I was planning on bagging my first ISS transit video this afternoon, but I got caught up in generating new transit reports and answering phone calls and emails, and before I knew it, I didn't have time to pack up the car for the relatively short trip, and get set up for it. So, I grabbed my binoculars and "the box" http://iss-transit.sourceforge.net/binocularCamera.html and a Radio Shack radio-controlled "atomic clock" and took off just in time to see the ISS zip straight across the middle of the Sun- several sunspots were clear & sharp. The way I was oriented, facing south and looking thru the "camera hole," the ISS came diving down from 12 o'clock high, split the solar disk neatly in half, and then disappeared out the bottom of the Sun. Given the height of the box, the solar image was about 2" in diameter. Not very long before the event, CalSKY was indicating a significant miss, using my location. Anyhow, subsequently I crunched the following TLEs thru SkyMap: ISSmcc 1 25544U 98067A 04169.13725307 .00020000 00000-0 20000-3 0 9007 2 25544 51.6318 326.0063 0005492 248.0712 111.9865 15.69077207 38310 ISSa 1 25544U 98067A 04169.10162037 .00014335 00000-0 12605-3 0 51 2 25544 51.6330 326.1792 0005233 243.4595 275.1775 15.69060507318303 ISSb 1 25544U 98067A 04169.27747323 .00014504 00000-0 12744-3 0 70 2 25544 51.6330 325.2825 0005320 243.3641 189.2726 15.69066692318333 ISSc 1 25544U 98067A 04169.47484569 .00016159 00000-0 14132-3 0 84 2 25544 51.6328 324.2771 0005306 244.4630 223.8287 15.69079576318368 ISSd 1 25544U 98067A 04169.68053241 .00016608 00000-0 14504-3 0 99 2 25544 51.6332 323.2284 0005332 246.0243 304.9107 15.69087566318391 using a center time of 19:08:57 UTC (Day-of-Year 169.797878); 34.8254° N, 81.8686° W (a point along my MCC computed center line); and an elevation of 202.5 meters. I should have added 31 meters to the latter, because I believe SkyMap treats elevation as distance above the WGS84 ellipsoid, rather than above the Geoid; locally, the Geoid is 31 meters below the ellipsoid: http://earth-info.nga.mil/GandG/wgsegm/intptW.html Consequently, SkyMap shows the MCC track very slightly above the center of the solar disk. The OIG TLEs were obtained from CalSKY, and represent all of the available (OIG) TLEs that preceded the transit, for DOY 169. http://j2ee-consultants.com/temp/17jun04-SunTransit.gif shows the results from SkyMap. The older OIG TLEs, ISSa and ISSb, are about 6' of arc off-center, while the most up-to-date TLE, ISSd, is about 4' off-center (allowing a bit for my 31 meter elevation error), corresponding to a ground track error of about 0.3 miles / 0.5 km. I guess it doesn't mean a whole lot- every TV ad says "Our product is the best"- but in any case there's the claim: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/Vector.html "You also have access to a Mission Control Center, or MCC, ephemeris during space shuttle missions and for the International Space Station. ... The state vectors that are generated in the MCC are far and above the most accurate available, and their use is highly recommended." At some point, even the MCC TLEs should become suspect, and fresh OIG TLEs should be preferred; however, I haven't come up with any "rule of thumb" for deciding when that is (and probably you'd need to take a look at recent solar activity to make a decision). Another of my "I hate OIG" anecdotes goes back 1 week before the Venus transit, on June 1. May 31 was Memorial Day holiday, and it had been 8 days since the MCC had updated their ISS ephemeris page. On May 24, I'd predicted a high-elevation ISS / Jupiter encounter thru Charlotte, NC, and I was going to have a go at it (~160 miles round-trip drive). The evening before (about 20 hours prior to the event), I got the most recent OIG TLE, and ran it thru WorldView- it showed that the ground track had moved only 100 meters from the MCC predicted ground track, a little more than a week before! The following day, I looked around with aerial-photo assisted DeLorme Topo USA and identified a couple good-looking observation spots. Then the MCC ... finally ... updated the ephemeris- the new MCC ground track was 1.5 miles from the OIG ground track computed about 10 orbits before! The weather was a bit iffy, and I didn't have time to try to identify a new observation site, so in frustration, I gave up (I did eyeball a beautiful ISS pass- nearly directly overhead- that evening from my front yard, however). A subscriber in Charlotte, who coincidentally was baby-sitting his grand-kid about halfway between the two tracks, later confirmed that the new MCC ground track was basically right- the ISS went slightly over Jupiter, whereas if the OIG track from the night before had been correct, it'd have passed slightly under Jupiter. It was a real shame that I let myself get so distracted as to miss being able to photograph today's transit. Though there were clouds here & there, the spot on the map I'd selected turned out to be just off a side road, in a big, recently mowed field near a church, and the Sun was nowhere near any cloud the whole time- it would have been a perfect place to set up my scope & other paraphernalia! http://iss-transit.sourceforge.net/firewire-webcam.html ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Thu Jun 17 2004 - 23:26:26 EDT
