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Satellite Tracking

Posted: 2 August 2015

On my 16 and 21 June 2011 Cassiopeia Observatory reports, I provided some notes on tracking the International Space Station (ISS) or any satellite with a telescope. I decided that I should collect those notes in a single article. I will add more as appropriate.

Some Notes on Tracking the ISS

Some telescope users wonder how they can track the International Space Station (ISS), or any satellite, using their telescope. The ISS moves rapidly across the sky, making it difficult to track it by manually slewing or moving a telescope by hand, even with low magnification. Some modern GOTO telescopes have software that will track satellites. There are also computer programs that will send slewing commands to your telescope to track satellites. Meade telescopes with the AutoStar can track satellites. You can download Two Line Elements (TLE) to the AutoStar using the AutoStar Update Application (Windows only) or manually enter the TLE data (from into the AutoStar. Since I don't use Windows, I manually enter the data for the ISS. The article "Tracking the International Space Station" has instructions on how to manually enter the TLE data. Once you have the current TLE added, the AutoStar will track the ISS. However, tracking is not perfect and you will have to make manual corrections with the AutoStar slew arrow keys to keep the ISS in an eyepiece (or camera) field-of-view (FOV). When the ISS (or any satellite) is near the horizon, the satellite moves slowly across the sky; tracking corrections are usually minimal and may not even be needed. However, as the satellite gets closer to you, getting higher in the sky, it moves ever faster and faster, and manual corrections become more frequent. As the satellite goes away from you in its orbit, its movement across the sky seems to slow down and tracking corrections become less. When the satellite is furthest away from you, especially when close to the horizon, it is at its smallest and atmospheric distortion will obscure details. When the satellite is at its closest to you and hence at its largest, it is also moving the fastest, requiring the most corrections. An Alt/Az mount is best for satellite tracking. A polar mounted telescope can be used but at some orientations of the telescope, depending on the orbital parameters, it can be difficult to look through the finderscope and/or eyepiece. And since the telescope is rapidly slewing at times, you don't want to stumble over your tripod as you move to follow the satellite. These factors make viewing and imaging details challenging.

When I first began tracking the ISS, I visually observed it with a 26mm (77X) eyepiece. That gave me a nice wide FOV with some reasonable magnification, allowing the ISS solar panels and central core to be seen. As the satellite moves in its orbit, it can change from a very bright silver to an orange or gold color on the solar panels. And as the perspective changes during the pass, you will get differing views of the station. Once I was comfortable with tracking the ISS at 77X, I began using a 15mm (133X) eyepiece. This was more challenging with the higher magnification and smaller FOV. But the views were more rewarding with the increased magnification.

Eventually, I began imaging attempts, first with an iPhone and later with a D7000 DSLR. The iPhone was mounted afocally with a 26mm eyepiece and a video recording made during the pass. I could then look at each frame of the recording on the computer, finding the best images to show. I also did afocal imaging with a 15mm eyepiece. I was never fully satisfied with these images, although some were pretty good and showed the overall structure of the ISS with the solar panels. I switched to using the D7000 DSLR at prime focus and later at prime focus + 2X Barlow Lens (or 3X TeleXtender). I used the HD video recording mode on the DSLR. I am now using a Nikon D7200 DSLR at prime focus and prime focus + 2X PowerMate using HD video recordings. The D7200 has a 1.3X crop factor when using 60 frames per second. So I get a little extra magnification and more frames per second with the D7200. With both the iPhone and DSLR, motion blur from the rapid slewing is the biggest problem, although keeping the ISS in the camera FOV is always a challenge. You can see some of my ISS images on my "Satellite Astrophotography" page.

Whether viewing the ISS with your eye or a camera, sometimes it will go out of the FOV. Then you will have to move to viewing through your finderscope and re-center the ISS. You then go back to the eyepiece and camera and hope that the ISS has not moved so rapidly as to be out of the FOV again. This game of "cat and mouse" is frustrating at times but the effort will be worth the views or images. Give it a try.

For another take on tracking the ISS, see this excellent YouTube video (thanks to Ray Gilchrist for the referral). The video discusses getting the ISS pass times, how to manually track the ISS, video recording using a webcam, and editing the video.

Some More Notes on Tracking the ISS

Dick Seymour posted the following information on ISS tracking with an AutoStar II (probably also applies to the other model AutoStars) on the LX200GPS Yahoo Group.

Subject:	[LX200GPS] Space station tracking
Sent:	Friday, June 17, 2011 10:03:49
From:	autostaretx (
How it works (very short edition):
During the "calculating pass" portion, the scope determines and
remembers about 100 or so *incremental* steps for the path.
(think "up two, over four at speed ZZZ", not "GoTo xx yy")
During the pass itself, it "plays back" that list of steps, about one
per second (for an ISS pass).
When you adjust the path to recenter the satellite, the next "up two,
over four" happens from *there*, and the subsequent steps likewise are
based upon that new spot.
The scope *does* change tracking speed during a pass... you can hear it
"rev up" as it nears the zenith.
If, however, you ever use ASCOM to drive your scope from a PC, that
(unless you tell it not to in the ASCOM driver setup dialog) may have
silently set your scope's Maximum Slew Speed to only 3 or 5 degrees per
second (instead of its usual 8 or faster)...  and that setting is
remembered across power off/on cycles.
You can check and change that from the Autostar's Setup/Telescope/Max
Slew Speed  menu setting.
Alt/Az -mounted scopes have problems keeping up with passes that go
within 10 degrees of zenith (straight overhead), and polar-mounted scope
have problems near the pole.  There is a limit to the speed a scope can
slew.  For that reason i may choose to mount my scope differently on an
evening i'm planning to follow a "high elevation" pass.
Recently (within the last week) other satellite chasers have been saying
that the TLEs (and Heavens-Above's predictions) have been a little "off"
for the ISS... sometimes as much as two minutes in error.
If you can't capture a satellite as low as the Autostar's "AOS"
prediction thinks, you should either change the Autostar's setup menu
AOS altitude to one that works for you, *or* start at the predicted AOS
position a few seconds *early*, let the scope rise to where you do
expect to see the ISS, and then tap Enter to pause.
(instead of slewing up and *then* starting the pass)
That will move you "down the list" of "steps", so that the Autostar will
be tracking at the speed of that higher elevation instead of at the
speed of the "beginning" of a pass.
The scope does not recalculate anything during a pass, since it would
always be behind.  It only "plays back" the original list if incremental

have fun

Dick also mentioned Brent Boshart's SatelliteTracker, available for download from the Files area of the satellitetracker Yahoo group.

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