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From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>

Date: Fri, 7 Dec 2018 00:32:09 -0500

Date: Fri, 7 Dec 2018 00:32:09 -0500

NROL-71 is scheduled for launch from VAFB on 2018 Dec 08 at 04:19 UTC, aboard a Delta IV-Heavy. Justin Ray will live-blog the launch for ULA: https://www.ulalaunch.com/missions/delta-iv-nrol-71 Stephen Clark will live-blog the launch for Spaceflight Now: https://spaceflightnow.com/2018/12/07/delta-382-mission-status-center/ Until a few days ago, I was certain that the payload is the first Block 5 large electro-optical satellite of KH-11 heritage. Doubts arose upon release of the launch time, which was inconsistent with either of the two established KH-11 orbital planes. Doubts grew when the NOTAM revealed that NROL-71 is targeting, and its second stage will de-orbit from, an approximately 74 deg orbit. KH-11 has always employed sun-synchronous orbits (SSO), inclined at approximately 97 deg or 98 deg, depending upon their altitude. The usually stated advantage is more or less constant solar illumination, except for seasonal variations. Virtually all optical Earth imagers are in SSO, so the last thing I expected was for NROL-71 to target a non-SSO, which results in extended periods of poor solar illumination for a given latitude, as the orbit precesses relative the sun. Why might a non-SSO be preferred? A search of the web for applications of non-sun-synchronous orbits for imaging turned up an unfamiliar concept, that just might explain a 74 deg KH-11 orbit: Multi Sun-Synchronous Orbit (MSSO). The plane of an SSO rotates once per Earth orbit around the sun. An MSSO rotates multiple times during the same period. The following paper states that MSSO orbits, "could be adopted by those Earth science missions whose aim is to study the local hour effects on the observed object." http://naca.central.cranfield.ac.uk/dcsss/2004/B11_MSSOb.pdf Could imaging a scene under a wide variety of illumination conditions aid photo interpretation? In recent years, KH-11s on their extended missions have operated in planes far removed from the primary western and eastern KH-11 planes. For example, when USA 186 (05042A / 28888) operated in the standard western plane, its MLTDN (mean local time of descending node) was near 9:42 AM. After it was replaced by USA 245 in 2013, it lowered its altitude and inclination, and moved to a plane with MLTDN near 8 AM, where the sun is much lower, and shadows are much longer. That it has spent four years there, suggests that the NGA (National Geospatial Agency) has found this orbit useful. Operating in an MSSO would be different, in that the sun-angles would constantly change. I have no idea whether the NGA would find that useful, or even tolerable, but it seems worth considering in planning to search for NROL-71. Equation 18 of the above paper relates the orbital inclination, planar rotations per Earth year, and the size and shape of the orbit: cos i = -4.774e-15 * Kn/Kd * a^(7/2) * (1-e^2)^2 where: i = inclination a = semi-major axis e = eccentricity The authors state: "In Kd Earth orbit periods, the orbit nodal line precesses (clockwise or counter-clockwise) |Kn| times." Setting Kn/Kd = 1 yields the inclination of an SSO. MSSOs have non-unity Kn/Kd. Figure 5 of the above paper plots the relationship between inclination and semi-major axis for various Kn/Kd. On a hunch, I wondered what the mean motion would be of a circular 74 deg orbit, that rotated twice per Earth year? Solving Equation 18 for semi-major axis, with i = 74 deg, e = 0, and Kn/Kd = -2, yields 7012 km, which has mean motion of 14.7859 rev/d. Look familiar? This has been the approximate KH-11 mean motion since 1986. Refining the calculation by including the corresponding typical KH-11 eccentricity of about 0.053, yields semi-major axis 7023 km and 14.75 rev/d. Meaningless coincidences abound in orbital mechanics, but this is interesting. I am by no