The launch took place on 2002 Jun 24 at 18:23 UTC - one minute later than planned. The elements appear to be converging as follows: 1 27453U 02032A 02176.10887502 -.00025567 00000-0 -11350-1 0 84 2 27453 98.7874 243.1851 0011378 331.9446 112.5061 14.23079723 48 Apart from being one minute late, the mean motion is somewhat greater than the one I derived using NOAA 16 as a proxy: 1 70000U 02176.10040685 .00000200 00000-0 11164-3 0 19 2 70000 98.7886 242.6421 0009705 275.1802 115.0094 14.10880075 40 The difference seems small, but by the time of Ralph McConahy's observations, the object was running about 4.8 min ahead and 5.5 deg from the sky-track predicted by my elements: http://satobs.org/seesat/Jun-2002/0177.html 27453 02 032A 8834 F 20020625054457200 17 35 1354237+181865 36 S+055 10 27453 02 032A 8834 F 20020625054508900 17 35 1342823+226114 36 S+055 10 27453 02 032A 8834 F 20020625054536000 17 35 1315075+329475 36 S+060 10 27453 02 032A 8834 F 20020625054544100 17 35 1305191+357900 36 S+060 10 Had the launch not been delayed by one minute, my prediction time error would have been one minute greater. Could I have done better? I decided to take a closer look at the launch circumstances and initial elements of all NOAAs that were launched into the present standard 850 km orbit. I have appended the results. Following the name of each satellite is its date and time of launch. I obtained the data from Jonathan McDowell's web site, at the URLs cited in my earlier post. As I compiled the data, a pattern quickly became apparent: - launches near 10:00 UTC had initial mean motion near 14.11 rev/d and inclination near 98.90 deg - launches near 15:52 UTC had initial mean motion near 14.22 rev/d and inclination near 98.75 deg This must be by design, and my guess is that it has to do with the perturbation caused by the sun's gravity, that strongly affects sun-synchronous orbits. This perturbation varies somewhat depending upon the orientation of the orbital plane with respect to the sun. In some cases, it causes orbital inclination may vary by more than one degree over a long period, causing the plane to wander away from its original location. Sun-synchronous satellites that must maintain a very precise orbital plane compensate for solar gravity and other perturbations by making frequent small manoeuvres. This is true of most Earth resources satellites, such as ERS, Landsat, Spot and RadarSat. The NOAA satellites are not manoeuvrable, probably because they are not required to maintain quite as precise planes as the Earth resources satellites. My guess is that their initial plane, mean motion and inclination are selected to achieve a mean plane-location during the planned mission that is close to some desired mean value. In any case, knowing that the planned launch time of NOAA 17 was closer to the 15:52 UTC time than to the 10:00 UTC launch times, it would have been prudent to use as my proxy one of the 15:52 UTC launches, such as: NOAA 15 1998 May 13 15:52:04 UTC 1 25338U 98030A 98134.12823554 +.00001287 +00000-0 +59324-3 0 00115 2 25338 098.7158 164.0280 0010413 339.2361 020.8393 14.22773088000061 Using the ten step process described in my earlier post, and taking into account the one minute late launch time, yields: 1 70000U 02176.23305037 .00000200 00000-0 89247-4 0 117 2 70000 98.7158 243.2926 0010413 339.2361 20.8393 14.22773088 64 At the time of Ralph McConahy's observations, this elset would have been just 3.3 s late and within 0.1 deg of the satellite's sky track. This lesson learned is that by making a more thorough study of past NOAA launches, I could have selected a proxy orbit that would have produced a much more accurate pre-launch elset - in this case, about 100 times more accurate. Ted Molczan Launch date and time, and initial elements of 850 km NOAA satellites prior to NOAA 17 NOAA 6 1979 Jun 27 15:51:59 UTC 1 11416U 79178.91752047 .00000926 00000-0 0 31 2 11416 98.7392 207.8071 0011227 280.8958 79.1177 14.22139740 40 NOAA 7 1981 May 29 10:53 UTC 1 12553U 81174.49904749 -.00000008 00000 0 0 18 2 12553 98.9001 129.0551 0012452 299.2665 60.7261 14.12002827 00 NOAA 8 1983 Mar 28 15:52 UTC 1 13923U 83087.91741999 -.00000009 00000-0 0 34 2 13923 98.7529 118.1785 0015621 262.2376 97.7036 14.22297001 37 NOAA 9 1984 Dec 12 10:42 UTC 1 15427U 84347.70411840 .00000082 49999-4 0 38 2 15427 98.9245 296.3557 0014661 268.2803 91.6724 14.11364999 31 NOAA 10 1986 Sep 17 15:52 UTC 1 16969U 86261.40966530 -.00000009 00000 0 0 25 2 16969 98.7498 289.4101 0012749 273.7777 85.9107 14.22475423 101 NOAA 11 1988 Sep 24 10:02:00 UTC 1 19531U 88270.30806841 .00001049 59671-3 72 2 19531 98.9044 210.0942 0011178 256.6974 103.3183 14.10632414 263 NOAA 12 1991 May 14 15:52 UTC 1 21263U 91135.19924078 .00026887 12316-1 39 2 21263 98.7337 164.8826 0011869 262.5003 97.3407 14.21373634 72 NOAA 13 1993 Aug 09 10:02 UTC 1 22739U 93 50 A 93222.31462920 -.00000019 00000-0 00000+0 0 109 2 22739 98.9084 163.5700 0008992 280.7119 79.3074 14.10849591 126 NOAA 14 1994 Dec 30 10:02 UTC 1 23455U 94089A 94364.88900946 -.00014416 00000-0 -79395-2 0 38 2 23455 98.8861 303.8954 0008755 314.2394 45.8013 14.11496091 67 NOAA 15 1998 May 13 15:52:04 UTC 1 25338U 98030A 98134.12823554 +.00001287 +00000-0 +59324-3 0 00115 2 25338 098.7158 164.0280 0010413 339.2361 020.8393 14.22773088000061 NOAA 16 2000 Sep 21 10:22 UTC 1 26536U 00055A 00265.76707352 -.00020078 00000-0 -11203-1 0 13 2 26536 98.7886 210.5136 0009705 275.1802 115.0094 14.10880075 42 ----------------------------------------------------------------- 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/sat/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Tue Jun 25 2002 - 02:43:32 PDT