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Astronomical Applications Department, U.S. Naval Observatory - NAO 150 (Page 5)

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Astronomical Applications Department, U.S. Naval Observatory - NAO 150
kinds of fundamental position catalogs. Figure 2 is an illustration showing
various reference frames, from the largest scale to the smallest, and the
kinds of processes or observational methods that they rely on. The first
column is a hierarchy of frames, from the size scale of the universe down
to the local and very practical question of "Where am I right now?". We
must attach the notion of time to that of position, since in any dynamical
frame the two are inextricably linked. The second column contains the
major input category or dynamics type that corresponds to the associated
reference frame. The third column lists the most important observation
types that determine the reference frame. The activities associated with
planetary ephemeris generation correspond to the shaded boxes, and these
are the areas we will concentrate on here. Figure 3 shows these same ref-
erence frames, but organized to show how they are related to each other
observationally.
1.4. Generating Precise Predictions of Planetary Positions
We have established the
need for accurate planetary
positions. Therefore, we
need to be able to generate
accurate predictions of
planetary positions. These
tabulated predictions we
call ephemerides.
5
How
does one generate an
ephemeris? This is a three-
stage process, as illustrated
in Figure 4.
First and foremost, we
must obtain accurate obser-
vations. Historically, ob-
servations consisted mainly
of ground-based optical po-
sitions of planets and their
satellites. Satellite posi-
MURISON: MODELING PLANETARY MOTIONS
5 of 20
5
From the Greek ephemeros, meaning daily. That is, a table of coordinates of a celestial
body at specific times.
Figure 4 -- Generating a planetary ephemeris. Nu-
merical integrations of a solar system model are com-
pared with observational data, resulting in O-C residu-
als. Based on these residuals, model parameters are
refined and the entire process repeated until conver-
gence.
Observations
Numerical
Integration
Parameter
Estimation
Done
Start
Solar System
Model
O-C

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