A rain radar system study carried out for the European Space Agency was completed in early 1992. It lead to the definition and comparison of two ambitious rain radar instruments providing higher accuracy in retrieving the rain rate profile: (1) a dual- frequency radar, operating simultaneously at 24 and 35 GHz, (2) a dual-beam radar, operating at 24 GHz and achieving a stereoscopic observation of the rain cells through two antenna beams (one forward, one aft). At last, ALCATEL ESPACE just won an ESA contract concerning the development of the critical subsystems of the radar instrument, in order to demonstrate their feasibility. In this project, an all digital pulse compression chain will be designed and manufactured. This paper presents the instrument configurations which were defined in the course of all ALCATEL ESPACE rain radar studies, as well as preliminary results concerning the digital pulse compression breadboard.

An eight-element of bread board model for TRMM (Tropical Rainfall Measuring Mission) precipitation radar has been developed in Communications Research Laboratory (CRL), Japan. The active array system, which consists of all solid components, such as SSPA, LNA, and PIN-diode phase shifter, is integrated to check the basic performance as the spaceborn rain radar. After describing the characteristics of major components, overall performance of antenna scanning is described.

ALCATEL ESPACE, Toulouse, France, is developing active antenna technology, and designing and manufacturing tiles for two civil radar projects. One is for SPOTRADAR, a CNES (French Space Agency) project, and consists of 1/64th (equals one tile) of a 100 MHz X-band radar active antenna. The tile consists of 96 one-Watt-T/R modules, 3 power supplies, one radiating panel, 2 corporate feeds (one for T/R, one for internal calibration), and one control unit. The development started in 1990 and will be completed by March 1994. The other one is for ASAR, an ESA (European Space Agency) project, and consists of 1/20th (equals one tile) of a 16 MHz C-band radar active antenna. The tile consists of 16 eight-Watt T/R modules, 4 power supplies, one radiating pane, 2 corporate feeds, and one control unit. The development started in September 1991 and will be completed by June 1993. All tiles are representative of flight hardware and will be tested under space environment. This paper describes: (1) missions and radars, (2) whole antenna characteristics and projected performances, and (3) on-going developments and available test data (T/R modules, radiating panels, ...).

Spaceborne wind scatterometers are designed principally to measure radar backscatter from the ocean's surface for the determination of the near-surface wind direction and speed. Although measurements of the radar backscatter are made over land, application of these measurements has been limited primarily to the calibration of the instrument due to their low resolution (typically 50 km). However, a recently developed resolution enhancement technique can be applied to the measurements to produced medium-scale radar backscatter images of the earth's surface. Such images have proven useful in the study of tropical vegetation as well as glacial and sea. The resolution enhancement technique is based on image reconstruction techniques and takes advantage of the spatial overlap in independent scatterometer measurements. This paper describes briefly describes some of the applications of this medium-scale K(mu) - band imagery for vegetation studies, hydrology, sea ice mapping, and the study of mesoscale winds.

The Seasat scatterometer (SASS) first demonstrated that winds could be measured from space using spaceborne scatterometers. Such measurements are crucial in understanding air/sea interaction and in studies of global climate. New scatterometer technologies promise lighter and cheaper instruments in the future. In this paper a scatterometer concept known as ASCAT (A Scanning Scatterometer) is presented. ASCAT is a conically- scanning pencil-beam scatterometer system designed for flight on a small satellite. Coupled with an advanced wind retrieval algorithm, the design concept is inherently very flexible and is suited for a variety of tradeoffs in its design and implementation. Some of these tradeoffs are discussed and associated performance estimates are presented.

ALCATEL ESCAPE, France, has designed, developed and manufactured a radar altimeter for CNES, the French Space Agency. This altimeter was integrated onto the TOPEX POSEIDON satellite as part of a cooperative program between CNES and NASA. TOPEX POSEIDON was successfully launched on 10 August 1992 by an ARIANE-4 rocket and the CNES/ALCATEL ESPACE altimeter turned on 21 August for the first time. Since then, the altimeter has been operated 15% of the time, the remaining 85% being dedicated to the NASA supplied APL altimeter (both altimeters share the same antenna and operate at the same frequency). Analysis of collected data have demonstrated exceptional performances of ALCATEL ESPACE altimeter. This paper includes the presentation of the altimeter objectives, the altimeter basic characteristics, and the in-flight measured performances. The low weight, low power, low TM rate and low cost altimeter was specifically designed to be flown on any satellite platform, including small satellites. A project of a small satellite using the ALCATEL ESPACE altimeter is also presented.

