W. L. Barnes

The Tropical Rainfall Measuring Mission (TRMM) Sensor Package

Abstract This note is intended to serve primarily as a reference guide to users wishing to make use of the Tropical Rainfall Measuring Mission data. It covers each of the three primary rainfall instruments: the passive microwave radiometer, the precipitation radar, and the Visible and Infrared Radiometer System on board the spacecraft. Radiometric characteristics, scanning geometry, calibration procedures, and data products are described for each of these three sensors.

The Status of the Tropical Rainfall Measuring Mission (TRMM) after Two Years in Orbit

The Tropical Rainfall Measuring Mission (TRMM) satellite was launched on 27 November 1997, and data from all the instruments first became available approximately 30 days after the launch. Since then, much progress has been made in the calibration of the sensors, the improvement of the rainfall algorithms, and applications of these results to areas such as data assimilation and model initialization. The TRMM Microwave Imager (TMI) calibration has been corrected and verified to account for a small source of radiation leaking into the TMI receiver. The precipitation radar calibration has been adjusted upward slightly (by 0.6 dB Z) to match better the ground reference targets; the visible and infrared sensor calibration remains largely unchanged. Two versions of the TRMM rainfall algorithms are discussed. The at-launch (version 4) algorithms showed differences of 40% when averaged over the global Tropics over 30-day periods. The improvements to the rainfall algorithms that were undertaken after launch are presented, and intercomparisons of these products (version 5) show agreement improving to 24% for global tropical monthly averages. The ground-based radar rainfall product generation is discussed. Quality-control issues have delayed the routine production of these products until the summer of 2000, but comparisons of TRMM products with early versions of the ground validation products as well as with rain gauge network data suggest that uncertainties among the TRMM algorithms are of approximately the same magnitude as differences between TRMM products and ground-based rainfall estimates. The TRMM field experiment program is discussed to describe active areas of measurements and plans to use these data for further algorithm improvements. In addition to the many papers in this special issue, results coming from the analysis of TRMM products to study the diurnal cycle, the climatological description of the vertical profile of precipitation, storm types, and the distribution of shallow convection, as well as advances in data assimilation of moisture and model forecast improvements using TRMM data, are discussed in a companion TRMM special issue in the Journal of Climate (1 December 2000, Vol. 13, No. 23).

MODIS Calibration: A Brief Review of the Strategy for the At-Launch Calibration Approach

Abstract The Moderate-Resolution Imaging Spectroradiometer (MODIS) is a key instrument on the NASA Earth Observing System. It is a multispectral sensor that will be used to track long-term global change in the land, atmosphere, and ocean components of the earth. Major advances are being made with MODIS over previous sensors in the form of improved on-orbit sensor characterization and calibration using a system of onboard calibrators. This article describes those calibrators and provides an early estimate of the expected accuracy for the MODIS calibrated datasets resulting from the use of these calibrators. The focus of the paper is the calibration approach that is being implemented at-launch for the top-of-the-atmosphere data products.

An Overview of the Visible and Infrared Scanner Radiometric Calibration Algorithm

Abstract The Visible and Infrared Scanner (VIRS) is one of the principal instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. VIRS measurements provide cloud-top temperatures and structures that complement the rainfall rates derived from other TRMM sensors. The VIRS radiometric calibration algorithm converts the digital data downlinked from the instrument into spectral radiances. VIRS has five bands: one in the visible, one in the shortwave infrared, and three in the thermal infrared. The calibration algorithm treats each band in the same manner except that the visible and shortwave infrared bands do not respond to the thermal radiation emitted by the instrument, and these bands do not have the nonlinear responses with input radiance found in the thermal bands. The calibration coefficients for the visible and shortwave infrared bands were determined in the laboratory before launch. VIRS carries a reference blackbody that is used to update the calibration coefficients for the ther...

Next-Generation MODIS for Polar Operational Environmental Satellites

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) protoflight model has been delivered to the NASA Earth Observing System AM-1 platform project to provide highly calibrated, near daily, global atmosphere, land, and ocean observation in 36 spectral channels. MODIS includes extensive in-flight calibration allowing improved environmental data products addressing the visible–infrared imaging Environmental Data Records required by the National Polar-Orbiting Operational Environmental Satellite System. NASA is considering Advanced MODIS concepts to dramatically reduce MODIS mass, power, and volume. Alternatively, next-generation MODIS Light options can substantially reduce MODIS cost, mass, power, and size but retain the core MODIS optical bench assembly spectroradiometric sensing subsystem to minimize both performance risk and changes to the data processing algorithms. These MODIS Light options range from in-flight calibration hardware removal to instrument repackaging and scanner redesign. Th...

First Results from the On-Orbit Calibrations of the Visible and Infrared Scanner for the Tropical Rainfall Measuring Mission

Abstract This work presents the first on-orbit calibration results using the Moon, the Sun, and cold deep space as inputs to the Visible and Infrared Scanner (VIRS) on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The authors have developed lunar reflectance curves using VIRS data for phase angles ranging from 1.6° to 82°. Comparisons with modeled reflectance curves show that the VIRS lunar data are as predicted. Specifically, the six-parameter model of Helfenstein and Veverka provides a good description of the VIRS 0.62-μm data. The lunar reflectance data will be used to discern long-term changes in VIRS response. Solar calibrations, performed using an onboard solar diffuser, show fluctuations of the VIRS responsivity of less than 1.3% and no indication of any systematic change during 11 months. Using the deep-space calibration implemented via a spacecraft maneuver on three different dates—7 January, 8 January, and 2 September 1998—the authors have measured the dependence of the scan mi...

Four Years of TRMM/VIRS On-Orbit Calibrations and Characterization Using Lunar Models and Data from Terra/MODIS

Abstract Four years of on-orbit solar calibration data have been used to quantify the temporal degradation of the two reflected solar bands of the Visible and Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite. Solar calibrations, performed using an onboard solar diffuser, show no significant changes in the VIRS 1.61-μm data. The responsivity (digital counts per radiance unit) of the 0.62-μm band has changed and, as a function of days since 1998, is given by R1(day) = 69.782 − 0.0022 × day, with an annual degradation of 1.15%. Four years of on-orbit lunar measurements were also examined in an attempt to quantify any degradation in the solar diffuser. The integrated lunar reflectance with phase angles ranging from 1.6° to 106° were fit with the lunar photometric model of Hapke. A good fit was made to the 0.62-μm data, but overall, the quality of the data did not support efforts to quantify any diffuser degradation. The quality of the VIRS radiometry was also examined by...

Ten Years of TRMM/VIRS On-Orbit Calibrations and Multiyear Comparisons of VIRS and MODIS

Abstract After 10 years of successful operation of the Tropical Rainfall Measuring Mission (TRMM)/Visible Infrared Scanner (VIRS), based on sensor performance, the authors have reexamined the calibration algorithms and identified several ways to improve the current VIRS level-1B radiometric calibration software. This study examines the trends in VIRS on-orbit calibration results by using lunar measurements to enable separation of the solar diffuser degradation from that of the VIRS Earth-viewing sensor and by comparing the radiometric data with two nearly identical Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the NASA Earth Observing System (EOS) Terra and Aqua satellites. For the VIRS, with spectral bands quite similar to several of the MODIS bands, the integrated lunar reflectance data were measured, from January 1998 to March 2007, at phase angles ranging from 0.94° to 121.8°. The authors present trending of the lunar data over periods of 4 yr (Aqua/MODIS), 6 yr (Terra/MOD...