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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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