The dual-frequency precipitation radar for the GPM core satellite

Abstract

This paper outlines the development of the dual-frequency precipitation radar (DPR) to be flown on the Global Precipitation Mission's spacecraft. I. INTRODUCTION In the Global Precipitation Mission (GPM), a dual- frequency precipitation radar (DPR) is planned to be flown on the spacecraft. The spacecraft serves as a high quality reference platform for training and calibrat- ing the rain retrieval algorithms used with the passive mi- crowave radiometers on the other constellation satellites. The dual-frequency radar is expected to provide accu- rate estimates of rainfall rate as well as drop size distribu- tion (DSD) parameters from the combination of Ku- and Ka-band radar returns. This paper outlines the present status of the DPR development. Following this introduc- tion, we discuss the critical issues that affect the designing of the DPR. II. DPR REQUIREMENTS A. Relevance of the DPR to GPM The relevance of the DPR to GPM lies in the radar's ca- pability of measuring storm structure, rainfall rates, drop- size distribution (DSD), path-integrated attenuation, and other useful parameters that cannot be obtained by pas- sive sensors. In the latest design, the DPR is composed of Ku-band and Ka-band channels. The Ku-band radar is approxi- mately the same as the TRMM Precipitation Radar (PR) with some improvements. The Ka-band radar provides high sensitivity to light rain and snow. The combination of data from two channels will provide accurate estimates of drop-size distribution parameters. The Ka-band radar will sample the echo data in two different modes simulta- neously. One is a high-sensitivity mode for light rain and snow detection, and the other is a matched-beam mode in which the sampling volumes of Ka- and Ku-band radar channels are matched for collecting dual-frequency echoes from the identical targets. The data collected in the lat- ter mode are used for the estimation of DSD parameters. In the matched-beam mode, a range resolution of 250 m is employed, while in the high-sensitivity mode, a range resolution of 500 m is planned. The current radar design adopts active phased array antennas in both radar chan- nels to make full use of TRMM experience. The DPR will provide three-dimensional information of hydrometeor distribution with high spatial resolution. Such data are very valuable for the study of storm struc- ture. The accurate rainfall estimates from the DPR are expected to be used for calibrating the corresponding esti- mates from the radiometer on the core satellite. The major importance of the DPR, however, lies in the fact that it can provide the regional and seasonal statistics of storm structure together with DSD parameters. Since rain re- trieval algorithms for passive microwave radiometers have to assume a vertical structure of storm either determinis- tically or statistically, reliable storm structure information is crucial for the accuracy of rain estimation. The statis- tics from the DPR can be used as a database in radiometer algorithms to reduce the uncertainties of the storm mod- els. How to utilize the information from radar data is a challenging issue. A possibility of improving the database used in a TMI rain retrieval algorithm by using TRMM's PR data is currently under examination. The DPR has three main roles in GPM. It will provide three-dimensional information of rain structure. The Ku- band radar is similar to, but not exactly the same as, the TRMM Precipitation Radar (PR). It is improved from the PR. The improvement is necessary because of three rea- sons. Firstly, the proposed orbit of the GPM core satellite is about 400 km and higher than the TRMM's orbit. This necessitates the improvement of the sensitivity to com- pensate for the increased range loss. The designed trans- mitting power is increased to 1000 W from PR's 500 W. (the actual Tx power of the PR turned out to be about 800 W.) Secondly, the orbital inclination is about 65 de- grees and larger than the TRMM's 35 degrees. Because of the oblate shape of the Earth, the altitude of the satel- lite changes more than 20 km at a 65-degree orbit which is much larger than 10 km at a 35-degree orbit. If we use a constant pulse repetition frequency (PRF) like the TRMM PR, we have to use a rather small PRF to absorb this large variation of rain echo range from the radar. The low PRF will result in a low signal-to-noise ratio. To max-