Development of a Water Vapor Tomography System Using Low Cost L1 GPS Receivers

A system of low cost Global Positioning System (GPS) stations is being developed to measure the threedimensional (3-D) structure of tropospheric water vapor over a small ground network. The concept is to deploy 20 GPS stations in a network spanning 10 km to 20 km. For each of the 20 stations, we will measure the integrated water vapor from the GPS antenna to each of the GPS satellites in view. These slant water vapor (SWV) observations will enhance current precipitable water vapor ( PWV) measurements by providing information on the spatial variability of water vapor above the network. In addition, they will be used as input for a tomographic estimation of the water vapor field. During the first year of this project, the primary focus has been in the development and testing of the prototype GPS station. Each station consists of a GPS receiver, a low multi-path GPS antenna, data communication capability through a 900-MHz radio modem equipped with Time Division Multiple Access (TDMA) technology, and a solar-charged battery for power. Eight of these GPS stations have been built. These instruments were tested near Platteville, Colorado, from August to September of 1998. This field deployment was used to verify the quality of the observations, to develop and test analysis software, and to evaluate SWV observations. We provide an overview of the system, present preliminary observations, and outline future tasks. A 20-station system will be deployed at the Cloud and Radiation Testbed (CART) facility in Lamont, Oklahoma, for the Water Vapor Intensive Observation Period (IOP) in the fall of 1999. Applications of the data obtained from this network include their use in comparison with other water vapor instruments at the CART site (lidars, radiosondes, profiling radiometers, etc.), as well as validating water vapor fields in single-column modeling ( SCM) parameterizations.