APPENDIX 2OVERVIEW OF
THE GLOBAL PRECIPITATION MEASUREMENT(GPM) ANDTHE PRECIPITATION MEASURING MISSION (PMM)

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1. Introduction“Precipitation” is one of most important environmental parameters. Changes in its amount anddistribution may affect our everyday life, and they may cause serious damages to human lives andproperties. Too much precipitation causes floods, and too less of it causes droughts. Agricultural
production depends on precipitation. It is one of the three foremost weather prediction variables
along with temperature and wind. Precipitation is a true global variable that determines the generalcirculation through latent heating, which is an "engine" for circumglobal winds, and reflects climatechanges. It is a key component of air-sea interaction and eco-hydrometeorological modeling.
Although there is no doubt that precipitation is such an important component of our environment, it
is one of the least known physics components of cloud, weather and climate prediction models.
Because of its large variability in space and time, its distribution over the globe is not accurately
known. Knowledge of the spatial and temporal distribution of global precipitation is a key toimproving our understanding of weather and climate systems.
The Tropical Rainfall Measuring Mission (TRMM) satellite, which archived tropical/subtropicalrainfall data more than 17 years, was a joint Japan-US mission. TRMM, launched in the end of
November 1997 by the Japanese H-II rocket, focuses on measuring tropical/subtropical rainfall andtheir diurnal variations, and covers latitude from 35S to 35N (Kummerow et al. 1998). TRMM had
three precipitation sensors: the Precipitation Radar (PR), the world first space-borne precipitation
radar developed by Japan (Kozu et al. 2001), and the TRMM Microwave Imager (TMI) and the
Visible Infrared Scanner (VIRS) developed by the U.S., which enabled observation of rainfall
structures by multiple sensors, simultaneously.
Because of the success of the TRMM satellite, several requirements for the successor mission
emerged from the science and operational user community. The Global Precipitation Measurement(GPM) mission was proposed to fulfill those requirements (Hou et al. 2014, Skofronick-Jackson etal. 2017). GPM is a satellite program to measure the global distribution of precipitation accurately
in a sufficient frequency so that the information provided by this program can drastically improve
weather predictions, climate modeling, and understanding of water cycles. Its feasibility has beenstudied at Goddard Space Flight Center of the National Aeronautics and Space Administration(NASA) and the Japan Aerospace Exploration Agency (JAXA). Accurate measurement ofprecipitation has been achieved using the Dual-frequency Precipitation Radar (DPR) installed on the
GPM Core Observatory (Kojima et al. 2012, Iguchi 2020). The DPR on the GPM Core Observatory
is being developed by JAXA and the National Institute of Information and Communications
Technology (NICT).
2. The Tropical Rainfall Measuring Mission (TRMM)
The Tropical Rainfall Measuring Mission (TRMM) satellite (Figure 1) was launched by H-II rocket
No. 6 in November 1997, and completed its mission in April 2015.
Major characteristics of the TRMM satellite are described in Table 1. TRMM is joint mission
between Japan (JAXA (former NASDA) and NICT (former CRL)) and the U.S. (NASA). The major
objective of TRMM is to determine accurate rainfall amount associated with tropical convective
activities, which is a drive source of global atmospheric circulation. To this purpose, the TRMM
satellite focuses on rainfall observation, and carries the world's first satellite-borne Precipitation
Radar (PR) developed by Japan, in addition to conventional instruments such as infrared imager andmicrowave imager (TRMM Microwave Imager: TMI). The combination use of PR and TMI has
greatly improved the estimation of rainfall amount and has succeeded in observing climate changes,
as with El Niño and La Niña. Since the three-dimensional structure of rainfall over the land andocean can be derived from PR, TRMM has also revealed the three-dimensional structure of typhoons
2-2over the ocean, which was rarely observed before TRMM. The success of TRMM shows thepotential of satellite remote sensing contributions for understanding the water cycle on Earth andimproving weather forecasts.
The TRMM satellite also targets rainfall observation in the tropics and sub-tropics. In order tomeasure tropical rainfall that has large diurnal variation, it flies in non-sun-synchronous orbit withan inclination angle of 35°. Although the designed lifetime of the satellite was about 3 years, thesatellite altitude was boosted from 350 km to 402.5 km in August 2001 to extend the lifetime by
reducing atmospheric drag. In March 2009, more than 11 years after the satellite’s launch, it
continues its excellent observation and provides valuable meteorological and climatological data
relating to precipitation, through long-term observation of the current status of rainfall in the tropics
and sub-tropics, for understanding water cycle mechanisms.

