David J. Dowgiallo

APMIR: airborne polarimetric microwave imaging radiometer

APMIR is a new instrument that is currently under construction at the U.S. Naval Research Laboratory in Washington, D.C. The system will include channels from C- to Ka-band (specifically 6.6, 6.8, 7.2, 10.7 or 10.65, 18.7 or 19.35, 22.23 or 23.8, and 37.0 or 36.5 GHz). Each channel will provide verticallyand horizontally-polarized brightness temperatures, and several (10.7, 18.7, and 37.0 GHz) will be fully polarimetric. This instrument is being built to provide calibration data for two satellite programs, as well as for supporting algorithm development. The first satellite is the Defense Meteorological Satellite Programs SSMIS. The second instrument is the Coriolis WindSat, a joint Navy, NPOESS, and Air Force mission. Additionally, APMIR has many of the same frequency bands as AMSR-E. Though the system has been developed primarily for the calibration effort, a great deal of flexibility has been incorporated, such that other missions can be accommodated. These satellite instruments have extremely tight error budgets, which in turn lead to correspondingly formidable error budgets for the instruments involved in calibration. A new aircraft system, planned and built from the ground up with the design philosophy of controlling the errors that are critical to the calibration of these particular instruments, was thought to be the best way to meet the error budget. An extra degree of care has been exercised in all areas, but particularly antenna pointing determination and control, system environmental control, EMI suppression and power supply stability.

Aerial Radiometric and Video Measurements of Whitecap Coverage

This paper presents the results of high-altitude microwave radiometric and video measurements in the presence of breaking waves made during the passage of Hurricane Dean on August 21, 2007, over the Gulf of Mexico. Previous measurements of foam fraction and radiometric brightness temperature have focused on the small scale, in which individual foam patches were of the same scale as the radiometer footprint. To work with data from spaceborne microwave radiometers, which have footprints on the scale of tens of kilometers, the knowledge of how the foam fraction sensitivity of brightness temperature scales when footprints increase from meters to kilometers is necessary. Video images of the sea surface recorded with a high-resolution monochrome digital camera were used to determine the foam fraction. Ocean-surface brightness temperature was measured with the Airborne Polarimetric Microwave Imaging Radiometer (APMIR) of the Naval Research Laboratory at frequencies of 6.6 [vertical and horizontal (VH) polarizations], 6.8 (VH), 7.2 (VH), and 10.7 GHz (V), with full polarimetric brightness temperatures measured at 19.35 and 37.0 GHz. Collocated nearly contemporaneous brightness temperatures were available from WindSat, Special Sensor Microwave Imager/Sounder, and Special Sensor Microwave/Imager satellite radiometer overpasses. Oceanographic and meteorological data were taken from buoys located along the flight track. There was good correlation between brightness temperatures measured with APMIR and satellite-borne radiometers with absolute differences largely within the expected uncertainty of the data. An analysis of the video imagery provided the fractional area coverage of the actively breaking waves on the ocean surface. The increase in brightness temperature from each of the microwave sensors was correlated with the whitecap coverage measured by the camera. The experiment not only serves as an important bridge between measurements made with spatial scales on the order of tens of meters and data collected from satellites with spatial scales of tens of kilometers but also provides guidance for improving future field measurements on this topic.

Whitecap lifetime stages from infrared imagery with implications for microwave radiometric measurements of whitecap fraction

Quantifying active and residual whitecap fractions separately can improve parameterizations of air-sea fluxes associated with breaking waves. We use data from a multi-instrumental field campaign on FLoating Instrument Platform (FLIP) to simultaneously capture the signatures of active and residual whitecaps at visible, infrared (IR) and microwave wavelengths using, respectively, video camera, mid-IR camera, and a radiometer at 10 GHz. We present results from processing and analyzing IR images and correlating this information with radiometric time series of brightness temperature at horizontal and vertical polarizations TBH and TBV. The results provide evidence that breaking crests and decaying foam appear in mid-IR as bright and dark pixels clearly distinguishing active from residual whitecaps. We quantify the durations of whitecap lifetime stages from the IR images and identify their corresponding signatures in TB time series. Results show that TBH and TBV vary in phase during the active and in anti-phase during the residual whitecap stages. A methodology to distinguish active and residual whitecaps in radiometric time series without a priori IR information has been developed and verified with corresponding IR and video images. The method uses the degree of polarization P (the ratio between the sum and difference of TBV and TBH) to capture whitecaps as prominent spikes. The maximum and zero-crossing of the first derivative of P serve to identify the presence of active whitecaps, while the minimum of dP marks the transition from active to residual whitecap stage. The findings have implications for radiometric measurements of active and total whitecap fractions. This article is protected by copyright. All rights reserved.

