# Re: Re: AO-40: more successful ARCJET operations

• Subject: Re: [amsat-bb] Re: AO-40: more successful ARCJET operations
• From: Phil Karn <karn@xxxxxxxxxxxx>
• Date: Mon, 25 Jun 2001 18:11:56 -0700 (PDT)
• In-reply-to: <3B373B8C.1427@jamsat.or.jp> (message from Mineo Wakita on Mon,25 Jun 2001 22:24:29 +0900)

```>I confirmed the equatorial radius of the oblate earth is 6378.140 km
>(6378140 m = 637814000 cm) by a astronomical journal of Japanese.
>As a result, this perigee height agrees to ~1 km with the results
>that Peter reported.

A minor nit: the equatorial radius defined in the WGS-84 coordinate
system is 6378137 meters, not 6378140.

But what *really* counts is that whatever figure you use is consistent
with the earth's other physical constants (flattening factor,
reference axes, etc), and that everyone uses the same set of physical
constants and orbit models in their measurements and computations.

WGS-84 is probably the best choice for physical constants. Because
it's the standard for GPS, much of your raw data is already referenced
to it, either explicitly or implicitly.

Even if everybody uses the same physical constants and orbit models,
there can be subtle differences in how apogee and perigee are
computed. One easy (and therefore common) technique is to use the
orbit model to find the spacecraft's closest and furthest approaches
to the earth's center and then compute altitude above the oblate earth
for these two points. But because the earth's surface is oblate and
lumpy, these points don't necessarily correspond to the actual closest
and furthest distances from the spacecraft to the earth's surface.  So
again it's important to ensure that everybody's doing it the same way
if you're going to compare results.

Ordinarily this level of detail would be nitpicking, but here they may
be significant since you're trying to spot orbit trends from some very
small changes.

To spot the effects of the plasma motor firing it would probably be
best to watch for changes in mean motion. That parameter directly
indicates the energy in the spacecraft's orbit, avoids any confusion
about the precise definitions of "apogee" and "perigee", is easily
measured with high precision and is given to many digits in the
element sets.

Phil
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