Is USA 276 targetting the orbit of ISS?

I am inclined to believe that the close conjunctions between USA 276 and ISS are intentional, but this remains unproven
and far from certain. The non-planar launch window is a major contrary clue, since it would have prevented a conjunction
within any reasonable time after launch on the original launch date, 2017 Apr 16. The possible relationship with the
Raven experiment on ISS continues to look interesting, but this remains far from proven. If offer the following
analyses.

1. Non-planar launch window

Did USA 276 target the orbital plane of ISS or did their small initial planar separation occur by chance? The main test
is to determine whether there was a planar launch window, which for a launch to ISS would have opened about 24 min.
earlier each day.

The launch was originally scheduled for 2017 Apr 16, but was postponed to Apr 30, then finally conducted on May 01. The
15 day delay would have shifted a planar window earlier by about 6 hours, which was 3 times the width of the announced 2
hour launch period. The launch period did not change at all, and the planned launch time on both Apr 30 and May was
11:15 UTC. Clearly, the window was not planar.

What is the significance of the non-planar window?

The launch on May 01 at 11:15 UTC placed the orbit plane of USA 276 about 11 deg east of the ISS plane. Their difference
in rate of precession, about 0.195 deg/d, will have reduced the planar separation to about 4 deg east at the time of the
conjunctions, projected to occur on Jun 03 (expected to be in progress as I write), just over 30 days after launch. That
seems a reasonable time to wait for an intended conjunction.

Had the launch occurred on Apr 16 at 11:15 UTC, then the initial RAAN of USA 276 would have been about 281 deg east of
the ISS plane. If the difference in rate of precession were the same, it would have taken 1420 days for the USA 276
plane to precess to 4 deg east of ISS. That would be intolerably long if the mission required close conjunctions with
ISS.

One solution might have been to increase the difference in rate of precession, by targeting an initial orbit of lower
inclination and altitude. As a guess, the lowest practical inclination and altitude of USA 276 would have been about 45
deg and 300 km. Such an orbit would have precessed at about 6 deg/d, or about 0.8 deg/d faster than ISS. With that
difference, it would have taken about 346 days to reduce the RAAN separation from 288 deg east to 4 deg east. That still
seems intolerably long; therefore, I conclude that Apr 16 would not have supported a launch that targeted the orbit of
ISS.

That the launch window was not planar, and the original launch date of Apr 16 would not have resulted in a close
conjunction within a reasonable time after launch, strongly suggests that the conjunctions set up by the May 1 launch
occurred by chance. Unless, it was never intended to launch on Apr 16, but to instead announce it as a ruse in an
attempt to make the intended conjunctions seem like chance occurrences. My impression is that range time is too scarce a
commodity to permit such tactics, but I am by no means well-versed in range operations.

Another way of looking at the problem, is to ask why, if ISS is not a target of USA 276, would the highly secretive NRO
have permitted a launch so close to its plane, let alone one that yielded such close conjunctions not long after launch,
which could only have increased public interest in the mission? It seems undesirable, unless it was decided that if
chance conjunctions could result in incorrect speculation about the mission, then that would be a good side-effect.

2. Possible relationship between USA 276 and Raven

If the primary mission of USA 276 is intended to test the Raven system on ISS, then it would be reasonable to expect
Raven to have been well along in its development when NRO awarded the NROL-76 launch contract. That occurred in the
summer of 2012, with launched planned for Dec 2015 as of spring 2013, as I documented here:

http://satobs.org/seesat/Apr-2017/0125.html

My web searches for RAVEN have yet to turn up articles older than the following from March 2014:

https://www.nasa.gov/mission_pages/station/research/news/satellite_repair_in_orbit

Start excerpt:

""After five years of ground-based testing, we're excited to test this system in space," says Reed. "It'll help us
examine how Raven's
sensors, avionics and algorithms work together as an integrated system."

Delivery to the space station is slated for early 2016 as part of the Department of Defense Space Test Program-Houston 5
(STP-H5) payload.

End excerpt.

That suggests that Raven, or its major components, were under development well before NROL-76 was contracted. If the
launch of NROL-76 depended on the launch of Raven, then it also implies at least a few months slippage in launch date in
the intervening year, which seems quite plausible. Based on this information, a relationship between USA 276 and Raven
seems possible, but far from proven.

Due to differences in altitude, the closest possible projected conjunction between the existing orbits of ISS and USA
276 is about 6 km. Typical projected values have been 2 to 3 times that. Are Raven's sensors functional at ranges of 6
km or greater? A technical paper on Raven, via NASA Technical Reports Server, is available here:

http://hdl.handle.net/2060/20150002731

Typical sensor to target ranges discussed are in the tens of metres. The greatest range discussed appears to be 1 km.
That does not mean that greater ranges are infeasible or would not yield useful data, but it raises doubts of a
connection with Raven, and that the conjunctions are anything other than chance occurrences. 

If USA 276 manoeuvres to rendezvous with the station, then much closer approaches would be feasible, which would
strengthen the case for a connection with Raven.

Another line of speculation is to ask what meaningful contribution USA 276 could make to the testing of Raven. From the
above paper, Raven will obtain its data over a period of two years, by observing spacecraft as they approach ISS for
docking. Many spacecraft will visit during that period. I imagine that USA 276 could add to the data set as follows. If
it can rendezvous, then it could keep station for long periods, during which it could change its attitude to present the
sensors with a variety of views, under a variety of lighting conditions. The total data collected could potentially far
exceed that from the other visiting spacecraft.

Ted Molczan


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