Satgen 535 SpaceStation Viewing by GM4IHJ (BID SGEN535) 1999-06-26 The physics of ISS Space Station visual tracking is relatively simple, but there are a few variations on what you see and when you see it , depending on your ground station latitude. The basic features of the ISS viewing problem are that, you see ISS only at night against a clear starry sky, and then only by reflected sunlight shining off it. Roughly 400 kms above the earth, ISS sees and therefore reflects sunlight long after the sun has set at your ground station, or , some time before the sun will rise above your ground station horizon. You do not get a viewing opportunity every day. You can usually only view when you have orbit passes against a clear relatively dark sky, for an hour or two after sunset or an hour or two before dawn. In practise this means that stations in the latitude belt 30 to 60 degrees north or south are overflown each day by a block of about 5 succesive orbits.These blocks produce useful post sunset orbits on each of several succesive nights every 60 days. While pre dawn orbit blocks are usable roughly 25 to 30 days after you have had post sunset orbit blocks. In all cases these orbits travel roughly west to east . You see the sunset orbits from about 10 degrees above the horizon and you lose them as the ISS loses the sun and goes into earth shadow a few minutes later. Pre dawn orbits are not seen until they are half way across your sky, from which point you can then see them as they descend to your eastern horizon. By contrast , equatorial stations have a different pattern of in range orbits. Whereby, passes near these stations are seen either as a pair of Ascending orbits or some 12 hours later seen as a pair of descending orbits. Further north or south, ie at stations located above latitude 50 the situtation has an added twist in your local summer. Here in Scotland for example (56N) it never goes completely dark at night during the months of May, June and July. So this midnight sun effect can provide you with a midnight view of a shining ISS as it comes north , such that while at low elevation to your south you see nothing . But as ISS elevation increases and the sub ISS latitude climbs up to near 50 degrees, it gets high enough to see the summer Sun in its midnight position over the Pacific, west of but at the same latitude as Hawaii. These special midnight Sun shots are brief but fun. Providing as they do a short but clear indication of the overlap of the Solar footprint and the ISS footprint. In addition , those who keep careful track of the dates and times of Space Shuttle visits to the ISS, should occasionally catch a Shuttle approach, or a docking , or a Shuttle withdrawal. Years of Salyut and Mir watching have produced some classic space duets of this kind. Dzanibekovs Soyuz as it approached for the Salyut 7 rescue mission, and the pas de trois as Kizim and Solovyev went backwards and forwards in Soyuz, between brand new Mir and the soon to be abandonned Salyut 7