NROL-71 search elements for 2018 Dec 21 UTC

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Wed, 19 Dec 2018 20:52:26 -0500
NROL-71 is scheduled for launch from VAFB aboard a Delta IV-Heavy on 2018 Dec 21 at 01:31 UTC.

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/

I believe that the payload is the first Block 5 KH-11. My payload speculation post is here:

http://satobs.org/seesat/Dec-2018/0058.html

1. Planar window

The launch times published to-date reveal that the window is planar, i.e. targeting a specific plane or range of planes:

2018 Dec 08 04:19 UTC
2018 Dec 09 04:06
2018 Dec 19 01:57
2018 Dec 20 01:44
2018 Dec 21 01:31

Over 13 days, the window moved 168 min. earlier, or about 12.9 min/d.

The rate of precession of the RAAN is:

= 360 * (1.00273790934 - 1440/(1440 - 12.9))

= -2.27 deg/d

The rate of precession is a function of inclination, mean motion and eccentricity, so this is a strong clue to the
planned final orbit of the satellite.

Analysis of the NOTAM data reveals the target inclination to be about 74 deg. My guess is that the uncertainty is less
than 0.5 deg.

The rate of precession is much more sensitive to inclination than to mean motion and eccentricity; therefore, even a
small uncertainty in inclination results in a large uncertainty in mean motion.

Below are the mean motion and eccentricity that yield -2.27 deg/d RAAN precession, that bracket the range of
inclination:

Inc     n0       e0     Per  Apo
73.5  15.5095  0.02246  259  564
74.0  15.7143  0.01386  259  446
74.5  15.9268  0.00498  259  325

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, which I used to constrain the above estimates.

The initial orbit could be a little lower or higher than tabulated above. The last two KH-11s made small apogee boosts a
few days after launch.

The orbit will be visible in the northern and southern hemisphere, as discussed in the following section. Based on
limited checks, it appears that northern hemisphere observers may initially have morning and evening visibility, and
that southern hemisphere observers will have only evening visibility. Prospective observers should check for visibility
from their location.

2. Search elements

The search elements are based on launch on 2018 Dec 21 at 01:31 UTC.

I intend to issue revisions in the event of material new information or launch delays.

Pairs of TLEs have been produced, for inclination 74.5 deg, 74 deg, and 73.5 deg. In each pair, one TLE has argument of
perigee that yields apogee passes near mid-latitudes; the other has argument of perigee that yields perigee passes. This
is intended to help bracket the minimum and maximum predicted elevation above the horizon.

The RAAN of each TLE could be off by at least 1 deg. Along-track prediction time uncertainty is at least a couple of
minutes.

The TLEs have been organized and labelled for northern hemisphere observers, but southern hemisphere observers can use
them by swapping evening and morning and apogee and perigee, as discussed in the following sections.

2.1 Morning passes in N. hemisphere

This section contains TLEs for morning passes in the northern hemisphere. Each TLE is labelled to indicate whether it
yields apogee or perigee passes at mid-northern latitudes. 

The same TLEs can be used by southern hemisphere observers, but for evening passes. Also, the apogee and perigee labels
must be swapped. For example, the 71901 TLE yields northern hemisphere morning passes near apogee, and southern
hemisphere evening passes near perigee.

apogee                                                   259 X 325 km
1 71901U          18355.06770835  .00000000  00000-0  00000-0 0    09
2 71901  74.5000 183.3235 0049782 235.0000 269.5000 15.92677149    04
perigee                                                  259 X 325 km
1 71902U          18355.06770834  .00000000  00000-0  00000-0 0    09
2 71902  74.5000 183.3085 0049782  55.0000  88.6000 15.92677149    08
apogee                                                   259 X 446 km
1 71903U          18355.06770833  .00000000  00000-0  00000-0 0    09
2 71903  74.0000 184.0935 0138590 235.0000 270.5000 15.71434437    02
perigee                                                  259 X 446 km
1 71904U          18355.06770832  .00000000  00000-0  00000-0 0    09
2 71904  74.0000 184.0885 0138590  55.0000  87.3000 15.71434437    01
apogee                                                   259 X 564 km
1 71905U          18355.06770831  .00000000  00000-0  00000-0 0    09
2 71905  73.5000 184.8535 0224609 235.0000 271.5000 15.50949585    02
perigee                                                  259 X 564 km
1 71906U          18355.06770830  .00000000  00000-0  00000-0 0    09
2 71906  73.5000 184.8835 0224609  55.0000  86.0000 15.50949585    05

2.2 Evening passes in N. hemisphere

This section contains TLEs for evening passes in the northern hemisphere. Each TLE is labelled to indicate whether it
yields apogee or perigee passes at mid-northern latitudes. These elements differ from their counterparts in the previous
section, mainly in argument of perigee. A different leg of the orbit will be visible, which requires different values of
argument of perigee to yield passes near perigee and apogee.

The same TLEs can be used by southern hemisphere observers, but for morning passes. Also, the apogee and perigee labels
must be swapped. For example, the 71907 TLE yields northern hemisphere evening passes near apogee, and southern
hemisphere morning passes near perigee.

apogee                                                   259 X 325 km
1 71907U          18355.06770829  .00000000  00000-0  00000-0 0    08
2 71907  74.5000 183.2835 0049782 323.0000 181.3000 15.92677149    04
perigee                                                  259 X 325 km
1 71908U          18355.06770828  .00000000  00000-0  00000-0 0    08
2 71908  74.5000 183.3085 0049782 110.0000  33.8000 15.92677149    08
apogee                                                   259 X 446 km
1 71909U          18355.06770827  .00000000  00000-0  00000-0 0    08
2 71909  74.0000 184.0535 0138590 305.0000 199.5000 15.71434437    02
perigee                                                  259 X 446 km
1 71910U          18355.06770826  .00000000  00000-0  00000-0 0    09
2 71910  74.0000 184.0885 0138590 115.0000  28.2000 15.71434437    09
apogee                                                   259 X 564 km
1 71911U          18355.06770825  .00000000  00000-0  00000-0 0    09
2 71911  73.5000 184.8535 0224609 305.0000 199.6000 15.50949585    07
perigee                                                  259 X 564 km
1 71912U          18355.06770824  .00000000  00000-0  00000-0 0    09
2 71912  73.5000 184.8735 0224609 125.0000  18.0000 15.50949585    04

This launch has already presented a huge surprise in the orbital inclination; there could well be more.

Happy hunting!
Ted Molczan


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Received on Wed Dec 19 2018 - 19:53:08 UTC

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