by alter-ego » Sat May 14, 2016 5:39 am
Thierry Legault wrote:Nitpicker wrote:I was also amazed when I first saw this one. Still amazed now. Mr Legault has a habit of amazing me. And I bet he put additional thought into his position, so as to avoid the possibility of the ISS lining up with Mercury and potentially obscuring it.
Actually I used the Calsky calculation for the transit of the ISS over
Mercury to have them as close as possible, and found the right place to set up, but I re-checked the calculation just before the ISS transit and noticed that the transit line had moved 200-300m East. Anyway, even the updated line was not exacty the right place too (Calsky calculations, based on orbital data, are very accurate but not perfect anyway!)
![Smile :)](./images/smilies/icon_smile.gif)
Unfortunately, TLE predictions (which Calsky uses) are not accurate enough for
reliable, precision conjunction assessments like a satellite / planet overlap, and ground track accuracy typically is better nearer, or even prior, to the TLE epoch. In addition, a large, LEO satellite like the ISS adds further difficulty to the calculation due to unmodeled forces or maneuver corrections. Ultimately, confident conjunction predictions require other orbit propagator calculations that are more real time than the TLE propagator. So in your case of an angular "miss" of 0.1°, I estimate the correct ground position for a bulls-eye conjunction with Mercury to be ≈1.5km
westeast of your location. Though I don't know the statistics, ground position errors of 100's of meters are probably fairly common.
Anyway, enough of this technical stuff.
You did a magnificent job with the ISS transit image and video - resolution and quality are outstanding, and imagery of the ISS and shuttle on your website are truly remarkable. I applaud your commitment to good (and honest!) work.
I'm sure we'll see an APOD from you again sometime.
[quote="Thierry Legault"][quote="Nitpicker"]I was also amazed when I first saw this one. Still amazed now. Mr Legault has a habit of amazing me. And I bet he put additional thought into his position, so as to avoid the possibility of the ISS lining up with Mercury and potentially obscuring it.[/quote]
Actually I used the Calsky calculation for the transit of the ISS over [u]Mercury[/u] to have them as close as possible, and found the right place to set up, but I re-checked the calculation just before the ISS transit and noticed that the transit line had moved 200-300m East. Anyway, even the updated line was not exacty the right place too (Calsky calculations, based on orbital data, are very accurate but not perfect anyway!)
:)[/quote]
Unfortunately, TLE predictions (which Calsky uses) are not accurate enough for [u]reliable[/u], precision conjunction assessments like a satellite / planet overlap, and ground track accuracy typically is better nearer, or even prior, to the TLE epoch. In addition, a large, LEO satellite like the ISS adds further difficulty to the calculation due to unmodeled forces or maneuver corrections. Ultimately, confident conjunction predictions require other orbit propagator calculations that are more real time than the TLE propagator. So in your case of an angular "miss" of 0.1°, I estimate the correct ground position for a bulls-eye conjunction with Mercury to be ≈1.5km [s]west[/s][color=#FF0000]east[/color] of your location. Though I don't know the statistics, ground position errors of 100's of meters are probably fairly common.
Anyway, enough of this technical stuff.
You did a magnificent job with the ISS transit image and video - resolution and quality are outstanding, and imagery of the ISS and shuttle on your website are truly remarkable. I applaud your commitment to good (and honest!) work.
I'm sure we'll see an APOD from you again sometime.