Post by Kiwi on Feb 9, 2010 6:17:10 GMT -4
I would like to get a really good idea of how far away the International Space Station's ground track is at different altitudes from the viewer. We know it's overhead at 90 degrees altitude, but at what distance is its ground track at all or most of the other angles down to zero degrees? In New Zealand we can often see it when it's only 5 or 6 degrees above the horizon, whereas the minumum altitude at which it apparently becomes visible in northern hemisphere countries is 10 degrees.
Are there any formulas that can be used to work this out, or can anyone offer advice or direct me to useful websites, other than Heavens-Above? I thought of posting this at BAUT, where I might find more experts in this subject, but wanted to try my favourite forum first. I realise the exact distance of the ground track may vary according the altitude of the countryside it passes over, but figures for the viewer and ground track being at sealevel should suffice.
The reasons I ask follow:
I often use Chris Peat's marvellous Heavens-Above website for sightings of the ISS and other satellites, particularly in summer when the sun is not far below our southern horizon at astronomical midnight, so the ISS can sometimes be seen on up to six consecutive orbits. The night of last 1-2 December was such an occasion for my area, but our sky was too cloudy. I had previously thought that five passes were the most that could be seen in one night, and did see that many passes on one occasion.
Whenever there are good close-to-overhead passes for much of New Zealand and skies are likely to be clear over a wide area, I post the details on a popular message board and often get grateful thanks from people who have been thrilled to see the ISS and sometimes have never seen it before.
Occasionally I have great difficulty convincing people how far away the ISS is from them. One guy, who uses a more basic website for pass details, recently claimed that on one particular pass it appeared above communities less than 25 kilometres from his city, Hamilton, when in fact it was over 640 km away above Greymouth then Christchurch. When I tried to enlighten him with the facts, he implied that only one of us knew what he was talking about and it wasn't me.
Heavens-Above gives the line-of-sight distance the ISS is from a viewing site, and supplies a small ground-track map, but it's too small to work out fairly precise distances when the track is far from obvious landmarks.
For those who are interested, one place that is useful for studying the ISS's ground track is Eden, Wyoming (42 deg. 3 min. north, 109 deg. 27 min. west). The reason for that is that more than quarter of the 10-degree-altitude circle around Eden neatly passes just inside most of the coastline from the Gulf of California up to Vancouver, so there are plenty of useful landmarks for passes to the south and up to to east-northeast of Eden. However, I'm not familiar enough with U.S. geography to pinpoint which communities the important parts of the ground track come near. I estimated that when the greatest altitude of the ISS was at 26 degrees altitude and southwest of Eden, the ground track was close to Panaca, Nevada, but was far from certain about it.
One other thing I noticed is that at Heavens-Above the point of greatest altitude of the ISS for any site is indicated by the point of the arrow-head on the ground track.
Are there any formulas that can be used to work this out, or can anyone offer advice or direct me to useful websites, other than Heavens-Above? I thought of posting this at BAUT, where I might find more experts in this subject, but wanted to try my favourite forum first. I realise the exact distance of the ground track may vary according the altitude of the countryside it passes over, but figures for the viewer and ground track being at sealevel should suffice.
The reasons I ask follow:
I often use Chris Peat's marvellous Heavens-Above website for sightings of the ISS and other satellites, particularly in summer when the sun is not far below our southern horizon at astronomical midnight, so the ISS can sometimes be seen on up to six consecutive orbits. The night of last 1-2 December was such an occasion for my area, but our sky was too cloudy. I had previously thought that five passes were the most that could be seen in one night, and did see that many passes on one occasion.
Whenever there are good close-to-overhead passes for much of New Zealand and skies are likely to be clear over a wide area, I post the details on a popular message board and often get grateful thanks from people who have been thrilled to see the ISS and sometimes have never seen it before.
Occasionally I have great difficulty convincing people how far away the ISS is from them. One guy, who uses a more basic website for pass details, recently claimed that on one particular pass it appeared above communities less than 25 kilometres from his city, Hamilton, when in fact it was over 640 km away above Greymouth then Christchurch. When I tried to enlighten him with the facts, he implied that only one of us knew what he was talking about and it wasn't me.
Heavens-Above gives the line-of-sight distance the ISS is from a viewing site, and supplies a small ground-track map, but it's too small to work out fairly precise distances when the track is far from obvious landmarks.
For those who are interested, one place that is useful for studying the ISS's ground track is Eden, Wyoming (42 deg. 3 min. north, 109 deg. 27 min. west). The reason for that is that more than quarter of the 10-degree-altitude circle around Eden neatly passes just inside most of the coastline from the Gulf of California up to Vancouver, so there are plenty of useful landmarks for passes to the south and up to to east-northeast of Eden. However, I'm not familiar enough with U.S. geography to pinpoint which communities the important parts of the ground track come near. I estimated that when the greatest altitude of the ISS was at 26 degrees altitude and southwest of Eden, the ground track was close to Panaca, Nevada, but was far from certain about it.
One other thing I noticed is that at Heavens-Above the point of greatest altitude of the ISS for any site is indicated by the point of the arrow-head on the ground track.