Because extensibility is built into the design of Newcomb, adding further
capabilities as they become necessary will involve minimal effort -- there
is no need, from a maintenance standpoint, to include capabilities that are
anticipated to go unused for a long time. That is, with a good object-
oriented design we do not have to worry so much about "making room"
for anticipated future capabilities. Figure 5 shows the observation types
hierarchy. Figure 12 shows the proposed corresponding object class hier-
archy used in Newcomb.
Each type of input data stream will contain embedded type information,
and instantiations of the appropriate data objects will handle the data. The
specific objects shown in Figure 12 encapsulate not only the correspond-
ing observational data but also the functionality required to reduce that
data type. For example, notice that all datatype objects have, via inheri-
tance from the base class
Observation
, platform information and the ability
to handle (say) aberration.
As with Figure 11, Figure 12 is intentionally not complete, especially re-
garding encapsulated data and method details. However, all the important
base classes, and their inheritance dependencies, are shown.
3. SUMMARY
We have given a brief description of the field of high-precision modeling
of solar system planetary and natural satellite motions. Motivations for
high-precision ephemerides stem from -- perhaps surprisingly to many --
military as well as astronomical requirements. The latter category includes
such areas as spacecraft navigation, celestial mechanics, occultation pre-
dictions, tests of General Relativity, etc. Several kinds of observations go
into determining high-precision ephemerides -- essentially, we use any-
thing we can get our hands on. We have also discussed in broad terms the
method of generating high-precision ephemerides, making use of both ob-
servational data sets and comprehensive models to solve for the "best"
model parameter values.
Given that the extant first generation of high-precision ephemeris pro-
grams is antiquated, the U.S. Naval Observatory has begun development
of a new, highly flexible ephemeris program called Newton. This modern
program takes full advantage of design and programming techniques de-
veloped in the 1980s and early 1990s and now available as a mature set of
technologies. An overview of the program design has been presented,
MURISON: MODELING PLANETARY MOTIONS
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