GPS applications that require precise phase solutions in difficult environments, where loss of lock is likely, would benefit from instantaneous (single-epoch) phase integer ambiguity resolution. Among these applications are aircraft positioning and attitude determination during a photogrammetric flight, engineering surveying, and large-scale mapping. This paper reviews current GPS on-the-fly (OTF) techniques; a single-epoch ambiguity resolution approach is discussed in detail. Test results from 48 h of static data and 105 min. of kinematic data are presented. Results are also provided to demonstrate the ability of the algorithm to determine the attitude of a platform. Comparison of the technique's performance with that of commercial software demonstrates subcentimeter precision results all of the time (100 percent success), provided that at least five satellites are available and multipath errors are small.