The Library tle_YYYYMMDD is a toolkit for processing Two Line Element
sets (TLE's). TLE's are files that provide descriptors and orbital
properties of objects in space at given epochs. The files are updated
and provided by the North American Aerospace Defence Command (NORAD)
multiple times a day (see: Hoots & Roehrich (1980); Hoots, et al. (2004)).
This toolkit takes these TLE files and computes the position and
velocities of the given space objects at a given epoch, i.e., the state
vectors. The state vectors can be computed at multiple epochs, be it
forward or backwards in time, and it doesn't necessarily have to match the
epoch defined on the TLE. Using the computed state vectors, this toolkit
can also be used to compute the look angles, azimuth and elevation, of the
space object from a location on the ground. Which for our interest is the
location of our radiotelescopes.

Most of the routines for the TLE ingestion are taken directly from David
Vallado. He took these, either directly from the aforementioned
references, or others influenced by them. Each such routine credits the
author ( David Vallado), explicitly states any changes made from their
original text, and also includes the reference by which the routine was
influenced.

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

