High frequency microwave and millimeter radiometers with low noise amplifier front-ends commonly suffer from gain instability, or so-called “flicker” noise. This noise has a 1/fenergy spectrum and hence is also commonly referred to as 1/fnoise. The effect of this noise on a passive instrument is to degrade its sensitivity and introduce post-processing calibration errors such as ‘striping’. In this paper we present a 1/fnoise mitigation technique using a combination of single frequency tone injection and high spectral resolution digital signal detection. This technique can be used in radiometers with sufficient information redundancy so that a limited portion of the detected signal spectrum can be dedicated to noise mitigation. A key requirement of implementing this technique is application specific integrated circuit (ASIC) or field programmable gate array (FPGA)-based spectral decomposition of the radio frequency energy. A proof-of-concept hardware setup and signal processing steps required to implement such a technique are presented in this paper. Measurements presented here show a reduction up to 87 % in 1/fnoise energy using this technique and are applicable to airborne and ground-based instruments.

We present a concept for UHF/L-band (0.5–2 GHz) remote sensing of Antarctic ice sheet internal temperature using a highly sparse synthetic aperture radiometer constellation. This concept leverages the relative stability of ice sheet thermal emission over long temporal periods to gradually assemble a collection of array baselines which are jointly transformed to develop large image facets. We formulate a calculation of minimum array complexity based on the desired sensitivity, spatial resolution, and time available for observations. We determine from this calculation that such a system can achieve 1–10-km spatial resolution (significantly finer than the program of record) over monthly to yearly timescales with as few as 10–20 elements; even fewer elements are required for observing only the ice sheet center. The inverse problem of reconstructing image facets from mixed-pointing and mixed-configuration observations is posed using a Fourier domain data constraint with a total variational regularization in the image domain. This approach enables image formation from heterogeneous observations while mitigating artifacts. We present a notional constellation design for three satellites which could accomplish the necessary baseline sampling by rotating the phase and semimajor axis of spacecraft relative positions in planar circular orbits (PCOs). We demonstrate image formation by observing system simulations leveraging predictions of Antarctica’s multiwavelength brightness temperature computed from ice sheet thermomechanical and radiative transfer models.

We present the STASIS concept, an innovative approach to developing high spatial resolution maps of Antarctic ice sheet thermal emission at P/L band. Rather than using a large real aperture system, the relative stability of ice sheet temperature over time implies that a sparse array system would be able to gradually build up spatial frequency sampling and generate images with 1K sensitivity at 1-10 km spatial resolution over monthly-seasonal time scales. This contrasts with the requirement for full snapshot spatial frequency coverage required by systems for monitoring soil moisture and ocean salinity Sensitivity heuristic calculations are presented, and simulated interferometric observations are generated incorporating a realistic ice sheet thermal emission model.

The Compact Ocean Wind Vector Radiometer (COWVR) is the first space-based radiometer to utilize electronic polarization basis rotation (EPBR) to measure a swath of polarimetric brightness temperatures without the need to scan the receiver electronics. The COWVR instrument also utilizes internal calibration with noise diode standards to facilitate implementation of the simplified conical-scanning radiometer design. Following successful Calibration of the instrument on-orbit by the multi-agency COWVR Cal/Val team, data provided by the mission Ground Data Processing System (GDPS) was assessed. An analysis of COWVR Wind Speed and Direction performance was performed by Aerospace utilizing matchups with wind retrievals from the Advanced Scatterometer (ASCAT) on Metop-B and -C. This analysis found that COWVR meets its Ocean Surface Vector Wind requirements established for the COWVR mission.

Venus holds the key to understanding rocky planet evolution in our solar system and beyond. As Earth’s "twin" in terms of size, mass and density and sitting within the habitable zone one might expect that the two planets would evolve very similarly. This expectation is quite erroneous with Venus surface and atmospheric conditions being very different than the Earth. To understand how Venus evolved so differently than the Earth ESA and NASA are sending three missions, EnVision, VERITAS and DaVinci to Venus in the 2030s. ESA’s EnVision mission has a synthetic aperture radar, VenSAR, provided by NASA designed to peer beneath the optically opaque atmosphere and provide high resolution imagery and topography of the surface. Here we describe the expected performance of the VenSAR instrument for its various modes of operation.

We present here the on-going design of a hyperspectral radiometer called HyperSounder operating near two Oxygen absorption lines at 60 and 118 GHz, and the water vapor absorption line at 183 GHz for airborne atmospheric sounding. This radiometer is designed to be installed onto a Gulfstream V (G550) aircraft operated by the National Oceanographic and Atmospheric Administration’s (NOAA) Office of Marine and Aviation Operations (OMOA). The key enabling technology used in this instrument is fast sampling ASIC based spectrometer chipsets that allow for wide-band width and high resolution signal detection.

The objective of this study is to compare microwave brightness temperature observations performed by the Temporal Experiment for Storms and Tropical Systems (TEMPEST) Space Test Program-Houston 8 (TEMPEST- H8) with those from the Global Precipitation Measurement (GPM) Microwave Imager (GMI). This study utilizes TEMPEST-H8 and GMI observations over tropical cyclones (TCs). Brightness temperature (TB) observations from the TEMPEST-H8 165 GHz channel and the GMI 166 GHz horizontal polarization channel were used in the cross-comparison study. Three tropical cyclones, including Tropical Cyclone Batsirai, Typhoon Mawar and Hurricane Hilary, occurring over a variety of oceanic regions, were analyzed. The cross-comparison results from all three cases showed that TC size and rain band structures appear similar in TEMPEST-H8 and GMI observations. The average correlation coefficient (r) values between the observations from the two instruments are 0.85 for TC Batsirai, 0.88 for Typhoon Mawar and 0.93 for Hurricane Hilary. These results demonstrate that the TEMPEST-H8 small-satellite sensor, which is nearly identical to TEMPEST-D, performs similarly to that of a traditional science mission sensor, i.e. GPM GMI.

We present the development of a hyperspectral microwave radiometer for high-altitude balloon deployment to better observe the Planetary Boundary Layer (PBL) with unprecedented sensitivity. The new hyperspectral radiometer is based on the existing High-frequency Airborne Microwave and Millimeter-wave Radiometer (HAMMR) instrument. Our key innovation from existing microwave sounders is increasing the number of channels around the oxygen and water-vapor absorption lines with high-resolution spectral sampling, thereby reducing profile retrieval errors and providing wide-band coverage over the frequency range of 18-200 GHz. In this paper, we present the overall system design, including the Stratollite subsystem, the full optical subsystem, and on-going RF and IF subsystems testing. We also show preliminary outdoor radiometric test results using a complete 48-72 GHz radiometer system. The hyperspectral HAMMR-HD balloon experiment will acquire up to 30 days of high-resolution PBL data, over a variety of weather conditions, and over land and ocean.

