Marcin Podgorski wrote: > It was 10:07pm when Thierry Marais wrote: > > TM> According to weather satellite pictures, Spanish observers have > TM> their chance in Gallicia, French ones in Brittany. > > We have clear skies here in Warsaw, Poland. Could you please > provide me with the map and timings or at least the current TLE? > Perhaps it's worth doing a short reentry observing walk tonight? Since most of us now obtain our TLEs and related data from Space Track, with its extremely restrictive User Agreement, we cannot share information in a timely manner. It is to be hoped that Space Track will develop a practical approval process for the timely sharing of data to facilitate discussions of this type. Until then, one way to cope would be to develop simple instructions and software for use by less experienced observers in situations such as this one, i.e. a potentially interesting observable event with very little advance notice. The following probably falls well short of what is required, but perhaps can serve as a first draft. 1. Obtain Most Recent TLE 1.1 Go to the Space Track website: http://www.space-track.org/perl/home.pl The site requires registration. Before you do so, please read the User Agreement, which is extremely restrictive. It forbids the "transfer [of] any data, including, but not limited to, the analysis of tracking data, or other information received through this website or any services described herein to any third party without the prior express approval of the Secretary of Defense or his delegatee." To my knowledge, no approval procedure exists at this time. 1.2 If you decide to register, then log in, and on the home page, in the section "Satellite Decay & Reentry Data", click "Current Decay Predictions (From TIP Messages)". Locate the object of interest in the table, and obtain its Latest Elset, by clicking the link provided. 2. Propagate elements to decay. Most prediction programs use the SGP4 propagator, which is ideal for most purposes, but does not handle decay predictions well. If the mean motion of the latest elset obtained from Space Track is less than, say, 16.4 rev/d, I recommend the use of Alan Pickup's Satevo to propagate it until just before decay: http://www.wingar.demon.co.uk/satevo/#Download When you run Satevo, have it save its results to a file, then open the file with a text editor, and copy the final elset before decay for use in the following steps. 3. Find the Likely Decay Ground Track(s) The decay is likely to begin when the satellite's altitude descends below about 100 km. To determine whether or not your location is near the path of the decay, use a ground track and altitude tabulation program, like my Gtrack.exe, available here: http://satobs.org/programs/GTrack.zip Run the program for the expected date of decay, and then scroll the output file, looking for the point where the altitude descends below 100 km. There is a good chance that decay will begin at about that point, especially if the predicted altitude goes well below 100 km after that point. Otherwise, decay could occur on a subsequent dip below 100 km. If the decay path seems likely to pass near you, proceed to step 4. 4. Run Satellite Prediction If you have a satellite prediction program that will report passes that occur within Earth's shadow, then use it to run a prediction for your location. Most such programs omit passes in shadow unless instructed to do so, so be careful to use the correct settings. You are likely to find suitable programs here: http://www.satobs.org/orbsoft.html I believe Mike McCants' Quicksat and Rob Matson's Skymap have the required features, but they are fairly full-featured programs that may have too long a learning curve for the casual user, so I am considering writing a much simpler program specifically for re-entries and other short-notice events. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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