L-Band data from the PALSAR-2 instrument on the ALOS-2 satellite have been collected globally following the basic observation plan. The sparse archive of either Stripmap or ScanSAR mode data is often an obstacle to obtain an adequately sampled interferometric phase history for applying multitemporal synthetic aperture radar interferometry (MTInSAR). In this study, we evaluate the feasibility of ALOS-2 Stripmap + ScanSAR MTInSAR, using both acquisition modes, to enhance the small baseline subset (SBAS) network by filling temporal gaps. We combined 11 Stripmap and 21 ScanSAR acquisitions during 2015–2020 to monitor ground subsidence in Busan, South Korea. This improved the observation frequency, thereby reducing errors due to geometric and temporal decorrelation. Due to the differences in the azimuth spectral patterns, common-band filtering was not performed when processing the Stripmap + ScanSAR dataset. The subsidence pattern of the surface deformation time series derived from each acquisition mode appeared similar. Although the mean coherence of the Stripmap + ScanSAR (~0.77) interferometric pairs was slightly lower than that of the Stripmap (~0.83) and ScanSAR (~0.84) only pairs, the standard deviation of the coherence in the Stripmap + ScanSAR dataset was the lowest. The mean difference in the displacement rate among the three datasets was 0.1 cm/year, with a standard deviation of 0.3–0.4 cm/year. The means of the standard deviation of the deformation, proportional to the residual phase in the time-series deformation of the Stripmap, ScanSAR, and Stripmap + ScanSAR dataset, were 0.7, 0.9, and 0.8 cm, respectively. The results show that the ALOS-2 Stripmap + ScanSAR SBAS applications provide a useful method to increase the frequency of observations and reduce errors in the deformation time series.

In an agricultural river valley in central Idaho, USA, we conducted a case study to investigate interferometric synthetic aperture (InSAR) coherence and phase response as an indication of soil moisture change. Throughout a 3-day observational campaign, repeated observations were acquired from a mobile vehicle with a multi-polarization L-band (1.6 GHz) InSAR system, first prior to controlled irrigation and then during a dry-out period. Here, we present results that show the time series of coherence and phase in coordination with in-situ soil moisture observations at two depths over the three days of the controlled experiment. During the subsequent dry out period, the time series of interferometric coherence shows an immediate degradation of the signal with a subsequent improvement. The in-situ soil moisture observations highlight this transition of controlled irrigation to subsequent drying out of the soils. We anticipate future work to include investigation of the interferometric times series of phase change as it relates to quantitative changes in soil moisture.

Ground-motion-related geohazards are omnipresent in alpine areas. Time series of spaceborne SAR data are routinely used to assess surface displacement with large coverage. However, there are also many cases and expositions for which terrestrial radar systems are more suitable to obtain measurements of ground motion. Observing a mountain slope from a moving car or a UAV and thereby spanning a much longer synthetic aperture allows using also lower frequencies, such as L-band, with good spatial resolution. In addition, if aperture synthesis using a mobile mapping SAR system is employed at higher frequencies, the azimuth resolution can also be further improved compared to the azimuth resolution of (quasi-) stationary radar system at larger range distances of several kilometers. In this contribution, we report case studies that we are conducting to further develop and consolidate ground motion mapping by means of our car-borne DInSAR system as an operational application.

Time series of SAR tomographic profiling measurements of seasonal snow in the Swiss Alps were acquired with the tower-mounted ESA Wideband Scatterometer (WBScat) during an entire snow season as part of the ESA SnowLab campaign. The wide range of frequency bands (1-40 GHz) covered in combination with the depth-resolving capability of the tomographic SAR measurements provides new insights into relative change of location and intensity of radar backscatter within the snowpack during melting and refreezing cycles as a function of time and various parameters obtained by auxiliary measurements such as snow accumulation, snow mass (SWE), snow surface temperature, and liquid water content. It is found that at both, Ku-band and Ka-band, the tomographic profiles provide evidence of substantial backscatter at melt/freeze crust interfaces within the seasonal snowpack. Varying penetration depths and (re-)appearing backscattering layers can be observed throughout several cycles of snow melting and gradual refreezing.