Figure 1 Overview of the TRMM Satellite and the Five on board Sensors

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Table 1 Major Characteristics of the TRMM Satellite
Orbit
Non-sun-synchronous circular orbit
Inclination
Approx. 35 degrees
Altitude
Approx. 350 km(402.5 km since August 24, 2001)
Launch dateNovember 28, 1997
6:54 AM (JST)Design life
3 years and 2 months
Mission instrument
Precipitation Radar (PR)TRMM Microwave Imager (TMI)
Visible Infrared Scanner (VIRS)
Lightning Imaging Sensor (LIS)
Clouds and Earth's Radiant Energy System (CERES)
3. The Global Rainfall Measurement (GPM)
1.

3.1 From TRMM to GPM

As accuracy of satellite precipitation estimates improves and observation frequency increases,
application of those data to societal benefit areas, such as weather forecasts and flood predictions, is
expected, in addition to research of precipitation climatology to analyze precipitation systems. Thereis, however, limitation on single satellite observation in coverage and frequency. Therefore, theGlobal Precipitation Measurement (GPM) mission was prpposed under international collaboration
to fulfill various user requirements that cannot be achieved by the single TRMM satellite.
One major characteristic of GPM as follow-on and expansion of TRMM is to operate the GPM Core
Observatory, which carries an active precipitation radar and a passive microwave radiometer, with anon-sun-synchronous orbit as a calibrator to other satellites. The other is a collaboration with aconstellation of several satellites developed by each international partner (space agency) that carriespassive microwave radiometers and/or microwave sounders, to increase observation frequency.
Although the TRMM satellite focused on observation of the tropics, the GPM mission covers
broader areas, including high latitudes.
3.2 Concept of the GPM Mission
TRMM is single satellite mission for scientific research. On the other hand, the GPM mission (Fig.
2) is an international mission to achieve high-accurate and high-frequent rainfall observation over a
global area. GPM is composed of a TRMM-like non-sun-synchronous orbit satellite (GPM Core
Observatory) and multi-satellites carrying microwave radiometer instruments (constellation
satellites). The GPM Core Observatory carries the Dual-frequency Precipitation Radar (DPR), which
is being developed by JAXA and NICT, and the GPM Microwave Imager (GMI) provided by NASA,and has been achieve more accurate but narrower observation as a calibrator to other constellation
satellites. Constellation satellites, which carry a microwave imager and/or sounder by each partner
agency for its own purpose, and can contribute to extending coverage and increasing frequency.
To take over the results that have been achieved by TRMM and to facilitate development of thoseresults, the GPM mission is planned to meet user requirements that cannot be achieved by TRMM
or are expected to be improved in GPM: 1) expansion of observation coverage; 2) increase ofobservation frequency; and 3) improvement of observation accuracy.

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Core Satellite
Objective:Ø Understanding the horizontal andvertical structure of precipitation
systemØ Drop size distribution measurement
Ø Improvement of precipitation rateaccuracy with constellation satellites• DPR (JAXA, NICT) (13.6, 35.5GHz)• GMI （NASA）• Launch in early 2014 by H-IIA• Non-Sun-synchronous orbit, inclination:
65deg., altitude: 407 km

Constellation Satellites
Objectives:
Ø Observation frequency
Ø Science, social applications• Cooperation with constellation
satellite providers; JAXA,NOAA, ISRO/CNES, etc.
• 3 hourly observation of 80% of
the globe.
• Launch around 2014 by eachorganization• Mainly sun-synchronous orbit
with altitude 600~800 km• Generation and dissemination of
global map of precipitation rate withhigh frequency and accuracy• Various applications such as weather,flood forecast, agriculture, etc.
Figure 2 Overview of the GPM Mission

3.3 Overview of the GPM Core Observatory
The GPM Core Observatory (Table 2 and Figure 3), which is being jointly developed by Japan andthe U.S., was launched in February 2014. The core satellite carries a Dual-frequency Precipitation
Radar (DPR) developed by Japan, and a GPM Microwave Imager (GMI) developed by U.S. The
orbit of the core satellite is non-sun-synchronous with an inclination angle of 65º. This orbit was
selected to meet certain requirements, such as to measure diurnal variation of rainfall in mid- andhigh-latitudes as well as the tropics for around 2 months.
Table 2 Major Characteristics of the GPM Core Observatory
Orbit

Non-sun-synchronous

Inclination

65 degrees

Altitude407 kmLaunch dateFebruary 28, 201403:36 AM (JST)Mission life

3 years (target: 5 years)

