This paper presents the simulations needed to assess the performance of the Radio Frequency Interference (RFI) processor for the Copernicus Imaging Microwave Radiometer (CIMR). A brief description of the common RFI processor used for all radiometer bands in CIMR and of the system parameters is given in sections 2 and 3, respectively. Hereafter follows a description of the simulations and how the performance can be obtained in sections 4 and 5. Conclusion is in section 6.

Radio Frequency Interference (RFI) has been an increasing problem for Earth observing radiometers - especially spaceborne - for many years as RFI adds to the power level measured by these radiometers and thereby affects the results derived from the measurements. Methods for removing RFI have been either ground processing (the only available option for older satellites like SMOS) or producing more output to improve ground processing capabilities for removing RFI as in SMAP. With the increasing demands for improved spatial and radiometric resolution the first method is insufficient and the second requires downloading too much data. During the design of the MetOp-Second Generation (MetOp-SG) satellite it has been demonstrated that on-board RFI detection is possible and for future radiometers on-board processing is the way forward. This paper introduces the RFI processor for the High Priority Copernicus Mission satellite CIMR (Copernicus Imaging Microwave Radiometer) which comprises 25 radiometer channels at five different radiometer bands.

In a traditional spaceborne microwave radiometer system with a scanning antenna, there is often a conflict between spatial and radiometric resolution. Integration over many beams per frequency might be necessary to improve radiometric resolution. Many beams may be generated using many classical feed horns or by a focal plane array (FPA) system. At C-band, horns are bulky and replacing several such horns with an FPA is an interesting option. The FPA concept uses many small antenna elements, many receivers, and powerful online digital processing. An FPA system that can replace four horns is evaluated, especially concerning power consumption.

As part of a validation of the SMOS sea ice thickness product, the DTU Space built L-band radiometer, EMIRAD-L (formerly EMIRAD 2), was participating in the MOSAiC campaign. Situated on P-deck of the German research vessel Polarstern and overlooking the sea or ice from the port side it recorded L-band radiometric data throughout the campaign. This paper briefly describes the installation, and provides some processing results as well as initial RFI findings.

For many years Radio Frequency Interference (RFI) has been an increasing problem especially for spaceborne radiometers; as a result, there has been an increasing effort on developing algorithms and hardware for counteracting RFI. Due to the amount of time it takes to develop and build spaceborne systems, this development has had a limited effect on current spaceborne systems. In spaceborne radiometer systems currently being built the need for onboard detection and blanking of RFI has been acknowledged. This paper addresses the efforts to implement RFI detection and blanking techniques on European operational spaceborne systems, MetOp Second Generation (MetOp-SG) satellites and the Copernicus High Priority Mission Passive Imaging Microwave Radiometer (CIMR) satellites.

Earth's cold regions are key elements of the planet's climate system: they have strong feedbacks with global change and they have a direct impact on human activities. Despite their importance, at present they are not adequately monitored by state-of-the-art instruments. In order to fill this gap, a dedicated spaceborne mission called Cryorad has been proposed in the framework of the ESA Earth Explorer 10 call. The mission would comprise a 0.4-2 GHz nadir-looking radiometer installed on a polar-orbit satellite. Scientific and technical studies are underway, as well as experimental campaigns in Greenland and Antarctica.

In order to evaluate a potential calibration target for spaceborne L-band radiometer systems, a 350 × 350 km area near the Concordia station on the East Antarctica plateau was mapped by an airborne L-band radiometer. Unexpectedly, the area showed significant brightness temperature spatial variations, well correlated with bedrock topography, hence ice thickness. Using SMOS data over a poorly known part of Antarctica, ice thickness in this area has been assessed, and an existing bedrock map has been improved.

An RFI processor breadboard has been designed and developed for future spaceborne microwave radiometer systems. RFI detection is based on the anomalous amplitude, kurtosis, and cross-frequency algorithms. These are implemented in VHDL code in an FPGA. Thus algorithm performance can be assessed by proper code simulation. The breadboard has been integrated with a Ku band radiometer subjected to RFI-like signals from a laboratory generator. Simulations show that the algorithms as implemented work according to theory when subjected to pulsed sinusoidal and QPSK signals. The laboratory measurements confirm the performance for pulsed signals.

Radio Frequency Interference (RFI) is an increasing threat to proper operation of space-borne Earth viewing microwave radiometer systems. There is a steady growth in active services, and tougher requirements to sensitivity and fidelity of future radiometer systems. Thus it has been decided that the next generation MetOp satellites must include some kind of RFI detection and mitigation system at Ku band. This paper describes a breadboard processor that detects and mitigates RFI on-board the satellite. Thus cleaned data can be generated in real time, and following suitable integration, downloaded to ground at the modest data rate usually associated with radiometer systems.

