Re:Satellite formation flying

Heyler (gene.heyler@jhuapl.edu)
Tue, 28 Sep 1999 08:10:51 -0400

Use of the term "Hill's orbits" is a misnomer; more than likely they are
referring to the use of Hill's equations to propagate the orbit of the
reference satellites. A better reference might Clohessy-Wiltshire
equations. With Hill's or C-W equations, one of the satellites is
designated the control point of the cluster, and by definition is at [0
0 0] location. The other sub-satellites are then navigated relative to
the control point in a rotating coord system. Thus distances are small
(km vs 1000's of km) and velocities are also small (m/sec vs km/sec).
Note that knowing the solution to these equations in no way tells you
where the cluster is in the sky, the control satellite is still
navigated by conventional means.

   The drawback of the equations is they are based on assumption of
circular drag-fire orbits, so decrease  in accuracy with increasing
eccentricity and drag, although there are more complex forms of the
equations that deal with these. An simple example use of the equations
would be docking of Shuttle to a space station. The space station would
be the reference point, and the shuttle would be brought in along a
'vbar' or 'rbar', the velocity or radial axes, which are two of the
three coordinates of the Hill's or C-W equations.

Gene Heyler
Columbia, MD, US


>The latest edition of the weekly New ScientisT ( Sept 26 in Australia),

>( Sept 24 in UK/elsewhere), has a article "Follow my leader" by
>Catherine Zandonella about FUTURE use of satellites and spacecraft
>in formation. Interestly no mention at all of NOSS objects!!!
>The next generation of radar spysats will only be meter sized
>cubes by the look of it, rather harder to see than Lacrosse!

>Mentions the use of "Hill's orbits" without further explanation.
>Can anyone give me a reference to such orbit families. I couldnt
>find one in title or keywords of papers in NASA's ADS database
>or the LANL preprint collection.

>Tony Beresford