Usage: /usr/bin/gdal_viewshed [--help] [--long-usage] [--help-general]
                              [-of <output_format>] [-ox <value>] [-oy <value>]
                              [-oz <value>] [-vv <value>] [-iv <value>]
                              [-ov <value>] [-co <NAME>=<VALUE>]...
                              [-a_nodata <value>] [-tz <value>] [-md <value>]
                              [-j <value>] [-cc <value>] [-b <value>]
                              [-om NORMAL|DEM|GROUND|ACCUM] [-os <value>]
                              [-sd <value>] [--quiet]
                              <src_filename> <dst_filename>

Note: /usr/bin/gdal_viewshed --long-usage for full help.