In this work, we introduce a novel InSAR processing routine for handling the low-coherence data from long temporal baseline (~ 4 months) L-band ALOS-2 InSAR pairs in the 2016–2017 snow season over the mountainous regions of Colorado, mostly covered by sparse forest. A physical InSAR scattering model was exploited to simulate the InSAR sensitivity of SWE retrieval for forest-covered sites. InSAR-retrieved SWE results are compared with SNOTEL in-situ measurements with a correlation of 0.5 (p-value of 0.01), however, with a much shorter dynamic range of 80 mm versus the actual SWE range of 400 mm. This paper sheds light on using long temporal baseline repeat-pass L-band InSAR data for forest-covered SWE retrieval.

This article experimentally investigates relationships between copol backscattering at a wide range of frequencies (L- to Ka-bands) and snow–ground state parameters (SPs) in different evolution phases during the full winter cycle of 2019/2020. Backscattering coefficients from 1 to 40 GHz,in situsnow–ground SPs, and meteorological data are measured at the Davos-Laret Remote Sensing Field Laboratory (Switzerland). Relative strengths of the snow–ground system’s three primary scattering elements (air–snow interface, snow volume, and snow–ground interface) on backscattering are assessed. An anticorrelation between reasonably high snow wetness and backscattering coefficient is found, especially at higher microwave frequencies. For small amounts of snow wetness, backscatter coefficients at L- and S-bands are intensified via increasing snow volume and snow surface scattering. Snow–ground SPs influence backscattering according to their characteristic time scales of temporal evolution. Under dry snow conditions and at low and intermediate frequencies, ground permittivity is the major influencer of backscatter at a time scale of roughly two weeks. Snowfall is the major influencer of backscatter at a time scale of a few hours to a few days. The findings of this article are valuable to the development of retrieval algorithms using machine learning while maintaining a grasp on the ongoing physical processes. Another key message is that multifrequency active microwave measurements are critical to maximize the number of retrievable SPs and their estimation accuracy. For example, while Ka-band performs well in the detection of snow cover, L-band measurements are more responsive to changes of snow water equivalent (SWE) under moist or wet snow conditions.

This article presents the bistatic operation mode and the performance analysis of KAPRI, a terrestrial frequency-modulated continuous-wave (FMCW) Ku-band polarimetric radar interferometer capable of acquiring bistatic full-polarimetric datasets with high spatial and temporal resolution. In the bistatic configuration, the system is composed of two independently operating KAPRI devices, one serving as a primary transmitter and receiver and the other as a secondary receiver. The secondary bistatic dataset is affected by possible offsets between the two devices’ reference clocks, as well as distortions arising from the bistatic geometry. To correct for this, we present a two-chirp bistatic FMCW signal model, which accounts for the distortions, and a reference chirp transmission procedure, which allows correcting the clock offsets in the deramped signal time domain. The second challenge of operation of a bistatic polarimetric system is polarimetric calibration since it is not possible to employ purely monostatic targets such as corner reflectors. For this purpose, we developed a novel active calibration device Variable-Signature Polarimetric Active Radar Calibrator (VSPARC), designed for monostatic and bistatic calibration of all polarimetric channels. VSPARC and its associated novel polarimetric calibration method were then used to achieve full calibration of both KAPRI devices with polarimetric phase calibration accuracy of 20° and 30-dB polarization purity in field conditions. This article thus presents a complete measurement configuration and data processing pipeline necessary for synchronization, coregistration, and polarimetric calibration of bistatic and monostatic datasets acquired by a real-aperture FMCW radar.

WBSCAT is a polarimetric scatterometer developed for the European Space Agency for measuring microwave signatures of snow in support of the SnowLab and SnowLab-NG projects. SnowLab-NG includes feasibility studies for ESA candidate satellite missions operating at L-Band such as ROSE-L and Hydroterra. The WBSCAT instrument is based on a vector network analyzer to cover the 1–40 GHz frequency range, including L-Band. Using ground-based data collected in the near-field to model data collected by satellites is challenging when there is volume scattering from a thick layer such as snowpack. WBSCAT has been constructed to use near-field aperture synthesis to permit focusing of backscatter data. Focusing mitigates the effects of near-field operation by synthesizing a narrow antenna beam that illuminates a smaller volume of the snowpack. Using aperture synthesis, an improvement in azimuth resolution by a factor of 6 is possible with WBSCAT at L-band. Initial results from phase calibration of a calibration target response show the potential for this approach.

