We have developed a GPS data processing software and demonstrate that by using receivers in an area of approximately 800 ¿ 400 km we can produce estimates of the atmospheric zenith total delay (ZTD) with a latency of a few seconds. It is based on a Kalman filter which simultaneously estimates clock offsets and satellite coordinates. The ZTD quality is typically 15 mm in terms of rms difference to post-processed ZTD time series. This level of uncertainty is useful in many navigation applications and if an additional reduction in the uncertainty can be obtained the usefulness for short term weather forecasting could be significant.