Global Precipitation Measurement (GPM) mission
From tropical to global rainfall observation
As the accuracy of satellite precipitation estimates improves and observation frequency increases, the data is expected to be applied to areas that benefit society, such as weather forecasts and flood predictions, in addition to research of precipitation climatology to analyze precipitation systems. There are, however, limitations on single satellite observation in coverage and frequency. Currently, the Global Precipitation Measurement (GPM) mission is scheduled under international collaboration to fulfill various user requirements that cannot be achieved by the single TRMM satellite.
One major characteristic of the GPM mission as follow-on and expansion of the TRMM satellite is operation of the GPM core satellite, which will carry an active precipitation radar and a passive microwave radiometer, with a non-sun-synchronous orbit as a “calibrator” to other satellites. The other is its collaboration with a constellation of several other satellites developed by each international partner (space agency), each of which will carry 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.
Concept of the GPM mission
The TRMM satellite is single satellite mission for scientific research. The GPM mission, on the other hand, is an international mission to achieve highly accurate and frequent global rainfall observation. The GPM mission is composed of a TRMM-like non-sun-synchronous orbit satellite (GPM core satellite) and multiple satellites carrying microwave radiometer instruments (constellation satellites). The GPM core satellite will carry the Dual-frequency Precipitation Radar (DPR), which is being developed by JAXA and the National Institute of Information and Communications Technology (NICT), and the GPM Microwave Imager (GMI) provided by NASA, and will achieve narrow but accurate observation as a calibrator to the constellation satellites. The constellation satellites will carry microwave imagers and/or sounders and are planned to be launched around 2013 by each partner agency for its own purpose and to extend coverage and increase frequency.
To surpass the results achieved by TRMM, and to facilitate development of those results, the GPM mission is planned to meet user requirements that cannot be met by the TRMM satellite or are expected to be improved in the GPM mission: 1) expansion of observation coverage; 2) increase of observation frequency; and 3) improvement of observation accuracy.
Overview of the GPM core satellite
The GPM core satellite, which is being developed jointly by Japan and the U.S., is scheduled to be launched in 2013. The core satellite carries a Dual-frequency Precipitation Radar (DPR) developed by Japan, and a GPM Microwave Imager (GMI) developed by the U.S. The orbit of the core satellite is non-sun-synchronous with an inclination angle of about 65 degrees. This orbit was selected to meet certain requirements, such as to measure diurnal variation of rainfall in mid- and high-latitudes as well as the tropics over 2 months.
The DPR consists of two radars: a PR-type precipitation radar with a frequency of 13.6-GHz, and a 35.5-GHz precipitation radar that was added to improve the sensitivity and accuracy of measurement. The GMI will be an improved model of the TRMM Microwave Imager (TMI), and carries four high-frequency channels about 166-GHz and 183-GHz in addition to nine microwave channels available in TMI.
The 13.6-GHz channel of the DPR has a swath width of about 245 km, and the 35.5-GHz channel observes a swath width of about 100 km. In the overlapping area, measurements will be performed synchronously. The GMI scans its antenna conically with a swath width of about 800 km.
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.
Dual-frequency Precipitation Radar (DPR)
The Dual-frequency Precipitation Radar (DPR) on board the GPM core satellite is composed of two radars: a Ku-band (13.6-GHz) Precipitation Radar (KuPR) and a Ka-band (35.5-GHz) Precipitation Radar (KaPR).
The KaPR instrument aims at sensitive observation, and can detect weak rainfall and snowfall that cannot be measured by KuPR. Since the KuPR instrument can detect heavier rainfall, simultaneous observation by the KaPR and KuPR will enable accurate measurement of precipitation from heavy rainfall in 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 by measuring precipitation particles at the same place simultaneously by dual-frequency, size of precipitation particles (raindrop size distribution) can be estimated by differences in precipitation attenuation.
This will improve the accuracy of precipitation estimation since the information could not be obtained by single-frequency radar, such as the TRMM’s PR. It is also expected to identify rainfall and snowfall by using differences in precipitation attenuation for dual-frequency.
GPM Microwave Imager (GMI)
The GPM Microwave Imager (GMI) instrument on board the GPM core satellite is a multichannel conical-scanning microwave radiometer developed by NASA, and is based on the TMI on board the TRMM satellite.
The major role of the GMI instrument is to improve the accuracy of rainfall/snowfall estimates by simultaneous observation with DPR, and to work as a bridge between highly accurate observation by the core satellite and frequent observations by the constellation satellites. The 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 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 about 166 GHz (“window” channel) and 183 GHz (water vapor channel). Addition of those high frequency channels is expected to improve accuracy of weak rainfall and snowfall estimates, especially for those over the ocean and land in high-latitudes. With a 1.2-m diameter antenna, the GMI will provide significantly improved spatial resolution over the TMI
Schematic flash of constellation observation
World’s missions for satellite precipitation observation (2005-2018) as of March 2009
Collaboration with constellation satellites
In cases of low orbital satellites, such as the TRMM and Aqua, a single-satellite cannot observe frequently at each local point. To overcome this weakness and achieve frequent observation, the GPM mission will work with other satellite missions around the world. The figure shows how the observation area covered in 3 hours by microwave radiometers on polar-orbiting satellites increases with the number of satellites. As the number increases, the coverage for a given time increases, and hence the sampling interval 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 and eight constellation satellites will produce 3-hour global precipitation maps that will be delivered to users in near real time.
A constellation of several satellites developed by each international partner (space agency) will carry passive microwave radiometers and/or microwave sounders and be in operation around 2013. The DPR and GMI instruments on board the core satellite will serve as a calibrator for data obtained by constellation satellites.