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.

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.

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.

Orbital observations of atmospheric temporal phenomena, require measurements with a cadence at least as frequent as the condition observed. This has basic architectural implications when one is designing a system to observe any particular atmospheric effect. We performed a study exploring the parameter space of low earth orbit constellations designed to retrieve temporal signatures, examining temporal resolution, constellation revisit time, regional coverage, spatial resolution, and data latency among other parameters, and assessed the results for mission cost, feasibility and technology maturity. We present the study results and discuss the implications for architectural trades particularly as they pertain to choices of geostationary versus low earth orbiting systems.

The next World Radio Conference (WRC) will be held in November 2019 in Geneva, Switzerland. This paper discusses WRC-19 agenda items that could impact scientific uses in Earth satellite remote sensing and radio astronomy.

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 -45° to +45°, 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 millimeter-wave 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 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.

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.

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.

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 SMAP RFI detecting digital backend performance is characterized using real-environment L-band RFI data from the SMAPVEX 2012 campaign. Various types of RFI signals are extracted from the airborne campaign dataset and fed to the SMAP radiometer using an Arbitrary Waveform Generator (AWG). The backend detection performance is tested, and missed-detections are further investigated. Initial results indicate RFI detection performance for the SMAP digital backend is acceptable.

The electromagnetic spectrum is a valued shared resource. Its scientific use allows us to learn about our universe, measure and monitor our planet, and communicate scientific data. The use of the spectrum is managed by national, regional, and global regulatory frameworks. There are increasing demands for new or extended allocations because of vast technological advances in the past few years. Understanding spectrum management is important in the successful planning and execution of missions and instruments, as well as in determining the potential source of radio frequency interference in existing data and instruments, and in working to ameliorate its impact. This paper provides a summary of this framework for radio scientists and engineers.

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.

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 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 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