The EMIRAD-2 radiometer is an airborne radiometer, which has been developed for calibration and validation of the ESA/SMOS mission data, as well as for data collection for development and evaluation of models for parameter retrieval, based on radiometric L-band data. The instrument has been applied in various campaigns in very different environments. Primary calibration sources are a traditional hot load along with a noise diode, and the calibration quality is verified prior to almost each flight using a liquid Nitrogen cooled target. EMIRAD-2 also features a commercial-off-the-shelf (Miteq) low noise amplifier (LNA), operated as an active cold load (ACL). This paper shows results for the ACL performance throughout several years of operation under very different conditions, and important parameters, such as stability, repeatability, and sensitivity to operating temperature variations are assessed.

The increasing problem with Radio Frequency Interference (RFI) in protected radiometer frequency bands has inspired the development and implementation of methods for detecting RFI. With increasing demands for next generation spaceborne radiometers, it becomes necessary to include RFI detection in such systems. Since input to these methods require much more data than traditional radiometer data, and since the downlink capacity for spaceborne systems are limited, RFI detection and mitigation must be implemented on-board thereby maintaining the low downlink data rate. The development of such a system is described in this paper.

A 350 × 350 km area near the Concordia station on the high plateau of Dome C in Antarctica has been mapped by an airborne L-band radiometer system. The area was expected to display a rather uniform brightness temperature close to the yearly mean temperature — well suited for calibration checks for spaceborne instruments like SMOS, Aquarius, and SMAP. The measured brightness temperatures shows unexpected variations like 8 K variation on an East-West profile through Concordia, and in certain cases a slope of almost 1 K per km. Comparing the measured brightness temperature map with bottom topography reveals a convincing correlation. Simulations show that variations in bedrock topography can indeed modulate the brightness temperature appropriately to explain the observed variations.

Design considerations concerning a scanning as well as a push-broom microwave radiometer system are presented. Strict requirements to spatial and radiometric resolution leads to a multiple-beam scanner achieving good sensitivity through integration over many beams, or to a push-broom system where sensitivity is not a problem. Strict requirements to land contamination leads to a dense feed array system. Resource demands, especially power, are important issues, and first estimates are presented.

This paper describes the EMIRAD L-band radiometer, developed in support of the ESA/SMOS mission. The instrument is a fully polarimetric, dual antenna system, built with special focus on antenna accuracy, receiver stability, and detection and mitigation of radio frequency interference (RFI). The EMIRAD system has been installed on three different airborne platforms for measurements of sea surface signatures and salinity, soil moisture, and the homogeneity of the Antarctic SMOS calibration site. The installations are shown in the paper, and some major results for ocean and ice observations are given.

The Soil Moisture and Ocean Salinity (SMOS) satellite with a passive L-band radiometer monitors surface soil moisture. In addition to soil moisture, vegetation optical thickness$\tau_{\rm NAD}$is retrieved (L2 product) from brightness temperatures ($T_{B}$, L1C product) using an algorithm based on the L-band Microwave Emission of the Biosphere (L-MEB) model with initial guesses on the two parameters (derived from ECMWF products and ECOCLIMAP Leaf Area Index, respectively) and other auxiliary input. This paper presents the validation work carried out in the Skjern River Catchment, Denmark. L1C/L2 data and the most sensitive algorithm parameters were analyzed by network and airborne campaign data collected within one SMOS pixel (44 km diameter). The SMOS retrieval is based on the prevailing low vegetation class. For the L1C comparison,$T_{B}$'s were calculated from in situ soil moisture using L-MEB. Consistent with worldwide findings, the initial/retrieved SMOS soil moisture captures the in situ dynamics well but with significant wet/dry biases and too large amplitudes in case of the latter. While the initial$\tau_{\rm NAD}$is in range with an in situ estimate for low agricultural vegetation, the retrieved$\tau_{\rm NAD}$is too high with too pronounced temporal variability. A filter based on L2 criteria removed radio frequency interference (RFI) and improved the$R^{2}$between retrieved and network soil moisture from 0.49 to 0.61, while the bias remained$(-0.092/-\!0.087\ \hbox{m}^{3}/\hbox{m}^{3})$. Likely error sources include the following: 1) still present RFI; 2) potential link between high retrieved$\tau_{\rm NAD}$and other L-MEB parameters, e.g., low roughness parameter$(H_{R})$; 3)$\sim$18% lower sand and$\sim$8% higher clay fractions while$\sim\!\!0.35\ \hbox{g/cm}^{3}$lower bulk density in SMOS algorithm than in situ; and 4) caveats in the Dobson dielectric mixing model implemented in the L-MEB model. A previous study at the Danish validation site had revealed superior performance of the Mironov dielectric mixing model at the 2$\times$2 km scale. Studies are ongoing to address the aforementioned issues, and the role of organic surface layers will be investigated.

