Since the force exerted by the Sun is much smaller than the central attraction of the Earth, it is not necessary to know its coordinates to the highest precision when calculating the perturbing acceleration acting on a satellite. For many purposes it is even sufficient to use simple equations for the solar coordinates that are accurate to about 0.1-1% and follow from more advanced analytical theories for the motion of the Sun (see e.g. van Flandern & Pulkkinen 1979, Montenbruck 1989, Montenbruck & Pfleger 2000 for further references).
Three different approaches are used in the test_SunPosition.m program for the computation of solar coordinates; NASA JPL Developement Ephemerides (DE436), high-precision analytical series (Brown's theory) and low-precision analytical series.
1. Montenbruck O., Gill E.; Satellite Orbits: Models, Methods and Applications; Springer Verlag, Heidelberg; Corrected 3rd Printing (2005).
2. Montenbruck O., Pfleger T.; Astronomy on the Personal Computer; Springer Verlag, Heidelberg; 4th edition (2000).
3. Vallado D. A; Fundamentals of Astrodynamics and Applications; McGraw-Hill; New York; 3rd edition(2007).
4. van Flandern T. C., Pulkkinen K. F.; Low precision formulae for planetary positions; Astrophysical Journal Supplement Series 41, 391 (1979).