Patrick wrote: > I've been trying to make accurate timings of satellites for a > while, and I > can't figure out how you guys do it. The closest I can get > is about two or > three seconds, because I have to look down at my stopwatch > and then back up > at the satellite, which takes some time. So my question is, > what's your > strategy? The strategy is to use a stopwatch referenced to an accurate time source, such as WWV. WWV broadcasts on several shortwave frequencies, including: 5 MHz, 10 MHz, 15 MHz, 20 MHz; or you can hear it by phoning 303-499-7111. My $30 Casio wristwatch has a stopwatch function with a memory that can record 30 split times. Shortly before I go out to observe, I start the stopwatch. Then I phone the WWV number, and record several splits at one minute intervals. I write down the actual UTC time of each split, so that I have a way to relate the elapsed time of the stopwatch to UTC. If I am reasonably alert, I should be able to consistently hit the stopwatch to within a range of a few hundredths of one second. I do not take the splits at the start of a minute, because WWV has a several second pause with no clock ticks prior to the long tone at the start of the minute. Instead, I count off 5 seconds after the minute, and then take a split. This should reduce the reaction-time error, but I can anticipate each tick of the clock. By this process, I have in effect calibrated the start time of the stopwatch. When I return from observing, I phone WWV again, and record a few more splits, to enable me to check how much the stopwatch has drifted. A cheap watch like mine will drift quite a bit over several hours, requiring that I correct my observed times. For a short observing session, during which negligible drift is expected, I can skip the pre-observing calibration against WWV. However, even if I don't plan a long session, I may do this calibration as a safety measure, in case I should accidentally stop the watch after I have recorded a few positions (it is all too easy to do that in the dark). Having done the pre-observation calibration means that even if I stop the watch, I can still compute the UTC time of each split that I have recorded. Without the calibration, the data would be lost. In any event, it is necessary to perform at least one calibration, either before or after observing. Most positional observers time the passage of the satellite between pairs of closely spaced stars. The closer, the better the positional accuracy, and the time accuracy. It is easier to accurately judge the moment that a satellite crosses the imaginary line between a pair of closely spaced stars, than more widely spaced ones. I try not to use stars separated by more than 1 deg, and preferably less than 0.5 deg. As a rough rule of thumb, an experienced and careful observer should be able to achieve an accuracy of about 5 percent of the separation between the stars, and 0.1 s in time. Ted Molczan ----------------------------------------------------------------- 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
This archive was generated by hypermail 2b29 : Fri Jun 02 2000 - 16:09:14 PDT