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    Re: automatic celestial navigation
    From: Frank Reed
    Date: 2007 Dec 07, 05:08 -0500

    Gary, you wrote:
    "O.K. but there are only  limited periods during the day when satellites are
    visible, shortly before sunrise and  shortly after sunset, say about an hour
    each. During sunlight you can't see them against the bright sky (or the
    stars either) and after the sun is well down the satellite is in the shadow
    of the earth and is not illuminated."
    
    Yes, it's definitely a night-time project! But the time period during which
    satellites are visible can be suprisingly long. It's typically longer than
    nautical twilight, sometimes much longer, so this is a more leisurely
    activity than the usual twilight round of star sights.
    
    And you wrote:
    "Here is a website that will predict when the space station of the space
    shuttle will be visible in your area.
    http://spaceflight1.nasa.gov/realdata/sightings/
    Here is a sample of its output for Simi Valley California. Notice that there
    are only three oportunities to see the interational space station in the
    next two weeks for a total viewing time of only 5 minutes. "
    
    Yeah, but Gary, that's just those TWO satellites. As I noted in my earlier
    message, the ISS would be a bad choice for this method of navigation because
    it maneuvers frequently. You can't count on calculating ephemerides for it
    unless you have access to very recent orbital elements.  Additionally it's
    orbit is decaying rapidly --which is the main reason it has to maneuver
    frequently. They have to give it a good shove every few weeks to counteract
    the effects of atmospheric drag. The station is in an unusually low orbit
    for a satellite which is also why its passes over any specific observaing
    location are rather rare. As for the space shuttle, those pass predictions
    on the NASA site today are probably garbage. The shuttle's maneuvers are
    frequently re-scheduled during its missions, and as you probably saw in the
    news today, the present mission, a critical one to launch the ESA Columbus
    laboratory, has just been delayed a day. So forget about those manned
    satellites. Luckily for this game, there aren't many others that maneuver.
    
    To get a better idea how many satellite passes are available for this
    navigation game, I recommend using a German web site: www.heavens-above.com
    which is much more comprehensive than the NASA site. It was honed to a fine
    state of near-perfection some seven to eight years ago. Select your location
    (there's a list) and then ask it to display all satellite passes with
    magnitudes brighter than 4.5 (easily visible from a dark site). For most
    twilights at most locations on the Earth, there are more than a dozen
    passes. The biggest problem that you may discover is that a majority are on
    roughly polar orbits which means that the LOPs from two different satellite
    observations will tend to cross at rather acute angles. Even so, you can
    usually find a pair that cross at a wider angle. Try experimenting with
    different locations on the Earth. For an interesting case, check for visible
    satellite's today up in the high Arctic, say around 80N, 90W. How many times
    during a given twelve hour period can you see satellites? Quite a few, huh!?
    It's interesting to note that they're mostly the same satellites repeating
    their appearances after a hundred minutes or so (that's just the orbital
    period, of course, but you don't often see this simple repeating in lower
    latitudes because the observer has rotated out from under the satellite's
    path in 100 minutes). At your location (well, I assume you mentioned Simi
    Valley because it's your location), you should be able to see at least a
    half-dozen satellites with binoculars every evening and every morning. Light
    pollution will take its toll.
    
    Just to reiterate the point of all of this, it's a method of simple, visual
    position-finding that requires no instruments except possibly binoculars. It
    should not be difficult to acheive one nautical mile accuracy from any
    location on Earth that has a dark night sky. It does require a computing
    device and specialized software.
    
    And I suppose I should add that this is very nearly the SAME method as the
    method of finding longitude and latitude by measuring lunar distances (at
    known GMT) which I was talking about last fall. We're accurately observing
    the position of a nearby object relative to distant objects and getting
    lines of position from the observations. In both cases, no horizon is
    necessary, no altitudes need to be measured. In both cases, we need a
    computer (or an awful lot of paperwork). In the case of the Moon, since it
    is some 230,000 miles away, we need a good sextant to fix its exact location
    among the stars. In the case of Low Earth Orbit satellites, a purely visual
    check is all that's needed.
    
    If you feel like throwing some technology at the problem, imagine observing
    those satellite passes with a night vision videocamera. If you could fix the
    satellite's position against the background stars to the nearest half a
    minute of arc, then the resulting LOP is accurate to better than a tenth of
    a nautical mile (assuming a nearly straight overhead pass of a satellite 500
    nautical miles high...  0.07n.m.=(0.5'/3438)*500n.m.).
    
     -FER
    
    
    
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