The NASA Scatterometer (NSCAT) is scheduled for launch aboard the Japanese Advanced Earth Observing Satellite (ADEOS) in early 1996. The NSCAT instrument is a K(mu) -band radar which measures global ocean surface wind speeds and directions at 50 km resolution. The wind measurement is accomplished by first measuring the normalized backscatter cross section ((sigma) ₀) of the ocean at three different azimuth angles and two different polarizations using eight fan-beam antennas. Wind vectors are then retrieved during ground data processing using an empirical geophysical model function which relates (sigma) ₀ to wind speed and direction. The accuracy with which the ocean surface wind can be estimated is a sensitive function of the radiometric accuracy of the (sigma) ₀ measurements. For this reason, NSCAT must be a highly calibrated and stable instrument. A considerable amount of effort has been invested by the NSCAT project in (1) analysis, to understand and quantify the effects of calibration errors on wind retrieval performance, and (2) the development of both a pre- and post-launch calibration approach to insure that (sigma) ₀ measurement accuracy requirements are met. This paper summarizes these efforts and their resulting conclusions.

Spaceborne scatterometers are radar systems designed specifically to measure the normalized radar backscatter coefficient ((sigma) ₀) of the ocean's surface in order to determine the near-surface wind vector. Precise calibration of the instrument sensor is required to determine (sigma) ₀ accurately (of the order of a few tenths of a dB). Although careful calibration of the instrument is performed before launch, a post-launch calibration must also be performed. Post-launch calibration may be performed using ground stations and/or extended-area natural targets. The most commonly used extended area target has been the Amazon tropical rainforest which exhibits a remarkably high degree of homogeneity in its radar response over a very large area. However, the rainforest does exhibit some spatial and temporal variability. In this paper we present a simple technique for post-launch calibration of spaceborne scatterometer data using tropical rainforests which accounts for the temporal and spatial variability of the forest response. We first illustrate the technique with Seasat scatterometer (SASS) data then apply the technique to ERS-1 Active Microwave Instrument (AMI) scatterometer data. Gains corrections of up to several tenths of a dB are estimated for SASS. ERS-1 data was found to be well calibrated so that no corrections are required.

The SIR-C/X-SAR experiment, a joint effort of NASA, DARA/DLR, and ASI, is a multi- frequency, multi-polarization synthetic aperture radar (SAR) system for spaceborne scientific Earth imaging scheduled for initial launch in April 1994. Its predecessors include the L-Band, single-polarization Seasat, SIR-A, and SIR-B missions of the late 70's and early 80's. Since SIR-C/X-SAR is intended to be the predecessor to the multi-frequency, multi-polarization EOS SAR satellite, it serves to demonstrate and validate various advanced SAR architectures and data products. This paper documents the architecture and performance of the SIR-C L-Band and C-Band active SAR arrays, with emphasis on RF test results and expected performance. Performance at the array level is extrapolated from measurements at the T/R module and antenna subarray levels.

The Shuttle Imaging Radar—C(SIR---C) is a synthetic aperture radar(SAR) designed to fly on the Space Shuttle as a payload instrument in the Shuttle Radar Laboratory(SRL). To fly together with SIR-C are the German/Italian X-band SAR(X-SAR) and the Data Processing Subsystem (DPS), built by the Applied Physics Laboratory of John Hopkins University. The first mission is scheduled in mid-April, 1994, for an 8-day duration, with one follow-up mission in planning. The instrument itself — its inheritance, mission profile, potential scientific advances, design and performance — has been published earlier"2

At the present time, the strip mode aircraft Synthetic Aperture Radar (SAR) processing techniques are relatively more mature than the spaceborne SAR processing techniques. In airborne SAR, the exact formulation for the point target response has been made in both the time and frequency domain. Efficient and accurate wave domain processing techniques such as the chirp scaling algorithm, have thus far been optimized for airborne SAR systems. With regards to spaceborne SAR processing, the existing analysis and processing algorithms are only applicable to narrow beam systems where the phase function is well approximated by the second order polynomial. There were also no detailed studies to show the applicability and the performance of the chirp scaling algorithm when applied to the spaceborne SAR systems. This paper provides a proposed solution and the supporting analysis for extending spaceborne SAR processing techniques to cases having wide beam angles as well as large squint angles. Detailed discussion are made on how to adapt the chirp scaling algorithm to spaceborne SAR, the requirement in updating processing parameters, and the approach for flexible pixel location and higher phase fidelity.