ESA's soil moisture and ocean salinity mission, SMOS, has been found to suffer much more from radio frequency interference (RFI) than expected, and methods for detecting RFI in SMOS data are of vital importance. This paper describes a method using the 3rdand 4thStokes parameters for the purpose. Obvious hot-spots are detected, but also smaller, yet still detrimental RFI, spreading out over for example the ocean, is detected. It is also discussed how detected and flagged samples statistically deviate from their surroundings.

Man-made Radio Frequency Interference (RFI) is an increasingly threatening problem for passive microwave radiometry from space. The problem is presently very evident in L-band data from SMOS, but it is realized that it is already now a problem at other traditional radiometer bands at C, X, and Ku bands. Studies of data from existing radiometer systems have revealed this, and clearly indicates increasing RFI intensity over time. Thus, future missions have to take this into consideration, and dedicated hardware and algorithms to safely detect and mitigate RFI must be included. The design of such an RFI processor is discussed, and resource demands on the spacecraft are indicated.

This paper describes the spectral kurtosis detector as an additional indicator for radio frequency interference, RFI in passive remote sensing systems. The estimator is based on continuous Fast Fourier Transformation of samples, followed by evaluation of each frequency bin in subsequent data blocks, using a kurtosis estimator. The spectral kurtosis response is simulated and tested in a laboratory setup for various RFI types, and the estimator response is evaluated and compared to the response of the traditional time domain kurtosis estimator. Results show that great benefits of spectral kurtosis is reached for continuous wave RFI or for RFI with high duty-cycle compared to the radiometer integration time. Typically the spectral kurtosis is superior for duty-cycles above 15%, while standard kurtosis is more efficient for lower duty-cycles, down to a few percent. In combination, the two estimators provide a very good indicator for most RFI types.

The Soil Moisture and Ocean Salinity (SMOS) mission delivers global surface soil moisture fields at high temporal resolution which is of major relevance for water management and climate predictions. Between April 26 and May 9, 2010, an airborne campaign with the L-band radiometer EMIRAD-2 was carried out within one SMOS pixel (44 × 44 km) in the Skjern River Catchment, Denmark. Concurrently, ground sampling was conducted within three 2 × 2 km patches (EMIRAD footprint size) of differing land cover. By means of this data set, the objective of this study is to present the validation of SMOS L1C brightness temperaturesTBof the selected node. Data is stepwise compared from point via EMIRAD to SMOS scale. From ground soil moisture samples,TB's are pointwise estimated through the L-band microwave emission of the biosphere model using land cover specific model settings. TheseTB's are patchwise averaged and compared with EMIRADTB's. A simple uncertainty assessment by means of a set of model runs with the most influencing parameters varied within a most likely interval results in a considerable spread ofTB's (5-20 K). However, for each land cover class, a combination of parameters could be selected to bring modeled and EMIRAD data in good agreement. Thereby, replacing the Dobson dielectric mixing model with the Mironov model decreases the overall RMSE from 11.5 K to 3.8 K. Similarly, EMIRAD data averaged at SMOS scale and corresponding SMOSTB's show good accordance on the single day where comparison is not prevented by strong radio-frequency interference (RFI) (May 2, avg. RMSE = 9.7 K). While the advantages of solid data sets of high spatial coverage and density throughout spatial scales for SMOS validation could be clearly demonstrated, small temporal variability in soil moisture conditions and RFI contamination throughout the campaign limited the extent of the validation work. Further attempts over longer time frames are planned by means of soil moisture network data as well as studies on the impacts of organic layers under natural vegetation and higher open water fractions at surrounding grid nodes.

