A major issue in many applications of GPS is the real-time estimation of the Zenith Tropospheric Delays (ZTD). Several authors have developed strategies that estimate ZTD, with a latency of one hour or more in order to compute Integrated Water Vapor (IWV), using local measurements of surface pressure, or to assimilate ZTD into Numerical Weather Predicition (NWP) models. These strategies require that data from a regional GPS network be processed in near real-time, using precise IGS orbits and partial orbit relaxation. Recently it has been shown that in Wide Area Differential GPS (WADGPS) networks of several hundred kilometers across, double-differenced carrier phase ambiguities can be computed on-the-fly, using a real-time tomographic model of the ionosphere obtained from the same GPS data. In this work we show how ambiguity resolution can help determine in real-time the ZTD for a WADGPS network user, only 10--20% worse than those of the post-processed solutions. ¿ 2001 American Geophysical Union