"Thomas" <alamyx@softhome.net> writes:
> Hvis man vil beregne en kurs fra start til slut i et planetsystem med
> bevægelige planeter, så er opgaven, så vidt jeg kan se, nærmest
> umulig. Hvis eksempelvis jeg er i kredsløb om Jorden og vil til Io
> eller ligende, så skal jeg bruge en kurs. Jeg skal have et
> afgangstidspunkt samt en retning og en acceleration og et tidsrum
> hvori jeg skal accelerere, men hvordan beregner jeg det?
En fyr fra NASA skrev tilfældigvis lidt om, hvordan de gør, på en
mailingliste for nyligt. Her er et uddrag:
,----[ Van Snyder]
|
| The core of spacecraft navigation, as you might imagine, is integrating
| Newton's equations of motion. We have an ODE integrator that was written
| in Fortran 77 and carefully refined during the course of two decades of
| deployment in many applications (not just spacecraft navigation). One of
| its useful features is that it solves second-order equations directly
| (rather than requiring to introduce auxiliary variables and reduce the
| system to first order). In addition to reducing the burden on the
| mathematicians who develop the models, it improves both speed and error
| propogation properties.
|
| (You may think the model business is trivial -- after all, we have only
| three second order equations, right? Well, the acceleration term isn't
| just gravity. We're concerned about solar pressure, battery outgassing,
| and numerous other features. Also, we're concerned about the gravitation
| of the Sun, Jupiter, and any other bodies near which the spacecraft
| passes. To make matters worse, we want to solve for masses and
| ephemerides of all of these bodies, and things like solar pressure and
| battery outgassing, which means we're integrating variational equations
| along with state equations. I.e., the trajectory integrator provides
| state and Jacobian for a nonlinear least-squares solver. The Galileo
| project navigation is solving for 500 parameters. So simplifying the
| model-maker's job is a significant win.)
`----
Det hele kan læses på:
<
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0111&L=comp-fortran-90&D=1&O=D&F=&S=&P=8428>