APPENDIX 2OVERVIEW OF
THE GLOBAL PRECIPITATION MEASUREMENT(GPM) AND
THE TROPICAL RAINFALL MEASURING MISSION(TRMM)

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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
climate changes. It is a key component of air-sea interaction and eco-hydrometeorologicalmodeling.
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 ofNovember 1997 by the Japanese H-II rocket, focuses on measuring tropical/subtropical rainfall andtheir diurnal variations, and covers latitude from 35S to 35N. TRMM had three precipitation
sensors: the Precipitation Radar (PR), the world first space-borne precipitation radar developed byJapan, 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. 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, andunderstanding of water cycles. Its feasibility has been studied at Goddard Space Flight Center of
the National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration
Agency (JAXA). Accurate measurement of precipitation has been achieved using theDual-frequency Precipitation Radar (DPR) installed on the GPM Core Observatory. 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-IIrocket 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 tropicalconvective activities, which is a drive source of global atmospheric circulation. To this purpose, theTRMM 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 and microwave imager (TRMM Microwave Imager: TMI). The combination useof PR and TMI has greatly improved the estimation of rainfall amount and has succeeded inobserving climate changes, as with El Niño and La Niña. Since the three-dimensional structure ofrainfall over the land and ocean can be derived from PR, TRMM has also revealed the2-2

three-dimensional structure of typhoons over the ocean, which was rarely observed before TRMM.
The success of TRMM shows the potential of satellite remote sensing contributions forunderstanding the water cycle on Earth and improving 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 byreducing atmospheric drag. In March 2009, more than 11 years after the satellite’s launch, itcontinues 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
AltitudeApprox. 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.
There is, however, limitation on single satellite observation in coverage and frequency. Therefore,
the Global 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 a non-sun-synchronous orbit as a calibrator to other satellites. The other is a collaboration witha constellation of several satellites developed by each international partner (space agency) thatcarries passive 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 aglobal area. GPM is composed of a TRMM-like non-sun-synchronous orbit satellite (GPM CoreObservatory) and multi-satellites carrying microwave radiometer instruments (constellationsatellites). 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 otherconstellation satellites. Constellation satellites, which carry a microwave imager and/or sounder
and are planned to be launched around 2014-2018 by each partner agency for its own purpose, andcan 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:➢ Understanding the horizontal andvertical structure of precipitation
system➢ Drop size distribution measurement➢ 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:➢ Observation frequency➢ Science, social applications• Cooperation with constellation
satellite providers; JAXA,NOAA, ISRO/CNES, etc.
• 3 hourly observation of 80% ofthe globe.
• Launch around 2014 by eachorganization• Mainly sun-synchronous orbit
with altitude 600~800 km• Generation and dissemination ofglobal map of precipitation rate withhigh frequency and accuracy• Various applications such as weather,flood forecast, agriculture, etc.
Figure 2 Overview of the GPM Mission

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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
and snowfall that cannot be measured by KuPR. Since KuPR can detect heavier rainfall,
simultaneous observation of KaPR and KuPR enables accurate measurement of precipitation fromheavy rainfall in the tropics to weak snowfall in high latitudes. Rain echo is affected byprecipitation 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, and2-5

measuring precipitation particles at the same place simultaneously by dual-frequency, size ofprecipitation 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 andsnowfall by using differences in precipitation attenuation for dual-frequency.
The GPM Microwave Imager (GMI) instrument on board the GPM Core Observatory is amulti-channel conical-scanning microwave radiometer developed by NASA, and it is based on theTMI on board the TRMM satellite. The major role of the GMI is to improve accuracy ofrainfall/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 theconstellation satellites. GMI is also expected to serve as a 'radiometric standard' for the othermicrowave radiometers on board the GPM constellation satellites, and to reduce differences in rain
rate estimation arising from biases of instruments. The GMI is characterized by thirteen microwave
channels ranging in frequency 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 vaporchannel). Addition of those high frequency channels is expected to contribute to improvements inaccuracy 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 theDPR and to provide calibration standards for the other microwave radiometers on the constellation
satellites.

GMIDPR (Dual-frequencyprecipitation radar)consists ofKuPR (13.6GHz radar)
andKaPR (35.5GHz radar)

KuPR
KaPR

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

3.

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 in the world. Figure 4 shows how the observation
area covered in 3 hours by microwave radiometers on polar-orbiting satellites increases with thenumber of satellites. As the number increases, the coverage for a given time increases, and hence
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the sampling interval at a given point decreases. In the GPM era, eight sun-synchronouspolar-orbiting satellites enable global observation of precipitation every 3 hours. In the GPM era,
one primary satellite and eight constellation satellites produces 3-hour global precipitation mapsthat are delivered to users in near real time.
Constellation of several satellites developed by each international partner (space agency) carriespassive microwave radiometers and/or microwave sounders and be in operation around 2014-2018.
The DPR and GMI instruments on board the core satellite serve as a ‘calibrator’ for data obtainedby constellation satellites.

Figure 4 Worldwide Missions for Satellite Precipitation Observation (2013-2020) as of April
2015.

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