Garmin GPS
GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks. GPS systems may include an input for differential corrections, using the RTCM SC-104 format. GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. Garmin GPS includes a (currently disabled) feature called Selective Availability (SA) that can introduce intentional, slowly changing random errors of up to a hundred meters (328 ft) into the publicly available navigation signals to confound, for example, guiding long range missiles to precise targets. GPS observation processing must also compensate for the Sagnac effect.