The Global Positioning System (GPS) carrier phase data are biased by an integer number of cycles. A successful strategy has been developed and demonstrated for resolving these integer ambiguities for geodetic baselines of up to 2000 km in length, resulting in a factor of 3 improvement in baseline accuracy, and giving centimeter-level agreement with coordinates inferred by very long baseline interferometry in the western United States. For this experiment, a method using pseudorange data is shown to be more reliable than one using ionospheric constraints for baselines longer than 200 km. An automated algorithm exploits the correlations between the many phase biases of a GPS receiver network to enable the resolution of ambiguities for very long baselines. A method called bias optimizing has been developed, which, unlike traditional bias fixing, does not require an arbitrary confidence test. Bias optimizing is expect to be preferable to bias fixing for poorly oorly configured networks. In order to enable ambiguity resolution for long baselines, it is recommended that future GPS networks have a wide spectrum of baseline lengths ranging from 1000 km and that GPS receivers be used which can acquire dual-frequency P code data. ¿ American Geophysical Union 1989