Search

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 a FPA is an interesting option. The FPA concept uses many small antenna elements, many receivers, and powerful on-line digital processing. A focal plane array system that can replace 4 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.

A next-generation real-aperture spaceborne radiometer system for high-quality ocean measurements is discussed. Instead of illuminating the antenna reflector by a classical feed array of horn antennas in a one-feed-per-beam configuration, a multi-feed-per-beam configuration is chosen. Each antenna beam is thus created by adding the outputs from many small antenna elements in the feed array, thus providing an antenna beam of unsurpassed quality. This solves the classical polarization purity and land/sea contamination issues. The concept requires many microwave receivers and fast analog-to-digital converters as well as fast digital signal processing onboard the satellite. This is discussed, and resource budgets, especially concerning power, are provided.

Anthropogenic radio frequency interference (RFI) within microwave radiometer bands is a serious problem in remote sensing. The problem is ever-increasing and thus, future spaceborne microwave radiometers will require efficient methods to mitigate the effects of RFI. In this paper, we present a solution: an RFI processor to detect and blank the RFI in real time. The processor was designed to be compatible with the 18.7-GHz channel of the Microwave Imager Instrument onboard the European MetOp Second Generation satellite system. The RFI processor developed here applies the following complementary detection algorithms: 1) anomalous amplitude detection; 2) kurtosis; and 3) two different cross-frequency algorithms. In the processing, the data are divided into subsamples with fine temporal and frequency resolution. The RFI processor detects and filters out RFI with this fine resolution in real time and then integrates the clean (noncontaminated) subsamples over time and frequency. Thus, a cleaned version of the radiometer data can be downlinked at a traditional low data rate. The processing is implemented in a reprogrammable field programmable gate array with high processing capacity, which provides high flexibility. The processing bandwidth that is applied is 200 MHz (+ 25-MHz transition bands on both sides). The detection limits of the system vary between various kinds of RFI but are in line with simulations. The power consumption of the RFI processor is ∼12 W (at room temperature), and its mass is ∼1.1 kg.

This paper describes the refurbishment of the DTU L-band radiometer EMIRAD-L (the former EMIRAD2) for the MOSAiC campaign. This includes the installation of EMIRAD-L on P-deck of the German research ship Polarstern wherefrom EMIRAD-L has a free view of the port side of Polarstern. Validation and early results including examples of RFI are also presented.

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works ESA’s Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is still in good health, providing valuable L-band observations of the Earth surface [1]. A number of products are obtained from these, including thin sea ice [2], frost/thaw soils [3], high winds [4], ocean surface wind [5] and Sun brightness temperature [6], besides the main mission measurements of soil moisture and sea surface salinity [7][8]. This paper deals with the description and early results of some technology activities conducted by ESA applying the lessons learnt by SMOS and in preparation of an advanced L-band radiometer mission

The Greenland ice sheet has experienced an average mass loss of 142 ± 49 Gt/yr from 1992 to 2011 (Shepherd et al. 2012), making it a significant contributor to sea-level rise. Part of the ice- sheet mass loss is the result of increased dynamic response of outlet glaciers (Rignot et al. 2011). The ice discharge from outlet glaciers can be quantified by coincident measurements of ice velocity and ice thickness (Thomas et al. 2000; van den Broeke et al. 2016). As part of the Programme for monitoring of the Greenland Ice Sheet (PROMICE; Ahlstrøm et al. 2008), three airborne surveys were carried out in 2007, 2011 and 2015, with the aim of measuring the changes in Greenland ice-sheet thicknesses. The purpose of the airborne surveys was to collect data to assess the dynamic mass loss of the Greenland ice sheet (Andersen et al. 2015). Here, we present these datasets of observations from ice-penetrating radar and airborne laser scanning, which, in combination, make us able to determine the ice thickness precisely. Surface-elevation changes between surveys are also presented, although we do not provide an in-depth scientific interpretation of these.

The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) has measured ice-sheet elevation and thickness via repeat airborne surveys circumscribing the ice sheet at an average elevation of 1708 ± 5 m (Sørensen et al. 2018). We refer to this 5415 km survey as the ‘PROMICE perimeter’. Here, we assess ice-sheet mass balance following the input-output approach of Andersen et al. (2015). We estimate ice-sheet output, or the ice discharge across the ice-sheet grounding line, by applying downstream corrections to the ice flux across the PROMICE perimeter. We subtract this ice discharge from ice-sheet input, or the area-integrated, ice sheet surface mass balance, estimated by a regional climate model. While Andersen et al. (2015) assessed ice-sheet mass balance in 2007 and 2011, this updated input-output assessment now estimates the annual sea-level rise contribution from eighteen sub-sectors of the Greenland ice sheet over the 1995–2015 period.

