Gene A. Poe

SSM/I instrument evaluation

The Special Sensor Microwave/Imager (SSM/I) instrument and scan geometry are briefly described. The results of investigations of the stability of the gain, calibration targets and spin rate, the radiometer noise and sensitivity, the coregistration, the beam width and main-beam efficiency of the antenna beams, and the absolute calibration and geolocation of the instrument are presented. The results of this effort demonstrate that the SSM/I is a stable, sensitive, and well-calibrated microwave radiometric system capable of providing accurate brightness temperatures for microwave images of the Earth and for use by environmental product retrieval algorithms. It is predicted that this SSM/I and the 11 future ones currently built or to be built will provide high-performance microwave measurements for determination of global weather and critical atmospheric, oceanographic, and land parameters to operational forecasters and users and the research community for the next two decades. >

The WindSat spaceborne polarimetric microwave radiometer: sensor description and early orbit performance

The global ocean surface wind vector is a key parameter for short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. In addition, it affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System Integrated Program Office. It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder, which is planned to provide wind vector data operationally starting in 2010. WindSat is the primary payload on the Department of Defense Coriolis satellite, which was launched on January 6, 2003. It is in an 840-km circular sun-synchronous orbit. The WindSat payload is performing well and is currently undergoing rigorous calibration and validation to verify mission success.

Advanced microwave sounding unit cloud and precipitation algorithms

[1]xa0Although the advanced microwave sounding unit (AMSU) on board the NOAA 15 and NOAA 16 satellites is primarily designed for profiling atmospheric temperature and moisture, the products associated with clouds and precipitation are also derived using its window channel measurements with a quality similar to those derived from microwave imagers such as the Special Sensor Microwave Imager. However, the AMSU asymmetry in radiance along the scan was found to be obvious at its window channels and could severely degrade the quality of cloud and precipitation products if not properly corrected. Thus a postlaunch calibration scheme is developed for these channels, and the causes of the asymmetry are analyzed from the AMSU instrument model. A preliminary study shows that the asymmetry may be caused by either the AMSU polarization misalignment or the antenna pointing angle error. A generic radiative transfer model is developed for a single-layered cloud using a two-stream approximation and can be utilized for the retrievals of cloud liquid water (L) and total precipitable water (V), cloud ice water path (IWP), and particle effective diameter (De). At the AMSU lower frequencies the scattering from cloud liquid is neglected, and therefore the retrieval of L and V is linearly derived using 23.8 and 31.4 GHz. However, for ice clouds the radiative transfer model is simplified by neglecting the thermal emission, and therefore the retrieval of IWP and De is analytically derived using the AMSU millimeter wavelength channels at 89 and 150 GHz. These cloud algorithms are tested for the AMSU on board the NOAA 15 and NOAA 16 satellites, and the results are rather promising. It is also found that the AMSU-derived cloud ice water path is highly correlated with the surface rain rates and is now directly used to monitor surface precipitation throughout the world.

Design and Evaluation of the First Special Sensor Microwave Imager/Sounder

The first Special Sensor Microwave Imager/Sounder (SSMIS) was launched in October 2003 aboard the Air Force Defense Meteorological Satellite Program (DMSP) F-16 Spacecraft. As originally conceived, the SSMIS integrates the imaging capabilities of the heritage DMSP conically scanning Special Sensor Microwave/Imager sensor with the cross-track microwave sounders Special Sensor Microwave Temperature and Special Sensor Microwave Humidity Sounder, SSM/T-2 into a single conically scanning 24-channel instrument with extended sounding capability to profile the mesosphere. As such, the SSMIS represents the most complex operational satellite passive microwave imager/sounding sensor flown while, at the same time, offering new and challenging capabilities associated with radiometer channels having common fields of view, uniform polarizations, and fixed spatial resolutions across the active scene scan sector. A comprehensive end-to-end calibration/validation (cal/val) of the first SSMIS initiated shortly after launch was conducted under joint sponsorship by the DMSP and the Navy Space and Warfare Systems Command. Herein, we provide an overview of the SSMIS instrument design, performance characteristics, and major cal/val results. Overall, the first SSMIS instrument exhibits remarkably stable radiometer sensitivities, meeting requirements with considerable margin while providing high-quality imagery for all channels. Two unanticipated radiometer calibration anomalies uncovered during the cal/val-sun intrusion into the warm-load calibration target and antenna reflector emissions-required significant attention during the cal/val program. In particular, the tasks of diagnosing the root cause(s) of these anomalies as well as the development of ground processing software algorithms to mitigate their impact on F-16 SSMIS and hardware fixes on future instruments necessitated the construction of extensive analysis and simulation tools. The lessons learned from the SSMIS cal/val and the associated analysis tools are expected to play an important role in the design and performance evaluation of future passive microwave imaging and sounding instruments as well as guiding the planning and development of future cal/val programs.

