Tetsuya Tagawa

Suppression of Surface Clutter Interference With Precipitation Measurements by Spaceborne Precipitation Radar

The sidelobe surface clutter along the nadir direction severely interferes with the rain echo in the off-nadir angle observations made using a spaceborne Precipitation Radar (PR). A new method to suppress this sidelobe clutter interference is introduced. A characteristic of the 1-D phased array antenna system is that high sidelobes arise along the beam scan plane. The proposed method tilts the antenna beam scan plane from the nadir such that these high sidelobes would not be directed along the nadir direction, along which a specular component of the backscattering radar cross section of the Earths surface is dominant. The simulation results using the designed parameters of a Ka-band spaceborne PR indicate the validity of this method, which is also quantitatively confirmed using Tropical Rainfall Measuring Mission/PR observation data sets

Improved Correction of Beam Mismatch of the Precipitation Radar After Orbit Boost of the TRMM Satellite

One effort in developing the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) data processing algorithm is to correct the PR received power for mismatched-beam data after the orbit is boosted. The current standard algorithm employs a beam-mismatch correction algorithm based on the rain-echo radar equation (Takahashis method). However, there remains a need for improving the estimated power of the mismatched-pulse echo returned from the Earths surface and weak rain because the echo is represented with the surface-echo radar equation and the noise-power level. The error in Takahashis method causes a negative bias (around 1 dB) in the PR received power near the main lobe clutter. This paper formulates an improved estimate of the mismatched pulse based on the rain-echo radar equation and the surface-echo radar equation for a flat-Earth surface. The improved estimate is then evaluated numerically and experimentally with the actual data obtained by TRMM/PR for observations over the ocean. The consistency of TRMM/PR data sets between preboost and postboost was improved by eliminating the negative bias (around 1 dB) in the PR received power. The mean rain rate near the surface-echo range increases by 10%, and the angle-bin asymmetry of the mean rainfall rate, which is present in the postboost data corrected by Takahashis method, is resolved.

Suppression of Surface Clutter Interference with TRMM Precipitation Radar Observation

A tilted antenna beam effect is analyzed with the Tropical Rainfall Measuring Mission/Precipitation Radar (TRMM/PR) observation datasets during the TRMM satellite orbit maneuver. A concept of the tilted antenna beam effect is devised to suppress surface clutter interference with precip- itation measurement by spaceborne precipitation radar. At the instant the antenna beam is scanning off-nadir direction, side lobe clutter interferes a rain echo severely because the strong side lobe illuminates the nadir direction, where the specular backscattering on sea/land surface occurs. The radiation pattern of the slotted waveguide phased array antenna, which is employed for TRMM/PR and GPM/DPR, is characteristic in that the region of the strong side lobe arises in crisscross. The devised method to suppress the side lobe clutter is to tilt the antenna beam by a few degrees in coordinate plane determined by the satellite flight direction and the nadir direction. By tilting the antenna beam, the strong side lobe illuminates the off nadir direction. The antenna of the TRMM/PR is fixed to the satellite, thus the antenna beam could not be tilted except for the situation that the TRMM satellite changes its attitude, in other word, changes its pitch angle. In August 2001, the TRMM satellite operating altitude was boosted from 350 kilometers to 402.5 kilometers to extend the satellite life time. The satellite attitude fluctuated from usual position to various roll/pitch/yaw angles during the orbit maneuver. The effect of tilted antenna beam is studied by analyzing TRMM/PR 1B21 received power datasets during the orbit maneuver. 1B21 is a data product including PR received power. In analysis of TRMM/PR 1B21 datasets, received power distribution in scan plane (in other word, vertical cross section) is averaged to increase Signal to Noise ratio (S/N), where the S means the received power from sea surface, and N means the receiver noise power. After averaging of signal, the observed backscattered power from sea surface is obtained. A numerical received power from sea surface is calculated with sea surface scattering coefficients, assuming the antenna pattern of Ku-band Precipitation Radar. Then the observed backscattered power from sea surface is compared with the calculated sea surface echo to verify the propriety of the tilted antenna beam effect. The results show that the observed backscattered power through an antenna side lobe becomes weaker as a pitch angle(of the TRMM satellite) becomes larger. The calculated sea surface echo through the antenna sidelobe also becomes weaker by increasing a pitch angle of the antenna in calculation.

Measurement of scattering coefficient dependence on soil moisture content and surface roughness by 35 GHz polarimetric scatterometer

Polarimetric measurements of scattering coefficient sigma0 of a bare soil were performed changing a roughness of a soil surface and a soil moisture content using the 35 GHz polarimetric scatterometer. At Ka band, a few experimental results of sigma0 data are available. One of the purpose of this experiment is to obtain the surface backscattering characteristics to evaluate surface clutter interference with precipitation measurement from space using the Dual frequency Precipitation Radar (DPR, 13.8 GHz and 35.5 GHz), which is planned to be onboard the Global Precipitation Measurement (GPM) Mission core satellite to observe precipitation globally. In evaluation of surface clutter interference, sigma0 data for various surface conditions are needed, especially sigma0 dependence on soil moisture content and surface roughness. Another purpose of this experiment is to apply the measured sigma0 data to estimate a soil moisture content globally after the launch of the GPM core satellite. An angular scan range of the DPR is from nadir direction though the incidence angle of 8.4 degrees (with the 35.5 GHz radar) or 17 degrees (with 13.6 GHz radar), so the measured data by the DPR is useful to observe the earth surface condition globally. In this study, the roughness of the soil surface was measured with a laser profile meter to determine the roughness dependence of the scattering coefficient. A soil moisture content was measured comparing the weight of the soil before and after the heating of the soil. The soil was heated enough to contain no water. To increase the number of the independent samples for each experimental conditions (soil moisture content/surface roughness/incidence angle), azimuthal angle is changed like clockwork using the turntable of 2 m diameter. The system of the scatterometer is network analyzer based polarimeter