means certain that NROL-71's payload will employ an MSSO, but it seems plausible, so I have added a fourth group of search TLEs to extend the lowest mean motion to 14.7 rev/d, as discussed below. This does not necessarily invalidate the higher mean motion orbits. The inclination is only approximately known. If it proves a degree or two greater than 74 deg, then the condition for an MSSO with Kn/Kd = -2, would be met within the range of the higher mean motion TLEs. I am interested in arguments pro and con for MSSO, as well as other possible explanations for NROL-71's 74 deg orbit. The following elements are consistent with the NOTAM information, on the assumption that the payload will initially be in about the same orbit as the second stage. They are based on launch on 2018 Dec 08 at 04:19 UTC: 350 X 351 km 1 71901U 18342.18402778 .00000000 00000-0 00000-0 0 06 2 71901 74.0000 213.3000 0001000 0.0000 142.0000 15.72000000 03 259 X 442 km 1 71902U 18342.18402777 .00000000 00000-0 00000-0 0 06 2 71902 74.0000 213.3000 0136000 235.0000 269.0000 15.72000000 03 259 X 442 km 1 71903U 18342.18402776 .00000000 00000-0 00000-0 0 06 2 71903 74.0000 213.3500 0136000 55.0000 85.0000 15.72000000 05 503 X 504 km 1 71904U 18342.18402775 .00000000 00000-0 00000-0 0 06 2 71904 74.0000 214.0500 0001000 0.0000 142.0000 15.20000000 02 259 X 748 km 1 71905U 18342.18402774 .00000000 00000-0 00000-0 0 06 2 71905 74.0000 214.0500 0355000 235.0000 272.0000 15.20000000 09 259 X 778 km 1 71906U 18342.18402773 .00000000 00000-0 00000-0 0 06 2 71906 74.0000 214.0500 0376000 55.0000 85.0000 15.15000000 09 579 X 581 km 1 71907U 18342.18402772 .00000000 00000-0 00000-0 0 06 2 71907 74.0000 214.5000 0001000 0.0000 142.0000 14.95000000 06 259 X 870 km 1 70908U 18342.18402771 .00000000 00000-0 00000-0 0 05 2 70908 74.0000 214.5000 0440000 235.0000 271.0000 15.00000000 03 259 X 901 km 1 71909U 18342.18402770 .00000000 00000-0 00000-0 0 06 2 71909 74.0000 214.5000 0461000 55.0000 82.0000 14.95000000 01 658 X 659 km 1 71910U 18342.18402769 .00000000 00000-0 00000-0 0 06 2 71910 74.0000 214.9000 0001000 0.0000 142.0000 14.70000000 07 259 X 1058 km 1 71911U 18342.18402768 .00000000 00000-0 00000-0 0 06 2 71911 74.0000 214.9500 0568000 235.0000 273.0000 14.70000000 06 259 X 1058 km 1 71912U 18342.18402767 .00000000 00000-0 00000-0 0 06 2 71912 74.0000 214.9500 0568000 55.0000 80.0000 14.70000000 03 Each TLE passes close to the launch site about 6.5 minutes after launch, and passes close to the centreline of the second stage de-orbit zone. The orbits that meet these criteria are strongly dependent upon RAAN; the further east, the smaller the required mean motion. There are four groups of TLE. Within each group, RAAN is constant or nearly so, but orbits vary in eccentricity and argument of perigee. The first TLE in a group is circular. If the payload is a KH-11, then given the disclosure that the new generation will retain the existing mirror diameter of 2.4 m, its perigee height should be near the existing value of 259 km. Accordingly, the second and third TLE in each group is elliptical, with that perigee height. The second TLE has argument of perigee near 235 deg, which results in near apogee passes around latitude 50 N; the third one has perigee passes around 50 N. The plane of the first group is a little west of the launch site, about 6.5 minutes after launch; the second group is roughly overhead; and the third and fourth groups pass progressively to the east. Based on history, the final two groups seem the more likely. I doubt passes will be much earlier than predicted by the first group, or much later than predicted by the final group. I hope the above will prove to reasonably bracket reality. This launch has already presented a huge surprise; there could well be more. Happy hunting! Ted Molczan _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-lReceived on Thu Dec 06 2018 - 23:33:13 UTC

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