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.

The Atmosphere-Space Interaction Monitor (ASIM) is an observatory mounted outside the Columbus module on the International Space Station. It has been operational since April 13th, 2018. It contains two instruments: The Modular X- and Gamma-ray Sensor (MXGS) and The Modular Multispectral Imaging Array (MMIA). The objective of ASIM is to monitor thunderstorms and auroras, including lightning discharges, especially discharges upwards above thunderstorms. This paper presents the instrument package and some first results.

The Modular Multispectral Imaging Array (MMIA) is a suite of optical sensors mounted on an external platform of the European Space Agency’s Columbus Module on the International Space Station. The MMIA, together with the Modular X- and Gamma- ray Sensor (MXGS), are the two main instruments forming the Atmosphere-Space Interactions Monitor (ASIM). The primary scientific objectives of the ASIM mission are to study thunderstorm electrical activity such as lightning, Transient Luminous Emissions (TLEs) and Terrestrial Gamma-ray Flashes (TGFs) by observing the associated emissions in the UV, near-infrared, x- and gamma-ray spectral bands. The MMIA includes two cameras imaging in 337 nm and 777.4 nm, at up to 12 frames per second, and three high-speed photometers at 180–230 nm, 337 nm and 777.4 nm, sampling at rates up to 100 kHz. The paper describes the MMIA and the aspects that make it an essential tool for the study of thunderstorms. The mission architecture is described in Neubert et al. (Space Sci. Rev. 215:26, 2019, this issue) and the MXGS instruments in Østgaard et al. (Space Sci. Rev. 215:23, 2019, this issue).

The Modular X- and Gamma-ray Sensor (MXGS) is an imaging and spectral X- and Gamma-ray instrument mounted on the starboard side of the Columbus module on the International Space Station. Together with the Modular Multi-Spectral Imaging Assembly (MMIA) (Chanrion et al. this issue) MXGS constitutes the instruments of the Atmosphere-Space Interactions Monitor (ASIM) (Neubert et al. this issue). The main objectives of MXGS are to image and measure the spectrum of X- and γ-rays from lightning discharges, known as Terrestrial Gamma-ray Flashes (TGFs), and for MMIA to image and perform high speed photometry of Transient Luminous Events (TLEs) and lightning discharges. With these two instruments specifically designed to explore the relation between electrical discharges, TLEs and TGFs, ASIM is the first mission of its kind. With an imaging system and a large detector area MXGS will, for the first time, allow estimation of the location of the source region and characterization of the energy spectrum of individual events. The sensors have fast readout electronics to minimize pileup effects, giving high time resolution of photon detection for comparison with measurements on μs-time scales of lightning processes measured by the MMIA and other sensors in space or on the ground. The detectors cover the large energy range of the relevant photon energies. In this paper we describe the scientific objectives, design, performance, imaging capabilities and operational modes of the MXGS instrument. publishedVersion

Anthropogenic Radio Frequency Interference (RFI) within radiometer bands is a serious problem in passive microwave remote sensing. Since this problem is ever-increasing, the next generation satellite radiometers will require efficient methods to mitigate the effects of RFI. In this paper, we present one solution: a spaceborne RFI processor to detect and blank the RFI in real time. The processor was designed to be compatible with the Microwave Imager (MWI) instrument, 18.7 GHz channel, onboard the European MetOp Second Generation satellite system. The developed RFI processor applies the following detection algorithms: (1) anomalous amplitude detection, (2) kurtosis, and (3) cross-frequency. In the processing, the data are divided into sub-samples in time and frequency with fine resolution. The RFI processor can detect and filter out RFI with this fine resolution in real time and then integrate the clean (non-contaminated) subsamples over time and frequency. Thus, a cleaned version of the radiometer data can be downlinked at traditional low data rate. The processing is implemented in a reprogrammable FPGA with high processing capacity, which provides high flexibility. The applied processing bandwidth is 200 MHz (+ 25 MHz transition bands at both sides). The measured performance of the RFI processor corresponds to the simulations and good overall detection capability has been achieved for narrow-band RFI. The power consumption of the RFI processor is approx.12 W (at room temperature) and the mass is approx. 1 kg.

