TriPoint VertexRSI 7200, Operation And Maintenance Manual

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T HEORY
3 - 1
3.0 THEO R Y
Th i s sect i on prov i des i n-depth i nformat i on on O P T and O r b i t s ca n , t w o of the more
advanced features of the 7 2 0 0 A C S.
3.1 O P T
3.1.1 O r b i t P r e d i c t i on
O P T i s the a l gor i thm used by the 7 2 0 0 A C S to pred i ct the pos i t i on of a spacecraft
based upon i ts prev i ous mot i on. The bas i c pr i nc i p l e proposes that i f the
spacecraft’s pos i t i on, ve l oc i ty, and a ll of the forces act i ng upon i t are kno w n, i ts
pos i t i on can be pred i cted at any t i me i n the future.
A n " OPT mode l " i s w hat i s referred to i n orb i ta l mechan i cs as an orb i ta l e l ement
set. A n orb i ta l e l ement set descr i bes the pos i t i on and ve l oc i ty of an orb i t i ng body
at some part i cu l ar t i me (the epoch t i me, or s i mp l y, the epoch). O P T ’s pos i t i on
pred i ct i ons are made by tak i ng the orb i ta l e l ement set and us i ng the data i t
prov i des, a l ong w i th the computed forces act i ng on the spacecraft, to pred i ct the
pos i t i on of the spacecraft at the des i red t i me.
The pred i ct i on i s performed us i ng an a l gor i thm ca ll ed a propagator. A propagator
takes an orb i ta l e l ement set at some epoch, and produces an orb i ta l e l ement set at
a l ater epoch. O P T uses t w o d i fferent propagators i n i ts operat i ons:
T w o-body propagator: W here on l y the earth’s force on the spacecraft i s
cons i dered and earth i s mode l ed as a sphere w i th un i form mass.
Mu l t i body propagator: W here the fo ll o w i ng effects on the spacecraft are
cons i dered:
- Earth’s grav i ty, i nc l ud i ng a geopotent i a l mode l (account i ng for the
nonun i form i ty of the earth’s grav i tat i ona l f i e l d).
- Sun’s grav i ty.
- Moon’s grav i ty.
- So l ar rad i at i on pressure on the spacecraft.
The t w o-body propagator i s the s i mp l est and fastest, and prov i des qu i ck so l ut i ons
w h il e account i ng for the most s i gn i f i cant factor affect i ng the spacecraft’s mot i on.
The mu l t i body propagator, a l though more comp l ex, prov i des a more accurate mode l
of the spacecraft’s actua l mot i on.