RE: NROL-82: targeting sun-synchronous orbit

From: Ted Molczan via Seesat-l <>
Date: Tue, 13 Apr 2021 09:23:31 -0400
With the news that NROL-82 is targeting a sun-synchronous orbit (SSO), apparently in the KH-11 eastern plane, here is my guess regarding the initial orbital configuration of the KH-11 Block 5 constellation:

NROL-71 73.6 deg, 400 km (USA 290, 19004A / 43941)
NROL-82 SSO KH-11 eastern plane
NROL-91 SSO KH-11 western plane?

I have always thought of KH-11 as an SSO spacecraft, so I was hugely surprised by the launch of NROL-71 to a non-SSO. But it seems less surprising when I consider the stealthy KH-11 variants, known as Misty, launched in 1990 and 1999 into quasi-65 deg orbits. 

Misty 1 (90019B / 20516) initially operated in a 65 deg 800 km orbit, where it was discovered in late 1990 by Russell Eberst, Daniel Karcher and Pierre Neirinck. It soon manoeuvred to a 66.2 deg, 730 km orbit. Russell observed it three times in that orbit, as an UNID, during 1996-97. The ground track of the initial Misty 1 orbit repeated every nine days. Its 66.2 deg orbit repeated every 3 days.

Misty 2 (99028A / 25744) was launched into a 63.4 deg 200 x 300 km parking orbit. It has never been identified among the UNIDs reported by hobbyists.

The main benefit of an SSO is that it guarantees that targets are always sunlit. With a non-SSO, targets will periodically be in shadow for weeks at a time.

An SSO yields fairly constant solar illumination of targets, i.e. direction and length of shadows. Two SSO spacecraft in different planes provide some diversity in solar illumination. A non-SSO yields far greater solar illumination diversity, due to widely varying solar illumination over time; my guess is that this aids in photo interpretation.

Operating a mixed SSO and non-SSO constellation takes advantage of the best features of both orbits. The emerging orbital configuration of Block 5 KH-11 appears to be an updating of what began in 1990; the main difference being that the non-SSO spacecraft is not stealthy.

Ted Molczan

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Received on Tue Apr 13 2021 - 08:24:44 UTC

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