Ku-band is used for measuring ocean wind velocities from meteorological satellites. Ku-band is also used for broadcasting DirectTV signals from satellites over the United States. The reflection of these signals are seen as RFI by the meteorological satellites and algorithms for detecting and blanking natural signals affected by RFI have to be developed. This paper presents a new algorithm targeted for detecting these DirectTV signals. The algorithm is implemented in firmware and its performance measured using a bread-board real time RFI processor developed for spaceborne radiometers and the DTU Space Ku-band radiometer POLRAD. It is shown that the new algorithm has a better performance than the traditional cross frequency algorithm for wideband RFI. It is also shown that the traditional cross frequency algorithm has a better performance than the new with respect to narrowband RFI. The new algorithm can thus augment but not substitute the tradition cross frequency algorithm.

A next generation spaceborne radiometer system for hi-quality ocean measurements is discussed. Instead of a classical horn, a focal plane array is used as antenna feed. The antenna beam is created by adding the outputs from many small antenna elements, thus providing an antenna beam of unsurpassed quality. This solves the classical polarization purity and land / sea contamination issues. The concept requires many microwave receivers and fast analog-to-digital converters as well as fast digital signal processing on-board the satellite. This is discussed, and resource budgets, especially concerning power, are provided.

A $350\,\text{km} \times 350\,\text{km}$ 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 (TB) close to the yearly mean temperature—well suited for calibration checks for spaceborne instruments like SMOS, Aquarius, and SMAP. The measured TBs show unexpected variations like 8-K variation over 240 km on an east–west profile through Concordia, and in certain local cases, a slope of about 0.7 K/km. Comparing the measured TB map with bottom topography reveals a convincing correlation. Simulations show that variations in bedrock topography can indeed modulate the TB appropriately to explain the observed variations. It is concluded that use of the Dome-C area for calibration check of spaceborne radiometers is indeed viable, but with caution—especially when comparing instruments with different spatial resolutions.

a r t i c l e i n f o a b s t r a c t The current deficit in Greenland ice sheet mass balance is due to both a decrease in surface mass balance (SMB) input and an increase in ice discharge (D) output. While SMB processes are beginning to be well captured by observationally-constrained climate modeling, insight into D is relatively limited. We use InSAR-derived velocities, in combination with ice thickness observations, to quantify the mass flux (F ) across a flux perimeter around the ice sheet at ∼1700 m elevation. To quantify D, we correct F for SMB, as well as changes in volume due to ice dynamics, in the area downstream of the gate. Using a 1961-1990 reference climatology SMB field from the MAR regional climate model, we quantify ice sheet mass balance within eighteen basins. We find a 2007-2011 mean D of 515 ± 57 Gt yr−1. We find a 2007-2011 mean total mass balance of −262 ± 21 Gt yr − 1

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.

Ice sounding with radar is a well-established technique for the retrieval of ice depth, and provides information on ice structures and layering. Airborne radar ice sounders suffer from off-nadir surface clutter that masks the signal from bedrock and ice layers with unwanted but simultaneously received surface reflections. This is of importance for future satellite ice-sounding missions, as the spaceborne geometry leads to strong surface clutter even for deep subsurface returns. This paper presents analysis and comparison of different clutter-suppression techniques applied to data acquired with the European Space Agency's P-band POLarimetric Airborne Radar Ice Sounder (POLARIS). The 4 m long antenna of POLARIS enables simultaneous reception of up to four across-track channels. It was operated in 2011 over Antarctica at a high flight altitude of 3200 m. Different coherent weighting techniques of the receive channels were used to suppress the surface ‘clutter’. However, with a channel spacing of 1.4 times the wavelength, the grating lobe imposes a limitation to the off-nadir angular range in which clutter can be effectively attenuated. Results of ice sounding over Jutulstraumen glacier are described, where we demonstrate a clutter suppression of up to 10 dB.