In this paper, we present examples of DInSAR-based measurement of surface displacements using a novel compact L-band SAR system that can be mounted on mobile mapping platforms such as a UAV or a car. The good DInSAR system performance is demonstrated and, particularly, we also show a use case in which a car-borne system setup is employed to map surface displacements of a fast-moving landslide and the surrounding area in Switzerland. Our results show that car-borne and UAV-borne interferometric displacement measurements at L-band are feasible with high quality over various natural terrain. This novel compact DInSAR system for agile platforms complements existing terrestrial, airborne, and space-borne radar interferometry systems in terms of its new combination of (1) radar wavelength (sensitivity to displacement/decorrelation properties), (2) spatial resolution, (3) (near-) terrestrial observation geometry, and (4) mobile mapping capability.

In this submission the bistatic operation mode of KAPRI, a polarimetric-interferometric, ground-based, real-aperture, Ku-band, FMCW radar is presented. Its possible configurations and the synchronization procedures needed to achieve distortion-free single-look complex bistatic images are described. The challenges of polarimetric calibration in the bistatic regime are outlined, and a novel active calibration device and the associated polarimetric calibration method which are used for bistatic calibration of KAPRI are presented. First results of observations of natural environments (vegetation, snow) using bistatic KAPRI are shown.

In this paper, we present first results of car-borne and UAV-borne mobile mapping of potential surface displacements with a compact repeat-pass interferometric FMCW SAR system at L-band: (1) glacier-flow-induced displacements were measured at Stein glacier in the Swiss alps in car-borne mode along a slightly curved road section; (2) a valley slope was observed repeatedly using the vertical-take-off-and-landing (VTOL) UAV Scout B1-100 flown by Aeroscout. The SAR raw data were focused directly to an image grid in map coordinates, involving a digital elevation model and accurate GNSS/INS navigation data, by using a time-domain back-projection (TDBP) approach. These geocoded complex SAR images then allow to directly form differential interferograms in map coordinates. The feasibility of repeat-pass interferometry using our novel FMCW L-band SAR on mobile platforms such as a car or a UAV is successfully demonstrated with several data examples.

WBSCAT is a new terrestrial 1-40 GHz polarimetric scatterometer. This instrument, built for the European Space Agency, with additional support from ETH WSL, is currently an element of the ESA SnowLab project for continuous microwave measurements of snowpack in Davos-Laret Switzerland. WBSCAT is based on a compact Vector Network Analyzer (VNA), combined with calibration standards and low-noise amplifiers to increase sensitivity. A pan/tilt positioner provides the angular and spatial diversity required for measurement of radar cross-section, 3D tomographic imaging, and measurements of interferometric coherence. The instrument can apply angular diversity and aperture synthesis to increase radiometric accuracy and suppress clutter. In Davos-Laret, WBSCAT is suspended from a 2.2-meter linear rail positioner that provides additional spatial diversity for high-resolution 3D tomographic imaging.

The aim of the ESA SnowLab project is to provide a comprehensive multi-frequency, multi-polarisation, multi-temporal dataset of active microwave measurements over snow-covered grounds to investigate the relationship between effective snow- and ground parameters and the resultant signals detected by microwave radar. An important part for the development of microwave models is the microstructural characterisation. This characterisation can only be done by repeated measurements by SnowMicroPen and more completely, but also much more expensive, by X-ray micro-tomography. Within this project we complemented the microwave measurements of Alpine snow in Switzerland with extensive effective snow- and ground parameters and meteorological data. Microwave backscatter measurements were conducted using the 9 - 18 GHz ESA SnowScat instrument and since December 2018 the recently built ESA WBScat instrument. WBScat allows to extend the spectral coverage to 1 - 40 GHz.

