Abstract: The correction method of additional time delay caused by ionosphere and atmosphere plays a more and more important role in satellite timing and positioning system. In many radio projects which using electromagnetic wave as a measuring tool have to deal with the additional time delay, such as GPS, LADGPS, WADGPS and WAAS. In this paper, a new additional time delay algorithm model for a sparsely observation station region in WAAS is established and studied.Based on the improved grid-based correction algorithm for ionospheric delay in WAAS[4] and the model given in this paper, after numerical simulating with GPS data, the result was obtained showing that the precision of new algorithm is better than original one in the case of sparsely observation station region.