  We describe the angular sensing and control of the 4 km detectors of the
Laser Interferometer Gravitational-wave Observatory (LIGO). The culmination of
first generation LIGO detectors, Enhanced LIGO operated between 2009 and 2010
with about 40 kW of laser power in the arm cavities. In this regime, radiation
pressure effects are significant and induce instabilities in the angular
opto-mechanical transfer functions. Here we present and motivate the angular
sensing and control (ASC) design in this extreme case and present the results
of its implementation in Enhanced LIGO. Highlights of the ASC performance are:
successful control of opto-mechanical torsional modes, relative mirror motions
of 1x10^{-7} rad rms, and limited impact on in-band strain sensitivity.