We discuss an approach to measuring high-resolution maps of Antarctic ice sheet temperatures using repeat-pass sparsely sampled microwave interferometry. This approach follows from the inference that the relative invariance of ice sheet temperatures on annual timescales obviates the need for high snapshot sensitivity imposed as a requirement for observing more variable regions of the Earth system with interferometers such as SMOS. Such measurements could hypothetically be conducted with spatial resolutions less than 10 km using a small constellation of satellites. We discuss specifically how modifications to sheet-base geothermal heat flux could manifest as observable thermal signatures and a strategy to form images from a mosaic of multiple heterogeneous sparsely sampled observations, and we conclude with comments on necessary areas for future investigations.

The principal objective of this study is to develop machine learning (ML) models, in particular a random forest (RF) classifier and an artificial neural network (ANN) regression model, for estimating surface rain rates on a global basis using brightness temperature (TB) observations from the Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) CubeSat. The accuracy of the models is assessed by comparing the estimated rain rates with the Integrated Multi-satellitE Retrievals for GPM (IMERG) final run rain rate product, which also serves as the ground truth for ML model development. The ML models are developed using a dataset consisting of TEMPEST-D observations of 12 tropical cyclones (TC) and the corresponding IMERG products from various locations around the globe, including the Atlantic, eastern/western Pacific and Indian Oceans. To evaluate the performance of the ML models, independent validation is conducted using Hurricane Isaac and Typhoon Hagibis. The structural similarity index measure (SSIM) is used to assess the similarity between the ML estimated rain rates and the IMERG products. For Hurricane Isaac, an SSIM score of 0.8 is achieved, indicating a strong resemblance to the IMERG product. Similarly, for Typhoon Hagibis, the SSIM score is 0.73, indicating quite good agreement with the IMERG rain rate.

We present the Forward Ocean Atmosphere Microwave (FOAM) radiative transfer model, a Python package designed to simulate passive microwave observations of ocean state, and discuss its basic architecture. FOAM is particularly useful for exploring concept architectures for future missions measuring ocean salinity from space. We discuss how FOAM can be used to design missions meeting a range of accuracy and revisit requirements, and we consider an example of ocean salinity retrieval with a wideband radiometer system.

For understanding englacial hydrology and its impact on ice sheet mass balance, observations of the liquid water content (LWC) within the ice sheets are needed. Earlier studies have shown the complementary nature of multi-frequency microwave radiometer measurements to detect subsurface LWC distribution in addition to surface LWC, which is critical for understanding the seasonal melt dynamics of ice sheets. In this study, we used 1.4 GHz brightness temperature (TB) measurements from the NASA Soil Moisture Active Passive (SMAP) satellite, and 6.9, 10.7, 18.9, and 36.5 GHz TB measurements from the JAXA Global Change Observation Mission-Water Shizuku (GCOM-W) satellite to investigate the multi-frequency response at pan-Greenland scale. The melt indications derived at different frequencies show trends consistent with persistent seasonal subsurface melt water and delayed subsurface refreezing of the seasonal melt water. The result suggests that the seasonal subsurface persistent melt water occurrences that are not captured by the high-frequency retrievals are both temporally and spatially very significant.

The ultra-wideband spectroradiometer instrument aims at measuring the brightness temperature gradient in the upper lunar regolith using a wideband passive microwave spectrometer covering a continuous band from 300 MHz to 6 GHz. As a part of the Development and Advancement of Lunar Instrumentation (DALI) program, the designed ultra-wideband spectrometer is expected to provide lunar heat flux measurements. Difficulty in RF matching across the ultra-wideband and lack of isolators covering the large bandwidth make it challenging to design and calibrate the instrument. The heat-flow spectroradiometer instruments employ internal calibration sources for tracking and detecting mismatch changes in addition to gain variations measurements for stable and reliable radiometric operation.

The objective of this study is to cross-validate observations of the Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) CubeSat mission with those observed by the Global Precipitation Measurement (GPM) Microwave Imager (GMI) [1] over precipitating systems. The purpose of this paper is twofold: first, to show consistency between TEMPEST-D and GPM/GMI, and second, if the measurements are consistent, to demonstrate the potential to enhance temporal sampling when the TEMPEST-D and GPM/GMI observations are merged. This paper demonstrates both objectives and shows good agreement between TEMPEST-D and GPM/GMI observations.

This paper presents some of the first nearly simultaneous observations between TEMPEST-D and RainCube, two CubeSat missions supported by NASA for on-orbit validation of technology for studying the Earth's atmosphere. This paper presents simultaneous observations by a Ka-band radar and multi-frequency millimeter-wave radiometers over precipitation systems, in three widely dispersed locations over the globe. The first storm was near Mexico's Pacific coast, whereas the second storm was over the South Pacific Ocean near the Solomon Islands, and the third storm was near Houston, Texas, USA. The comparisons showed good physical consistency between the TEMPEST-D and RainCube observations.

Temporal Experiment for Storms and Tropical Systems-Demonstration (TEMPEST-D) is a 6U CubeSat satellite with a cross-track scanning millimeter-wave radiometer measuring at five frequencies from 87 to 181 GHz. It employs a direct-detection architecture with InP HEMT monolithic microwave integrated circuit (MMIC) low-noise amplifiers and related new technologies. An end-to-end two-point external calibration is performed every 2-s rotation of the scanning mirror, based on observations of the cosmic microwave background and an internal blackbody calibration target, with three thermistors to monitor the target physical temperature. Corrections for antenna pattern effects and cross-scan biases based on prelaunch measured values were updated using data from an on-orbit calibration pitch maneuver. Validation of the observed brightness temperatures ( TB) is performed by comparing to coincident nonprecipitating ocean observations from five well-calibrated on-orbit instruments, including Global Precipitation Measurement (GPM) mission Microwave Imager (GMI) and four Microwave Humidity Sounder (MHS) sensors on board NOAA-19, MetOp-A, MetOp-B, and MetOp-C satellites. Absolute calibration accuracy is within 1 K for all channels, well within the 4-K requirement. Calibration precision, or stability over time, is within 0.6 K for all channels, also well within the 2-K requirement. The intrinsic noise of TEMPEST-D is lower than MHS, resulting in similar on-orbit noise equivalent differential temperatures (NEDTs), even though TEMPEST-D has a much shorter integration time of 5 ms as compared to 18 ms for MHS. As a result, although the TEMPEST-D radiometer is substantially smaller, lower power, and lower cost than similar current operational radiometers, it has comparable or better performance in terms of instrument noise, calibration accuracy, and calibration stability or precision.

