Update to version 6.1

Author: DavidFCrouse

3rd_Party_Libraries/SGP4/Related Documentation/Spacetrack Report No. 3- Models for Propagation of NORAD Element Sets.pdf

@@ -1,18 +1,11 @@
 David Vallado's original distribution came with implementations in C++,
 Fortran, Microsoft Excel, Matlab, and Pascal. Only the C++ implementation
 is retained here and is called as an external library by a mex function to
-run the SGP 4 propagator. The code has been slightly modified to eliminate
-all warnings when compiled with the -Weverything option under Clang, except
-for the Wcovered-switch-default warning. For example, a few unused
-variables were eliminated. Also, the debugging code and the IO code as well
-as files containing helper functions for the IO code have been removed as
-they is not used in the Tracker Library. The definition of pi had to be
-moved from sgp4unit.h to sgp4unit.cpp to avoid a namespace conflict with
-one of Matlab's headers. The documentation files appearing in AIAA journals
-have been removed. They have been superseded by the publication:
+run the SGP 4 propagator.  The documentation files appearing in AIAA
+journals have been superseded by the publication:
 D. A. Vallado, P. Crawford, R. Hujsak, and T. S. Kelso "Revisiting
 Spacetrack Report #3," AIAA/AAS Astrodynamics Specialist Conference and
 Exhibit, Guidance, Navigation, and Control and Co-located Conferences,
 Keystone Colorado, 21-24 August 2006.
 
-December 2014 David F. Crouse, Naval Research Laboratory, Washington D.C.
+December 2024 David F. Crouse, Naval Research Laboratory, Washington D.C.