 However, positional accuracy could usually be computed to within 100 meters for a slow-moving ship, even with reception of just one two-minute Doppler curve.This was the navigation criterion demanded by the U.S. Navy, since American submarines would normally expose their UHF antenna for only 2 minutes to obtain a usable Transit fix.The U.S. submarine version of the Transit system also included a special encrypted, more accurate version of the downloaded satellite's orbital data This enhanced data allowed for considerably enhanced system accuracy [not unlike Selective Availability (SA) under GPS].