The Temporal Experiment for Storms and Tropical Systems Technology Demonstration (TEMPEST-D) instrument is a five-frequency millimeter-wave radiometer operating from 87 to 181 GHz. The cross-track scanning radiometer has been operating on a 6U CubeSat in low Earth orbit since September 5, 2018. The direct-detection architecture of the radiometer reduces its mass and power consumption by eliminating the need for a local oscillator and mixer, also reducing system complexity. The instrument includes a scanning reflector and ambient calibration target. The reflector rotates continuously to scan the antenna beams in the cross-track direction, first across the blackbody calibration target, then toward the Earth over the full range of incidence angles, and finally to cosmic microwave background radiation at 2.73 K. This enables precision end-to-end calibration of the millimeter-wave receivers during every 2-s scan period. The TEMPEST-D millimeter-wave radiometers are based on 35-nm indium phosphide (InP) high-electron-mobility transistor (HEMT) low-noise amplifiers. This article describes the instrument and its characterization prior to launch.

TEMPEST-D is a 6U CubeSat with a payload of a 5-channel millimeter wave cross-track scanning radiometer. It is a technology demonstration mission with requirements of 2 K precision and 4 K absolute calibration. Since its deployment from the International Space Station in July of 2018, the TEMPEST-D team has focused efforts on validating the calibration of the instrument by comparing with similar well-calibrated operational sensors. Such comparisons have shown the instrument to be very well calibrated and stable, with very low noise, well within the requirements. Efforts have subsequently focused on demonstrating that the data can be used for various science applications, including water vapor and cloud water/ice retrievals and data assimilation.

A new wideband radiometer covering P/L-band was developed at the Jet Propulsion Laboratory for polar ocean salinity and seasonal sea-ice thickness measurements. The instrument was deployed on the US Coast Guard Cutter Healy for an Arctic Ocean research cruise from September 13, 2018 to October 20, 2018. This work shows the first results relating sea ice thickness obtained from the measurements taken with the wideband P/L-band radiometer during the campaign. Results from the Artic cruise campaign were also used to study wideband spectral properties of salinity. In addition to this paper, Salinity and wide-band calibration challenges are presented in two other companion papers.

A new ultra-wide P/L-band radiometer instrument has been developed at the Jet Propulsion Laboratory for polar ocean salinity and seasonal sea-ice thickness measurements. The Arctic field campaign performed with the instrument deployed on the US Coast Guard Cutter Healy from September 13, 2018 to October 20, 2018. A new calibration strategy is developed for the ultra-wide band instrument to minimize the mismatch related effects. A noise-wave model is built to analyze and validate the instrument behavior for the calibration. The calibration strategy is analyzed using the results from the cruise campaign. Sea-ice thickness and sea surface salinity are presented in two other companion papers.

The demand for microwave spectrum for commercial and industrial use has been increasing rapidly over the last decade, putting stress on the limited spectral resources for passive microwave remote sensing. Radio frequency interference from man-made sources is expected to become worse over the coming years. At 1.4 GHz, the SMAP mission has implemented and demonstrated advanced interference detection algorithms for its microwave radiometer. This scheme will not be feasible at higher microwave frequencies (above 6 GHz) due to much larger radiometer bandwidths used and the limited downlink data volume available to implement RFI filtering algorithms in the ground processing. In this paper, we present the design, development, and test of an advanced on-board interference detection and RFI filtering digital back-end that is capable of operation for a 1 GHz-radiometer bandwidth. We describe the combined RFI detection algorithms implemented in the digital backend's firmware and the on-board RFI filtering of interference-corrupted data that will be necessary to limit downlink rate requirements for future high-frequency microwave missions.

Juno is a New Frontiers mission to study Jupiter and carries as one of its payloads a six-frequency microwave radiometer to perform atmospheric sounding of the Jovian atmosphere to pressures of approximately 250 bars [1]. Juno was launched from Kennedy Space Center on August 5, 2011 and reached Jupiter orbit on July 4, 2016. The Microwave Radiometer (MWR) operates from 600 MHz to 22 GHz and was designed and built at the Jet Propulsion Laboratory. Because the mission is operating in the harsh Jovian radiation environment, it required an ambitious radiometer design that pushed the limits for an internally calibrated radiometer. The MWR uses noise diodes and a PIN-diode Dicke switch located inside the receiver. Typically, internally calibrated radiometers are designed to minimize the loss and the temperature gradient between the antenna and the noise calibration sources. However, for the MWR, the receivers with the calibration sources needed to reside inside a centralized radiation vault and the large antennas were up to 1.5 m away on the outward facing side of the spacecraft. Producing calibrated antenna temperatures requires a correction for up to 3 dB of front-end loss with a 150 K temperature gradient. In other words, 50% of the received signal at the internal calibration plane originates in the front-end of the radiometer. Additionally, the MWR is operating on a spacecraft rotating through the strong Jovian magnetic field, requiring targeted magnetic shielding of the ferrite isolator inside the receiver.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and filtering technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel and will demonstrate on-board real-time RFI processing. The system is currently under development, with an expected launch date in mid-2018 followed by a one year period of on-orbit operations. CubeRRT spacecraft and radiometer instrument testing as well as the mission concept of operations are described in this paper.

In this paper we discuss the necessary technology required to enable the future of spectrum resource constrained missions. We discuss the CubeSat Radiometer Radio Frequency Interference Technology (CubeRRT) validation mission and the development of its digital backend, necessary for performing on-board RFI detection and filtering for wideband high frequency radiometry. The CubeRRT mission will validate the on-board RFI filtering technology solving technological challenges such as bandwidth, data downlink volume, and RFI types. We present a few initial results of the backend spectrometer leading to full-system integration and test.

The Tropospheric Water and Cloud ICE (TWICE) millimeter and sub-millimeter radiometer instrument is being developed to enable global observations of upper tropospheric/lower-stratospheric water vapor and ice particle size distribution in clouds. Global observations using the TWICE instrument are critically needed to reduce uncertainties in weather and climate models. A low-noise, power-efficient command and data handling (C&DH) subsystem has been designed and tested to control TWICE data acquisition and other subsystems. Considering the limited power resources available on such platforms, a highly-efficient power regulation board has been designed to minimize power losses and reduce system noise. Furthermore, heavy-ion radiation testing has been performed for some critical commercial-off-the-shelf components to analyze radiation tolerance in low-Earth orbit. The C&DH prototype board meets the functional, noise and size, weight and power (SWaP) requirements for deployment on a 6U -Class satellite.

