Text 12947, 84 rader
Skriven 2014-03-19 13:38:11 av mark lewis (1:3634/12.71)
  Kommentar till text 12937 av Joe Delahaye (39525.fidonews)
Ärende: satellites, tracking and scopes (was: Kyoto anyone?)
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On Tue, 18 Mar 2014, Joe Delahaye wrote to mark lewis:

 ML> the GPS satellites are not in geosync orbit... they are in 
 ML> constellations and move around the earth overlapping others' foot 
 ML> prints... yes, a minimum of 3 are needed for basic pinpointing of 
 ML> one's location... more gives better accuracy... generally 
 ML> speaking, there are 8 or 9 GPS satellites visible for use at any 
 ML> time and for any point on the earth... the number of visible 
 ML> satellites can be as high as 12 as they orbit into and out of 
 ML> range...

 ML> the information above mainly pertains to the US' NAVSTAR 
 ML> satellites... there are other GPS satellite constellations in use 
 ML> and being put in place these days...

 JD> I would have thought that the GPS sats would be geosynchronous. 

nope... they cannot cover the areas well enough... geosync is around the
equator... the signal will be extremely weak at the poles...

 JD> Seems logical to me that if in a stand still orbit, you can 
 JD> compute your position easier then from a sat that moves.

you don't care if the sat is moving... all you care about is how long it took
the signal from that sat to arrive at your device... the signal carries an
accurate timestamp so looking at your own timestamp with the signal arrives and
comparing the contained timestamp gives you the time separation from which the
distance to the satellite can be calculated... you do this with several
satellites and because their position is known, even though they are moving,
you can then plot your location...

 JD> I've been wrong before though <G>

i thought i was once but i was wrong about that O:)

 ML> yes, i do track satellites and have written a few private 
 ML> tracking and  prediction packages that make use of the available 
 ML> two-line element (TLE) files... i also watch the visible ones fly 
 ML> overhead when the sky is clear in the evenings and mornings ;)

 JD> I have two different programs I use at times.  Used to use them at
 JD> every shuttle launch.

i have no less than four here... some give me only earth footprint plots where
the path of the satellite over the earth is see with its current coverage
area... others i have give a sky view from a chosen location and show the
stars... when a satellite comes into view, then it is plotted... some of them
will also show the satellite's path across the sky to make finding them
easier...

of the groups that i track with, some of them use only binoculars... one fellow
uses 20x80 binoculars mounted on a fluid pan-head tripod... others use similar
mounted binoculars that may only be 10x70 or larger than the 20x80s... some use
telescopes with tracking mounts on them and cameras attached to record to
digital media so that the satellites can be seen and their path measured from
the start... accurate timing is necessary as is a good star chart... some
observers use stopwatches for the timing of a satellite's position near one
star to a position near another star... with an audible time source, they can
get pretty accurate times for when they start and stop the stopwatches...

i do a lot of naked eye viewing so i tend to see only those that are visible to
mag 4.5 or so depending on how dark my skys are... i have several ccd cameras
here that will be fitted to (at least) one of the 8inch telescopes i'm
building... i've ground the mirrors myself and just need to build the mounts
and tracking systems for them... an 8in scope should get me to mag 13.5 or
so... with cameras and longer photo times, i may be able to get as dim as mag
14.xx or even mag 15.xx... mag 13-15 will alow me to see and report on geosync
sats...

i'm undecided as to whether i want to keep the two scopes as separate devices
to be able to track multiple sats at once or if i want to build them into one
mount for a nice bino-scope ;)

i'm also considering building a 12in to 15in scope but the cost of glass goes
up quite a bit... if i build my own kiln, then i can melt my own glass and make
my own mirrors from little to nothing ;)

)\/(ark

One of the great tragedies of life is the murder of a beautiful theory by a
gang of brutal facts. --Benjamin Franklin

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