Suppression of surface clutter interference with precipitation measurement from space by the Dual frequency Precipitation Radar

A new method to suppress the surface clutter interference with precipitation measurement from space by the Dual frequency Precipitation Radar (DPR, 13.8 GHz and 35.5 GHz) is introduced for the Global Precipitation Measurement (GPM) mission, which is planned in succession to the Tropical Rainfall Measuring Mission (TRMM). The DPR has very high sensitivity and its minimum detectable rain rate is designed to be 0.3mm/h(attained by the 35.5 GHz radar) at the rain top. In this study, the radiation pattern of the slotted wave guide planar phased array antenna was calculated by considering the Taylor distribution with random errors in excitation current. The signal (S) to clutter (C) power ratio (S/C ratio) was evaluated for the antenna pattern given by the Taylor distribution (designed peak side lobe level=-35 dB, n=6, ; these values are same as the TRMM PR), where the S means received power from rain scattering volume, and the C means the backscattered power from sea/land surface. A uniform rain rate of 0.3 mm/h was assumed for the calculation of signal S at 35.5 GHz and 0.5 mm/h for S at 13.6 GHz. A side lobe clutter interferes the rain echo severely when the strong side lobe illuminates the nadir direction, where the specular component of the scattering coefficient of sea/land surface is dominant. The introduced method to suppress the side lobe clutter is to tilt the antenna beam a few degrees in coordinate plane determined by the satellite flight direction and the nadir direction. The radiation pattern of the phased array antenna is characteristic in that the region of the strong side lobe arises in crisscross. By tilting the antenna beam, the strong side lobe illuminates the off nadir direction. So that makes it possible to suppress the side lobe clutter. Calculation results show that the surface clutter interference is suppressed well at the main beam tilt angle 2 degrees.

Calculations of surface clutter interference with precipitation measurement from space by 35.5 GHz radar for Global Precipitation Measurement Mission

Surface clutter interference with precipitation measurement from space using 35.5 GHz radar was evaluated for the Global Precipitation Measurement (GPM) Mission. The GPM Mission is unique in that it consists of a core satellite with dual-frequency precipitation radar (13.6 GHz and 35.5 GHz) and eight small companion satellites that are equipped with microwave radiometers. The 35.5 GHz precipitation radar has very high sensitivity; its designed minimum detectable rain rate at the rain top is 0.3 mm/h. In this study, a Taylor distribution with random errors in the excitation current is considered in calculating the radiation pattern of a 35.5 GHz slotted waveguide planar phased array antenna. The signal-to-clutter power ratio (S/C) was evaluated for the antenna pattern given by the Taylor distribution (peak side lobe level=-35dB, n=6; the same values as for the TRMM PR), where S is the received power from the rain scattering volume and C is the backscattered power from sea surface. Uniform rain rates of 0.3 and 1.0 mm/h were assumed in the calculation of S. We show that the interference of surface clutter with precipitation measurement can be suppressed more at 35.5 GHz than at 13.6 GHz because of the short wavelength. The calculated S/C ratio distribution showed that the effect of the side lobe clutter is not negligible, especially for low rain rates (less than 1.0 mm/h), but it is negligible for heavier rain (over 1.0 mm/h). The calculations also show that the effect of the main lobe clutter is severe and not negligible for either light or heavy rain. The conclusion is that 35.5 GHz precipitation radar can accurately observe rain with a planar phased array antenna fed with a Taylor distribution (n=6, peak side lobe level=-35 dB).

Modification of the beam mismatch correction algorithm

In August 2001, the orbit of the Tropical Rainfall Measuring Mission (TRMM) satellite has been boosted from the altitude of 350 km to 400 km to extend its life time. The time delay between transmission of pulses and reception from rain/surface echoes occurs by one inter pulse period (IPP) (Takahashi and Iguchi, 2004). Due to this time delay, one pulse transmitted at N - 1th angle bin is received at the receiving time gate of the next angle bin N. So the mismatch occurs between the transmitting antenna beam direction and the receiving antenna beam direction. This causes antenna gain reduction by 6 dB and leakage of rain/surface echo from the N - 1th angle bin to the Nth angle bin. In this paper, the modified beam mismatch correction algorithm is tested numerically, then its applied to the standard algorithm of TRMM/precipitation radar to reduce the correction error of the current beam mismatch correction algorithm.

Measurement of scattering properties of vegetation at Ka-band by 35GHhz polarimetric scatterometer

Polarimetric measurements of scattering properties of vegetation were performed using the 35 GHz polarimetric scatterometer. The miniature forest is constructed inside the experimental laboratory to measure the scattering dependence on forest biomass at Ka-band by changing the degree of density of trees. A soil is underlying the trees and soil moisture content is also changed to find out if soil moisture content can be observed to what extent leaf area index (LAI) becomes large. As a result of increasing density of trees, the volume scattering in vegetation layer becomes larger as LAI increases and the scattered wave from soil is reasonably more attenuated. Polarization ratio HH/VV of received power is calculated to study polarization difference in attenuation. The results show that HH polarized electric field penetrates into vegetation and reaches soil surface more strongly than VV polarized electric field.