r.horizon(1grass) on Linux
The text b2ea003d32c4, as Debian 6.0 (Squeeze) shipped it.
Horizon angle computation from a digital elevation model. Computes horizon angle height from a digital elevation model. The module has two different modes of operation: 1. Computes the entire horizon around a single point whose coordinates are given with the 'coord' option. The horizon height (in radians). 2. Computes one or more raster maps of the horizon height in a single direction. The input for this is the angle (in degrees), which is measured counterclockwise with east=0, north=90 etc. The output is the horizon height in radians.
Flags it lists
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-d