Paper: Perturbations In Perigee And Eccentricity At The Critical Inclination

From: Ted Molczan via Seesat-l <>
Date: Thu, 30 Jul 2015 16:26:35 -0400
I long wondered whether the increasing eccentricity of the NOSS orbit was intended or a tolerated side-effect. A few
years ago, I came across the abstract of a 1978 report that reveals it was the latter, and totally unexpected. The full
text is now available:

"A cluster of three satellites showed large, unpredicted orbital perturbations shortly after launch. There was immediate
concern for the success of its mission which depends on the stability of the cluster geometry. The Naval Surface Weapons
Center found the zonal harmonics of degree five and seven to cause most of the observed perturbations. Subsequently, the
three orbits were computed for the time frame June 1976 to July 1984. It was also shown that the cluster configuration
remains unaffected by the large orbital perturbations, and the mission appears safe."

So, the increasing eccentricity is a bug, not a feature.

The 3rd gen NOSS employ a partial mitigation. They start out in a 1000 x 1200 km orbit, with argument of perigee near
180 deg. The latter causes the eccentricity to initially decrease to near zero, while the argument of perigee shifts to
90 deg, which takes about four years. Over the subsequent four years, the eccentricity increases, as the argument of
perigee moves toward zero deg, and the orbit is once again 1000 x 1200 km. This strategy maintains a reasonably circular
orbit for ~eight years, which apparently is the nominal mission life. The orbit continues to become more eccentric, so
any operation beyond 8 years will be with a gradually increasing eccentricity.

Ted Molczan

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Received on Thu Jul 30 2015 - 15:27:24 UTC

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