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Tracking Satellites:

This is a map of the path of a Satellite.
Each point represents the point on earth directly below the satellite at a given
time.
(Side point: If its in a circular orbit i.e. constant speed, Why do the points
get further spaced apart near the poles? hover mouse over picture for
answer.)
Satellites travel in *great circular orbits, the kinds of circles the equation
deals with. In other words, if the Earth were not spinning (relative to
the satellite) then we would get the same boring graph as before. Adding
motion to the earth will make things much more complicated though.
It ends up that I needed to derive an
entirely new equation in parametric form (using the same diagram (not
pictured here), because everything needs to be in terms of time (in
minutes).
x(t)=arctan(cos(Phi)*tan(a*t)) ± (1/4)*t
y(t)=arcsin(sin(Phi)*sin(a*t))
Notice: . . . ± (1/4)*t this takes
into account the earth spinning 360 degrees in 24 hours or 1440 minutes.
it is really reduced from ± (360/1440)*t. + if the Earth is spinning away
from the satellite, - if its traveling with the satellite. I forgot about
this for quite a while and it drove me absolutely crazy, I had the Earth
spinning the wrong way!
a*t is how fast the satellite is going around earth, like its angular speed,
this depends on how far up it is.
Finding Phi:
This particular picture was taken from
The Champ
Sattelite's Home Page. It is used to map Earth's gravitational and
magnetic fields. I researched the satellite on the wonderful
heavens-above.com
(Champ
satellite detail) and found the value of Phi is 87.2 degrees.
Finding 'a':
I could cheat and look it up on the website and calculate that
a=3.920632233, or I could be more creative:
The site is updated every 5 mins so I think that each Dot is is about 5 mins
apart. I counted the number of dots it took to cross 60 degrees
latitude near the equator (because it looks like a pretty good linear
approximation there) to get 'a' approximately (2/3) or 2 degrees in 3
minutes. That is pretty far off, it could be because the dots were
more that 5 mins apart, but that is not the only thing. I think I know
the other reason, if you you can figure it out I'd be happy to talk to you
about it.
I then tried
using the map to approximate how far the Earth had spun while the satellite
went 360 degrees, its full orbit. The earth spun about 25 degrees, and
I know the Earths angular speed so the time taken was around 100 mins. The
satellite's angular velocity was then near 360/100=3.6, not a bad
approximation.
plugging in Phi=87.2, a=3.92 we get:
x(t)=arctan(cos(87.2)*tan(3.92063*t))+(1/4)*t
y(t)=arcsin(sin(87.2)*sin(3.92063*t))
Dot Mode:
I tried graphing it in Dot Mode, t-step at 5 min, the dots were spaced the
same as this graph, very exciting. Unfortunately I was using GraphCalc and it
won't let you increase the dot size so I didn't post it here. NJIT makes
us use so many different math programs I can't use any of them efficiently.
I simply added the red line like I usually do. The "Previous" black line,
flows right into the "Predicted" blue line in continuation of the map, just like
it should.
Note: Click Picture or
Here, to get an unbroken version
in new window. |