A solution using GPS measurements acquired during the CASA UNO campaign has resulted in 3--4 mm horizontal daily baseline repeatability and 13 mm vertical repeatability for a 729 km baseline, located in North America. The agreement with VLBI is at the level of 10--20 mm for all components. The results were obtained with the GIPSY orbit determination and baseline estimation software and are based on five single-day data arcs spanning the 20, 21, 25, 26, and 27 and January, 1988. The estimation strategy included resolving the carrier phase integer ambiguities (bias fixing), utilizing an optimal set of fixed reference stations (fiducials), and constraining GPS orbit parameters by applying a priori information (derived from initial multi-day trajectory fits to broadcast ephemeris data). A multi-day (January 20--27) GPS orbit and baseline solution has yielded similar 2--4 mm horizontal daily repeatabilities for the same baseline, consistent with the contrained single-day arc solutions. The application of weak constraints to the orbital state for single-day data arcs produces solutions which approach the precise orbits obtained with unconstrained multi-day arc solutions. This work suggests that the general availability of nominal ephemerides based on many days of global tracking data would be very valuable and practical for regional geodesy, allowing for high precision with single-day network solutions when combined with a continental scale fiducial network. ¿ American Geophysical Union 1990