Atmospheric profiles can be retrieved with techniques utilizing measurements of the propagation delay of signals from Global Positioning System (GPS) satellites observed by one or more receivers in low earth orbit. The observations are made in a limb-sounding geometry when the radio path between the transmitter and receiver passes through the atmosphere. The various components of the microwave refractivity of the earth's atmosphere are described and in particular the effect of liquid water suspended in the atmosphere is examined. We briefly describe the retrieval technique and the information it provides. We then discuss the impulse response of the retrieval technique and provide an intuitive description of the effect of horizontal structure on the retrieval process. We conclude with examples of the ability to separate temperature and water vapor densities in measurements made in the lower troposphere.

The Advanced Microwave Sounding Unit - B (AMSU-B) is a five channel radiometer. It provides two 'window' channels centered on 89 GHz and 150 GHz, and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. The paper addresses the principal requirements for the instrument and presents the results obtained for the Engineering Model instrument.

The U.K. Meteorological Office is procuring the humidity sounding element of the Advanced Microwave Sounding Unit (i.e. AMSU-B). This consists of a five channel microwave radiometer with channels centered at 89, 150, 183+/- 1, 183+/- 3, & 183+/- 7 GHz with a field of view of nominally 1.1 degree(s) (i.e. 15 km footprint at nadir). To characterize the radiometric behavior of AMSU-B an extensive series of tests have been performed on the engineering model in a thermal-vacuum chamber where the instrument can view an earth target and space target. Results showing the sensitivity, absolute calibration accuracy and linearity of the five channels are presented and show the instrument is within specification.

The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer due to be flown in conjunction with AMSU-A on the TIROS-N series of satellites. The instrument provides two 'window' channels centered on 89 GHz and 150 GHz and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. AMSU-B is a total power radiometer which is required to achieve a temperature sensitivity of 1 K to 1.2 K according to channel. It has a scan period of 8/3 seconds; during each scan it acquires 90 Earth view pixels, 4 cold calibration pixels and 4 hot calibration pixels. The calibration data are, in effect, used to determine the gain of the radiometer during that scan period. The sources used for calibration are an on-board, ambient temperature black- body target for the hot calibration, and deep space for the cold. The radiometric performance of the instrument is dependent on the accuracy to which it is calibrated during each scan period. The required calibration accuracy is +/- 1 K with a random component due to short term fluctuations of not more than +/- 0.2 K. This paper addresses the assessment of in-orbit error sources which effect calibration over the mission duration. The emphasis has been placed on errors in the bias values over life and errors associated with orbital variations of the hot and cold calibration target. The conclusions of the assessment are presented.

The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer. The instrument provides two 'window' channels, one at 89 GHz and one at 150 GHz. Three channels centered around 183.31 GHz are used to determine the atmosphere's humidity profile. Double sideband operation improves the instrument's temperature sensitivity by reducing its effective noise temperature. However, because this implies that the atmosphere is sounded over two separate frequency bands it is necessary to determine how the sensitivity of the instrument varies across each passband, and also the difference between the sensitivities of the lower and upper sidebands of each channel. This is especially important for the 183.31 GHz channels where the sounding frequency relates to altitude in the atmosphere. This paper describes a test technique which was developed to characterize the variation in sensitivity of each channel of AMSU-B.

A powerful and versatile approach to the design of three-dimensional Quasi-optical systems, which makes use of a number of complimentary predictive tools, is presented. The tools, and the method for combining them, are described. Examples of their effectiveness individually, and as a whole, are given.

The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer due to be flown in conjunction with AMSU-A on the TIROS-N series of satellites. The instrument provides two 'window' channels centered on 89 GHz and 150 GHz and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. AMSU-B is specified to have an antenna pattern half power beamwidth (HPBW) of 1.1$DEG +/- 10%, beam efficiency of >=95% over the main beam (2.5 times HPBW) and to control mispointing of the antenna beam to within +/- 0.1$DEG with a knowledge of +/- 0.05$DEG. The paper describes the techniques used to measure the performance of the AMSU-B antenna system and the results obtained. Using the techniques described, the antenna patterns of AMSU-B were measured over a dynamic range greater than 76 dB. Beam pointing was measured to within +/- 0.02$DEG.

