NROL-76 elements from observations

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Thu, 1 Jun 2017 10:23:39 -0400
The following elements have been derived from observations by Leo Barhorst, Russell Eberst, David Hopkins, Marco
Langbroek, Alberto Rango and Mike Waterman:

USA 276                                                  389 X 410 km
1 42689U 17022A   17151.89933359  .00008690  00000-0  12038-3 0    08
2 42689  50.0027 124.1765 0015587 118.2881 241.9663 15.56230102    02
Arc 20170527.04-0531.92 WRMS resid 0.054 totl 0.018 xtrk

The object was several seconds early last night, due to increased drag, or perhaps a small manoeuvre. The above is what
seems to be the most likely of a couple solutions I found.

Marco Langbroek has discovered what may well be the operational connection between USA 276 and ISS, which he blogged a
short time ago:

https://sattrackcam.blogspot.nl/2017/06/the-plot-thickens-ball-aerospace-usa.html

RAVEN arrived at ISS in Feb 2017, not long before the launch of USA 276. It seems reasonable to speculate that NRO and
NASA may have jointly funded RAVEN to share in technology development. NASA's interest is in autonomous orbital
rendezvous: 

https://www.nasa.gov/feature/goddard/2017/nasa-to-launch-raven-to-develop-autonomous-rendezvous-capability

Perhaps the NRO's interest is to rendezvous for close satellite inspection, similar to the supposed mission of Prowler,
launched in 1990. This is mostly speculation, but perhaps worthy of further research.

Below are updated conjunction estimates for the closest approaches on 2017 Jun 03 with range <= ~100 km.

                   ISS sub-satellite point     USA 276 sub-sat point
                  -------------------------  ------------------------- 
  TCA      Range    Lat      Lon      Alt      Lat      Lon      Alt
  UTC       km      deg      deg      km       deg      deg      km
10:56:37   96.68  43.82 S   1.47 E   418.32  44.19 S   0.47 E   423.22
11:43:09   65.70  43.36 N 170.70 E   410.57  43.65 N 170.09 E   392.21
12:29:13   30.88  43.83 S  22.08 W   418.31  43.94 S  22.39 W   423.12
13:15:31   18.67  43.83 N 146.14 E   410.73  43.82 N 146.17 E   392.27
14:01:51   35.87  43.77 S  45.48 W   418.28  43.63 S  45.11 W   423.01
14:47:53   71.19  44.30 N 121.57 E   410.89  43.98 N 122.24 E   392.34
15:34:27  101.72  43.78 S  69.03 W   418.27  43.37 S  67.99 W   422.91

The time and range of closest approach is fairly sensitive to the orbital eccentricity and decay rate of USA 276, which
may not yet have settled down to their actual values, due to the still short observational arc. That would be especially
true if there has been a recent small manoeuvre.

For USA 276, I used the above TLE. For ISS, I used NASA's current estimated TLE for June 3:

https://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html

1 25544U 98067A   17154.59563174  .00016717  00000-0  10270-3 0  9050
2 25544  51.6397 106.0758 0004758 233.6745 126.3969 15.53986584 19633

If the intention is to perform an actual rendezvous, instead of, or in addition to a series of fly-bys, then USA 276
could manoeuvre to match the plane of ISS near one of the above times. If delta-V is a significant constraint, then the
minimum energy manoeuvre would occur on Jun 24, when USA 276 and ISS would have the same RAAN, leaving only a 1.6 deg
change of inclination.

Ted Molczan


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Received on Thu Jun 01 2017 - 09:24:17 UTC

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