A paper on GPS-based moisture assimilation has been published

Seko, H., T. Miyoshi, Y. Shoji, and K. Saito, 2011: Data Assimilation Experiments of Precipitable Water Vapor using the LETKF System: Intense Rainfall Event over Japan 28 July 2008. Tellus, 63A, 402-414. doi:10.1111/j.1600-0870.2010.00508.x