The ocean plays an important role in modulating the mass balance of the polar ice sheets by interacting with the ice shelves in Antarctica and with the marine-terminating outlet glaciers in Greenland. Given that the flux of warm water onto the continental shelf and into the sub-ice cavities is steered by complex bathymetry, a detailed topography data set is an essential ingredient for models that address ice–ocean interaction. We followed the spirit of the global RTopo-1 data set and compiled consistent maps of global ocean bathymetry, upper and lower ice surface topographies, and global surface height on a spherical grid with now 30 arcsec grid spacing. For this new data set, called RTopo-2, we used the General Bathymetric Chart of the Oceans (GEBCO_2014) as the backbone and added the International Bathymetric Chart of the Arctic Ocean version 3 (IBCAOv3) and the International Bathymetric Chart of the Southern Ocean (IBCSO) version 1. While RTopo-1 primarily aimed at a good and consistent representation of the Antarctic ice sheet, ice shelves, and sub-ice cavities, RTopo-2 now also contains ice topographies of the Greenland ice sheet and outlet glaciers. In particular, we aimed at a good representation of the fjord and shelf bathymetry surrounding the Greenland continent. We modified data from earlier gridded products in the areas of Petermann Glacier, Hagen Bræ, and Sermilik Fjord, assuming that sub-ice and fjord bathymetries roughly follow plausible Last Glacial Maximum ice flow patterns. For the continental shelf off Northeast Greenland and the floating ice tongue of Nioghalvfjerdsfjorden Glacier at about 79° N, we incorporated a high-resolution digital bathymetry model considering original multibeam survey data for the region. Radar data for surface topographies of the floating ice tongues of Nioghalvfjerdsfjorden Glacier and Zachariæ Isstrøm have been obtained from the data centres of Technical University of Denmark (DTU), Operation Icebridge (NASA/NSF), and Alfred Wegener Institute (AWI). For the Antarctic ice sheet/ice shelves, RTopo-2 largely relies on the Bedmap-2 product but applies corrections for the geometry of Getz, Abbot, and Fimbul ice shelf cavities. The data set is available in full and in regional subsets in NetCDF format from the PANGAEA database at doi:10.1594/PANGAEA.856844.

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.

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.

The Greenland ice sheet has been losing mass at a dramatic rate in recent years, raising political concern worldwide due to the possible impact on global sea level rise and climate dynamics (Luthcke et al. 2006; Rignot & Kanagaratnam 2006; Velicogna & Wahr 2006; IPCC 2007; Shepherd & Wingham 2007). The Arctic region as a whole is warming up much more rapidly than the globe at large (ACIA 2005) and it is desirable to quantify these changes in order to provide the decision-makers with a firm knowledge base. To cover this need, the Danish Ministry of Climate and Energy has now launched a new Programme for Monitoring of the Greenland Ice Sheet (PROMICE), designed and operated by the Geological Survey of Denmark and Greenland (GEUS) in collaboration with the National Space Institute at the Technical University of Denmark and Asiaq (Greenland Survey). The aim of the programme is to quantify the annual mass loss of the Greenland ice sheet, track changes in the extent of local glaciers and ice caps, and track changes in the position of the ice-sheet margin.

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.

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.

Ice deformation and basal motion characterize the dynamical behavior of the Greenland ice sheet (GrIS). We evaluate the contribution of basal motion from ice deformation measurements in boreholes drilled to the bed at two sites in the western marginal zone of the GrIS. We find a sustained high amount of basal motion contribution to surface velocity of 44–73% in winter, and up to 90% in summer. Measured ice deformation rates show an unexpected variation with depth that can be explained with the help of an ice-flow model as a consequence of stress transfer from slippery to sticky areas. This effect necessitates the use of high-order ice-flow models, not only in regions of fast-flowing ice streams but in all temperate-based areas of the GrIS. The agreement between modeled and measured deformation rates confirms that the recommended values of the temperature-dependent flow rate factor A are a good choice for ice-sheet models., Journal of Glaciology, 60 (222), ISSN:0022-1430, ISSN:1727-5652

We present ice thickness and bed topography maps with high spatial resolution (250 to 500 m) of a and-terminating section of the Greenland Ice Sheet derived from combined ground-based and airborne radar surveys. The data have a total area of ~12000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of the Isunnguata Sermia Glacier is over-deepened and reaches an elevation of several hundreds of meters below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The covered area is one of the most studied regions of the Greenland Ice Sheet with studies of mass balance, dynamics, and supraglacial lakes, and our combined dataset can be valuable for detailed studies of ice sheet dynamics and hydrology. The compiled datasets of ground-based and airborne radar surveys are accessible for reviewers (password protected) at doi.pangaea.de/10.1594/pangaea.830314 and will be freely available in the final revised paper.

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.

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 3rd and 4th Stokes 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.