Several soil moisture and sea salinity campaigns, including airborne L-band radiometer measurements, have been carried out in preparation for the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission. The radiometer used in this context is fully polarimetric and is capable of detecting radio frequency interference (RFI) using the kurtosis method. Analyses have shown that the kurtosis method generally detects RFI in an efficient manner, particularly concerning pulsed, low duty cycle signals, but it has some shortcomings when it comes to more continuous wave signal types. Hence, other detection methods have been investigated as well. In particular, inspection of the third and fourth Stokes parameters shows promising results—possibly as a complement to the kurtosis method. The kurtosis method, however, cannot be used with SMOS data. Since SMOS is fully polarimetric, the third and fourth Stokes parameter method is an option, and a first assessment using a fully polarimetric SMOS data set looks promising. Finally, a variable incidence angle signature algorithm is introduced, and the possibility of using this as an RFI indicator is discussed.

Several soil moisture and sea salinity campaigns, including airborne L-band radiometer measurements, have been carried out in preparation for the ESA Soil Moisture and Ocean Salinity (SMOS) mission. The radiometer used in this context is fully polarimetric and is capable of detecting Radio Frequency Interference (RFI) using the kurtosis method. Analyses have shown that the kurtosis method generally detects RFI in an efficient manner, even though it has its shortcomings. Hence, other detection methods have been investigated as well. In particular, inspection of the 3rdand 4thStokes parameters shows promising results possibly as a complement to the kurtosis method. The kurtosis method, however, cannot be used with SMOS data. Since SMOS is fully polarimetric, the 3rd and 4th Stokes parameter method is an option, and this has been used on a recent, fully polarimetric SMOS data set. Finally, a discussion of the variable incidence angle signature algorithm, and the possibility of using this as RFI indicator, is carried out.

In support of the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission, a number of soil moisture and sea salinity campaigns, including airborne L-band radiometer measurements, have been carried out. The radiometer used in this context is fully polarimetric and has built-in radio-frequency-interference (RFI)-detection capabilities. Thus, the instrument, in addition to supplying L-band data to the geophysicists, also gave valuable information about the RFI environment. Campaigns were carried out in Australia and in a variety of European locations, resulting in the largest and most comprehensive data set available for assessing RFI at L-band. This paper introduces the radiometer system and how it detects RFI using the kurtosis method, reports on the percentage of data that are typically flagged as being corrupted by RFI, and gives a hint about geographical distribution. Also, examples of polarimetric signatures are given, and the possibility of detecting RFI using such data is discussed.

Two semiconductor active cold loads (ACLs) to be used as cold references in spaceborne microwave radiometers have been developed. An X-band frequency was chosen, and the target noise temperature value was in the 50-100-K range. The ACLs are characterized in the operating temperature range of 0degC-50degC, and long-term stability is assessed. To this end, a test bed has been developed. This test bed is actually a stable radiometer, and its design and performance are discussed. The test setup is described, and test campaign results indicate output temperatures of 77 and 56 K for the two ACLs. The temperature sensitivity is slightly below 0.4 K/degC for the units, and long-term stability within 2 K/year is observed.

ESA has assigned the Technical University of Denmark to develop an airborne P-band ice sounding radar demonstrator. The intention is to obtain a better understanding of the electromagnetic properties of the Antarctic ice sheet at P-band and to test novel ice sounding techniques in preparation for a potential spaceborne ice sounding radar. The airborne system is a coherent, high-resolution and fully polarimetric radar. Aperture synthesis is applied in the along track direction and an experimental surface clutter suppression technique based on a multi-aperture antenna can be applied in the across track direction. In May 2008, a proof-of-concept campaign was organized in Greenland, where data were acquired over the ice sheet. The system proved capable of detecting the bedrock under 3 km thick ice and of mapping the internal ice layers down to a depth of at least 1.3 km. In this paper, the system concept is outlined and first results are presented.

The CAROLS, L band radiometer, is built and designed as a copy of EMIRAD II radiometer of DTU team. It is a Correlation radiometer with direct sampling and fully polarimetric (i.e 4 Stockes). It will be used in conjunction with other airborne instruments (in particular the C-Band scatterometer (STORM) and IEEC GPS system, Infrared CIMEL radiometer, one visible camera), in coordination with in situ field campaigns for SMOS CAL/VAL. The instruments are implemented on board the French research airplane ATR42. A validation campaign with four flights was made over south west of France, Hourtin Lake and Bay of Biscay (Atlantic Ocean) in September 2007. In order to qualify the radiometer data, different types of aircraft movements were realized: circle flights, wing and nose wags. Simultaneously to flights, different ground measurements were made over continental surfaces and ocean. First results show a good quality of data over ocean surfaces. For continental surfaces, important Radio-Frequency Interferences (RFI) were observed over a large part of the studied region.

