This project would continue operation of the Greenland GPS Network (GNET) for four years. GNET has proven highly successful in "weighing" the Greenland ice sheet by tracking the earth's elastic response to changing surface loads. The PIs propose to extend the life of GNET so that it can continue to sense space-time changes in ice mass, including spatial shifts in the position of the centers of ice loss, and accelerations in mass change rates. GNET is now resolving seasonal oscillations in the vertical position of the earth's crust as well as longer-term trends. These oscillations reflect seasonal changes in the loads placed on the solid earth by both the atmosphere and the ice sheet. The atmospheric pressure signal will be removed, using state-of-the-art weather models, in order to isolate the signal due to changes in the ice sheet. As part of the proposed pressure analysis, they would also produce a time series of integrated water vapor for each station in GNET, with benefits for weather and climate predictions. Broader impacts include the training of a graduate student and support of a postdoctoral researcher as co-investigator. Scientifically, GNET is addressing a problem of great interest and also considerable societal importance: the trajectory and magnitude of Greenland mass balance. With current and potential additional gaps in satellite geodesy coverage of Greenland, GNET is a cornerstone of our observations of the cryosphere. The data are openly accessible and are available in near real-time through UNAVCO.