Yes. I had orbital mechanics in grad school, although I already knew
about Keplerians before I took the course. I'm going to gloss over some
of the details because things like the catalog number and the element
set number have an obvious meaning and the decay rate has an obvious
meaning that happens to be wrong, but which isn't important to most people.
In general, there are seven numbers that make up the parameters that are
known as "Keplerians" The number of parameters is easy to remember
because in order to specify the position of a satellite at any time, you
need to record the position and velocity vectors in three dimensional
space for six of those parameters, and then you have to record the time
at which it was at that position. That form is equivalent to what you
get in either AMSAT or NORAD form, but it's not convenient, so what you
actually get is in a different form.
There are two parameters that describe the shape of the orbit, they are
the mean motion (which is equivalent to, that is it can be converted
into, the period and the semimajor axis) and the eccentricity. The mean
motion is how many orbits are made by that satellite per day. All
closed orbits are ellipses, and the eccentricity is how squished the
ellipse is. A circle has an eccentricity of 0 and that number gets
closer and closer to 1 as it gets more and more squished.
Then, you have to position the plane which holds the ellipse. That's
defined by the right ascension (sometimes called the "longitude") of the
ascending node and the inclination. In an orbit, a "node" is a crossing
of the equator. Unless the orbit is precisely on the equator, the orbit
crosses the equator twice. The "ascending node" is the equator crossing
when the orbiting object is traveling north. The inclination is the
angle that the plane of the orbit makes with the plane of the equator at
the ascending node, measured counterclockwise from the east side of the
equator to the orbit. The difference between the "right ascension" and
the "longitude" is that "right ascension" is fixed to the celestial
sphere, while the longitude is fixed to the earth. I believe that
"longitude" is technically incorrect, because an orbit is fixed in space
not relative to the earth, but people insist on using it. Go figure.
To understand the next two parameters, you have to understand the term
"anomaly" as it relates to orbital mechanics. The anomaly at any time
is the angle the line of periapsis (or perigee for earth-relative
orbits) makes with the position of the orbiting object at the center of
the mass of the system in the plane of the orbit at that point in time.
Like any angle, it ranges from 0 to 2 pi radians.
The next parameter describes the direction of the orbits axes relative
to "north". It is called the argument of periapsis (or argument of
perigee for earth-relative orbits) and gives the anomaly of the
ascending node.
The last parameter is the mean anomaly, which describes the position of
the satellite in the orbit at "epoch." "Epoch" is what they call the
time that I already mentioned is part of the Keplerian data.
Orbital mechanics uses some unusual words and uses some more common
words in unusual ways, and I'm sure I've made some mistakes here, so if
some word seems off, please ask about it and I'll try to explain better.
On 1/20/20 9:34 AM, Alexander Park via BVARC wrote:
I am interested in satelite operating and I have wanted to know what
the numbers mean in the arrl's keplerian data that is published
weekly. Does anyone in the club know what that is?
-Thanks and 73
Alex
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--
Jonathan Guthrie
ARS KA8KPN
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Brazos Valley Amateur Radio Club
Get on the air - 146.94 Repeater
Volunteer now for the Houston Hamfest
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