Re: Tracking ISS with Horizons #APCC #Mach2GTO

Ray Gralak

Hi David,

1-minute intervals are not accurate enough.

You might try to do a 1-second interval. You can do this by specifying a start and end time and specifying an "equal interval (unitless)" step size. Then, specify a count that results in 1-second intervals. For example, if the range is one hour, the count to produce 1-second intervals would be 3600, because there are 3600 seconds in one hour.


-----Original Message-----
From: [] On Behalf Of David Johnson
Sent: Monday, March 15, 2021 5:35 PM
Subject: [ap-gto] Tracking ISS with Horizons #APCC #Mach2GTO

Okay, I'm trying to use A-P's Horizons to track the ISS, and I have a couple of questions. I was able to get the
ephemeris data from JPL and load it into A-P's Horizons. So I have data loaded in there at 1 minute intervals.

I tried the Test Tracking, and my first question is about the following. It appears that it does the test tracking using
the data for the time period you select, but the mount does not use that time period for pointing purposes but the
current time. What I mean is, for example, if I try to track using the data on 3/16, starting at, say, 9:28 p.m., it says
that the ISS is at a certain celestial coordinate in my sky at that time. The mount goes to that coordinate, but, of
course, that coordinate is not at the same place in the sky now as it will be on 3/16 at 9:28 p.m., so the altitude and
azimuth shown in the ephemeris data will not be where the telescope points now. I noticed this because I naturally
started trying to track as the ISS was coming up over the horizon and therefore the first altitude shown was
something like 1.7 (degrees, I assume), but the telescope was obviously not pointing that low, and, indeed, APCC
says that it was not that low. If this is how Horizons works when doing Test Tracking, that's fine, but I just wanted
to make sure I wasn't doing something wrong.

The second question is on the tracking itself. I noticed that the mount was not able to keep up with the tracking
when the apparent movement of the ISS became relatively fast, even though watching it, it was obvious the tracking
was well within the slewing speed of the Mach2 on 24V, which is what I'm using. The tracking appeared to fall well
behind, as in 2H of RA for RA Delta and something equivalent (a few degrees) in Dec Delta. Maybe this has
something to do with the closed loop correction not being fast enough? I also noticed that there is no check box for
disabling the closed loop correction, as is shown in the APCC documentation.

Anyway, if I can't track the ISS with the Mach2, that's fine. I just want to make sure it's not because I'm doing
something wrong. I have a real pass of the ISS coming over at 9:21 p.m. local time tonight, so I'm going to try "real
tracking", albeit in my basement since it's raining outside.


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