KeyHoles: latest elements and discussion of perigee height

From: Ted Molczan (molczan@rogers.com)
Date: Thu Dec 26 2002 - 16:44:47 EST

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    I have derived the following from Greg Roberts' recent observations:
    
    For arc 2002 Dec 19 - 26 UTC:
    
    USA 116         15.0  3.0  0.0  4.7 v
    1 23728U 95066A   02359.96644676  .00004453  00000-0  23878-3 0    05
    2 23728  97.9377 115.8193 0305137  56.2524 306.7481 14.82883852    05
    
    For arc 2002 Dec 18 - 25 UTC:
    
    USA 129         15.0  3.0  0.0  5.3 v
    1 24680U 96072A   02359.82819444  .00031720  00000-0  35959-3 0    03
    2 24680  97.8482  60.4176 0496428 214.4363 142.3850 14.82700976    07
    
    For arc 2002 Dec 18 - 25 UTC:
    
    USA 161         15.0  4.0  0.0  5.4 v
    1 26934U 01044A   02359.98802083  .00004090  00000-0  22134-3 0    01
    2 26934  97.9099 109.7178 0308042  89.7821 273.9004 14.82107661    05
    
    All have WRMS error less than 0.01 deg
    
    The western plane object, 96072A, continues to maintain the roughly 270 km perigee height that has been the standard ever since the
    launch of the first KH 11 in 1976.
    
    The eastern plane objects, 95066A and 01044A have adopted orbits of nearly identical dimensions. Both have a mean perigee height of
    411 km (relative Earth's mean radius, 6371 km).
    
    The move to higher perigee heights among primary KeyHoles, i.e. those in one of the two standard planes, has been gradual, and has
    taken place entirely in the eastern plane.
    
    For approximately its first three years in orbit, 87090A's perigee height was about 280 km. In late 1990 or early 1991, it raised
    its perigee height to about 333 km, where is remained until it was de-orbited in June 1992.
    
    During its first 3.4 years in orbit, 92083A's perigee height was about 270 km. In the spring of 1996, it pioneered the use of the
    411 km perigee height. This was about 6 months after it had been replaced by 95066A, so it was no longer in the standard eastern
    plane. Its perigee remained near 411 km altitude until it was de-orbited in June 2000.
    
    For approximately its first three years in orbit, 95066A's perigee height was about 270 km. In late 1998 or early 1999, it raised
    its perigee height to about 323 km, were is remained through the summer of 2002, before manoeuvring to its present 411 km perigee
    height. This was about one year after it had been replaced by 01044A, by which time it had drifted about 6 deg east of the standard
    eastern plane.
    
    During its first three months in orbit, 01044A's perigee height was about 270 km. In January 2002, it manoeuvred to its present 411
    km perigee height.
    
    84122A was the only western plane Keyhole to have operated at perigee heights significantly greater than the 270 km standard, but
    only after its replacement by 88099A. In August 1989, it raised its perigee height to about 340 km. In November 1990, it raised its
    perigee to about 550 km, where it remained until it was de-orbited in November 1994. (This final manoeuvre almost certainly was for
    the purpose of supporting Operations Desert Shield and Desert Storm, by re-establishing the operational mean motion, albeit in a
    much more nearly circular orbit, and in a non-standard plane.) 
    
    It appears that 84122A's use of the 340 km perigee height after it primary mission had ended, paved the way for the use of similar
    perigee heights by 87090A and 95066A during the latter stages of their primary missions.
    
    Likewise, it appears that 92083A's use of the 411 km perigee height after it primary mission had ended, paved the way for its use by
    95066A after its primary mission ended and 01044A's use of it during most of its primary mission to-date.
    
    Higher perigee orbits reduce atmospheric drag, which conserves propellant required to maintain the orbit. The 411 km perigee height
    of 95066A and 01044A results in about one eighth the rate of decay of 96072A's 278 km perigee height.
    
    With the next generation optical imaging satellites (called FIA, or Future Imagery Architecture) reportedly running two years behind
    their planned 2006 first launch, it may be prudent to conserve propellant now to reduce the risk of coverage gaps.
    
    Higher perigee orbits also improve near-perigee coverage at latitudes near the equator - an advantage given the present geopolitical
    situation.
    
    The disadvantage of higher perigee orbits is the reduction in maximum resolution. At a range of 411 km, resolution is about one
    third less than at 270 km. Perhaps the Keyhole's electro-optical systems have been improved over the years, enabling the use of a
    higher perigee height without loss of resolution compared with the earlier satellites in their orbits.
    
    Ted Molczan
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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