In February 2011, the Polarimetric Airborne Radar Ice Sounder (POLARIS) was flown in Antarctica in order to assess the feasibility of a potential space-based radar ice sounding mission. The campaign has demonstrated that the basal return is detectable in areas with up to 3 km thick cold ice, in areas with up to several hundred meters thick warm shelf ice, and in areas with up to 700 m thick crevassed glacier ice. However, major gaps in the basal return are observed, presumably due to excessive absorption, scattering from ice inclusions in the firn, low basal reflectivity, and the masking effect of the surface clutter. Internal layers are observed down to depths exceeding 2 km. The polarimetric data show that the internal layers are strongly anisotropic at a ridge, where the ice flow is supposed to be highly unidirectional. In case of space-based ice sounding, the antenna pattern cannot offer sufficient surface clutter suppression, but improved clutter suppression has been demonstrated with novel multi-phase-center techniques.

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.

Leading up to the ESA Soil Moisture and Ocean Salinity (SMOS) mission, a series of campaigns have taken place, including airborne L-band radiometer measurements using the EMIRAD-2 radiometer - a fully polarimetric radiometer with Radio Frequency Interference (RFI) detection capabilities by means of the kurtosis method. An RFI detection scheme combining kurtosis and polarimetry has been evaluated, and it has been established that the two methods complement each other well. Following the addition of a digital filter bank to the radiometer system, a new RFI detection scheme has been developed and tested. The RFI removal capabilities of the two detection schemes are similar, however, the new detection scheme results in fewer data samples being discarded due to RFI.

The ice flux divergence of a glacier is an important quantity to examine because it determines the rate of temporal change of its thickness. Here, we combine high-resolution ice surface velocity observations of Nioghalvfjerdsfjorden (79north) Glacier, a major outlet glacier in north Greenland, with a dense grid of ice thickness data collected with an airborne radar sounder in 1998, to examine its ice flux divergence. We detect large variations, up to 100 m/yr, in flux divergence on grounded ice that are incompatible with what we know of the glacier surface mass balance, basal mass balance and thinning rate. We examine the hypothesis that these anomalies are due to the three-dimensional flow of ice around and atop bumps and hollows in basal topography by comparing the flux divergence of three-dimensional numerical models with its surface equivalent. We find that three-dimensional effects have only a small contribution to the observed anomalies. On the other hand, if we degrade the spatial resolution of the data to 10 km the anomalies disappear. Further analysis shows that the source of the anomalies is not the ice velocity data but the interpolation of multiple tracks of ice thickness data onto a regular grid using a scheme (here block kriging) that does not conserve mass or ice flux. This problem is not unique to 79north Glacier but is common to all conventional ice thickness surveys of glaciers and ice sheets; and fundamentally limits the application of ice thickness grids to high-resolution numerical modeling of glacier flow. Citation: Seroussi, H., M. Morlighem, E. Rignot, E. Larour, D. Aubry, H. Ben Dhia, and S. S. Kristensen (2011), Ice flux divergence anomalies on 79north Glacier, Greenland, Geophys. Res. Lett., 38, L09501, doi: 10.1029/2011GL047338.

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 3rd and 4th 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 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 2005 an airborne survey was carried out from a Twin Otter aircraft at Pâkitsup Akuliarusersua (Paakitsoq) near Ilulissat in West Greenland. The survey aimed to measure ice thickness with a 60 MHz coherent radar and surface elevation with a scanning laser altimeter. Positioning information came from multiple on-board differential GPS units and an inertial navigation system. The region surveyed covers >80km along the ice margin and has a total area of ~2700km2 with varying density of measurements: the between-track distance was ~1 km near the margin, increasing to ~3km away from the margin. Regional high-resolution maps of basal topography under the Greenland ice sheet are useful for resolving important glaciological and hydrological questions and for enhancing related process studies, such as the influence of basal meltwater on ice dynamics. The ice-sheet margin in this region is also currently under consideration for hydropower development and has a long and continuing history of glaciological investigations, lately with emphasis on the connection between surface meltwater formation and surface velocity of the ice sheet. Here we present a new regional map of the surface and basal topography of the ice-sheet margin and discuss some of the implications for reported observations at Swiss Camp.

Transitional and and turbulent thermo-convective flows in a cylinder heated from below are studied numerically. The governing equations for mass, momentum and energy conservation are solved in vorticity-streamfunction-temperature variables utilizing the Boussinesq approximation and a sub-grid-scale turbulence model. The simulations are performed by a finite-difference code employing a combined 2nd and 4th order accurate spatial discretization. The resulting set of equations is factored into one-dimensional operators that are applied in alternating order. Calculations have been run on the CRAY C92 vector processor as well as on the SP2 parallel machine. The results show good performance, especially on the CRAY C92 where up to 550 Mflops were achieved.