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.

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.

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.

This paper explores the application of modern image processing techniques in retrieving high-resolution passive microwave images of the polar ice regions on Earth from sparsely sampled interferometric array measurements. Such observations, sensitive to ice sheet temperature, would be valuable benchmark measurements for ice process models. In this paper, we propose to use a total variation-based method that addresses the challenges associated with large sidelobes and blurry maps resulting from long baseline interferometry. We present a robust algorithm that employs total variation (TV) minimization and the split Bregman optimization. This technique effectively deconvolves images, preserves edges, and minimizes noise amplification without introducing artifacts. To evaluate the algorithm’s performance, we performed tests on a simulated image and a real satellite image of Antarctica. Additionally, we assessed the algorithm’s performance using different interferometric array configurations, including both dense and sparse arrays with varying numbers of elements.

The Smart Ice Cloud Sensing (SMICES) is an active/passive sensor. SMICES is sponsored by NASA Earth Science Technology Office (ESTO) under Instrument Incubator Program 19 (IIP-19) awarded to Northrop Grumman Corporation (NGC) and Jet Propulsion Laboratory (JPL). The instrument is designed to measure upper tropospheric and lower stratospheric cloud ice and water vapor. SMICES uses a suite of passive radiometers that are constantly conically scanning to locate ice clouds. The ice clouds are located using an artificial intelligence controller that identifies key labels related to the ice cloud. Once an ice cloud is identified, the artificial intelligence controller activates and targets the on-board radar. While the SMICES instrument is currently being developed for an airborne demonstration, the final goal is to deploy it as a small satellite (SmallSat) instrument in low-Earth orbit (LEO). The onboard AI controller will significantly reduce DC power consumption of the satellite mission. This will enable the SMICES system to be hosted on a smaller platform with fewer solar cells and significantly drive down mission costs while maintaining the quality of scientific data. This work presents the latest development on the SMICES microwave radiometer.

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.

Three newly developed microwave and millimeter-wave radiometers which employ the pseudo-correlation architecture are discussed. This architecture-also described as the “continuous comparison” radiometer [1]-employs a hybrid power splitter to divide antenna and reference signals between two receiver chains which are later recombined by a second hybrid to again separate the antenna and reference signals. This scheme offers an alternative to the traditional Dicke switched radiometer design which otherwise places a reference switch between the antenna and receiver. We have applied the pseudo-correlation design to three unrelated radiometers to solve three (also unrelated) problems of (1) how to share telecommunication and radiometer functions in a single receiver without interrupting the communication channel with a Dicke switch; (2) how to maintain valid reference signals in the presence of strong radio interference; and (3) how to improve the noise figure of an internally calibrated millimeter-wave radiometer-by eliminating an electrically lossy Dicke switch.

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.

Earthquakes occur when stress in the Earth exceeds the strength of the surrounding host rock. Earthquakes can occur naturally or be induced by human activity [1]. In this paper, we address earthquakes occurring on fault systems that are driven by plate tectonics. Motion of the Earth's tectonic plates causes deformation of the Earth's crust. As this deformation occurs, strain accumulation can be released seismically in earthquakes, or aseismically as creep along faults and in bulk deformation of the crust. Understanding how strain is distributed along fault systems, how stress transfers through the crust along fault zones between earthquakes, and the fraction of aseismic versus seismic deformation, is key to assessing earthquake hazard for mitigating losses from future events [2].

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 Jet Propulsion Laboratory has developed a new airborne sensor, the Microwave Temperature and Humidity Profiler (MTHP), by adding a 183 GHz receiver to the current Microwave Temperature Profiler (MTP). The instrument scans ahead of the aircraft flight path sampling at multiple angles, allowing for atmospheric retrievals above and below the aircraft, to generate vertical profiles. Liquid water path and precipitable water vapor can also be retrieved above the aircraft in the absence of scattering. The instrument has had shakeout flights on the GII and C130 in 2016. In March 2017, the MTHP will be modified to be installed in a standard PMS canister to fly on the GV.

