The TLE Library is a toolkit for processing ephemerides of Earth
Orbiting satellites in the so-called, two-line element (TLE) set
format. North American Aerospace Defence Command (NORAD) maintains the
publicly available database of TLE's and updates them several times a day
(Hoots and Roehrich (1980); Hoots, et al. (2004)). This toolkit creates a
table of satellite cartesian positions and velocities, i.e., state
vectors, for an arbitrary epoch. Using the computed state vectors, the
TLE library can also be used to compute the look angles, i.e. azimuth and
elevation, of the space object from the specified location on the ground.  

The TLE library was mostly derived from Vallado (2019) with minor changes 
to the Fortran version of the code. Any such changes are explicitly
stated in the routines in this toolkit..

References:
1.  Hoots, F. R. and Roehrich, R. L. (1980), "Models for Propagation 
    of NORAD Elements", Spacetrack Report No. 3.
2.  Hoots, F. R., Schumacher Jr, P. W., and Glover, R. A., (2004).
    "History of Analytical Orbit Modeling in the U.S. Space 
    Surveillance System", Journal of Guidance, Control, and Dynamics,
    Vol. 27, No. 2, p 174-185, doi:10.2514/1.9161
3.  Vallado, D. (2019), "Fundamentals of Astrodynamics and Applications 4th Ed.",
    Microcosm Press, Hawthorne, CA, p. xvi