A study was undertaken to determine the accuracy of DMSP SSM/T-2 water vapor sounder brightness temperature measurements by independent comparison with co-located aircraft measurements. Five underflights of the SSM/T-2 were made by NASA ER-2 research aircraft which carried the MIR, an instrument with similar channels and scan characteristics to the SSM/T-2 and a stated accuracy of 1 K. The flights occurred on both coasts of the U.S. with both water and land surfaces targeted for measurements. Comparisons of the SSM/T-2 and MIR 183 GHz measurements over water fields of view (FOVs), which provide the most accurate estimate of the true instrument bias, display RMS differences of 0.9 to 1.6 K, roughly within the accuracy limits of the calibrating MIR instrument. Larger differences occur for regions where surface emissivity variations are significant (up to 11 K for coast and land FOVs). The overall conclusion is that the SSM/T-2 suffers no significant bias in its calibration.

Total Precipitable Water (TPW) calculations using the Special Sensor Microwave Water Vapor Sounder (SSM/T-2) launched November 1991 on the Defense Meteorological Satellite Program (DMSP) F-11 satellite are compared with TPW values obtained from analysis of the collocated Special Sensor Microwave Imager (SSM/I). The four data sets used were collected over the ocean. The different characteristics of these instruments are described. Their response to the ambient conditions indicate that the two independent measures of TPW generally agree within 20 percent over the range of TPW observed. Some direct measurements of TPW obtained from radiosondes at the time and in proximity to the satellite overpasses are presented for independent comparison.

In support of the AMSU-B program, the UK Meteorological Office (UKMO) in collaboration with Laboratoire de Meteorologie Dynamique (LMD) have developed the Microwave Airborne Scanning Radiometer System (MARSS) which operates at 89 and 157 GHz, near the 'window' channels of AMSU-B. This total power radiometer is flown on board the C-130 aircraft of the UKMO which is well- equipped with sensors measuring thermodynamical and cloud microphysical parameters up to a height of 9 km. The instrument has a scanning cycle time of approximately 3 seconds, during which time the radiometer takes 9 upward and 9 downward views as well as two views of internal calibration targets. It has been found that the Liebe MPM model gives more consistent agreement with the observed brightness temperatures than other published transmission models.

The Advanced Microwave Precipitation Radiometer (AMPR) has just completed a number of successful field deployments and will be refitted with a number of improvements to allow for improved calibration measurements and enhanced data processing. The AMPR is sponsored by NASA at the Marshall Space Flight Center for the investigation of precipitation using passive microwave brightness temperatures from the NASA ER-2 high altitude aircraft. The primaiy goal of the AMPR is the exploitation of the scattering signal of precipitation at frequencies near 18, 37, and 85 GHz together to unambiguously retrieve storm precipitation structure and intensity information from high in the storm (85 GHz) to deep within the storm (18 GHz). The instrument is a total power radiometer using a cross track scanning technique to gather data at four separate frequencies. The four center frequencies, 10.7, 19.35, 37.10, and 85.50 GHz. are separated into two feedhorns. The 10.7 GHz. being the larger diameter horn, drives the placement and position of the loads and the three remaining frequencies are contained in a multifrequency feedhom The previous load material used in the initial instrument deployments for the hot and cold loads were found to.be difficult to maintain at a uniform elevated temperature, causing gradients through the material. With the uncertainty in sensed temperatures versus radiometer brightness temperatures a set of new loads were developed and installed. Also, with the harsh environment in which the instrument flies, the data system needed to be revised. With the current data rates, a tape system was not selected to help minimi7.e the mechanical suscepubility to the high condensation seen by the instrument and electronics on landing. A flash RAM approach will be used to store the data during each flight at a modest rate of 160 words/sec. Future growth in the data system will allow increased data throughput and capacity for J<OSS1Dle dual-polari7.ation applications. Ground station computers will take :flash RAM cards and ingest the data for quick view presentation and initial data analysis for next day flight preparation.