GeoSTAR-III, a 180 GHz prototype for the Precipitation and All-weather Temperature and Humidity Sounder (PATH), is the culmination of a decade of technology development funding. The interferometric radiometer comprises 144 receivers operating from 165-183 GHz and utilizes a 192×192 input, ASIC based mixed signal correlator. The demonstration of this instrument raises the technology readiness of the radiometer subsystem to level 6 (TRL 6) and the correlator subsystem to TRL 5. We demonstrate the full functionality of this system with observations of the Sun and Moon as well as nearby thermally emissive objects. This represents the final milestones in the development effort of pre-mission technologies for this decadal survey mission.

In its “Decadal Survey” of earth science missions for NASA published in 2007 [1] the U.S. National Research Council (NRC) recommended that a geostationary microwave sounder be developed for a Precipitation and All-weather Temperature and Humidity (PATH) mission and recommended that it be implemented as an “array spectrometer”. That was largely based on a synthetic-aperture concept then under development at the Jet Propulsion Laboratory (JPL). At the time the required technology was not perceived as being sufficiently mature, and PATH was therefore put in the “third tier” group of missions. Now, under the NASA Earth Science Technology Office's (ESTO) Instrument Incubator Program (IIP), the key technology has been developed and has been brought to Technology Readiness Level (TRL) 6, required for mission implementation, thus enabling the PATH mission.

Proposed approaches to the NRC Earth Science Decadal Survey's Precipitation, All-Weather Temperature, and Humidity (PATH) mission involving synthetic aperture arrays require massively parallel, high speed correlators implemented on a geostationary satellite platform. We present testing methodology for a coarse digital correlator chip using a low-power ASIC architecture. The chip was designed in the Electrical Engineering and Computer Science Department of the University of Michigan. These tests precede the integration of the chip into a Geostationary Synthetic Thinned Aperture Array (GeoSTAR) instrument prototype in development at NASA's Jet Propulsion Laboratory.

A new geometry for synthetic aperture radiometers is presented which increases the distance between adjacent elements in the array without changing the visibility sample density in the u-v plane. This provides room for higher elemental antenna gain, which improves both the overall system sensitivity and alias rejection in the synthesized image-both critical requirements for the Earth observing application. The geometry is derived from the simple Y -array geometry by shifting alternate elements within an otherwise linear array arm into two or more rows of antennas. The resulting system largely retains the same hexagonal sample grid in the u-v plane of the visibility function, yet allows for an elemental antenna aperture that is physically larger than the u-v sample spacing. Only the shortest visibility baselines are lost, and a small dedicated low-gain array must be added to the system to recover these baselines. The radiometer is thus divided between a large high-gain array and a small low-gain array. Since the sensitivity (delta-T) of the system is dominated by that of the large array, this approach greatly improves the overall system sensitivity-in this letter, by a factor of 9 (or, equivalently, factor 81 integration time).

We describe the design and performance of a 180 GHz correlation radiometer suitable for remote sensing. The radiometer provides continuous comparisons between a the observed signal and a reference load to provide stable radiometric baselines. The radiometer was assembled and tested using parts from the GeoSTAR-II instrument and is fully compatible with operation in a synthetic aperture radiometer or as a standalone technology for use in microwave sounding and imaging . This new radiometer was tested over several days easily demonstrating the required 6 hour stability requirement for observations of mean brightness temperature for a geostationary instrument.

The Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR) team recently concluded its second Earth Science Technology Office (ESTO) IIP-07, “GeoSTAR technology development and risk reduction for PATH”. The major accomplishments during this project at JPL were:1) Demonstrate performance and scalability of the 183 GHz receivers 2) Local oscillator phasing architecture and technology 3) Subarray design validation including feedhorns, manifolds and alignment 4) System demonstration of signal distribution topology and measurements. Significant progress has been made to retiring risk of the various subsystems.

