I recently did some comparison calculations relating to the SGP4 model; for easier viewing, this is also online at http://j2ee-consultants.com/iss-transit/sgp4.htm When observing a transit of the space station that's nearly overhead, the "transit radius" is about 2 miles in diameter (the sun & moon have angular diameters of about 1/120 radian, and the ISS is about 250 miles up). If you want to be within a bullseye of 20% of the diameter of the sun or moon, you therefore can deviate laterally from the transit path by only about +/- .2 miles, or 1000 feet -> 300 meters (proportionately more, if the transit occurs at a low elevation angle). Below, I ran some comparisons. I found significant differences between the ancient CelesTrak reference, and my own values, though my values (computed by Java code) compare quite well with some tests done by Bill Gray (C++ code). More worrisome are the differences I found between my computations, and those given on the NASA web page, since they amount to much of the allowable error (only errors that are lateral to the line of sight matter, of course). Does anyone else have any calculations, for comparison? TSINCE X (km) Y Z 0. 2328.96594238 -5995.21600342 1719.97894287 http://celestrak.com/NORAD/documentation/spacetrk.pdf 2328.97070292 -5995.22083201 1719.9706582 WorldView; EARTH_RADIUS = 6378.135, EARTH_FLATTENING = 1 : 298.26 2328.97070411 -5995.22083267 1719.97065591 Bill Gray http://www.projectpluto.com/sat_code.zip (TEST2.OUT) 360.00000000 2456.00610352 -6071.94232177 1222.95977784 2456.1078549 -6071.93868494 1222.89555536 2456.10787315 -6071.93868109 1222.89554062 720.00000000 2567.39477539 -6112.49725342 713.97710419 2567.56292733 -6112.50381173 713.96185309 2567.56296196 -6112.50380107 713.96182567 1080.00000000 2663.03179932 -6115.37414551 195.73919105 2663.09012478 -6115.48276372 196.39911433 2663.09017487 -6115.48274400 196.39907429 1440.00000000 2742.85470581 -6079.13580322 -328.86091614 2742.55433733 -6079.66987202 -326.39144567 2742.55440172 -6079.66984111 -326.39149805 XDOT (km/s) YDOT ZDOT 2.91110113 -0.98164053 -7.09049922 CelesTrak 2.91207225 -0.98341531 -7.09081697 WorldView 2.91207225 -0.98341531 -7.09081697 Project Pluto 2.67852119 -0.44705850 -7.22800565 2.67938906 -0.4482884 -7.22879264 2.67938906 -0.44828838 -7.22879265 2.43952477 0.09884824 -7.31889641 2.44024486 0.09811113 -7.31995951 2.44024485 0.09811117 -7.31995951 2.19531813 0.65333930 -7.36169147 2.19612078 0.65241689 -7.36282421 2.19612076 0.65241695 -7.36282421 1.94707947 1.21346101 -7.35499924 1.94849699 1.21107414 -7.35619305 1.94849696 1.21107421 -7.35619305 Below, from http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/ orbit/ISS/SVPOST.html ISS TRAJECTORY DATA Lift off time (UTC) : N/A Area (sq ft) : 3325.0 Drag Coefficient (Cd) : 2.36 90 day mean solar flux (jansky) : 172.4 12 month mean earth geomagnetic index : 2.35 Maneuvers contained within the current ephemeris are as follows: IMPULSIVE TIG (GMT) M50 DVx(FPS) LVLH DVx(FPS) DVmag(FPS) IMPULSIVE TIG (MET) M50 DVy(FPS) LVLH DVy(FPS) Invar Sph HA DT M50 DVz(FPS) LVLH DVz(FPS) Invar Sph HP --------------------------------------------------------------------- --- There are no maneuvers. Coasting Arc #1 (begining on orbit 1211) --------------------------------------- Vector Time (GMT): 2003/111/04:16:12.228 Vector Time (MET): N/A Weight (LBS) : 396133.0 M50 Cartesian M50 Keplerian ----------------------------------- -------------------------- ------ X = 4825000.71 A = 6776030.81 meter Y = -4469367.23 meter E = .0020221 Z = 1585585.64 I = 51.40937 XDOT = 2329.384698 Wp = 56.30435 YDOT = 4560.772825 meter/sec RA = 306.09898 deg ZDOT = 5725.744991 TA = 321.14417 MA = 321.28937 Ha = 216.422 n.mi Hp = 206.252 * My results (with differences from NASA's values): * * Calendar is: 21 Apr 2003 / 4:16:12.228 UTC * julianDay = 2452750.67791931 * year = 2003, dayOfYear = 111.17791931 * timeElapsed since TLE epoch = -0.06098661 (fraction of day = -1.5 hours, or about -1 orbit) * * Raw values (epoch of date) J2000 M50 * * x = 4869714.11 meters x = 4866972.63 diff. = 71.96 x = 4825071.69 diff. = 70.98 * y = -4411611.02 y = -4415204.63 -11.99 y = -4469379.80 -12.57 * z = 1610483.26 z = 1608921.70 -211.30 z = 1585374.09 -211.55 M50 Cartesian J2K Cartesian ----------------------------------- -------------------------- ------ X = 15830054.83 X = 4866900.67 Y = -14663278.32 feet Y = -4415192.64 meter Z = 5202052.64 Z = 1609133.00 XDOT = 7642.338249 XDOT = 2250.405630 YDOT = 14963.165437 feet/sec YDOT = 4586.372359 meter/sec ZDOT = 18785.252594 ZDOT = 5736.872024 The mean element set is posted at the UTC for which position is just north of the next ascending node relative to the above vector time TWO LINE MEAN ELEMENT SET ISS 1 25544U 98067A 03111.23890592 .00041100 00000-0 34010-3 0 9003 2 25544 51.6314 306.3227 0006560 42.4273 317.7388 15.58334166 12129 Here, I computed the values for the start of the TLE epoch, day of year 111.23890592: x = 4035830.69 y = -5432723.81 z = 210.36 As indicated by the NASA note, the space station is slightly north (210 meters) of the earth's equatorial plane at that time. EARTH_RADIUS 6378.135 km - http://dgfi2.dgfi.badw-muenchen.de/geodis/REFS/ellipsoid.html 6378.137 - http://dgfi2.dgfi.badw-muenchen.de/geodis/REFS/grs80.html 6378.1363 - http://www.ngs.noaa.gov/GRD/GPS/DOC/files/ac_noaa.html (9 Feb 2003) EARTH_FLATTENING 1 : 298.26 - http://dgfi2.dgfi.badw-muenchen.de/geodis/REFS/ellipsoid.html 1 : 298.2572 - http://dgfi2.dgfi.badw-muenchen.de/geodis/REFS/grs80.html ----------------------------------------------------------------- To unsubscribe from SeeSat-L, send a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@satobs.org List archived at http://www.satobs.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Wed Apr 23 2003 - 15:58:51 EDT