Allen Thomson wrote: > The apparent divergence between USG estimates and SatEvo persists. > Hopefully enough tracking data will be collected to throw > light on where the difference comes from. Visibility windows for various latitudes are available here: http://satobs.org/seesat/Feb-2008/0205.html The object has entered a brief period of visibility near 30 N, which will gradually move northward over the coming days. Beginners can useful contribute to the tracking of USA 193, especially when it has not been tracked for several days, as this case now. It was last observed with precision on Feb 11, and its orbital elements were updated, but due to its rapid and uneven rate of decay from orbit, those elements are rapidly losing accuracy for predictions. Tonight, it could easily be early or late, by at least 1 minute. In these circumstances, an observation timed to within one or two seconds, and accurate in position to within one degree or so, would make a valuable contribution to tracking the object, by enabling us to approximately revise the orbital period and rate of decay. Here is how to make a simple, but reasonably accurate observation. 1. Obtain a Prediction Heavens-Above provides predictions of nearly all satellites, including star charts showing the time and path. Due to the interest in USA 193, the site has a special link at the top of the main page. http://www.heavens-above.com/ I recommend allowing for at least 60 s prediction time uncertainty, and 2 min to be safe. 2. Observe Go outside at least 10 min before the satellite is due, to give your eyes time to adapt to the darkness, and to locate the predicted point of arrival of the satellite shown on the Heavens-Above chart. If you are not all that familiar with sky, or how to use a star chart, then you may need 30 min or longer. Wait for the satellite to appear, allowing at least 1 or 2 minutes, in case it is early or late. Beware of imposters! There are many satellites in orbit, so it is all too easy to follow the wrong one. The one you want will closely follow the predicted path. When you spot it, observe it as it crosses the sky, and when you see that it is about to pass close to a star that you can identify, get ready to make your observation. At the moment of closest approach, note whether the satellite was above, below, right or left of the star, and the approximate separation. For example, "one half degree below Procyon". (For reference, the moon's apparent diameter is 1/2 deg.) The closer the separation the better, but 1 or 2 degrees is acceptable for a rough observation. If you do not know the star by name, then circle it on the Heavens-Above chart, or other star chart. At the moment of closest approach you must also measure the time. If all you have is a wrist-watch, then turn to read it as quickly as you can after noting the position of the satellite. You may wish to subtract a second or two to allow for the delay in taking the reading. The goal is to achieve 1 or 2 sec accuracy. You will need to calibrate the watch to a precise time source. Perhaps the simplest method is to phone radio station WWV (303-499-7111), and note how many minutes and seconds your watch is early or late, and then correct your observed time accordingly. Or, you may wish to set your watch to synchronize as closely as possible to WWV. Keep in mind that watches tend to drift, so this should be repeated each day you observe. If you have a stop watch handy, then that is the preferred timing method. The usual approach is to hit the start button at the moment of the observation, then stop it at the start of a minute using WWV. Subtracting the elapsed time from the WWV time, yields your observed time. 3. Report Your observation report can be as simple as "passed one half degree below Procyon" on 2008 Feb 15 at 7:12:43 PM CST. If you are familiar with Universal Time, aka UTC or GMT, that is preferred, but local civil time is acceptable, as in the above example. If you know the time zone, that would help. If you can roughly estimate the accuracy of your position and time, please include it. Last, but not least, we need to know your location on Earth. Typically, we report latitude and longitude to 0.0001 deg accuracy, and height above sea level to within a few metres. If you have GPS, use it, else you can use Google Maps. For low-precision observations, 0.1 deg accuracy should suffice; height can be within 100 m. Happy Hunting! Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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