Brightness Temperature

Overview

The brightness temperature is a measurement of the radiance of the microwave radiation traveling upward from the top of the atmosphere to the satellite, expressed in units of the temperature of an equivalent black body. The brightness temperature (or TB)是由被动的基本参数测量microwave radiometers. The brightness temperatures, measured at different microwave frequencies, are used at Remote Sensing Systems to derive wind, vapor, cloud, rain, and SST products. Despite differences in sensor frequencies, channel resolutions, instrument operation and other radiometer characteristics, RSS produces high-quality, carefully intercalibrated data,using uniform processing techniques, with a brightness temperature data record spanning multiple instruments over several decades. At the bottom of this page, we include information on access to our brightness temperature data and links to more detailed information.

What is Brightness Temperature?

卫星被动微波辐射仪测量原始天线计数,从中确定天线温度,然后计算地球的亮度温度。大天线用于辐射计的各种通道,并且在操作期间,每个天线进料堆通过热和冷的目标,以便提供一致的校准的原始计数。亮度温度(也称为TB)是从地球大气层顶部向上行驶的微波辐射辐射的辐射的量度。从辐射计数到大气层TB的转换称为校准过程。需要几个校准处理步骤来导出TB值。微波辐射计结核病被认为是一种基本的气候数据记录,是我们派生风速,水蒸气,云液体水,雨率和海面温度的海洋测量的价值。万博吧manbet客户端2.0

We obtain antenna temperature data files for each microwave radiometer from data sources such as NASA, NOAA, DMSP, or NRL. To ensure a climate-quality, inter-calibrated dataset of ocean products, we first reverse engineer the data in these files to raw radiometer antenna counts. This process removes corrections or adjustments that may have been added by the data provider. Once we have the raw counts, we move forward as described below.

Processing Approach

Calculating TB from raw radiometer counts is a complex, multi-step process in which a number of effects must be accurately characterized and adjustments made to account for them. These effects include radiometer non-linearity, imperfections in the calibration targets, emission from the primary antenna, and antenna pattern adjustments. RSS TB are consistently calibrated so that the TB measurements for all sensors can be used to construct a multi-decadal time series. A rain-free ocean is used as the absolute calibration reference and our state-of-the-art radiative transfer model (RTM) of the ocean and intervening atmosphere in the absence of rain can predict the top-of-the-atmosphere TB to a high degree of accuracy. A completedescription of the calibration of all SSM/I可用。虽然文档描述了SSM / I传感器,但该方法适用于其他辐射计。

Several of the steps necessary are summarized in the table below by microwave radiometer and are discussed further below.

Calibration Steps for Microwave Radiometers

Geolocation Analysis

Attitude Adjustment

Along-scan Correction

Absolute Calibration

Hot Load Correction

Antenna Emissivity

SSM/I NRL / RSS. No Yes APC. No 0
SSMIS RSS No Yes APC. Yes 0.5-3.5%
TMI. RSS Dynamic Yes APC. No 3.5%
风Sat NRL / RSS. Fixed Yes APC. Yes 0
AMSRE RSS Fixed Yes APC. Yes 0
AMSR2 RSS No Yes APC. Yes 0

The first step is geolocation. Knowing the exact location of each measurement is required for any subsequent collocations or comparisons performed. We use ascending minus descending values and look at small ocean islands and ensure they do not 'move'. Geolocation is not always performed by RSS as is shown in the table [ NRL = Naval Research Lab, GSFC = Goddard Space Flight Center]. The correction for geolocation is different from a correction for instrument mounting errors (also called roll/pitch/yaw corrections) which must also be addressed.

通过将天线温度与由我们的辐射转移模型模拟的那些进行比较来执行表中列出的剩余校正。海洋表面万博体育app网页注册和中间气氛的遥感系统的大气辐射转移模型(RTM)已经不断开发和精制30多年,在1-100 GHz(微波)谱中是对海洋观测的高度准确。海面模型组件包括偏振风速和方向,方向具有表面发射率和散射的依赖性。我们的RTM的大气组分依赖于最新和相关的氧气和蒸汽的测量。万博吧manbet客户端2.0

Attitude adjustment includes correcting for spacecraft pointing errors. Spacecraft pointing is determined by a number of different methods, the preferred being a star tracker. Another method is horizon balancing sensor. For SSM/I no pointing information was given, so it was assumed to be correct. TMI has a dynamic pointing correction that changes within an orbit because the horizon sensor used prior to the orbit boost is not as accurate as a star tracker. After orbit boost, the horizon sensor was disabled and pointing was determined from two on-board gyroscopes, also not as accurate as a star tracker. AMSR-E had no pointing problems, as the AQUA satellite had a star tracker. The AMSR on ADEOS-II needed a dynamic correction, while WindSat needed a simple fixed correction to the roll/pitch/yaw.

