What does it really mean? Some guy on a TV program was talking about satelite resolution and he said that one with 6-inch resolution could identify a grapefruit. Does this mean that a grapefruit would be plainly recognizeable on someone’s patio table? Can a satelite “read” a license plate on a car? Recognize people?
“Six-inch resolution” means that objects of six inches or larger in size can be individually distinguished from other objects, assuming they are separate. Put a grapefruit out in a field, and it could be seen (though it could not necessarily be determined that it was a grapefruit, just that it was a small, roundish object). Put it in a big pile of other grapefruit, and you probably wouldn’t be able to single one out.
Which pretty much means that you wouldn’t be able to read a license plate or recognize specific people (especially since you’d only be seeing the top of their heads anyway).
6 inch resolution. yes, this means anything that is 6x6 inches–in length and width–i.e. grapefruit can be seen from that satelite. so the license plate thing … yes, if the plate were to be laid facing the sky.
I always thought resolution was measured by how close two things could be and still be distinguished as separate objects.
Resolution is a measure of how close two objects can get and still be distinguished and is usually a measure of the pixel size of the image. Six inch resolution simply means that if you placed two grapefruit side by side on a table 7 inches apart the sattelite could distinguish two separate objects. Place them five inches apart and the satellite will see one large object. It doesn’t mean that objects six inches or larger can be seen. Two objects three miles wide, yet standing five inches apart would show up as one object. An object 10 x 8 that happened to fall in the corner of four pixels would not show up at all unless it was very highly contrasted, and then it would show up as a solid square object 12 x 12. Conversely a particularly bright object the size of a pinhead, such as say a laser, could be seen quite clearly but would only show up as a six inch solid mass of something bright.
In reality resolution is only meaningful in terms of contrasting light/dark images. Two yellow metal blocks one foot to a side on a yellow metal runway made of the same material may not show up because although they meet the resolution requirement they lack contrast. Basically your grapefruit might show up against a black surface, but it’s highly unlikely it would against any other background.A six inch resolution is pretty impressive but it would almost certainly not show up a grapefruit unless it contrasted heavily with the background. Then it would show one square pixel that is a different colour. It certainly wouldn’t show shape or even necesaarily size. The general rule of thumb is that to get any idea of shape you need an object 10x the resolution of your target. So for a six inch diameter grapefruit you’d need a resolution of .6 of an inch to know that it was round. Reading licence plates at this resolution is right out. The characters on a licence plate are only about 2 inches wide at best, meaning that to read them you would need a resolution of about .2 inches. Recognising people would probably require about 1 inch resolution, and only then if they were looking straight into the camera.
Resolution is just one way of measuring the quality/utility of satellite images and is often meaningless without also knowing a range of other factors such as what part of the spectrum your looking at, what angle you’re looking from etc.
Gaspode-
Wow. Thanks.