Re: how to find out what sat..

From: John A. Dormer 2 (jad@texas.net)
Date: Sun May 25 2008 - 15:50:49 UTC

  • Next message: Steve Newcomb: "8539 obs May 25"

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    The vehicle doesn't have to be directly overhead, but there are limits
    which include the usable field of view along the satellite's ground
    track and the resolution of the image plane.
    
    The preferred angle is normal to the Earth's surface, viewing from
    "nadir." However, this can consume a lot of maneuvering power to keep
    the satellite bus appropriately angled, so things like gravity gradient
    booms have been used in the past to keep a vehicle pointing relatively
    close to local nadir.
    
    There are photogrammetric techniques which allow an image to be
    "orthorectified:" make it look like it's all on a flat plane. However,
    this results in odd things like chimneys and radio towers painted across
    the landscape. You can only trust the base of such structures to match a
    specific latitude and longitude, and even then compensation for terrain
    should be performed if possible. Knowing exactly where the vehicle was
    and how it was pointing fits in to the equation as well.
    
    This all allows a satellite image to have a fairly wide path, usually a
    few hundreds of kilometers at most, before distortion can become an
    issue. The image is designed to satisfy a particular mission type. An
    image plane is only so many pixels across, after all. The Landsat
    program probably contains the most well-studied vehicles:
    
    	http://landsat.gsfc.nasa.gov/
    	http://en.wikipedia.org/wiki/Landsat_program
    
    It's a "thematic" system which is designed to snarf up a large path,
    rather than an intelligence system which might be designed to grab a few
    square kilometers in a spot image.
    
    	John
    
    
    Bjoern Gimle@GlocalNet wrote:
    | Hi Yk,
    |
    | my guess is that commercial satellites that also take "wide" angle shots
    | are
    | most likely, and that these are taken vertically.
    | But I don't know the angle.
    |
    | OTOH, I got a message that Swedish students were doing this Friday
    | 8/ 5/23 12:24:36  0:14:04  Autotrack 292 76 25994 TERRA
    | 8/ 5/23 11:56:46  0:14:04  Autotrack 291 76 25682 LANDSAT 7
    | (local times = UTC+2)
    |
    | -- Björn Gimle                                            --
    | -- COSPAR 5917, STAR,  +18.05447 (E), +59.34185 (N), 33 m --
    | -- COSPAR 5918 WGS84,  +18.10127 (E), +59.29813 (N), 44 m --
    | -- COSPAR 5919, MALMA, +18.6206  (E), +59.2615  (N), 33 m --
    |
    |
    |
    | ----- Original Message ----- From: "ykchia" <chiayk1@singnet.com.sg>
    | To: <seesat-l@satobs.org>
    | Sent: Saturday, May 24, 2008 4:56 AM
    | Subject: how to find out what sat..
    |
    |
    |> Hi folks:
    |>
    |> A few days ago, groups of student were holding out a combined 15m x 15m
    |> and a NASA satellite was supposed to pass by and took a picture of this
    |> reflection during noon time.  Knowing the date, time bracket and TLEs  I
    |> am interested to do a elimination approach to find which sat qualify in
    |> this task. One question - Does the sat needs to be directly overhead ( or
    |> a minimum elevation anglet) to do this imaging? And what class of sats
    |> will fit the bill and do this imagining  job? Commercial sats or military
    |> ones? Plan to do this in Rob's Skymap.  Comments welcome.
    |>
    |>
    |> Thanks
    |>
    |> rgds
    |> ykchia
    |>
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    |
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