As part of the ESA SnowLab campaign the SnowScat device, a terrestrial stepped-frequency continuous-wave (SFCW) scatterometer which supports fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz, was operated in tomographic profiling mode. In this tomographic profiling mode the SnowScat device is subsequently displaced in elevation direction to obtain a high-resolution not only in range direction but also along elevation. This leads to two-dimensional vertical profiles of a snowpack, which means that radar backscatter, co-polar phase difference, interferometric phase and coherence can be distinguished also along the vertical dimension of the snowpack. In this paper, we provide a summary and a few examples of a time series of tomographic measurements of snow obtained within the ESA SnowLab campaign at two different locations in the Swiss Alps during three snow seasons.

Terrestrial radar systems are used operationally for area-wide measurement and monitoring of surface displacements on steep slopes, as prevalent in mountainous areas or also in open pit mines. One limitation of these terrestrial systems is the decreasing cross-range resolution with increasing distance of observation due to the limited antenna size of the real aperture radar or the limited synthetic aperture of the quasi-stationary SAR systems. Recently, we have conducted a first experiment using a car-borne SAR system at Ku-band, demonstrating the time-domain back-projection (TDBP) focusing capability for the FMCW case and single-pass interferometric capability of our experimental Ku-band car-borne SAR system. The cross-range spatial resolution provided by such a car-based SAR system is potentially independent from the distance of observation, given that an adequate sensor trajectory can be built. In this paper, we give (1) an overview of the updated system hardware (radar setup and high-precision combined INS/GNSS positioning and attitude determination), and (2) present SAR imagery obtained with the updated prototype Ku-band car-borne SAR system.

Differential interferometry using ground-based radar systems permits to monitor displacements in natural terrain with high flexibility in location, time of acquisition, and revisit time. In combination with polarimetric imaging, discrimination of different scattering mechanisms present in a resolution cell can be obtained simultaneously with the estimation of surface displacement. In this paper, we present the preprocessing steps and the calibration procedure required to produce high-quality calibrated polarimetric single-look complex imagery with KAPRI, a new portable Ku-band polarimetric radar interferometer. The processing of KAPRI data into single look complex images is addressed, including the correction of beam squint and of azimuthal phase variations. A polarimetric calibration model adapted to the acquisition mode is presented and used to produce calibrated polarimetric covariance matrix data. The methods are validated by means of a scene containing five trihedral corner reflectors. Data preprocessing is assessed by analyzing the oversampled response of a corner reflector, and the polarimetric calibration quality is verified by computing polarimetric signatures and residual calibration parameters.

SnowScat is a terrestrial stepped-frequency continuous-wave (SFCW) scatterometer which supports fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz. Recently, the hardware has been upgraded by adding a tomographic profiling mode. This tomographic approach allows to retrieve high-resolution information about a snowpack via observables, such as radar backscatter, co-polar phase difference, interferometric phase and coherence. Since the tomographic imaging itself is also affected by the refraction occurring at the air-snow interface and within the snowpack the two problems, 1) the production of well-focused and correctly located tomographic profiles, and 2) the retrieval of snow structure parameters are inherently linked. In this contribution, a tomographic inversion scheme to retrieve the refractive index of snow through an autofocus approach is presented. The current autofocus-based retrieval relies on using an aluminium sphere of a test target deployed in the scene. The refractive indices and accompanying snow density measurements obtained at four dates during a cold period in January during the ESA SnowLab 2016/2017 campaign are compared to an empirical model by Matzler and Wiesmann that describes the relation between snow density and the real part of the relative permittivity for dry snow.

The Calaveras fault passes directly through Coyote Dam located near Gilroy, California. The earthen structure of the dam was constructed to withstand the expected deformation due to fault creep at a rate of 10 to 15 mm/year. As part of a possible dam retrofit, the Santa Clara Valley Water District initiated a series of measurements using a Ku-Band terrestrial interferometer to accurately localize the fault trace through the dam. Measurements over the period 12-May 2016 to 18-Nov-2016 were acquired from 4 different positions situated around the down-stream face. Time series of measurements from each position were obtained after performing corrections for variable tropospheric phase delay. These measurements were combined using least-squares estimation to generate three-dimensional maps delineating both stable and rapidly deforming regions.

