Dec 2008

Credits ...

Revisiting Spacetrack Report #3
David A. Vallado
Center for Space Standards and Innovation, Colorado Springs, Colorado, 80920
Paul Crawford
Crawford Communications Ltd., Dundee, DD2 1EW, UK
Richard Hujsak
Analytical Graphics, Inc., Exton, PA, 19341
T. S. Kelso
Center for Space Standards and Innovation, Colorado Springs, Colorado, 80920

Message of Dec19/08

The biggest visible feature is rather a silly one -- eye-candy but not very useful, yet.  Three toolbar buttons to alter how fast time passes!  The left button puts time in reverse, repeated touching making time go back faster and faster -- the right button makes time go forward faster and faster -- the center button restores normal time (abruptly).  It's actually quite fun visually.  Incomplete

It would have been a bit more useful to have been able to slow time down once you got to tomorrow so you could focus on a sighting opportunity, but this 'feature' is mostly a test of my having separated the display process from the computational one -- prior to this the orbital calculations have been invoked from inside the classes that maintain the display views.

The actually useful version of this will use the 'forward/backward' buttons to spin things forward to the next viewing opportunity (or, if no forthcoming sighting, to some other sensible time).

This shot taken as I typed this with ISS passing to my North -- but the sky is full of snow and nothing is to be seen (actually the previous orbit, to which I can now go, was the better one -- ISS was closer and the sky darker).

This version is also shipped with elements for ISS which allows the application to work when it is first run, even if there in no Internet connection to obtain fresh elements -- the sample will get out of date, but that's better than nothing (I think).

I'm also trying to settle on some color scheme that makes sense and is clear.  The current globe seems almost OK (white ambient light from the Sun, and a teal diffuse surface with a faint dark blue glow for the globe.  The current ISS location, footprint and track are all white today -- if I was showing two satellites, I'd want different colors.

Which reminds me, if the Shuttle is flying (more accurately, if there is an element set for an object containing "STS-" anywhere in the collection), the application will track that also.  Just for fun, I was tracking the dropped tool bag from the last Shuttle mission -- you need clear sky, good binoculars, and knowing where/when to look to see it (for me two out of three wasn't enough).

Speaking of elements, I've stopped hammering the element server for every run I do -- I now seek new elements no more than once an hour.

next version will hook up the Preferences (ignored so far) so that you can make your own choices about some parameters.

Speed Up & Slow Down ...

More internal changes have further isolated the display process from the computational one. This is exemplified in the new ability to speed up and reverse time! Controls in the new toolbar influence the passage of time -- the central button brings everything back to now and real time.

This capability will be used to make predictions of visibility. Speaking of which, ISS has passed the time when it’s constantly in sunlight and is now visible again in North America, though now in the mornings.

This is available as ad-hoc release

Permanent sunlight

Mid-winter in the Northern hemisphere and the ISS orbital plane has rotated to lie parallel to the terminator -- all being a long way of saying that ISS is orbiting in sunlight all the time right now. This also makes it impossible to see because the sky is too bright when it passes overhead.

Message of Dec08/08

The rotation and zooming of the Earth are now a lot smoother and more responsive.

There is now an accurate track of the ISS (from a few minutes behind where it is to almost a complete orbit later). For those that care there is a difference between the orbit and the track -- the orbit doesn't move (discernibly) in the inertial frame that the iPhone models, but the Earth rotates in that frame and so the track (the path relative to the ground) and the orbit are different.  The track drifts West as the Earth rotates to the East.

The track is also decorated (slightly bigger dots) to show when the ISS is in sunlight.  Note those times when it is in sunlight while the ground below is dark -- if you're standing in that dark area, and the ISS is over the horizon, you'll see it.

In this example from tonight (the clock shows GMT -- UT1 actually), ISS has just passed North of the Falkland Islands. If you were in Port Stanley at the time, you'd see ISS appear in the dark sky in the North West and head to the Eastern horizon.  It's nearly mid-Summer there so the sky doesn't get very dark, but ISS is bright and it gets dark enough.

My next version will hook up the Preferences (ignored so far) so that you can make your own choices about some parameters.

Now, if we could have some clear evening skies in South-West Michigan, I'd be able to say I'd stood outside with ISS in my hand while it arced across the sky.  The orbit I said almost doesn't move actually drifts slowly Westwards and opportunities to see it are getting fewer in my neck of the woods, but there is a transit directly overhead on Tuesday evening so my fingers are crossed.

Looking good ...

The changes of the last few days have taken iSat to a pretty good place ...

Additions include:
  • smooth zooming via pinching
  • rotation by touch feels better
  • the satellite track (a little behind and nearly one orbit ahead) is now placed over the right part of the earth. The previous attempt drew out the orbit which doesn’t actually move in the inertial space of the model. Because the Earth rotates under the orbit the track draws out a path constantly drifting west of the ‘orbit’.
  • the terminator no longer has a dotted line on it -- it was too easily confused with other great circles and I’ve managed to get the lighting such that the terminator is pretty obvious, also
  • the satellite track is highlighted when the satellite would be in sunlight.
The images below are taken from the ‘ec078’ version of the program

Earth pinched smaller, showing the ISS heading North-East over the South Indian Ocean. Note that the track disappears behind the Earth and reappears over the South Pacific off the coast of Chile. When it does, it is in shadow (smaller dots) and only comes back into sunlight near the Falkland Islands.

This is a pinched out detail from about 30 seconds earlier. Were you to be an observer at this time in Port Stanley, you’d see the ISS appear in the dark sky and brighten as it passed from the North East to the Eastern horizon.

This is ad-hoc release

Latest Changes

iSat continues to improve. The slider that controlled the zoom factor has been removed and replaced with a ‘pinch’ effect multi-touch ...

It’s actually quite difficult to get the ‘pinch’ gesture right since the timing of your fingers landing on the glass is not easily controlled and the code to filter little bits of random movement and a finger ‘bounce’ again during a pinch is tricky to get right. In fact, I don’t have it right yet, but I’m working on it.