The Temporal Experiment for Storms and Tropical Systems Technology Demonstration (TEMPEST-D) instrument is a five-frequency millimeter-wave radiometer capable of observing thermal radiation from the Earth at 89, 165, 176, 180, and 182 GHz. The direct-detection architecture of the radiometer reduces its power consumption and eliminates the need for a local oscillator and mixer, reducing complexity. The instrument includes an ambient blackbody calibration target and a scanning reflector. The reflector rotates to scan the antenna beams in the cross-track direction so that the TEMPEST-D feed horn and receiver view first the blackbody calibration target, then the Earth over a range of nadir angles from −45ºto +45º, and finally the cosmic microwave background radiation at 2.73 K. This enables precision end-to-end calibration of the millimeter-wave receivers every scan period. The TEMPEST-D millimeterwave radiometers are based on 35-nm InP HEMT MMIC low-noise amplifiers and related technology developed under extensive investment by the NASA Earth Science Technology Office (ESTO).

The Temporal Experiment for Storms and Tropical Systems (TEMPEST) mission concept consists of a constellation of five identical 6U-Class nanosatellites observing at five millimeter-wave frequencies with five-minute temporal sampling to observe the time evolution of clouds and their transition to precipitation. The TEMPEST concept is intended to improve understanding of cloud processes, by providing critical information on the temporal development of cloud and precipitation microphysics and by improving our understanding of some of the largest sources of uncertainty in cloud process models. TEMPEST millimeter-wave radiometers are able to perform observations inside the cloud to observe changes as the cloud begins to precipitate or ice accumulates inside the storm. The TEMPEST Technology Demonstration (TEMPEST-D) mission is intended to reduce risk and demonstrate measurement capabilities for 6U-Class satellite constellations for Earth Science. The capabilities to be demonstrated include differential drag maneuvers to provide desired time separation in a 6U-Class satellites constellation. In addition, TEMPEST-D millimeter-wave radiometers will be cross-calibrated with space-borne radiometers with similar frequency channels. TEMPEST-D will provide radiometric observations at five millimeterwave frequencies from 89 to 183 GHz using a low-power, compact instrument that is highly suitable for deployment on 6U-Class satellites.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and filtering technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel and will demonstrate on-board real-time RFIS processing. The system is currently under development, with an expected launch date in mid-2018 followed by a one year period of on-orbit operations. Development of the CubeRRT spacecraft, radiometer instrument, and concepts of operation are described in this paper.

Small satellites offer the capability to deploy constellations of instruments to monitor the time evolution of dynamic weather phenomenon through rapid repeat observations. There are several planned missions that employ passive microwave sensors to study precipitation and storm dynamics, including TEMPEST and TROPICS. Recently, the HAMSR sensor on the Global Hawk aircraft was used to acquire rapid repeat observations of tropical cyclones as a part of the 2016 NOAA SHOUT campaign. This paper describes these time resolved observations of the storm and offers perspectives for these future missions.

Existing spaceborne radiometer receivers, particularly below 40GHz, have typically utilized analog direct detection, but recently have been pushing toward heterodyne systems with digital signal processing to deal with the problem of radio frequency interference (RFI). But, radar and radiometer receivers have yet to be integrated in an optimized manner for spaceborne systems. Fast sampling analog-to-digital converters (ADCs) have been evolving to where we can consider in the 2025 time frame digitizing broad IF signals containing the desired radar signal, natural brightness emission and extraneous man-made RF signals (e.g. RFI) which can then be distributed and processed by radar and radiometer specific digital processors. In this way, a common RF front-end architecture can interface with an agile reconfigurable backend and can be used across several systems (e.g. small, large class sensors). In this study, we designed and demonstrated a multi-purpose receiver that can serve both the needs for the radar and radiometer, first focusing on Ka-band.

The advanced microwave radiometer (AMR) is a critical payload on the recently launched Jason-3 mission, designed to provide the electrical range delay of the radar altimeter signal due to tropospheric water vapor and cloud liquid water [1]. The errors in the wet tropospheric path delay measurements have a direct impact on the record of global mean sea level (GMSL) and could lead to uncertainty in derived trends if spurious drifts in the radiometer are not accounted for. Therefore, it is imperative to quantify and correct radiometer calibration drift, enabling producing of a high-quality stable record of wet tropospheric path delay for use in the development of GMSL.

In this paper we discuss a polarimateric calibration technique applied on the Soil Moisture Active Passive (SMAP) L-band radiometer. We take advantage of the SMAP antenna rotation and varying incidence angle during pitch maneuvers performed by the spacecraft for periodic cold-sky calibration. We present initial comparisons between the polarization corrected ocean signal at various incidence angles and the expected polarization signal. The difference between the two signals is utilized to back-out cross-polarization coupling parameters. We also discuss factors such as Faraday rotation that impact cross-polarization calibration at low L-band frequencies.

In this paper we discuss the design and development of the Radiometer Digital Backend (RDB) of the CubeSat Radiometer Radio Frequency Interference Technology (CubeRRT) validation mission. We present a brief introduction of the mission and the Radio Frequency Interference (RFI) detection and mitigation algorithm. The digital backend developed for the CubeSat is presented. The digital backend has unique capabilities of taking in wide bandwidths of up to 1GHz and can perform on-board complex operations to detect and filter out RFI in real-time. We present a few initial results of the backend spectrometer leading to full-system integration and test.

Global measurements of upper tropospheric/lower-stratospheric water vapor and ice particle size distribution in clouds are critically needed to reduce uncertainties in global weather and climate models. To address this need, the conically scanning Tropospheric Water and Cloud ICE (TWICE) millimeter and submillimeter radiometer instrument is being developed. A low-noise, power-efficient command and data handling (C&DH) subsystem has been designed to control TWICE data acquisition and other subsystems. The C&DH prototype board meets functional, noise and size, weight and power (SWaP) requirements for deployment in a 6U-class satellite. Considering the limited power resources available on such platforms, a highly-efficient power regulation board has been designed to minimize power losses and reduce system noise. Furthermore, all of the components have been tested for radiation tolerance in low-Earth orbit.

The Temporal Experiment for Storms and Tropical Systems (TEMPEST) mission concept consists of a constellation of five identical 6U-Class nanosatellites observing at five millimeter-wave frequencies with five-minute temporal sampling to observe the time evolution of clouds and their transition to precipitation. The TEMPEST concept is designed to improve the understanding of cloud processes, by providing critical information on the time evolution of cloud and precipitation microphysics and by improving our understanding of the largest sources of uncertainty in cloud models. TEMPEST millimeter-wave radiometers are able to perform observations inside the cloud to observe changes as the cloud begins to precipitate or ice accumulates inside the storm. The TEMPEST Technology Demonstration (TEMPEST-D) mission will be deployed to demonstrate measurement capabilities required for a constellation of 6U-Class nanosatellites to directly observe the temporal development of clouds to understand the conditions that control their transition from non-precipitating to precipitating clouds. TEMPEST-D will provide observations at five millimeter-wave frequencies from 89 to 183 GHz using a single compact instrument that is well suited for the 6U-Class architecture.

