Raoul Behrend asks some critical questions about decay times ands positions. Here some general informations. >Well, I always asked me how can the localisations of impact be given at 10 or 1 > km (0.1, 0.01degree) for a satellite which travel at the speed of 30Mm/s... (snip) It is state of the art for more then 2 decades for the ballistic missile people to calculate the impact point of a warhead (also for a space object) based on the initial conditions with an accuracy better then 1.5 km. The software that they and I use considers a lot of parameters like up- and crosstrack lift, changes of the drag coefficient with altitude, winds ect. I start my REENTRY program when the perigee is below 120 km. The quality of the decay and reentry prediction depends strongly from the atmosphere density model. MSIS-90 is the most reliable and also used by SpaceCom. >... and in counterpart estimate the epoch of the scratch only at minutes or hours levels ? I saw uncertainties of more than 12 hours on these datas > (corresponding to many times the earth's size), but with always the > localisation of the impact at less the 10km, according to these datas. (snip) What means "decay time" in programs used by SpaceCom and myself? It means the final point of the reentry trajectory. The output shows the seconds, but there is a lot of uncertainty what's going on below 70 km (disintegration ect.). Is is a matter on convention to give the decay time at the minute level and also the position of the "impact point" with one decimal. (Sometimes I show 2 decimals.) Large uncertainties - as Raoul mentioned above - are mostly related to predictions in advance 4 - 2 days before decay. If SpaceCom shows in its FINAL REPORT for example a decay time with +/- 02 hours decay window reflects it the fact that there is no sensor contact in this time span. But the time and position could be accurate. Harro Harro.Zimmer@t-online.de Berlin, Germany ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
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