y ago
  Do we have any good theories explaining why the plane would have even traveled in that direction? AFAICT there is nothing in that direction on the map. What were they flying towards?