All channels of the Aquarius radiometer were observed to have calibration instability consisting of a drift in the antenna temperature during the first couple of months of the mission and pseudo-periodic oscillations of the antenna temperature over the mission life. For the version 4 Aquarius processing, both of these anomalies were corrected by removing a time variable bias in the Aquarius measurements relative to a seven-day global average from a salinity model. In order to accurately track long-term variation of salinity on climate scales it is necessary to decouple Aquarius radiometric calibration from ocean salinity models. In this paper, a new technique is used to investigate the nature of anomalies using nonocean vicarious external sources such as Antarctic ice or Amazonian rain forests. Two completely different solutions are developed to correct the pseudo-periodic oscillations as well as the drift of the Aquarius radiometers, decoupling the Aquarius measurements from salinity model.

The Compact Ocean Wind Vector Radiometer (COWVR) is new type of conical sensor ideal for small satellite implementation. This paper provides an overview of the COWVR sensor, mission and provides perspectives for the future of this technology to enable low-cost sustainable passive microwave observations into the next decade.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and mitigation technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6-40 GHz using a 1 GHz bandwidth tuned channel, and will demonstrate on-board real-time RFI processing. The system is currently under development, with launch readiness expected in 2018 followed by a one year period of on-orbit operations. Project plans and status are reported in this paper.

TEMPEST-D will demonstrate technology for 6U-Class nanosatellites to advance NASA's Earth Science Goals. It will also reduce risk, cost, and development time for future constellations of small satellites to perform Earth Science measurements. It will raise the TRL of a millimeter-wave radiometer instrument from 6 to 7, representing the first on-orbit demonstration of 35-nm InP HEMT-based millimeter-wave radiometer front ends.

WindSat has been providing accurate ocean vector winds (OVWs) for over a decade. WindSat makes polarimetric brightness temperature measurements of Earth looking forwards and backwards. However, because the overlap of these two swaths is relatively narrow, the benefit of two-look polarimetric (2LP) retrieval accuracy has not been utilized. This paper derives OVW from WindSat 2LP measurements using a radiative transfer model and maximum-likelihood estimation. The purpose of this paper is a comparison of WindSat 2LP wind direction accuracy with WindSat one-look, QuikSCAT, and Advanced Scatterometer (ASCAT) wind directions. Retrievals are compared with anemometer measurements on collocated moored buoys. Statistics are examined for both the first-ranked wind direction ambiguity and the selected ambiguity after median filtering initialized with a numerical weather prediction wind field. For winds above 8 m/s, WindSat 2LP retrievals have the most accurate first-ranked direction compared with all other sensors. For winds of 6-9 m/s, the standard deviation relative to buoys is 17°, and for 9-20 m/s, it is less than 10°. One-look standard deviations are nearly twice as large. At low winds, QuikSCAT provides the most accurate wind directions, for first-ranked and selected ambiguity. Thus, scatterometer and radiometer OVW measurements provide complementary capabilities. The accuracy of 2LP OVW is particularly relevant now that new internal calibration technology allows for a 360° conical scan of earth observations. Moreover, new low-cost designs would make it possible to affordably deploy a constellation of OVW sensors capable of providing accurate winds under a wide range of conditions, described herein.

A method is applied to maintain the long-term calibration of a microwave radiometer through intersatellite calibration and is used to mitigate an observed calibration drift of the Advanced Microwave Radiometer (AMR) on Jason-2/Ocean Surface Topography Mission. The AMR provides a correction for the wet tropospheric path delay (PD) of the radar altimeter signal, and it is critical that any drift in the radiometer be estimated and removed to enable studies of global mean sea-level variability. The intersatellite calibration method transfers the long-term calibration from other satellite microwave radiometers using a transfer function to map the other sensor's brightness temperature (TB) observations to those of the AMR. Intersensor mapping functions are derived separately for ocean observations and observations over the Amazon rainforest. This provides a warm and cold TB calibration reference to enable the distinction between long-term gain and offset drifts. A database of co-incident observations is generated between the AMR and conically scanning microwave sensors, namely, AMSR-E, TMI, and SSMIS. Monthly averaged differences are found between the AMR and the AMR equivalent TBs computed from the reference sensors. The apparent change in the AMR calibration determined from the three reference sensors is intercompared between the sensors and compared to that determined using natural on-Earth references. It is found that apparent trends in the AMR TBs between the reference sensors and the natural on-Earth references agree within a month to better than 0.4 K. The AMR 18.7- and 23.8-GHz channels are found to be stable to 0.5 K over the first three years of the mission, and the calibration 34.0-GHz channel is found to drift downward by approximately 6 K. In all channels, the calibration change is determined to be a series of offset jumps (independent of TB). These calibration changes in each AMR channel are estimated and removed using the comparisons to the reference sensors. The uncertainty in the PD long-term stability after recalibration is estimated to be less than 0.5 mm/year from July 2008 to August 2011.

The Jet Propulsion Laboratory's High-Altitude Monolithic Microwave Integrated Circuit (MMIC) Sounding Radiometer (HAMSR) is a 25-channel cross-track scanning microwave sounder with channels near the 60- and 118-GHz oxygen lines and the 183-GHz water-vapor line. It has previously participated in three hurricane field campaigns, namely, CAMEX-4 (2001), Tropical Cloud Systems and Processes (2005), and NASA African Monsoon Multidisciplinary Analyses (2006). The HAMSR instrument was recently extensively upgraded for the deployment on the Global Hawk (GH) unmanned aerial vehicle platform. One of the major upgrades is the addition of a front-end low-noise amplifier, developed by JPL, to the 183-GHz channel which reduces the noise in this channel to less than 0.1 K at the sensor resolution (~2 km). This will enable HAMSR to observe much smaller scale water-vapor features. Another major upgrade is an enhanced data system that provides onboard science processing capability and real-time data access. HAMSR has been well characterized, including passband characterization, along-scan bias characterization, and calibrated noise-performance characterization. The absolute calibration is determined in-flight and has been estimated to be better than 1.5 K from previous campaigns. In 2010, HAMSR participated in the NASA Genesis and Rapid Intensification Processes campaign on the GH to study tropical cyclone genesis and rapid intensification. HAMSR-derived products include observations of the atmospheric state through retrievals of temperature, water-vapor, and cloud-liquid-water profiles. Other products include convective intensity, precipitation content, and 3-D storm structure.