Intersensor calibration of DMSP SSM/I's: F-8 to F-14, 1987-1997

The Defense Meteorological Satellite Program (DMSP) operational special sensor microwave imager (SSM/I) marked its ten-year anniversary on the launch date of the first SSM/I (F-8), June 19, 1987. After F-8, the DMSP has launched five more SSM/Is, F-10 (December 1990), F-11 (November 1991), F-12 (August 1994), F-13 (March 1995), and F-14 (April 1997), leaving the last SSM/I for a candidate launch in 1999. Built by Hughes Aircraft Co., these instruments have proven to be the most reliable and well-calibrated, space-based, passive microwave imaging radiometers to date, allowing the data to be used quantitatively for both operational and climatological applications. The remarkable stability of the SSM/I sensors also provides the opportunity to quantify the incremental brightness temperature differences to which the SSM/Is can be intercalibrated, thus establishing the noise floor for intercomparisons. This paper summarizes the prelaunch and postlaunch performances of each new sensor determined during calibration and validation (cal/val), starting with the formal, multiyear cal/val effort conducted by both government and public institutions under the direction of the Naval Research Laboratory (NRL) and sponsored by the joint Air Force/Navy DMSP. Sensor-specific components, orbital configuration, and systematic relative errors are examined that contribute to the total system calibration. In particular, a large (1-3 K) but correctable left-right scan asymmetry of SSM/I brightness temperatures was observed in the data and traced to an antenna field-of-view (FOV) intrusion by the spacecraft (start of scan) and a glare suppression sensor (end of scan). These effects were found to be correctable to first order using a pixel-dependent spillover correction. Empirical statistical distribution functions for rain-free ocean pixels were constructed for the entire set of SSM/Is and formed the basis for assessing intersensor calibration. Manufacturer-derived sensor-specific antenna pattern correction (APC) coefficients were found to be the source of large intersensor differences for several channels, e.g., 1-2 K for the 22-V channel.

Special Sensor Microwave Imager Sounder (SSMIS) Radiometric Calibration Anomalies—Part I: Identification and Characterization

Two calibration anomalies of the Defense Meteorological Satellite Programs (DMSP) Special Sensor Microwave Imager Sounder (SSMIS) radiometer are examined by using several sources of data. Early orbit mode data from the SSMIS are used to create radiometric images of the warm calibration load that evolve over an entire orbit to elucidate the effects of direct and reflected solar illumination of the warm-load (WL) emissive surface. Analysis of the radiometric gain and apparent WL radiometric brightness temperature observed during the solar intrusion events show the impact of these events on the SSMIS calibration. A graphical simulation of the SSMIS and DMSP spacecraft is used to define the regions where solar intrusion occurs and to characterize the WL anomalous regions for the specific DMSP F-16 orbit. The graphical simulation is also used to determine the cause of additional calibration errors that were identified by using comparisons to numerical weather prediction (NWP) models, as emission from the SSMIS reflector antenna. Mitigation of these calibration anomalies is critical if the operational SSMIS radiometers achieve their full utility in NWP, climate monitoring, forecasting, and other emerging applications. A detailed characterization of the SSMIS calibration provides a basis for this process.