Mission instrument

Dual-frequency Precipitation Radar (DPR)
GPM Microwave Imager (GMI)The Dual-frequency Precipitation Radar (DPR) on board the GPM Core Observatory is composedof two radars: a Ku-band (13.6-GHz) Precipitation Radar (KuPR) and a Ka-band (35.5-GHz)
Precipitation Radar (KaPR). KaPR aims at sensitive observation, and can detect weaker rainfall andsnowfall that cannot be measured by KuPR. Since KuPR can detect heavier rainfall, simultaneous
observation of KaPR and KuPR enables accurate measurement of precipitation from heavy rainfall
in the tropics to weak snowfall in high latitudes. Rain echo is affected by precipitation attenuation,and its amount depends on radar frequency and raindrop size. By matching position of radar beams
and timing of transmitted pulses for KuPR and KaPR, and measuring precipitation particles at the
2-5same place simultaneously by dual-frequency, size of precipitation particles (raindrop size
distribution) can be estimated by differences in precipitation attenuation. This information cannot be
obtained by single-frequency radar, such as TRMM’s PR, and can improve accuracy of precipitation
estimation. It is also expected to identify rainfall and snowfall by using differences in precipitation
attenuation for dual-frequency.
The GPM Microwave Imager (GMI) instrument on board the GPM Core Observatory is a multichannel conical-scanning microwave radiometer developed by NASA, and it is based on the TMI
on board the TRMM satellite. The major role of the GMI is to improve accuracy of rainfall/snowfall
estimates by simultaneous observation with the DPR, and to work as a bridge between highly
accurate observation by the core satellite and frequent observations by the constellation satellites.
GMI is also expected to serve as a 'radiometric standard' for the other microwave radiometers on
board the GPM constellation satellites, and to reduce differences in rain rate estimation arising frombiases of instruments. The GMI is characterized by thirteen microwave channels ranging infrequency from 10 GHz to 183 GHz. In addition to carrying channels similar to those on the TRMM
Microwave Imager (TMI), the GMI carries four high frequency, millimeter-wave, channels of about
166-GHz (‘window’ channel) and 183-GHz (water vapor channel). Addition of those high frequency
channels is expected to contribute to improvements in accuracy of weak rainfall and snowfall
estimates, especially over the ocean and land in high-latitudes. With a 1.2 m diameter antenna, the
GMI can provide significantly improved spatial resolution over TMI.
The roles of the GPM primary satellite are to collect as much microphysical information as possible
for accurate rain estimation by performing synchronous observation with the GMI and the DPR and
to provide calibration standards for the other microwave radiometers on the constellation satellites.

GMI

DPR (Dual-frequency
precipitatio n radar)co nsists of
KuPR (13.6GHz radar)an d
KaPR (35.5GHz r ad ar )

KaPR

KuPR

Figure 3 Overview of the GPM Core Satellite and Concept of Precipitation Observation

3.4 Collaboration with Constellation Satellites
In the case of low orbital satellites, such as TRMM and Aqua, single-satellite cannot observe
frequently at each local point. To overcome this weakness and achieve frequent observation, theGPM mission works with other satellite missions with microwave radiometers in polar-orbiting
satellites. As the number increases, the coverage for a given time increases, and hence the samplinginterval at a given point decreases. In the GPM era, eight sun-synchronous polar-orbiting satellites
enable global observation of precipitation every 3 hours. In the GPM era, one primary satellite andeight constellation satellites produces 3-hour global precipitation maps that are delivered to users in
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near real time.
Constellation of several satellites developed by each international partner (space agency) carriespassive microwave radiometers and/or microwave sounders. The DPR and GMI instruments onboard the core satellite serve as a ‘calibrator’ for data obtained by constellation satellites.
4. Precipitation Measuring Mission (PMM)
Measurements of Convection (vertical atmospheric motion) by Doppler velocities from spaceborne
radar is desirable for understanding of cloud-precipitation process. On the other hand, estimating
Doppler velocities from space is very challenging because of satellite motion, velocity folding, andnonuniform beam filling.
World's first satellite-based Ku-band (13GHz) doppler precipitation radar (KuDPR) is planned in the
JAXA’s PMM. The KuDPR will be two-antenna system that adopts Displaced Phase Center
Antenna (DPCA) approach (Durden et al. 2007, 2023, Tanelli et al. 2016, Nakamura and Furukawa
2023). The DPCA approach can lead to more accurate Doppler measurement.
In June 2023, JAXA’s Precipitation Measuring Mission (PMM) Project Team was established on
for the PMM observatory carrying the Ku-band Doppler Precipitation Radar, with participation inNASA AOS mission.
In December 2022, Implementation Plan of the “Basic Plan on Space Policy” noting the Precipitation
Radar Satellite Phase B activity targeting the launch of JFY2028 (April 2028 to March 2029) was
released from Cabinet Office of the Japanese government.

Figure 4 A concept of the PMM observatory
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