The primary objective of the National Research Council (NRC) Decadal Survey recommended SWOT (Surface Water and Ocean Topography) Mission is to measure the water elevation of the global oceans, as well as terrestrial water bodies (such as rivers, lakes, reservoirs, and wetlands), to answer key scientific questions on the kinetic energy of ocean circulation, the spatial and temporal variability of the world's surface freshwater storage and discharge, and to provide societal benefits on predicting climate change, coastal zone management, flood prediction, and water resources management. In this paper, we present the overall concept of the SWOT mission, as well as the scientific rational, objectives and development status of the technology items currently under development.

The Precipitation and All-weather Temperature and Humidity (PATH) mission is one of the NASA missions recommended by the NRC in its recent Earth Science “Decadal Survey.” The focus of this mission is on the hydrologic cycle in the atmosphere, with applications from weather forecasting to climate research. PATH will deploy a microwave sounder, a passive radiometer that measures upwelling thermal radiation, in geostationary orbit and will for the first time provide a time-continuous view of atmospheric temperature and all three phases of water under nearly all weather conditions. This is possible because microwave radiation is sensitive to but also penetrates both clouds and precipitation, as has been demonstrated with similar sensors on low-earth-orbiting satellites. Data from those sensors, despite observing a particular location only twice a day, have had more impact on weather prediction accuracy than any other type of satellite sensor, and it is expected that PATH will have a similar impact with its ability to continuously observe the entire life cycle of storm systems. Such sensors have also played an important role in climate research and have been used to estimate long-term temperature trends in the atmosphere. An important application of PATH data will be to improve the representation of cloud formation, convection, and precipitation in weather and climate models, particularly the diurnal variation in those processes. In addition to measuring the three-dimensional distribution of temperature, water vapor, cloud liquid water, and ice, PATH also measures sea surface temperature under full cloud cover. Such observations make a number of important applications possible. Depending on the application focus and the geostationary orbit location, PATH can serve as anything from a hurricane and severe-storm observatory to an El Niño observatory. A geostationary orbit offers many advantages, as has been demonstrated with visible and infrared imagers and sounders deployed on weather satellites, but those sensors cannot penetrate clouds. It has not been possible until now to build a microwave radiometer with a large enough antenna aperture to attain a reasonable spatial resolution from a GEO orbit. A new approach, using aperture synthesis, has recently been developed by NASA at the Jet Propulsion Laboratory, and that is what makes PATH possible. Key technology enabling the large array of receivers in such a system has been developed, and a proof-of-concept demonstrator was completed in 2006. The state of the art in this area is now such that PATH mission development could start in 2010 and be ready for launch in 2015, but the actual schedule depends on the availability of funding. An option to fly PATH as a joint NASA-NOAA mission is being explored.

An algorithm is developed to retrieve wet tropospheric path delay (PD) near land from a satellite microwave radiometer to improve coastal altimetry studies. Microwave radiometers are included on ocean altimetry missions to retrieve the wet PD, but their performance has been optimized for retrievals in the open ocean. Near land, the radiometer footprint contains a mixture of radiometrically warm land and radiometrically cold ocean. Currently, the radiometer retrievals in the coastal region are flagged as invalid since large errors result when the open-ocean retrieval algorithm is applied to mixed land/ocean scenes. The PD retrieval algorithm developed in this paper is applicable to both open-ocean and mixed land-ocean scenes, thus enabling retrievals in the coastal zone. The performance of the algorithm is demonstrated with detailed simulations and application to measurements from the Advanced Microwave Radiometer on the Jason-2/Ocean Surface Topography Mission. The algorithm error is estimated to be less than 0.8 cm up to 15 km from land, less than 1.0 cm within 10 km from land, less than 1.2 cm within 5 km from land, and less than 1.5 cm up to the coastline.

The Precipitation and All-weather Temperature and Humidity (PATH) mission is one of 15 Earth space missions that the U.S. National Research Council recently recommended that NASA undertake in the next decade. The PATH mission will place a microwave atmospheric sounder, operating in the same temperature and water vapor bands used by the low-earth-orbiting Advanced Microwave Sounding Units (AMSU), into geostationary orbit. The objective is to enable time-continuous observations of severe storms, tropical cyclones and atmospheric processes associated with the hydrologic cycle under all weather conditions. The ultimate goal is to improve models in these areas, provide initial conditions and assimilation data for improved forecasts, and develop long time series to support climate studies. Both NOAA and NASA have long sought to develop such a sensor, but it is only recently that new techniques have emerged that enable such a mission. The Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR) is a microwave sounder concept based on aperture synthesis that has been developed at the Jet Propulsion Laboratory. A small proof-of-concept prototype was completed in 2006 under the NASA Instrument Incubator Program, and this demonstrator proves that the aperture synthesis method is a feasible approach for attaining the very large aperture required for adequate spatial resolution. The performance of the prototype and projections to a full-scale space version indicate that GeoSTAR, unlike alternative approaches, can meet all measurement requirements. It is therefore now considered the baseline PATH payload and is expected to be implemented by NASA in the next decade.

An end-of-mission recalibration effort was recently completed for Topex Microwave Radiometer to generate climate data records of precipitable water vapor and cloud liquid water for 1992–2005. The TMR climate data is analysed for trends. The global trend in precipitable water vapor is found to be 0.9 ± 0.06 mm/decade. Regional precipitable water vapor trends are found to be highly correlated with regional sea surface temperature trends. The cloud liquid water trends are observed to be generally negative outside the tropics and positive in the tropics.

The Jet Propulsion Laboratory's High Altitude MMIC Sounding Radiometer (HAMSR) is a 25 channel microwave sounder with channels near the 60 GHz and 118 GHz oxygen lines and near the 183 GHz water vapor line. It participated in three hurricane field campaigns, CAMEX-4, TCSP and NAMMA The absolute calibration of the HAMSR brightness temperatures is shown to be better than 1.5 K. A non-linear iterative optimal estimation based retrieval algorithm is developed to retrieve atmospheric temperature and absolute humidity profiles. Comparisons of the retrieved profiles with coincident dropsonde profiles during NAMMA show excellent agreement at all altitudes, with the exception of a 30% residual dry bias in the absolute humidity profile above 4 km. The warm core structure of Hurricane Erin in 2001 and Hurricane Emily in 2005 is retrieved. The 60/118 GHz channels which have matched clear air weighting functions are used to assess convective intensity in the eye wall through the relative scatter darkening between the two channels.

