Greenland's outlet glaciers exhibit variations in flow on a wide range of time scales. These major outlet glaciers can both accelerate and decelerate more rapidly than previously appreciated. Some abrupt accelerations at the largest outlet glaciers, including Jakobshavn Isbræ and Helheim and Kangerdlugssuaq Glaciers, are linked to large-scale calving events and glacial earthquakes. Other variations are observed on daily and subdaily time scales, associated with meltwater flux and tidal forcing. Some of these variations may also be linked to seasonal and interannual changes in glacier flow speed and strain rate. Here, we present results from a GPS network we have operated at Helheim Glacier, in East Greenland, during four summers and a limited fall/spring operation period, as well as preliminary results from a more limited deployment on on Kangerdlugssuaq Glacier. Some of the data were collected using a low-cost, single-frequency GPS receiver system developed in collaboration with UNAVCO.