Spherical trigonometry, Yule's PARCOR identity, and FRLS fully normalized lattice

Yule's PARCOR identity is recognized as the cosine law of spherical trigonometry. The six PARCORs propagated by the fully normalized FRLS lattice filter are the cosines of the six elements of a spherical triangle, and this lattice algorithm is one solution to a spherical triangle problem that arises naturally in navigation and astronomy. Exploiting this new geometric interpretation yields unnoticed (and potentially useful) recursions among FRLS quantities.