In this paper, the L-band Microwave Emission of the Biosphere (L-MEB) model used in the Soil Moisture and Ocean Salinity (SMOS) Level 2 Soil Moisture algorithm is calibrated using L-band (1.4 GHz) microwave measurements over a coniferous (pine) and a deciduous (mixed/beech) forest. This resulted in working values of the main canopy parameters optical depth (tau), single scattering albedo (omega), and structural parameterstt(H) andtt(V), besides the soil roughness parametersHRandNR. Using these calibrated values in the forward model resulted in a root mean-square error in brightness temperatures from 2.8 to 3.8 K, depending on data set and polarization. Furthermore, the relationship between canopy optical depth and leaf area index is investigated for the deciduous site. Finally, a sensitivity study is conducted for the focus parameters, temperature, soil moisture, and precipitation. The results found in this paper will be integrated in the operational SMOS Level 2 Soil Moisture algorithm and used in future inversions of the L-MEB model, for soil moisture retrievals over heterogeneous, partly forested areas.

The National Airborne Field Experiment 2005 (NAFE'05) and the Campaign for validating the Operation of Soil Moisture and Ocean Salinity (CoSMOS) were undertaken in November 2005 in the Goulburn River catchment, which is located in southeastern Australia. The objective of the joint campaign was to provide simulated Soil Moisture and Ocean Salinity (SMOS) observations using airborne L-band radiometers supported by soil moisture and other relevant ground data for the following: (1) the development of SMOS soil moisture retrieval algorithms; (2) developing approaches for downscaling the low-resolution data from SMOS; and (3) testing its assimilation into land surface models for root zone soil moisture retrieval. This paper describes the NAFE'05 and CoSMOS airborne data sets together with the ground data collected in support of both aircraft campaigns. The airborne L-band acquisitions included 40 km × 40 km coverage flights at 500-m and 1-km resolution for the simulation of a SMOS pixel, multiresolution flights with ground resolution ranging from 1 km to 62.5 m, multiangle observations, and specific flights that targeted the vegetation dew and sun glint effect on L-band soil moisture retrieval. The L-band data were accompanied by airborne thermal infrared and optical measurements. The ground data consisted of continuous soil moisture profile measurements at 18 monitoring sites throughout the 40 km × 40 km study area and extensive spatial near-surface soil moisture measurements concurrent with airborne monitoring. Additionally, data were collected on rock coverage and temperature, surface roughness, skin and soil temperatures, dew amount, and vegetation water content and biomass. These data are available at www.nafe.unimelb.edu.au.

This paper presents the top-level design of an airborne, P-band ice sounding radar under development at the Technical University of Denmark. The ice sounder is intended to provide more information on the electromagnetic properties of the Antarctic ice sheet at P-band. A secondary objective is to test new ice sounding techniques, e.g. polarimetry, synthetic aperture processing, and coherent clutter suppression. A system analysis involving ice scattering models confirms that it is feasible to detect the bedrock through 4 km of ice and to detect deep ice layers. The ice sounder design features a digital signal generator, a microstrip antenna array, a conventional RF-architecture with a central transmitter, four receivers, and internal calibration loops. In 2008 the first data acquisition campaign will take place in Greenland.

The performance of a previously developed algorithm for radio frequency interference (RFI) detection and mitigation is experimentally evaluated. Results obtained from CoSMOS, an airborne campaign using a fully polarimetric L-band radiometer are analyzed for this purpose. Data is collected using two separate integration times, as a result of which sensitivity of the detection algorithm is measured. The impact of RFI on remotely sensed data over land and sea is also presented.

The CoSMOS (Campaign for validating the Operation of the Soil Moisture and Ocean Salinity mission) campaign was conducted during November of 2005 in the Goulburn River Catchment, in SE Australia. The main objective of CoSMOS was to obtain a series of L-band measurements from the air in order to validate the L-band emission model that will be used by the SMOS (Soil Moisture and Ocean Salinity) ground segment processor. In addition, the campaign was designed to investigate open questions including the sun-glint effect over land, the application of polarimetric measurements over land, and to clarify the importance of dew and interception for soil moisture retrievals. This paper summarises the campaign activities, and presents progress on the analysis of the CoSMOS data set.

This paper describes the experimental activities carried out during the international experiment CAPMOS’05, which was carried out on an off-shore platform in Katsiveli, Ukraine, in May-June 2005. During the experiment, the sea surface was continuously observed by synchronous active and passive microwave instruments, combined with contact and optical observations, in order to retrieve the wave parameters and to characterize the spectral properties of the waved surface. An additional airborne campaign was carried out on the North Sea to investigate the Radio Frequency Interferences (RFI) effects which seriously may hamper the L-band measurements. Predictions of a two scale emissivity model of waved sea surface resulted in agreement with the radiometric data at S and Ka bands. A method for retrieving the wave spectrum parameters from angular radiometric measurements was developed, and compared with three different spectrum models. An inversion algorithm for retrieving the horizontal wind speed component from radiometric data at S and Ka band was implemented and validated with the experimental acquisitions.

