What Determines the Latency of IMERG?
1. Most of the low-orbit microwave satellites downlink their data once or twice an orbit (which is about 90 minutes long) to the operating agency. [Except GMI comes down in 5-min. granules.] The agencies compute the Level 1B files, package them, and ship the data around. PPS gets all the NOAA and EUMETSAT data through NOAA, DMSP from NRL Monterey, and the Japanese AMSR2 through NASA, SAPHIR (now intermittently and generally not in near-real time) through ISRO, and GPM DPR and GMI within GPM. The global merged geo-IR data are assembled by NOAA/NWS/CPC, and the Precipitation Processing System (PPS) accesses GEOS FP forecasts through the Goddard NCCS and Autosnow from NOAA. By about 3 hours after observation time the geo-IR and about 85% of the microwave (depending on how systems feel that day) have arrived at PPS. PPS converts the microwave data to precip estimates and computes the IMERG Early Run, which only uses forward propagation morphing (plus IR in the Kalman filter) by about 4 hours after observation time. [One subtle point is that the IR half-hour data fields come in pairs, so both halves of the hour are processed together, meaning the second half hour almost always has shorter latency than the first. Another is that IMERG uses forecasts to get the analysis of vertically integrated vapor, which is what IMERG V06 uses to estimate system motion for the morphing. So, hitches in delivery of the latest GEOS FP are not a big deal – IMERG just uses the most recent forecast sequence, whatever that is.]
2. To compute the IMERG Late Run, which entails both forward and backward propagation morphing (plus IR), processing has to wait long enough that the following microwave overpass has a chance to occur and then follow the delivery chain described above. So, PPS waits 11 hours to capture a reasonably complete set of "next overpass" data, and then start processing. The nominal latency stated in the documentation is 14 hours, but it's more like 12 in recent data.
3. The IMERG Final Run uses MERRA2 for the vertically integrated vapor, GPCC monthly Monitoring Analysis for gauge, and revised precipitation retrievals that depend on ERA-5 (and ERA-I up through mid-2019). The pacing items are the GPCC and ERA-5, usually giving a latency of about 3.5 months.