The geostationary synthetic thinned aperture radiometer (GeoSTAR) is a new instrument design that has been under development at the Jet Propulsion Laboratory in the form of a proof-of-concept prototype. It is intended to fill a serious gap in our Earth remote sensing capabilities - namely the lack of a microwave atmospheric sounder in geostationary orbit. Such sensors have long been part of low-earth-orbiting (LEO) operational weather satellites and research satellites and have had a major impact ranging from numerical weather prediction to climate research. A similar capability in GEO is highly desired because of the advantageous observing point GEO offers, with continuous views of the entire visible Earth disc - crucial for the observation of hurricanes and other rapidly evolving atmospheric phenomena. GEO also enables full resolution of the diurnal cycle, which is particularly important in the study of atmospheric processes and climate variability where clouds and convection play a role, since those phenomena are known to have strong diurnal variability and are difficult to sample properly with sun synchronous LEO satellites. The GeoSTAR prototype produced the first interferometric radiometric images obtained at sounding frequencies in early 2005, and subsequent tests have demonstrated that the system exhibits excellent stability, accuracy and sensitivity and performs even better than predicted. This can be characterized as a breakthrough development. The technology required to implement GeoSTAR is at a level of maturity that a space mission can be contemplated. Such a mission is recommended by the U.S. National Research Council in its recent Decadal Survey of Earth missions and is being considered by both NASA and NOAA for the coming decade. Recent studies indicate that it is indeed feasible to implement a GeoSTAR mission in the 2014-16 time frame. We discuss possible mission scenarios as well as the science benefits that would ensue. The benefits are particularly significant in the area of tropical cyclones and severe storms, where there currently is a dearth of observations. With a geostationary microwave sounder it is possible to obtain the 3-dimensional distribution of temperature, water vapor and liquid water continuously and regardless of cloud cover, and atmospheric stability indices such as lifted index (LI) and convective available potential energy (CAPE) can be derived nearly everywhere. That will make it possible, for example, to detect severe-storm precursor conditions even if the area is under cloud cover. Recent progress in radiative transfer models now also makes it possible to obtain those parameters in the presence of moderate precipitation, and rain rates and snow rates can be derived as well. Aircraft based field campaign observations have also shown that a microwave sounder can be used to derive measures of convective intensity and precipitation in deep-convective systems from scattering due to ice particles formed by such systems. This can be used to estimate the intensity of tropical cyclones and can be used to detect sudden intensification and weakening in near-real time.

A new and improved L-Band version of a programmable digital noise source has been developed to aid in the characterization of microwave radiometers. The system consists of a commercial Arbitrary Waveform Generator (AWG), “RF Head” frequency upconversion modulators with integral calibration reference sources, and a local oscillator. It is being used to evaluate the performance of two upcoming spaceborne microwave radiometers - the Aquarius polarimetric radiometer (a low earth orbiting ocean salinity mission) and the Juno microwave radiometer (a Jupiter orbiter for atmospheric sounding). For each of these radiometer evaluations, the programmable noise source can generate signals that: a) simulate the expected observations and test the radiometer’s response; and b) exercise the radiometer’s response to variations in the observations in such a way that its overall behavior can be more fully characterized.

Ground based tests of the GeoSTAR (Geostationary Synthetic Thinned Array Radiometer) demonstrator instrument are reported which simulate the view of the Earth from geosynchronous Earth orbit (GEO). The test used a 4-meter target disk mounted on a tower above the instrument to simulate the brightness of the Earth with a contrasting cold background. Continuous observations at 50.3 GHz for over 100 hours, along with simultaneous atmospheric measurements from independent radiometers, yielded an excellent data set with which to test all aspects of the GeoSTAR calibration. This paper presents a preliminary look at these data, and presents an algorithm to remove the aliased background from the synthesized image.

The Geostationary Synthetic Thinned Array Radiometer represents a promising new approach to microwave atmospheric sounding from geostationary orbit based on passive interferometry. One of the major concerns about the feasibility of this new concept is related to the ability of the sensor to cope with the failure of one or several of its single receivers/antennas. This letter shows that the inclusion of a small percentage of additional antennas significantly reduces the degradation of radiometric resolution caused by such receiver failure. Impact of antenna failure is analyzed, taking into account two test images with very different spatial harmonic content. A tradeoff analysis of several array topologies is performed so as to minimize the number of additional antennas while keeping worst case radiometric error within a reasonable level

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.

Results are presented from the long-term monitoring and calibration of the National Aeronautics and Space Administration Jason Microwave Radiometer (JMR) on the Jason-1 ocean altimetry satellite and the ground-based Advanced Water Vapor Radiometers (AWVRs) developed for the Cassini Gravity Wave Experiment. Both radiometers retrieve the wet tropospheric path delay (PD) of the atmosphere and use internal noise diodes (NDs) for gain calibration. The JMR is the first radiometer to be flown in space that uses NDs for calibration. External calibration techniques are used to derive a time series of ND brightness for both instruments that is greater than four years. For the JMR, an optimal estimator is used to find the set of calibration coefficients that minimize the root-mean-square difference between the JMR brightness temperatures and the on-Earth hot and cold references. For the AWVR, continuous tip curves are used to derive the ND brightness. For the JMR and AWVR, both of which contain three redundant NDs per channel, it was observed that some NDs were very stable, whereas others experienced jumps and drifts in their effective brightness. Over the four-year time period, the ND stability ranged from 0.2% to 3% among the diodes for both instruments. The presented recalibration methodology demonstrates that long-term calibration stability can be achieved with frequent recalibration of the diodes using external calibration techniques. The JMR PD drift compared to ground truth over the four years since the launch was reduced from 3.9 to -0.01 mm/year with the recalibrated ND time series. The JMR brightness temperature calibration stability is estimated to be 0.25 K over ten days.