Satellite calibration and validation utilizing the Airborne Polarimetric Microwave Imaging Radiometer (APMIR)

The Airborne Polarimetric Microwave Imaging Radiometer (APMIR) was designed and built by the Naval Research Laboratory, Washington, D.C., as a tool for calibration and validation of the Coriolis WindSat and Defense Meteorological Satellite Program SSMIS satellite radiometer sensors. The satellite sensor data aids in short-term weather modeling and general climatic information via measurements of ocean winds, water vapor, soil moisture, rain rates, and ice/snow characteristics. The APMIR sensor was designed to assist in the fulfillment of the airborne portion of the calibration/validation effort. By underflying a satellite, the airborne sensor data is coincident in space and time with the satellite, allowing an accurate comparison of the viewed scene. Extensive testing of the APMIR system included tip curves, pool views, and radiative transfer model comparisons. The encompassing test methods achieve a high degree of confidence in the APMIR system. APMIR underflew the SSMIS satellite during a March/April 2004 field campaign. Based on the high confidence of the APMIR sensor, and the excellent data comparison of the SSMIS satellite, the APMIR instrument demonstrates itself as a precision tool for calibration and validation activities. The results of the underflights during the March/April 2004 field campaign for the SSMIS satellite are presented.

APMIR: an airborne polarimeter designed for high accuracy

The Airborne Polarimetric Microwave Imaging Radiometer (APMIR) has been developed at the Naval Research Laboratory. This instrument was designed primarily as a calibration tool for satellite sensors. As such, the system design began with a challenging error budget. The design and construction followed from the error budget. The system has flown several times. This paper focuses on the design of the instrument and preliminary results.

Millimeter wave interferometric radiometry for passive imaging and the detection of low-power manmade signals

Millimeter wave detection and imaging is becoming increasingly important with the proliferation of hostile, mobile millimeter wave threats from both weapons systems and communication links. Improved force protection, surveillance, and targeting will rely increasingly on the interception, detection, geo-sorting, and the identification of sources, such as point-to point communication systems, missile seekers, precision guided munitions, and fire control radar systems. This paper describes the Naval Research Laboratorys (NRL) demonstration broadband passive millimeter wave (mmW) interferometric imaging system. In addition to limited active signal detection, the Ka-band system will provide the potential for detecting the passive signature of non-transmitting hostile systems along with a capability for meter-precision geolocation for imaged objects. The interferometer uses a distributed array of 12 antenna elements to synthesize a large aperture. Each antenna is packaged into an individual receiver, from which a baseband signal is recorded. The correlator is software-based, utilizing signal processing techniques for visibilities, and image formation via beamforming methods.

Azimuthal Variation of the Emissivity of Foam From C and X Band Polarimetric Measurements

Sea foam increases surface emission and brightness temperature at microwave frequencies. Together with the surface roughness, it is a key component of the signal used to obtain surface wind vector with satellite-borne radiometric and polarimetric instruments. Current knowledge of foam emissivity, however, is incomplete, particularly in regard to azimuthal effects. Since breaking waves on the open ocean are intermittent and highly variable, radiometric measurements of reproducible breaking waves were performed in an outdoor salt water wave tank in 2002 and 2004. The wave tank was configured to create foam-producing breaking waves using an underwater shoaling beach. Four radiometers, operating at frequencies of 6.8, 10.8, 18.7, and 37 GHz, were positioned at several incidence angles and azimuth look angles to record microwave emission from the breaking waves. A bore-sighted video camera recorded images of foam fraction within radiometers footprints. Auxiliary information on the wave field and breaking events (such as void fraction, bubble size spectrum, foam layer thickness, wave height, and subsurface turbulent dissipation) was provided by a suite of additional instruments. The results of wave tank measurements of the azimuthal dependence of foam emissivity at 6.8 and 10.8 GHz are presented, and possible contributors to the observed azimuthal changes of foam emissivity are discussed

Calibration and validation activities of the Airborne Polarimetric Microwave Imaging Radiometer - APMIR

The Airborne Polarimetric Microwave Imaging Radiometer (APMIR) was designed and built by the Naval Research Laboratory as a tool for calibration and validation (cal/val) of the Coriolis WindSat and Defense Meteorological Satellite Program SSMIS satellite radiometer sensors. During March and April of 2004, APMIR underflew both WindSat and SSMIS a number of times as part of the respective cal/val campaigns. A subset of the results is presented here.

Radiometric measurements of the microwave emissivity of foam

Radiometric measurements of the microwave emissivity of foam were conducted in May 2000 at the Naval Research Laboratorys Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were made at 36.5 GHz; horizontal, vertical, +45, -45, left circular, and right circular polarization measurements were obtained at 10.8 GHz. Surface foam was generated by blowing compressed air through gas permeable tubes supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine beam-fill fraction of the foam generator.

An economical and highly accurate attitude and position determination system for airborne polarimetric sensors

An economical and highly accurate Attitude and Position Determination System (APDS) has been investigated for aircraft polarimetric science. The trend towards smaller and less expensive aircraft platforms makes such a system attractive and cost efficient. The list price is a fraction of the cost of popular gyroscope based systems typically used for attitude determination, but with comparable attitude accuracy. The APDS incorporates four GPS antennas in a baseline configuration that, when coupled with horn mounted inclinometers for ground based alignment, and resolvers on the positioning system, achieves a system angular accuracy of better than +/- 0.07/spl deg/, and positional information to better than 10 meters. Additionally, GPS time tagging and velocity information is available.