Create a SOFA Interface

[bbercoviciUspace]

The Standards of Fundamental Astronomy (SOFA) is a C-library provided by the international Astronomical Union that has potential to complete the use of Spice in Basilisk.

Of particular note are functions that return the Earth's translational and rotational state relative to inertial frames of reference. For instance, the function iauEpv00 whose docstring is available below.

int iauEpv00(double date1, double date2,
 double pvh[2][3], double pvb[2][3])
/*
** − − − − − − − − −
** i a u E p v 0 0
** − − − − − − − − −
**
** Earth position and velocity, heliocentric and barycentric, with
** respect to the Barycentric Celestial Reference System.
**
** This function is part of the International Astronomical Union’s
** SOFA (Standards Of Fundamental Astronomy) software collection.
**
** Status: support function.
**
** Given:
** date1,date2 double TDB date (Note 1)
**
** Returned:
** pvh double[2][3] heliocentric Earth position/velocity
** pvb double[2][3] barycentric Earth position/velocity

Another function of interest is iauPlan94, which allows one to compute the heliocentric state of the other planets. The docstring also provides a quantification of the position and velocity errors compared to spice.

int iauPlan94(double date1, double date2, int np, double pv[2][3])
/*
** − − − − − − − − − −
** i a u P l a n 9 4
** − − − − − − − − − −
**
** Approximate heliocentric position and velocity of a nominated major
** planet: Mercury, Venus, EMB, Mars, Jupiter, Saturn, Uranus or
** Neptune (but not the Earth itself).

...

Comparisons of the present function with the JPL DE200 ephemeris
** give the following RMS errors over the interval 1960−2025:
**
** position (km) velocity (m/s)
**
** Mercury 334 0.437
** Venus 1060 0.855
** EMB 2010 0.815
** Mars 7690 1.98
** Jupiter 71700 7.70
** Saturn 199000 19.4
** Uranus 564000 16.4
** Neptune 158000 14.4
**
** Comparisons against DE200 over the interval 1800−2100 gave the
** following maximum absolute differences. (The results using
** DE406 were essentially the same.)
**
** L (arcsec) B (arcsec) R (km) Rdot (m/s)
**
** Mercury 7 1 500 0.7
** Venus 7 1 1100 0.9
** EMB 9 1 1300 1.0
** Mars 26 1 9000 2.5
** Jupiter 78 6 82000 8.2
** Saturn 87 14 263000 24.6
** Uranus 86 7 661000 27.4
** Neptune 11 2 248000 21.4

I think it would be fairly easy to integrate sofa into Basilisk through a mechanism akin to spiceInterface, but this time leveraging a new class dubbed sofaInterface.

Potential benefits include

• no need to provide data kernels
• SOFA seems thread-safe

What are your thought on this ? =)

[schaubh]

I looked at SOFA a while back but never had the time to integrate into BSK. I think it would be a nice alternative to SPICE. I would recommend keeping the same planet state message type, even though the name contains "Spice", at least for now. That way the user can readily switch between using Spice, the planetEphemeris module, or this SOFA module to specify the planet states.