David Kunkee

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.

Achieving satellite instrument calibration for climate change

For the most part, satellite observations of climate are not presently sufficiently accurate to establish a climate record that is indisputable and hence capable of determining whether and at what rate the climate is changing. Furthermore, they are insufficient for establishing a baseline for testing long-term trend predictions of climate models. Satellite observations do provide a clear picture of the relatively large signals associated with interannual climate variations such as El Nino-Southern Oscillation (ENSO), and they have also been used to diagnose gross inadequacies of climate models, such as their cloud generation schemes. However, satellite contributions to measuring long-term change have been limited and, at times, controversial, as in the case of differing atmospheric temperature trends derived from the U.S. National Oceanic and Atmospheric Administrations (NOAA) microwave radiometers.

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 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).

Stabilization of the Brightness Temperature of a Calibration Warm Load for Spaceborne Microwave Radiometers

We present the results of a study that shows that a simple design modification is sufficient to avoid a major shortcoming in the layout of external warm loads commonly used in the calibration of spaceborne microwave radiometers. The modification consists of placing a layer of Plastazote, a polyethylene foam, over the opening of the warm load enclosure. The foam is transparent at micrometer and millimeter wavelengths, is opaque in the infrared and visible, and isolates the warm load from the environment, keeping the temperature of the radiometric warm load constant. The proposed solution can be easily implemented and is suitable even for retrofitting on instruments that have already been built but not yet launched, and the material presents no obvious shortcomings that could prevent its intended application in space.

Spectrum management for the NPOESS Conical-scanning Microwave Imager/Sounder (CMIS)

The Conical-scanning Microwave Imager/Sounder (CMIS) currently under development by the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office utilizes segments of microwave spectrum from /spl sim/6 GHz to above 190 GHz. During the development and optimization of the current CMIS baseline design, the Earth Exploration Satellite Service (EESS) (Passive) frequency allocations, and estimates of the expected RF environment played a substantial role in determining channelization. This will allow CMIS performance requirements to be met while minimizing the potential for radio frequency interference occurrences to CMIS.

Evaluation of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager Sounder (SSMIS)

The defense meteorological satellite program (DSMP) successfully launched the F-16 spacecraft carrying the first special sensor microwave imager sounder (SSMIS) on October 18, 2003. SSMIS provides for the first time, atmospheric soundings that are derived by an instrument with a constant viewing geometry, in lieu of the more traditional cross-track, dwell and step-stare instruments such as the heritage DMSP and NOAA AMSU instruments. In addition, the conical scan features constant pixel resolution yielding exciting high resolution imagery. On-orbit SSMIS data indicates that the instrument is generally performing to specification. The presence of unexpected residual calibration anomalies was found during the sensor calibration and validation (Cal/Val) period and attributed to solar illumination of the warm load tines and reflector self-emission. The warm load errors have been partially mitigated and in progress for the reflector emissions. Mitigation appears to be necessary in order to achieve the full utility of SSMIS data, particularly for the atmospheric sounding EDRs and numerical weather prediction.

Ocean surface foam modeling for passive remote sensing of ocean surface wind vectors

A method of incorporating ocean surface foam into a combined asymmetric wave geometrical optics (AWGO) ocean surface emission model using dielectric mixing theory is investigated. Dielectric mixing represents a more direct method of denying the emissivity of ocean foam within in the AWGO emission model compared with the current AWGO foam emissivity values which are based upon models derived from airborne brightness temperature measurements of ocean foam. Unfortunately, these observations may not reflect the proper sea state or other relevant conditions that are desired within the model. Using dielectric mixing, knowledge of the void fraction within a foam-covered area is used to compute the emission characteristics of a surface facet. This latter technique perhaps will allow better fidelity in ocean foam emission modeling provided that data concerning the bubble density, bubble size distributions, and foam patch locations are well known. Comparison of AWGO model brightness temperatures using the above two methods of modeling foam emissivity illustrate the sensitivity to foam in the computed azimuthal wind direction signatures.

SNPP VIIRS thermal emissive band performance after three years on-orbit

The Suomi National Polar-orbiting Partnership (SNPP) spacecrafts primary sensor is the Visible-Infrared Imaging Radiometer Suite (VIIRS) which launched on October 28, 2011. It has 22 total bands with 7 thermal emissive bands (TEBs), a high dynamic range monochromatic Day Night Band (DNB) and 14 reflective solar bands (RSBs). The TEB gain and noise performance is tracked on-orbit using an On-Board Calibrator BlackBody (OBCBB) as a thermal source. The TEBs view the OBCBB every scan allowing gain correction roughly every 1.7 seconds. Long term trending of the F factor (inversely proportional to gain) and Noise Equivalent delta Temperature (NEdT) allows the stability and uncertainty in the TEB thermal model to be evaluated. This paper will discuss the impacts of the thermal model uncertainties on the VIIRS calibration and how those impact the long term performance of VIIRS. It will also show the stability of the TEBs over 3 years on-orbit.

Foreword to the Special Issue on Radio Frequency Interference: Identification, Mitigation, and Impact Assessment

The 14 papers in this special issue cover a combination of software and hardware solutions to the Radio Frequency Interference (RFI) problem, detail the challenges in monitoring RFI, and attempt to quantify the impact that interference has on measurements.