Two semi-conductor Active Cold Loads (ACLs) to be used as cold references in spaceborne microwave radiometers have been developed. An X-band frequency has been chosen, and the target noise temperature value is in the 50 to 100 K range. The ACLs are to be characterized in the operating temperature range 0-50degC, and long term stability must be assessed. To this end a Test Bed has been developed. This Test Bed is actually a stable radiometer, and its design and development is discussed. The test setup is described, and preliminary test campaign results indicate output temperatures of 73 K and 51 K for the two ACLs. The temperature sensitivity is slightly below 0.4 K/degC for the units, and excellent stability seems to prevail.

Mapping of sea surface salinity, based on L-band radiometric measurements, is presently investigated as a preparation for space missions. Special concern is on correction for effects caused by the sea surface roughness, and this paper will address two campaigns, LOSAC and CoSMOS, with the aim of investigating these effects. Conclusions from LOSAC are presented, and open issues to be investigated during the presently ongoing CoSMOS campaign are outlined. Finally, the installation and campaign plan for CoSMOS are presented.

Accurate measurement of the antenna voltage patterns of large-aperture synthesis radiometers is critical in order to achieve good radiometric accuracy, and a very time consuming and expensive task. Measurement requirements and a tradeoff study relating radiometric accuracy degradation and number of elements to be measured are presented.

Microwave radiometers can measure sea surface salinity from space using L-band frequencies around 1.4 GHz. However, requirements to the accuracy of the measurements, in order to be satisfactory for the user, are so stringent that the influence of the intervening atmosphere cannot be neglected. The present paper will describe and quantify the effect of losses in the atmosphere caused by oxygen, water vapor, clouds, and rain, and indicate possible correction actions to be taken.

The spectral window at L-band (1.413 GHz) is important for passive remote sensing of surface parameters such as soil moisture and sea surface salinity that are needed to understand the hydrological cycle and ocean circulation. Radiation from celestial sources (mostly galactic) is strong in this window, and an accurate accounting of this background radiation is often needed for calibration. This paper presents a comparison of the background radiation predicted by a model developed from modern radio astronomy measurements with measurements made with several modern L-band remote sensing radiometers. The comparison validates the model and illustrates the magnitude of the correction necessary in remote sensing applications.

L-band radiometers can measure ocean salinity and soil moisture from space. A synthetic aperture radiometer system, SMOS, is under development by ESA for launch in 2007. A real aperture push-broom system, Aquarius, has been approved by NASA for launch in 2008. Pros et cons of the two fundamentally different imaging concepts behind the two missions are discussed

The polarimetric EMIRAD radiometer, based on novel digital down conversion and detection techniques, has been installed on a C-130 aircraft from the Royal Danish. Air Force during the L-band Ocean Salinity Airborne Campaign (LOSAC) in 2001 and 2003. Full 360/spl deg/ circle flight patterns around the same target area as well as clover leaf patterns have been measured, and both provide an azimuth signature of the ocean at a constant incidence angle. The resulting azimuth signatures show significant variations in all the three first Stokes parameters, and a correlation is found between the downwelling galactic background signal and the measured results. The measured 3/sup rd/ Stokes parameter has variations of the same order of magnitude as the two linear polarizations, and to verify this result, an experiment for direct observation of the sky over long time is set up. This experiment confirms the presence of a polarized galactic background signal, and conclusions are made with respect to the necessity for polarimetric corrections in future measurements over the sea at L-band.

Radiation from the celestial sky in the spectral window at 1.413 GHz is strong and an accurate accounting of this background radiation is needed for calibration and retrieval algorithms. Modern radio astronomy measurements in this window have been converted into a brightness temperature map of the celestial sky at L-band suitable for such applications. This work presents a comparison of the background predicted by this map with the measurements of several modern L-band remote sensing radiometers.

