This document provides a basic set of documentation for the data products available from the GPM Ground Validation System (GVS) Validation Network (VN). In the GPM era the VN performs a direct match-up of GPM’s space-based Dual-frequency Precipitation Radar (DPR) data with ground radar data from the U.S. network of NOAA Weather Surveillance Radar-1988 Doppler (WSR-88D, or “NEXRAD”). Ground radar networks from international partners are also part of the VN. The VN match-up will help evaluate the radar reflectivity attenuation correction algorithms of the DPR and will identify biases between ground observations and satellite retrievals as they occur in different meteorological regimes. A prototype of the required capability was developed using a match-up of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data with ground-based radar (GR) measurements from a set of WSR-88D sites along the southeastern coast of the U.S.  TRMM data and their matching GR observations continue to be part of the VN operations in the GPM era. Volume 1 of the Validation Network Data User’s Guide describes the legacy TRMM-specific VN data set.  Refer to Volume 2 of the Validation Network Data User’s Guide for a description of the GPM core and constellation VN data set.