WHAT'S "-S" VERSION RTK FOR ?
-S version having RTK position stamp feature is mainly for UAV mapping application. It takes external positive edge camera trigger signal occurring on the TRIG pin and compute RTK position within closest internal 1msec tic and output in STI,005 message. For 50Km/hr flying UAV, max position error is 50e5/3600/1000 = 1.39cm on top of 1cm + 1ppm RTK error.
Many RTK receivers on the market runs 10Hz RTK with input triggered precision time stamp to interpolate the camera trigger position, with UAV flying at 50Km/hr that would mean trying to interpolate position between two 10Hz 139cm position movement points. Unless the UAV is moving in a straight line at constant speed, interpolating linearly using time will not be correct.