A number of studies have shown the feasibility of estimating surface soil moisture from L-band passive microwave measurements. Such measurements should be acquired in the near future by the Soil Moisture and Ocean Salinity (SMOS) mission. The SMOS measurements will be done at many incidence angles and two polarizations. This multiconfiguration capability could be very useful in soil moisture retrieval studies for decoupling between the effects of soil moisture and of the various surface parameters that also influence the surface emission (surface temperature, vegetation attenuation, soil roughness, etc.). The possibility to implement N-parameter (N-P) retrieval methods (where N = 2, 3, 4, ..., corresponds to the number of parameters that are retrieved) was investigated in this study based on experimental datasets acquired over a variety of crop fields. A large number of configurations of the N-P retrievals were studied, using several initializations of the model input parameters that were considered to be fixed or free. The best general configuration using no ancillary information (same configuration for all datasets) provided an rms error of about 0.059 m/sup 3//m/sup 3/ in the soil moisture retrievals. If a priori information was available on soil roughness and at least one vegetation model parameter, the rms error decreased to 0.049 m/sup 3//m/sup 3/. Using specific retrieval configurations for each dataset, the rms error was generally lower than 0.04 m/sup 3//m/sup 3/.

To retrieve soil moisture over vegetation-covered areas from microwave radiometry, it is necessary to account for vegetation effects. At L-band, many retrieval approaches are based on a simple model that relies on two vegetation parameters: the optical depth (/spl tau/) and the single-scattering albedo (/spl omega/). When the retrievals are based on multiconfiguration measurements, it is necessary to take into account the dependence of /spl tau/ and /spl omega/ on the system configuration, in terms of incidence angle and polarization. In this paper, this dependence was investigated for several crop types (corn, soybean, wheat, grass, and alfalfa) based on L-band experimental datasets. The results should be useful for developing more accurate forward modeling and retrieval methods over mixed pixels including a variety of vegetation types.

The objective of this work is to evaluate soil moisture retrieval methods over different canopy, with the view of developing an operational algorithm (for the SMOS project for instance). We tested different inversion process of the /spl tau/-/spl omega/ model, using or not using ancillary information, to estimate soil moisture and /spl tau/-/spl omega/ parameters (vegetation parameters and soil roughness) simultaneously under different crop types. The best general method (same for all data sets) provides a RMSE=0.059 m/sup 3// m/sup 3/ using no ancillary information and a RMSE=0.048 m/sup 3// m/sup 3/ if we use soil and vegetation a priori information. Moreover, using specific methods to each data set provide RMSE values between 0.04 and 0.05m/sup 3// m/sup 3/.

A synthetic aperture radiometer system, SMOS, is under development for launch in 2007. The synthetic aperture concept requires calibration activities of novel nature in addition to traditional radiometer calibration exercises. Especially very accurate antenna pattern measurements are an issue.

This paper describes preliminary results from field measurements of polarimetric azimuth signatures with the EMIRAD L-band polarimetric radiometer, performed over a land test site at the Institut National de la Recherche Agronomique in Avignon, France. Scans of 180 degrees in azimuth were carried out in order to identify an eventual dependence of the Stokes vector on the look-direction. Results indicate a clear signature, for bare soil as well as for the crop-covered surface, and variations of more than 10 K are observed.

A series of circle flights have been carried out over the sea surface, using the EMIRAD L-band polarimetric radiometer. Motion compensation is applied, and polarimetric azimuth signatures are generated. Single tracks show geophysical noise, typically about 2 K, but averaging decreases the noise, producing signatures with variations below 500 mK. A harmonic analysis of the results provides no clear signature, and for the 2/sup nd/, 3/sup rd/, and 4/sup th/ Stokes parameters it is shown, that a significant part of the signal must be due to Gaussian noise. The 1/sup st/ Stokes parameter is shown to have a 500 mK 2/sup nd/ harmonic, but a comparison of the signature to the downwelling galactic background radiation indicates, that the signature may not origin from the wind driven sea surface pattern.

Modern spaceborne radiometer systems feature an almost perfect on-board calibration, hence the primary calibration task to be carried out before launch is a check of radiometer linearity. This paper describes two ways of measuring linearity of microwave radiometers only requiring relatively simple equipment.

A series of circle flights have been carried out over the wind driven sea, using the EMIRAD L-band polarimetric radiometer, described in J. Rotboll et al. (2001). Data are calibrated and corrected for aircraft attitude, and 360 degrees azimuth profiles are generated. The results show some variation over a full circle, typically about 1 K, and no clear, repeated azimuth signature from circle to circle is identified. Averaging reduces the variations, and frequency analysis of the profiles show an almost flat spectrum, which excludes a simple extrapolation of wind signatures, known at higher frequencies.