The SnowScat device is a ground-based stepped-frequency continuous-wave (SFCW) scatterometer supporting fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz. It was originally designed to support the investigation and validation of Snow Water Equivalent (SWE) retrieval algorithms in the context of the development of the deselected COld REgions Hydrology High-resolution Observatory (CoReH20) candidate Earth Explorer 7 mission. Recently, the SnowScat hardware has been enhanced to also provide a tomographic profiling mode which allows to obtain high-resolution 2-D vertical profiles that may provide further insight into the electromagnetic interaction within layered snow packs. In winter 2014/2015, a first test campaign was carried out yielding a successful proof of concept of the enhanced hardware, tomographic measurement, and basic processing concept. In Nov/Dec 2015, the SnowScat device was then installed as a part of the SnowLab experiment at a test site on 1700m altitude close to the Grimsel pass in Switzerland. A comprehensive time series of tomographic profiles of a snow pack was acquired until end of March, 2016. In this paper, we present and discuss first results of this new time series of tomographic profiles including 2-D vertical profiles of backscatter, phase difference between the co-polar channels, and interferometric phase difference.

Sentinel-1 IWS interferometric time-series analysis (SBAS and PSI) is discussed and results over Mexico City, a mega-city subject to very substantial ground deformation, are presented. In the early steps the processing needs to be adapted to the organization of the data in sub-swaths and burst, and for the SLC co-registration an extremely accurate co-registration is required for interferometry, because of the strong along-track Doppler centroid variation. Furthermore, a deramping of the co-registered SLC for an along-track phase ramp present in each burst is applied. After that interferometric time-series techniques (SBAS, PSI) can be applied in the same way as for stripmap mode data.

The GPRI2 Ground-Based Real-Aperture radar was deployed in Seattle over a 6-week period during March and April 2015 to measure sub-millimeter scale subsidence related to construction of the Alaskan Way Viaduct and Seawall Replacement Program (AWVSRP). The radar was located on the mid-level roof of the Smith Tower overlooking Pioneer Square at an altitude of about 75 meters above street level. Radar images at 2 hour intervals were processed to obtain Line-of-Sight (LOS) deformation maps. Point target analysis was used to identify stable targets and to mitigate variations in propagation path length due to atmospheric changes. Geolocation and rectification of the more than 100,000 monitored points was carried out using a LiDAR DEM.

SAR tomography as an extension to persistent scatterer interferometry (PSI) approaches has the potential to improve the level of detail of retrievable information, in particular, in the case of layover scenarios in urban areas. In this paper, a processing approach is sketched that eventually allows for retrieving a 3-D point cloud of point-like scatterers based on subsequent PSI and SAR tomography processing of an interferometric stack of high-resolution spaceborne TerraSAR-X data acquired over the city of Barcelona between the years 2008 and 2012. Experimental results are presented in the form of (1) vertical tomographic slices of high-rise buildings and (2) a 3-D point cloud of a larger district of the city of Barcelona retrieved from the tomograms.

Agile SAR platforms such as an automobile require a flexible SAR processing scheme to account for nonlinear sensor trajectories during the synthetic aperture. In this contribution, a parallelized implementation of a time-domain back-projection SAR focusing algorithm based on NVIDIA's CUDA GPU computing framework is presented and discussed using a car-borne SAR data set. The processing performance is assessed using different hardware. In addition, a pre-processing scheme is described that allows for full 3-D motion compensation, yet staying conveniently in conventional slant-range/azimuth geometry of single-look complex SAR images.

In this contribution, a car-borne SAR and InSAR experiment is described. The slope of a valley was imaged by means of a single-pass InSAR system mounted on a car driving on roads along the bottom of the valley. The GAMMA portable radar interferometer GPRI-II hardware with a modified antenna configuration was used for data acquisition. The experimental setup (1), SAR imagery focused along a slightly curved sensor trajectory (2), and first interferometric results (3) obtained using this configuration are presented.

