RE: NROL-76 elements from observations

From: Ted Molczan via Seesat-l <>
Date: Wed, 24 May 2017 12:01:49 -0400
The following elements are derived from observations by Leo Barhorst, Kevin Fetter, Sergey Guryanov and Scott Tilley:

USA 276                                                  390 X 409 km
1 42689U 17022A   17144.45083662  .00000000  00000-0  00000-0 0    05
2 42689  49.9984 162.7662 0014150  91.2112 269.0477 15.56119177    09
Arc 20170523.76-0524.47 WRMS resid 0.059 totl 0.021 xtrk

Sergey described it as "brighter than 3 mag," which based on the circumstances of his observation, suggests a standard
visual magnitude of about 4 (1000 km range, 90 deg phase angle).

The ground track nearly repeats after 3 days and 46 revs. U.S. imagery intelligence satellites tend to employ orbits
with ground tracks that repeat, or nearly so, after 2, 3 or 4 days.

The orbital solution is still fairly preliminary, but it is interesting to note that the argument of perigee is near 90
deg. If this is confirmed by subsequent observations, and if it remains more or less constant long-term, then the orbit
is of the frozen type.

Frozen orbits are intended to maintain as nearly a constant altitude above geoid as possible around the entire orbit.
The argument of perigee may be frozen near 90 deg or 270 deg.

Below is a partial of list of satellites that have employed frozen orbits that I compiled in Jan 2007:

GEOS-3         radar altimeter
Seasat 1       SAR, radar altimeter
GEOSAT         radar altimeter
Spot           Earth resources imaging
Lacrosse       SAR
UARS           various Earth sensors
JERS 1         SAR
Topex/Poeidon  radar altimeter
ERS            SAR, radar altimeter
RadarSat       SAR
GEOSAT FO      radar altimeter
Landsat 7      Earth resources imaging
Terra          spectroradiometers, radiometer
EO-1           Earth resources imaging
Jason 1        radar altimeter
Envisat        SAR, radar altimeter, radiometers
Aqua           radiometers, microwave sounder
Icesat         altimeter, radiometer
ALOS           SAR, radiometer
USA 193        classified

The known sensors of the above payloads predominantly employed radio waves, but some were optical. A frozen orbit is
strong evidence of an imaging payload, but not all imagers employ frozen orbits.

To the above list, we can add the three Worldview optical imaging satellites built on the Ball Aerospace BCP-5000 bus.
Ball Aerospace is believed to have built USA 276, the payload of NROL-76, which adds to our interest in the possibility
that its orbit may be frozen.

Ted Molczan

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Received on Wed May 24 2017 - 11:03:00 UTC

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