The design, error budget, and preliminary test results of a 50-56-GHz synthetic aperture radiometer demonstration system are presented. The instrument consists of a fixed 24-element array of correlation interferometers and is capable of producing calibrated images with 1deg spatial resolution within a 17deg wide field of view. This system has been built to demonstrate a performance and a design which can be scaled to a much larger geostationary Earth imager. As a baseline, such a system would consist of about 300 elements and would be capable of providing contiguous full hemispheric images of the Earth with 1 K of radiometric precision and 50-km spatial resolution. An error budget is developed around this goal and then tested with the demonstrator system. Errors are categorized as either scaling (i.e., complex gain) or additive (noise and bias) errors. Sensitivity to gain and/or phase error is generally proportional to the magnitude of the expected visibility, which is high only in the shortest baselines of the array, based on model simulations of the Earth as viewed from geostationary Earth orbit. Requirements range from approximately 0.5% and 0.3deg of amplitude and phase uncertainty, respectively, for the closest spacings at the center of the array, to about 4% and 2.5deg for the majority of the array. The latter requirements are demonstrated with our instrument using relatively simple references and antenna models, and by relying on the intrinsic stability and efficiency of the system. The 0.5% requirement (for the short baselines) is met by measuring the detailed spatial response (e.g., on the antenna range) and by using an internal noise diode reference to stabilize the response. This result suggests a hybrid image synthesis algorithm in which long baselines are processed by a fast Fourier transform and the short baselines are processed by a more precise (G-matrix) algorithm which can handle small anomalies among antenna and receiver responses. Visibility biases and other additive errors must be below about 1.5 mK on average, regardless of baseline. The bias requirement is largely met with a phase-shifting scheme applied to the local oscillator distribution of our demonstration system. Low mutual coupling among the horn antennas of our design is also critical to minimize the biases caused by crosstalk of receiver noise. Performance is validated by a three-way comparison between interference fringes measured on the antenna range, solar transit observations, and the system model.

The geostationary synthetic thinned array radiometer represents a promising new approach to microwave atmospheric sounding from geostationary orbit based on passive interferometry. Distortion due to mechanical or thermal constraints produces a displacement of the ideal antenna positions in the array that causes sampling errors. In this paper, the impact of array distortion on radiometric error is analyzed in detail so as to identify the dominant sources of error. A preliminary analysis showing that array distortion can be well corrected by means of an external phase reference is also presented.

The Naval Research Laboratory WindSat polarimetric radiometer was launched on January 6, 2003 and is the first fully polarimetric radiometer to be flown in space. WindSat has three fully polarimetric channels at 10.7, 18.7, and 37.0 GHz and vertically and horizontally polarized channels at 6.8 and 23.8 GHz. A first-generation wind vector retrieval algorithm for the WindSat polarimetric radiometer is developed in this study. An atmospheric clearing algorithm is used to estimate the surface emissivity from the measured WindSat brightness temperature at each channel. A specular correction factor is introduced in the radiative transfer equation to account for excess reflected atmospheric brightness, compared to the specular assumption, as a function wind speed. An empirical geophysical model function relating the surface emissivity to the wind vector is derived using coincident QuikSCAT scatterometer wind vector measurements. The confidence in the derived harmonics for the polarimetric channels is high and should be considered suitable to validate analytical surface scattering models for polarized ocean surface emission. The performance of the retrieval algorithm is assessed with comparisons to Global Data Assimilation System (GDAS) wind vector outputs. The root mean square (RMS) uncertainty of the closest wind direction ambiguity is less than 20/spl deg/ for wind speeds greater than 6 m/s and less than 15/spl deg/ at 10 m/s and greater. The retrieval skill, the percentage of retrievals in which the first-rank solution is the closest to the GDAS reference, is 75% at 7 m/s and 85% or higher above 10 m/s. The wind speed is retrieved with an RMS uncertainty of 1.5 m/s.

Absolute calibration of WindSat's third and fourth Stokes brightness temperatures (T/sub 3/ and T/sub 4/) is needed at the tenth of Kelvin level in order to adequately resolve their dependence on wind direction. Previous aircraft based fully polarimetric microwave radiometers have generally relied on "circle flights", during which a single area of the ocean is observed at all azimuth angles, to estimate residual biases in the calibration of its polarimetric channels. WindSat, the first spaceborne fully polarimetric microwave radiometer, operates in low Earth orbit and thus cannot execute this traditional calibration technique. A new method is presented to estimate the residual biases that are present in WindSat's T/sub 3/ and T/sub 4/ estimates. The method uses a vicarious cold reference brightness temperature applied to measurements made by WindSat at /spl plusmn/45/spl deg/ slant linear (T/sub P/ and T/sub M/) and left- and right-hand circular (T/sub L/ and T/sub R/) polarization. WindSat derives the third and fourth Stokes brightness temperatures by the differences T/sub P/-T/sub M/ and T/sub L/-T/sub R/, respectively. The method is demonstrated by applying it to the 10.7-GHz WindSat observations. Calibration biases of 0.2-0.6 K are determined with a precision of 0.04 K.

A retrieval algorithm has been developed which simultaneously estimates the over ocean near-surface wind speed and rain rate profile using data from a 10.7 GHz microwave radiometer and a dual-frequency Doppler radar. The algorithm uses the radar backscatter measurements to estimate two parameters of the gamma drop size distribution (GDSD) at each range gate. The parameterized GDSD can be integrated to determine the rain rate profile. The wind speed is estimated from the 10.7 GHz brightness temperatures (TBs) by removing the contribution from the atmosphere and isolating the contribution from the surface wind speed. The atmospheric optical depth at 10.7 GHz is estimated by integrating the extinction coefficient determined at each radar range gate using the parameterized GDSD and Mie theory. Results of wind speed and rain rate retrievals are presented from a field campaign in June of 2003 in which several precipitation overflights were made with a NASA DC-8 equipped with the PR-2 radar and the LRR-X microwave radiometer

Three aspects of WindSat performance are presented. (1). End-to-end system calibration is tested by comparisons between WindSat measurements and a robust model of the Earth T/sub B/ using the vicarious cold reference method. (2). In an effort to determine probable causes for any calibration biases or scale errors that are identified, a refined Antenna Pattern Correction algorithm has been developed that includes main reflector aperture illumination and edge diffraction effects, in addition to the feed spillover effects that are already accounted for in WindSat's nominal flight processing algorithm. (3). A wind vector retrieval algorithm has been developed that is based on an empirical geophysical model function relating the four Stokes components of ocean surface emissivity to wind speed and direction. In order to implement an emissivity-based retrieval, decomposition of the radiative transfer equation is performed into surface and atmospheric contributions.

Brightness temperatures (TBs) from satellite microwave radiometers need to be calibrated on-orbit at the hottest and coldest ends of the dynamic TB spectrum. A method is developed to determine hot reference calibration TBs from SSM/I TB data for other radiometers operating between 18-40 GHz and 0-55 degrees incidence. Regions of optically thick vegetation in the Amazon rainforest are chosen as calibration targets. Hot reference temperatures for the on-orbit calibration of microwave radiometers are determined for these regions.

The Jason microwave radiometer is calibrated using hot and cold on-Earth theoretical brightness temperature references. The retrieved path delay values are validated using collocated Topex microwave radiometers and radiosonde values. The calibrated path delay values are demonstrated to have no significant bias or scale errors. The absolute accuracy of the individual path delay values is demonstrated to have no significant bias or scale errors. The absolute accuracy of the individual path delay measurement exceeds the mission goal of 1.2 cm RMS.