Precise terrain-corrected georeferencing of synthetic aperture radar (SAR) images and derived products in range–Doppler coordinates is important with respect to several aspects, such as data interpretation, combination with other geodata products, and transformation of, e.g., terrain heights into SAR geometry as used in differential interferometric SAR (DInSAR) applications. For georeferencing, a lookup table is calculated and then refined based on a coregistration of the actual SAR image to a simulated SAR image. The impact of using two different implementations of such a simulator of topography-induced radar brightness, 1) an approach based on angular relationships and 2) a pixel-area-based method, is discussed in this letter. It is found that the pixel-area-based method leads to considerable improvements with regard to the robustness of georeferencing and also with regard to radiometric normalization in layover-affected areas.

In this contribution Persistent Scatterer Interferometry (PSI) processings done in the framework of the FP7 Project PanGEO over a number of European sites are presented. After a short introduction of the PanGEO project, we present the data used, the processing done and the results of the PSI analysis. The influence of specifics as the presence of large non-urban areas, significant topography, seasonal effects, special deformation characteristics, and the size of the available data stacks on the processing are highlighted.

Arctic and mountainous periglacial areas are subject to intense freezing cycles and characterized by remarkable surface displacement. We used TerraSAR-X interferometry to analyze the seasonal surface displacement on two regions in Alaska and Switzerland. In both cases, TerraSAR-X interferograms with an 11 days time interval are computed in series in order to maximize coherence. Seasonal subsidence maps highlight surface displacements of several centimeters while time-series show non-linear behaviors of the movements.

Azimuth spectrum band-pass filtering has been applied successfully for estimation of along-track ground displacement [1] as well as for other applications, such as the identification of directional scattering [2] and the coherence estimation for long-baseline pairs [3]. Particularly L-band split-beam interferograms have shown another phase component related to along-track variations in the ionospheric path delay. In our work we present methodologies to identify and quantify ionospheric path delays affecting an interferogram using the corresponding split-beam interferogram.

In this letter, the requirements to derive topography from a portable terrestrial radar interferometer are introduced, the instrument design and the relationship between interferometric phase shift and surface topography are explained, and two examples of topographic maps from measurements at the Rhone glacier and Grabengufer rock glacier in Switzerland are presented. In the first case, an external digital elevation model was used to assess the error of topography mapping with the portable radar interferometer and to analyze ice surface changes of the glacier in the last 14 years. We found that the height error standard deviation is about 3 m within a distance of 2 km from the sensor and observed massive thinning of the Rhone glacier. In the second case, we used the terrestrial radar interferometer in order to measure the height difference between August 2009 and March 2010 over the rock glacier as a consequence of its destabilization.

ERS-ENVISAT cross-interferometry is a unique tool for a number of applications since it combines a short repeat-pass interval (28 minutes) with a long perpendicular baseline (2 km). In the present work a methodology to use the cross-interferogram phase to map the ice thickness of frozen tundra lakes and northern rivers. After presenting the methodology used examples over sites in northern Canada and Siberia are shown. For one site the results could be compared to in-situ measurements and showed good correspondence.

The COld REgions Hydrology High-resolution Observatory (CoReH20) Mission proposes a dual frequency radar operating at 9.6 and 17 GHz utilizing VV and VH polarization [1]. By combining Xand Ku-Band with both coand cross-polarization diversity it is possible to estimate the Snow Water Equivalent of dry snow. To support this proposed mission, ESA has sponsored the development of a ground-based coherent polarimetric scatterometer operating over the 9-18 GHz frequency range. ESA is supporting campaigns to acquire and process data using SnowScat for validation of Snow Water Equivalent (SWE) retrieval algorithms.

In the past, the application of Persistent Scatterer Interferometry (PSI) was primarily possible in the case of slow (less than a few centimeters per year) uniform movements. In this paper, we show how PSI permits the monitoring of relatively fast (including rates up to > 50 cm/year) and nonuniform movements using TerraSAR-X repeat observations over deep-level mining. To enable this, parts of the PSI methodology were adapted to the special characteristics of the example studied. Apart from a description of the methodology used and the result achieved, error considerations and a validation of the result with in situ measurements are included.

