[amsat-bb] Re: Planet Pseudo-Keps

Ken Ernandes n2wwd at mindspring.com
Sat Feb 27 09:27:41 PST 2010

Your statement is correct, however there is an issue of context.  Most
satellite tracking software uses Simplified General Perturbations No. 4
(SGP4) as an orbit propagator.  SGP4 is one of several orbit propagation
algorithms that presume the Earth to be the central attracting body (as both
origin and major attracting mass).  As such, the associated Keplerian
elements are in the context of an Earth-orbiting satellite as are the
corresponding output positions and velocities.  This is a convenient
simplifying assumption for Earth-orbiting satellites and also since Earth
station coordinates use the center of the Earth as the origin.

Certainly, there are orbit propagation (and ephemeris interpolation) methods
that have a different (or a flexible choice of) attracting body (and
therefore a different origin and central mass).  In using such an algorithm,
the software must also (1) know the mass of the central attracting body, (2)
compute the Earth's position relative to the origin (the attracting body)
and (3) also determine its orientation relative to (for example) the Sun's
coordinate system.  This requires an additional translation (i.e., vector
subtraction) and a re-orientation (i.e., coordinate transformation).  These
are fairly simple to code into a library and thus simple to use.  However,
the software needs to have that additional logic already embedded.

I do not know of a method in which the Keplerian elements of a planet
orbiting the Sun could be used to produce any sort of meaningful answer in
software that propagates orbit using an Earth-centered propagator.  Indeed,
there's nothing wrong with the physics, but one must pay attention to the
simplifying assumptions underlying the implemented algorithms.

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of ve4yz
Sent: Saturday, February 27, 2010 11:41 AM
To: amsat-bb at amsat.org
Subject: [amsat-bb] Re: Planet Pseudo-Keps

Keplerian Elements have nothing to do with the Earth.   To say it only
applies to earth orbiting objects would be like saying Newton's laws of
motion only apply to the earth.

They are mathematical descriptions of the motion of one body orbiting
another in an unperturbed orbit using the 6 degrees of freedom of the
orbiting body - i.e the 6 key elements of the keps. 

They predate computers and software; circa 1605. 

This not to say that they are universally applicable.  Larger or smaller as
in galactic or subatomic with perturbed orbits - all bets are off. 


Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

More information about the AMSAT-BB mailing list