We describe the development and progress of the GeoSTAR-II risk reduction activity for the NASA Earth Science Decadal Survey PATH Mission. The activity directly addresses areas of technical risk including the system design, low noise receiver production, sub-array development, signal distribution and digital signal processing.

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 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.

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.

A proposed instrument for deployment on next generation Geostationary Operational Environmental Satellite (GOES) platforms is the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR) [1,2]. A high resolution full earth disk model has been developed to aid in the development of the instrument design and to characterize sensor performance. A variety of publicly available geophysical fields are used as data inputs into a full radiative transfer model that also accounts for the propagation and viewing geometries from GEO. The resulting model simulates full disk microwave images with the highest known resolution. The model can be used in concert with an instrument simulator to conduct design tradeoff studies. With the capability of generating high resolution brightness images at different frequencies, atmospheric profile retrievals can be evaluated.

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.

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.

GeoSTAR is a radiometer concept to provide high resolution microwave images of the Earth from geostationary Earth orbit (GEO) in bands from 50 to 183 GHz. The system consists of a Y-array of correlation interferometers, and uses aperture synthesis to achieve high resolution hemispheric coverage of the Earth. A ground-based 50 GHz demonstration instrument has been built and tested at the Jet Propulsion Laboratory which has now validated the calibration approach and error analysis. These analysis show that the antenna gain of the original design is marginal, since only about 42 percent of the received energy originates in the Earth disk as viewed from GEO. This degrades signal-to-noise (delta-T), and poses a problem for the 183 GHz bands where receiver noise and resolution requirements are greatest. This paper presents a new approach to the array geometry which solves this problem by arranging the majority of elemental antennas along two rows within each of the three array arms. The new geometry provides a factor of SQRT(3) times more distance between adjacent elements, and therefore enough physical space to raise the gain of the antenna elements by a factor of 3. The visibility sample grid and number of elements are unchanged. Only the shortest baselines retain the original design.

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

Two tropospheric calibration systems have been developed at the Jet Propulsion Laboratory (JPL) using different technologies to achieve different levels of accuracy, timeliness, and range of coverage for support of interplanetary NASA flight operations. The first part of this paper describes an automated GPS-based system that calibrates the zenith tropospheric delays. These calibrations cover all times and can be mapped to any line of sight using elevation mapping functions. Thus they can serve any spacecraft with no prior scheduling or special equipment deployment. Centimeter-level accuracy is provided with 1-h latency and better than 1-cm accuracy after 12 h, limited primarily by rapid fluctuations of the atmospheric water vapor. The second part describes a more accurate line-of-sight media calibration system that is primarily based on a narrow beam, gain-stabilized advanced water vapor radiometer developed at JPL. We discuss experiments that show that the wet troposphere in short baseline interferometry can be calibrated such that the Allan standard deviation of phase residuals, a unitless measure of the average fractional frequency deviation, is better than 2times10-15on time scales of 2000 to approximately 10 000 s.

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 GeoSTAR demonstrator can be characterized at close range by means of a simple near-to-far-field phase correction. This reduces the test set-up configuration to reasonable dimensions. In order to simulate the Earth as seen from GEO, the target consists of a disc of absorbent material at ambient temperature placed against the sky. This work presents the details of the near-to-far-field correction as well as some preliminary results that confirm its suitability to characterize the demonstrator.

Geo STAR represents a new approach to microwave atmospheric sounding that is now under development. It has capabilities similar to sensors currently operating on low earth orbiting weather satellites but is intended for deployment in geostationary orbit - where it will complement future infrared sounders and enable all-weather temperature and humidity soundings and rain mapping. The required spatial resolution of 50 km or better dictates an aperture of 4 meters or more at a sounding frequency of 50 GHz, which is difficult to achieve with a real aperture system - this is the reason why it has until now not been possible to put a microwave sounder on a geostationary platform, GeoSTAR is instead based on a synthetic aperture imaging approach. Among the advantages of such a system are that there are no moving parts, and the size of the aperture is easily expandable to meet future needs. A ground based prototype of GeoSTAR is currently under development in an effort led by the Jet Propulsion Laboratory