The Soil Moisture and Ocean Salinity (SMOS) space mission is currently undergoing phase-B studies at the European Space Agency. The SMOS payload is an L-band interferometric radiometer based on a two-dimensional aperture synthesis concept. This paper presents the first images obtained by a demonstrator of the SMOS instrument over land surfaces at the Avignon test site in 1999.

The speed and direction of winds over the ocean can be determined by polarimetric radiometers. This has been established by theoretical work and demonstrated experimentally using airborne radiometers carrying out circle flights and thus measuring the full 360/spl deg/ azimuthal response from the sea surface. An airborne experiment, with the aim of measuring wind direction over the ocean using an imaging polarimetric radiometer, is described. A polarimetric radiometer system of the correlation type, measuring all four Stokes brightness parameters, is used. Imaging is achieved using a 1-m aperture conically scanning antenna. The polarimetric azimuthal signature of the ocean is known from modeling and circle flight experiments. Combining the signature with the measured brightness data from just a single flight track enables the wind direction to be determined on a pixel-by-pixel basis in the radiometer imagery.

Ocean wind speed and direction can be assessed with a polarimetric radiometer system measuring the full set of Stokes parameters. There are two fundamentally different ways of implementing the polarimetric radiometer system: polarization combining and correlation radiometer systems. Also, within each fundamental category there are significant tradeoffs. This paper discusses the different implementation possibilities. Paramount issues are potential instrument stability and sensitivity as well as the tradeoff between increased microwave hardware complexity and fast digital correlator circuitry. It is concluded that each implementation has its role to play, but for a given application the choice of system configuration is important. Following these considerations, a second-generation airborne, imaging, polarimetric radiometer system, used by the authors for ocean wind vector sensing, is described and discussed.

EMISAR is a high-resolution (2/spl times/2 m), fully polarimetric, dual-frequency (L- and C-band) synthetic aperture radar (SAR) system designed for remote-sensing applications. The SAR is operated at high altitudes on a Gulfstream G-3 jet aircraft. The system is very well calibrated and has low sidelobes and low cross-polar contamination. Digital technology has been utilized to realize a flexible and highly stable radar with variable resolution, swath width, and imaging geometry. Thermal control and several calibration loops have been built into the system to ensure system stability and absolute calibration. Accurately measured antenna gains and radiation patterns are included in the calibration. The processing system is developed to support data calibration, which is the key to most of the current applications. Recent interferometric enhancements are important for many scientific applications.

The Technical University of Denmark (TUD) Synthetic Aperture Radiometer (SARad) is a two-channel demonstration model that can simulate a two-dimensional (2D) thinned array radiometer having an unfilled aperture populated with several small antenna elements. Aperture synthesis obtained by interferometric measurements using the antenna elements in pairs, followed by an image reconstruction based on an inverse Fourier transform, results in an imaging instrument without the need of mechanical scan. The thinned aperture and the nonscanning feature make the technique attractive for spaceborne radiometer systems, especially at low frequencies. The TUD SARad demonstration model consists of a two-channel K/sub u/-band correlation radiometer with two horn antennas and an antenna mounting structure enabling the horns to be mounted in relevant positions within a certain aperture. A total aperture synthesis is obtained by sequentially placing the two antenna elements in all required pairs of positions and measuring the corresponding samples of the visibility function. The system has been used to demonstrate 2D synthetic aperture imaging of complex targets in outdoor ground experiments, a special feature of the system is that it uses a focused antenna system, thus enabling a short distance to the target. Set still utilizing image reconstruction algorithms identical to those used in a normal far-field situation. The aperture synthesis theory is discussed, with special emphasis on focused systems; the radiometer system is described; and images suitable for demonstration of resolution and other imaging properties are presented and discussed.

Antenna reflector losses play an important role in the calibration budget for a microwave radiometer. If the losses are small, they are difficult to measure by traditional means. However, they can be assessed directly by radiometric means using the sky brightness temperature as incident radiation. This paper describes how such measurements are carried out as well as a suitable experimental setup. The main reflector of the European Space Agency's MIMR system is used to demonstrate the principle.

In January 1986, the design of a high-resolution airborne C-band synthetic aperture radar (SAR) started at the Electromagnetics Institute of the Technical University of Denmark. The initial system test flights took place in November and December 1989. The authors describe the design of the system, its implementation, and its performance. They show how digital technology has been utilized to realize a very flexible radar with variable resolution, swath-width, and imaging geometry. The motion-compensation algorithms implemented to obtain the high resolution and the special features built into the system to ensure proper internal calibration are outlined. The data processing system, developed for image generation and quality assurance, is sketched, with special emphasis on the flexibility of the system.>