The Tropical Rainfall Measuring Mission (TRMM) is a joint space mission between NASA and Japan's National Space Development Agency designed to monitor and study tropical and subtropical precipitation and the associated release of energy. The mission uses 5 instruments: Precipitation Radar (PR), TRMM Microwave Imager (TMI), Visible Infrared Scanner (VIRS), Clouds & Earths Radiant Energy System (CERES) and  Lightning Imaging Sensor (LSI).  The TMI and PR are the main instruments used for precipitation.  These instruments are used in an algorithm that forms the TRMM Combined Instrument (TCI) calibration data set (TRMM 2B31) for the TRMM Multi-satellite Precipitation Analysis (TMPA), whose TMPA 3B43 monthly precipitation averages and TMPA 3B42 daily and sub-daily (3hr) averages are probably the most relevant TRMM-related products for climate research. 3B42 and 3B43 are available in 0.25° spatial resolution, covering 50°N to 50°S for 1998-present.

As discussed in the Expert Guidance, the TMPA combines microwave data from multiple satellites including SSMI, SSMIS, MHS, AMSU-B and AMSR-E, each inter-calibrated to the TCI.   Coverage gaps in space and time are filled in with calibrated infrared (IR) data (which are generally available with near-global coverage every 3 hours); coefficients are derived from co-located IR brightness temperatures and the microwave-based precipitation estimates. The final data products reflect scaling the multi-satellite estimates to rain gauge data on a monthly basis, and ensuring that the 3-hourly averages in 3B42 sum to the monthly totals in 3B43.  

Due to the fact that TMPA timeseries are bulit up with estimates from  constantly varying data sources, it is likely that inhomegeneities in the temporal record exist.  Each precipitation field should be interpreted as the best estimate of precipitation in each grid point at the observation time.  In general, as a high-resolution precipitation product, TMPA does not have the same homogeneity goals as a climate dataset like GPCP v2.2.  Indeed, significant discrepancies in the interannual variability of rainfall over the ocean have been reported between TMPA  version 7 and GPCP v2.2; see the Figures below for illustration.  

After 17 years (1997-4/2015)  the TRMM mission came to an end. The Global Precipitation Mission (GPM) is the successor mission. New products to supersede the TMPA datasets are being produced under the Global Precipitation Measurement (GPM) umbrella with the Integrated Multi-satellitE Retrievals for GPM (IMERG) algorithm. TRMM-era data have been reprocessed using the IMERG algorithm for 2000-present, creating a long-term continuous record. Likewise, the TMPA 3B42 and 3B43 analyses have continued into the GPM era, but are slated to end production at the end of 2019.