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*To*: orekit-users@orekit.org*Subject*: Re: [Orekit Users] Computation of reflectivity coefficient for SphericalSpacecraft*From*: Christophe Le Bris <chris.lebris@gmail.com>*Date*: Thu, 25 Sep 2014 12:06:39 +0200*Dkim-signature*: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :content-type; bh=hMveoK+91qecSTpvAmjLibBKjBGpMMCW96BPbtDqXYw=; b=G+MKaZxXFJflHjnHINXOzGnSiqt48WAqk3b0THPTr5W5y/yXD7SnMNSn0E1DFKTcL/ /g3D+c0Ek+FpZwz/Ai9ZunGOix8pru1XUi70+76kirUlv6LuBCuUzp+8yGe3w1vRwY8F TJyBrrcNCc2IL4noGaKyfrXujqff8SPqODxN+ezDiHawLjYfL8BRS0uk+5BceCmCI1Yh UZq22KtlvMPPORsjLEJf+JCUQXUu6JAjVYwoq4vGF3JNfui8wwDYZZlJ8aWuAw7j5JMZ Y6OcOO9n/tiY6INsLp5RCiVMjsUg9tt2B4saJ5GMNbuyp1Qbr3hfm7KN3H1q6GL40YOY f9jw==*In-reply-to*: <5423DACD.20807@c-s.fr>*References*: <CAAv=LsMTWLNPvB+0tRyQL8AVb7qVNvv1QG1ySY3W03MqFoqH0Q@mail.gmail.com> <5423DACD.20807@c-s.fr>

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.

Christophe

2014-09-25 11:05 GMT+02:00 Luc Maisonobe <Luc.Maisonobe@c-s.fr>:

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

**Follow-Ups**:**Re: [Orekit Users] Computation of reflectivity coefficient for SphericalSpacecraft***From:*Luc Maisonobe <Luc.Maisonobe@c-s.fr>

**References**:**[Orekit Users] Computation of reflectivity coefficient for SphericalSpacecraft***From:*Christophe Le Bris <chris.lebris@gmail.com>

**Re: [Orekit Users] Computation of reflectivity coefficient for SphericalSpacecraft***From:*Luc Maisonobe <Luc.Maisonobe@c-s.fr>

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