Geolocation and pointing accuracy analysis for the WindSat sensor

Geolocation and pointing accuracy analyses of the WindSat flight data are presented. The two topics were intertwined in the flight data analysis and will be addressed together. WindSat has no unusual geolocation requirements relative to other sensors, but its beam pointing knowledge accuracy is especially critical to support accurate polarimetric radiometry. Pointing accuracy was improved and verified using geolocation analysis in conjunction with scan bias analysis. Two methods were needed to properly identify and differentiate between data time tagging and pointing knowledge errors. Matchups comparing coastlines indicated in imagery data with their known geographic locations were used to identify geolocation errors. These coastline matchups showed possible pointing errors with ambiguities as to the true source of the errors. Scan bias analysis of U, the third Stokes parameter, and of vertical and horizontal polarizations provided measurement of pointing offsets resolving ambiguities in the coastline matchup analysis. Several geolocation and pointing bias sources were incrementally eliminated resulting in pointing knowledge and geolocation accuracy that met all design requirements.

POLARIMETRIC EMISSION MODEL OF THE SEA AT MICROWAVE FREQUENCIES AND COMPARISON WITH MEASUREMENTS

Atwo-scale scattering model of the sea developed in terms of wind-generated stochastic processes of the surface-the elevation spectral density of the small-scale structure and the probability density of slopes of the large scale roughness-is combined with the Durden/Vesecky (1) wave height spectral model to analyze recent polarimetric measurements. Ad hoc parameter values are found for the wave model that allow the two-scale model to account for essentially all of the azimuthal features, amplitude and phase, appearing in all four Stokes parameters for the Jet Propulsion Laboratory (JPL) aircraft measurements at 19.35 and 37 GHz (2) and recent Naval Research Laboratory (NRL) aircraft measurements at 10.7 GHz. The excellent agreement provides support for the validation of the approximations of the two-scale model for the range of conditions encountered. The ad hoc parameters of the wave model are developed using the 19.35 and 37.0 GHz data and then tested with 10.7 GHz data. The two- scale model should be useful in studies dealing with simulations and retrievals of surface wind direction from satellite-based polarimetric measurements.

Geolocation Error Analysis of the Special Sensor Microwave Imager/Sounder

Geolocation errors in excess of 20-30 km have been observed in the special sensor microwave imager/sounder (F-16 SSMIS) radiometer observations when compared with accurate global shoreline databases. Potential error sources include angular misalignment of the sensor spin axis with the spacecraft zenith, sample timing offsets, nonuniform spin rate, antenna deployment offsets, spacecraft ephemeris, and approximations of the geolocation algorithm in the Ground Data Processing Software. An analysis methodology is presented to automate the process of quantifying the geolocation errors rapidly in terms of partial derivatives of the radiometer data in the along-scan and along-track directions and is applied to the SSMIS data. Angular and time offsets are derived for SSMIS that reveal the root cause(s) of the geolocation errors, while yet unresolved, are systematic, correctable in the ground processing software, and may be reduced to less than 4-5 km (1-sigma).

Calibration and Validation of DMSP SSMIS Lower Atmospheric Sounding Channels

The Special Sensor Microwave Imager Sounder (SSMIS), a new type of conically scanning microwave sounder, was launched by the Defense Meteorological Satellite Program in October 2003. Performance of the instrument and retrieval software was characterized in an extensive calibration/validation campaign. This paper describes results based on comparisons between SSMIS Lower Atmospheric Sounding (LAS) channel measurements and radiative transfer calculations based on conventional synoptic radiosondes, numerical weather prediction models, and special observations campaigns including dedicated lidar measurements and scientific radiosonde and dropsonde measurements. Retrieved lower atmospheric profiles were also directly compared with these data sources. Two significant sources of bias were identified. The emissivity of the primary reflector contributes to measured brightness temperatures, and the warm load calibration source is susceptible to uncompensated solar heating. Otherwise, it was determined that LAS channels are locally stable and accurately track atmospheric changes. Polarization of some channels was found to differ from the design. Several approaches were identified to mitigate sources of bias.