Weather forecasters and disaster managers want to know how much precipitation has recently fallen in various parts of the world. Rain gauges and ground radars only make observations over land, and individual satellites can see only a small fraction of the earth at a single moment.

To address these issues, NASA Goddard stitches together, in near real-time, the observations from a fleet of satellites that can estimate precipitation. These satellites were built by various countries and are operated by various organizations.

The “core” satellite of this constellation of precipitation-observing satellites is Global Precipitation Measurement (GPM). It was built by NASA and the Japan Aerospace Exploration Agency (JAXA) and was launched in February 2014.

NASA distributes the resulting “merged” estimates in real time and in various file formats. The merged precipitation estimates displayed by Cesium are stored in NASA Goddard’s Integrated Multi-satellitE Retrievals for GPM (IMERG).
As of the fall of 2016, the GPM science team is making available these IMERG precipitation estimates using the Cesium viewer on the Precipitation Measurement Missions website.

Previously, the real-time IMERG data was made available using the Google Earth plugin on the PMM website. There were several disadvantages to the Google Earth approach. First, the end-user had to modify their browser by downloading and installing the Google Earth plugin. Cesium, on the other hand, is pure JavaScript, so no plugin is required. Second, Cesium works with a number of browsers and operating systems, while Google Earth support was uneven for machines not running Microsoft Windows. Third, during the Google Earth era, the IMERG data was formatted as KML SuperOverlays with KML NetworkLinks providing updates to the real-time imagery. Some KML-enabled display software other than Google Earth had trouble displaying NetworkLinks. In contrast, IMERG is stored as a quadtree of images for display in Cesium. The url “pattern” that defines this quadtree can be expressed in a single line of code, making it easy for other software packages to display it if they understand quadtrees.

For more information about the Cesium real-time IMERG viewer, please contact the Helpdesk of NASA’s Precipitation Processing System (PPS) at For more information about the IMERG algorithm, please see the the theoretical basis document: IMERG ATBD.