当镜子旋转时,在地球场景的边缘,将开始含有诸如卫星本身或冷镜的一部分的障碍物。另外,在扫描期间,天线侧链图案可能导致来自航天器的不同部分的贡献。每个仪器都需要沿扫描校正。

Next we perform an antenna pattern correction (APC). The APC is determined pre-launch and consists of spill over and cross-polarization values. After launch, the spill over and cross-polarization values are adjusted so that the measured antenna temperatures match the RTM simulated antenna temperatures. This correction is needed for all instruments.

Only some of the radiometers need hot load thermal gradient corrections. The determination of TB from counts for microwave radiometers is completed using two known temperatures to infer the Earth scene temperature. For each scan, the antenna feedhorns view a mirror that reflects cold space (a known temperature of 2.7 K) and a hot absorber measured by several thermistors. Assuming a linear response, the Earth scene temperatures are then determined by fitting a slope to these two known measurements (hot and cold). This 2-point calibration system continuously compensates for variations in the radiometer gain and noise temperatures. This seemingly simple calibration methodology is fraught with subtle difficulties. The cold mirror is relatively trouble-free as long we note when the moon intrudes on the cold space view and remove moon-affected values. The hot absorber has been more problematic. The thermistors often do not adequately measure thermal gradients across the hot absorber. For example, a hot load correction is required for AMSR-E because of a design flaw in the AMSR-E hot load. The hot load acts as a blackbody emitter and its temperature is measured by precision thermistors. Unfortunately, during the course of an orbit, large thermal gradients develop within the hot load due to solar heating making it difficult to determine the average effective temperature from the thermistor readings. The thermistors themselves measure these gradients and may vary by up to 15 K. Several other radiometers have had similar, but smaller, issues.

Lastly, the main reflector is assumed to be a perfect reflector with an emissivity of 0.0, but this is not always the case. A bias in the TMI measurements was attributed to the degradation of the primary antenna as atomic oxygen present at TMI’s low altitude (350 km) led to rapid oxidization of the thin, vapor-deposited aluminum coating on the graphite primary antenna. The measured radiation is therefore comprised of the reflected Earth scene and antenna emissions. Emissivity of the antenna was deduced during the calibration procedure to be 3.5%. The antenna emissivity correction utilizes additional information from instrument thermistors to estimate the antenna temperature, thereby reducing the effect of the temporal variance. This emissivity is constant for all the TMI channels. The SSMIS instruments also has an emissive antenna where the emissivity appears to increase as a function of frequency, changing from 0.5 to 3.5 %.

Data Availability and Access

Brightness temperatures are treated as an intermediate product, not a typical Earth Systems Data Record (ESDR). Our brightness temperature data for various instruments are available via different data centers listed in the table below.

仪器/Satellite RSS Brightness Temperature Data Availability
Table of Brightness Temperature Access by Instrument (updated Aug 2014)

SSMI on DMSP
(F08, F10, F11, F13, F14, F15)

RSS V7 TBs distributed by NOAA NCDC in netCDF format

SSMIS on DMSP
(F16, F17)

RSS V7 TBs distributed by NOAA NCDC in netCDF format
风Sat on Coriolis RSS V7 TBs not publicly available
TMI.on TRMM RSS V7 TBs not publicly available
AMSR-E on Aqua RSS V7 TBs distributed by NSIDC(注意:NSIDC在其系统中使用不同的版本号)
AMSR2 on GCOM - W1 RSS V7 TBs not publicly available

There are two documents available that further describe the contents of the netCDF RSS V7 TB data products for SSM/I and SSMIS (see links at left).

Brightness temperature data are available for the SSM/I and SSMIS sensors during the following time periods:

仪器 Start Date 截止日期
F08 SSM/I 1987年7月 Dec 1991
F10 SSM/I Dec 1990 Nov 1997
F11 SSM/I Dec 1991 2000年5月
F13 SSM/I May 1995 2009年11月
F14 SSM/I 1997年5月 Aug 2008
F15 SSM/I Dec 1999 present (do not use after Aug 2006 for climate study)
F16 SSMIS Oct 2003 present
F17 SSMIS Dec 2006 present
F18 SSMIS Oct 2009 present (data are NOT currently available at RSS)
F19 SSMIS Apr 2014 present (data are NOT currently available at RSS)