The development of a microstrip-based L-band Dicke radiometer with the long-term stability required for future ocean salinity measurements to an accuracy of 0.1 psu is presented. This measurement requires the L-band radiometers to have calibration stabilities of les 0.05 K over 2 days. This research has focused on determining the optimum radiometer requirements and configuration to achieve this objective. System configuration and component performance have been evaluated with radiometer test beds at both JPL and GSFC. The GSFC test bed uses a cryogenic chamber that allows long-term characterization at radiometric temperatures in the range of 70 - 120 K. The research has addressed several areas including component characterization as a function of temperature and DC bias, system linearity, optimum noise diode injection calibration, and precision temperature control of components. A breadboard radiometer, utilizing microstrip-based technologies, has been built to demonstrate this long-term stability

Preliminary details of a 2-D synthetic aperture radiometer prototype operating from 50 to 55 GHz will be presented. The laboratory prototype is being developed to demonstrate the technologies and system design needed to do millimeter-wave atmospheric soundings with high spatial resolution from Geostationary orbit. The concept is to deploy a large thinned aperture Y-array on a geostationary satellite, and to use aperture synthesis to obtain images of the Earth without the need for a large mechanically scanned antenna. The laboratory prototype consists of a Y-array of 24 horn antennas, MMIC receivers, and a digital cross-correlation subsystem

A future mission for a new microwave atmospheric temperature sounder radiometer in a middle Earth orbit (MEO) at 11,000 km altitude is described. The MeoSTAR design uses a stationary 1-dimensional Synthetic Thinned Array Radiometer in the 50-60 GHz microwave sounding band, to provide a "pushbroom" image as the satellite orbits. The advantage of this concept is an image with a high spatial resolution and a wide swath with no scanning antenna to disturb the visual and IR sensors on the same satellite

An NEDT analysis of a Dicke radiometer with noise diode injection is presented. The analysis is formulated for a calibration that would form separate running averages of receiver noise temperature and of gain in order to minimize the NEDT and maximize the antenna observation duty cycle relative to the reference and noise diode duty cycles. Results are applied to the Aquarius ocean salinity radiometer problem to show that near ideal total-power radiometer performance is possible.

The NASA/JPL Airborne Rain MApping Radar (ARMAR) was operated on the NASA DC-8 aircraft during TOGA COARE in early 1993. On 12 flights ARMAR observed stratiform precipitation associated with mesoscale convective systems. The statistics of 16 melting layer parameters, including maximum reflectivity, cooling rate, Doppler velocity, LDR, and the HH-VV correlation coefficient are presented and discussed.

Pulse compression allows a substantial reduction in the peak transmitted power of a radar and is attractive for spaceborne remote sensing applications. In the case of a downward looking rain measuring radar, however, the range sidelobes associated with surface return can mask return from rain and must be kept to a minimum. The authors describe the pulse compression system for the NASA/JPL Airborne Rain Mapping Radar. This system uses time-domain weighting of the transmitted pulse and is able to achieve a range sidelobe level of -55 dB or better in flight tests. This is significantly lower than other values reported in the open literature.>

Calibration algorithms for a synthetic aperture microwave radiometer are presented. The calibration is geared to Earth remote sensing applications and is demonstrated on an airborne prototype thinned array imager. Two approaches to the system calibration are presented. The first utilizes commonly available reference brightness temperature scenes, such as open water, and the second utilizes data collected on the antenna range. Both algorithms yield spatial response information which is cast in matrix form and inverted to obtain the image reconstruction formula. Experimental results are examined, and errors in some reconstructed images are linked to the present prototype antenna design. Algorithms for improving the synthesized antenna pattern sidelobe performance are also presented. In one solution, the pattern efficiency is optimized by minimizing the pattern outside a defined beam. In another solution, the patterns are matched to a desired model pattern by the method of least squared errors. Both techniques offer an attractive alternative to aperture weighting.>