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-boun...@amsat.org [mailto:amsat-bb-boun...@amsat.org] On Behalf Of ve4yz Sent: Saturday, February 27, 2010 11:41 AM To: amsat-bb@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...@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 _______________________________________________ Sent via amsat...@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