A research program has been initiated at NASA Langley Research Center to investigate the critical technologies for developing advanced microwave radiometers suitable for Earth science observations. A significant objective of this research is to enable microwave measurements with adequate spatial resolutions for a number of Earth science parameters, such as sea ice, precipitation, soil moisture, sea surface temperature, and wind speed over oceans. High spatial resolution microwave sensing from space with reasonable swath widths and revisit times favor large real aperture radiometer systems. However, the size requirements for such systems are in conflict with the need to emphasize small launch vehicles. This paper describes a tradeoff between the science requirements, basic operational parameters, test configurations, and expected sensor performance for a satellite radiometer concept. The preliminary designs of real aperture systems utilizing novel light-weight compact-packaging techniques are used as a means of demonstrating this technology.

The desire for passive microwave measurements of improved spatial resolution and the development of radiometer and antenna technologies have resulted in several studies to investigate large aperture radiometer systems. These systems may utilize aperture synthesis, phased array, or reflectors and phased array feeds. The in-flight calibration and thus the stability of these systems is an important consideration in any realistic study. Thus, estimates of stability and calibration requirements for these complex radiometer systems are of interest. A statistical method to characterized the stability of microwave radiometer components and subsystems which could aid in the development of numerical models to predict the stability of these radiometer systems is presented in this paper. Preliminary measurements results to demonstrate the utility and limitations of the approach are also presented.

This paper deals with the measurement method on how to obtain the useful data in microwave passive remote sensing. The first case is, in order to obtain the microwave antenna radiative temperature of a real object (usually it is bigger) with radiometer at a real height, one can take the imitation method to get the data by measuring a smaller imitating model of the real object with the radiometer at a lower height, the relation between the real surveying and the imitation surveying is given in this paper. The second case is, the relation between the antenna radiative temperature of an object measured with the radiometer at a height and that of the same object measured with the radiometer at another height is obtained, and here the antenna radiative temperature of a plate similar to the down-surveying area of that object is required. The experimental results are in agreement, and it shows that these methods are available in the microwave radiative measurement.

The following paper describes an airborne multispectral mm-wave imaging technique that has been developed for remote sensing ocean and atmosphere. The paper discusses design and salient characteristics of this instrument. The results of field experiment on microwave studying of the Kurosio ring are presented.

A concept for a high spatial resolution passive microwave instrument is presented. In a low Earth orbit, the Synthetic Aperture Imaging Radiometer (SAIR) instrument would provide microwave images with a 5 km spatial resolution, that is 10 times better than current spaceborne passive microwave sensors. The scientific applications would be for high spatial resolution measurements of sea and land ice, snow cover, and rainfall. The SAIR would be a low cost, low mass, non-scanning instrument that could be launched with a small launch vehicle.

The United States Air Force Defense Meteorological Satellite Program (DMSP) provides state-of-the-art tactical weather forecasts in support of a broad range of military missions. To enhance current capability, the Air Force will equip the block 5D-3 spacecraft with a Special Sensor Microwave Imager Sounder (SSMIS) currently being designed and fabricated by Aerojet Electronic Systems Division in Azusa, California. Aerojet is responsible for the development of the sensor, the uplinkable flight processing software and the SSMIS ground processing software which includes the processing algorithms. The SSMIS will provide lower atmospheric temperature and humidity profiles with greatly enhanced spatial resolution compared to existing microwave sensors and provide twelve additional surface related environmental parameters including wind speed over the ocean and rain rates. In addition the SSMIS will provide temperature profiles of the upper atmosphere, adding a new capability to the DMSP. The SSMIS ground processing software will be installed at the Air Force Global Weather Center and at the Navy Fleet Numerical Oceanography Center. This paper will describe the hardware, software and the status of the SSMIS program.

This paper provides details of the configuration, measured performance and calibration of the Advanced Microwave Sounding Unit-A (AMSU-A) for the Advanced TIROS-N spacecraft.

The Special Sensor Microwave water vapor profiler (SSM/T-2) is a five channel passive microwave sensor that operates in the 90 - 190 GHz frequency band. The instrument was developed by Aerojet Electronic Systems Division (AESD) of GenCorp Aerojet under a contract to the Defense Meteorological Satellite Program (DMSP). The first in a series of these instruments was successfully orbited in November 1991. This paper addresses details of the instrument configuration, as well as relevant information on the status of the project. A block diagram of the instrument is described in relation to its electrical, environmental and reliability requirements. Performance data measured in laboratory conditions is presented along with data from the operating unit in orbit.