Le 24/09/2014 18:29, Christophe Le Bris a écrit :
> Hello,
Hi Christophe,
You have found one of the design choices in Orekit we regret the most...
>
> 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?
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