Re: [Orekit Users] Computation of reflectivity coefficient for SphericalSpacecraft

[Luc Maisonobe <Luc.Maisonobe@c-s.fr>]

Le 25/09/2014 12:06, Christophe Le Bris a écrit :
> Thank you for the reply and the explanation.
> 
> In fact, it's not obvious because, when I looked in the
> BoxAndSolarArraySpacecraft, this class seems to use the K{a,s,d}
> coefficients and not alpha/tau, like in the SphericalSpacecraft class.

Yes, the coefficients are different (and it is part of the problem since
you cannot directly replace one class by the other one, despite they
have the same names. This is a documentation error too.

Luc

> 
> Christophe
> 
> 
> 2014-09-25 11:05 GMT+02:00 Luc Maisonobe <Luc.Maisonobe@c-s.fr
> <mailto:Luc.Maisonobe@c-s.fr>>:
> 
>     Le 24/09/2014 18:29, Christophe Le Bris a écrit :
>     > Hello,
> 
>     Hi Christophe,
> 
>     >
>     > I have some doubt about the computation of the reflectivity
>     coefficient
>     > used in SRP acceleration for a spherical spacecraft.
>     >
>     > We have: Ca (absorption) + Cs (specular) + Cd (diffuse) = 1
>     > and, for a spherical spacecraft, after integration on the sphere, I
>     > expect the reflectivity coefficient Cp to be:
>     > Cp = 1 + 4/9 * Cd
>     >
>     > So, if using only Ca and Cs, I would expect to have: Cp = 1 + 4/9
>     * (1 -
>     > Ca - Cs).
>     >
>     > But, in the SphericalSpacecraft class, the value is:
>     > 1 + 4/9 * (1 - Ca) * (1 - Cs)
>     >
>     > Is it an error in Orekit or am I missing something on reflectivity
>     > computation?
> 
>     You have found one of the design choices in Orekit we regret the most...
> 
>     It is not an error, it is a problem of definition of the coefficients.
> 
>     When developing this part, we used as a reference the book Spaceflight
>     Dynamics (part I), which was a collective work done in 1995 at CNES.
>     This book is sometimes called "the 95 book". In this book, section 5.2
>     delas with radiative forces. In section 5.2.2.1.3.1 (page 296 of the
>     English edition), the absorption coefficient alpha and specular
>     reflexion coefficient tau are defined. These are the coefficients we use
>     in Orekit. Later in section 5.2.2.1.3.2 there is a comment (comment 1
>     page 299 of the English edition) that reads:
> 
>       Some authors prefer to express thermo-optical properties for surfaces
>       using the following coefficients:
>         Ka = alpha, Ks = (1-alpha)tau, Kd = (1-alpha)(1-tau)
> 
> 
>     These are the coefficients you are used too and it explains the
>     different formulas. Experience showed that in fact these coefficients
>     are the ones almost everyone else use, so we should have selected these
>     ones for better compatibility with existing systems.
> 
>     We are *really* not happy with our previous choice and your question
>     raises this concern again.
> 
>     So I would like to ask a question to the community at large: do you want
>     us to deprecate the older SphericalSpacecraft class that uses this
>     uncommon definition and replace it with another class (name to be found)
>     using the more common K{a,s,d} coefficients?
> 
>     best regards
>     Luc
> 
>     >
>     > Thank you
>     >
>     > Christophe
> 
>