Four TerraSAR-X stripmap mode scenes acquired during the late summer of 2008 in the Oberwallis region in the Swiss Alps have been interferometrically analyzed for the survey of the activity of unstable slopes. Differential interferometry was applied using a high precision external Digital Elevation Model (DEM). Position, extent, contour, and approximate velocity of unstable slopes were determined for a large area. Selected results representing rockglacier movement and landslide activity are discussed and compared with in-situ information and interferograms derived from ENVISAT ASAR, JERS-1 SAR and ALOS PASLSAR data. This application strongly benefits from the higher spatial resolution of the TerraSAR-X data in comparison to the C- and L-band sensors used in the past, because many of the instabilities are of relatively small size.

Before the summer of 2007 58 square Trihedral Corner Reflectors (TCR) were installed in salt marshes within the Venice Lagoon where anthropogenic structures completely lack or few constructions are scattered at a distance from one another too large to reliably resolve the radar phase ambiguity on ERS and ENVISAT interferometric point target analysis. An optimal TCR network has been established taking into account the location of ¿natural¿ point targets in ERS and ENVISAT SAR interferometric analyses and keeping to a value of about 1 km the maximum distance between the TCR or between an ¿artificial¿ and the adjacent ¿natural¿ reflectors. In this contribution we discuss the set-up of the TCR, their backscattering intensity response on ENVISAT ASAR and TerraSAR-X images, and the approach followed to measure the ground displacement in areas of particular interest within the Venice Lagoon.

ERS-ENVISAT cross-interferometry is a unique tool for a number of applications since it combines a short repeat-pass interval (28 minutes) with a long perpendicular baseline (2 km). Temporal decorrelation effects are limited and the sensitivity to topographic features is strongly enhanced. In this contribution the focus is on problems encountered during the coherence estimation in ERS-ENVISAT cross-interferometry. Because of the ERS-2 Doppler Centroid variations the azimuth common band available is often only a relatively small fraction of the PRF. Similarly, in the case of not ideal baselines, the common range bandwidth is often much smaller than the chirp bandwidth. Furthermore, high phase gradients in the cross-interferograms can significantly affect the coherence estimates. In our contribution we propose methodologies to reduce these problems in the coherence estimation.

ERS-ENVISAT cross-interferometry is a unique tool for a number of applications since it combines a short repeat-pass interval (28 minutes) with a long perpendicular baseline (2 km). Temporal decorrelation effects are limited and the sensitivity to topographic features is strongly enhanced. The short repeat-pass also allows monitoring of fast-moving displacements. In this paper we show a number of examples on Digital Elevation Map (DEM) generation in flat areas, crop height retrieval and displacement monitoring using data from the recent tandem mission dedicated to ERS-ENVISAT interferometric acquisitions. The examples demonstrate the feasibility of the applications, highlighting at the same time some limitations, which are due primarily to the very specific configuration of the interferometric system.

In our contribution we present an initial assessment of the applicability of TerraSAR-X for repeat-track interferometry. A series of TerraSAR-X repeat observation in stripmap mode over an test site that includes different land use classes as well as fast and slow deformation phenomena was selected for our investigation. From our investigation we conclude that long-term interferograms can mainly be interpreted over urban areas. For forested areas and for denser agricultural vegetation such as fully grown crops the coherence levels are very low even for 11 day intervals. For sparse vegetation as present in late winter the coherence is high, over many fields even over several 11-day cycles. The high spatial resolution is ideal to measure localized deformation patterns. For slow deformation at larger scale confusion with atmospheric effects is strong. Multi-temporal approaches can be used to resolve the deformation and atmospheric phase components if sufficient numbers of observations are available.

Satellite interferometry has been used extensively for ground-motion monitoring with good success. In the case of landslides, for example, space-borne SAR interferometry has a good potential to get an overview on the slope stability. The role of a space-borne INSAR as an element in a landslide or rock fall warning system is constrained by the specific space-borne SAR imaging geometry, the typical multiple-week repeat-interval, and uncertainties in the data availability. Most of these limitations can be overcome with an in-situ radar imaging system. GAMMA has developed a portable radar interferometer that utilizes real-aperture antennas to obtain high azimuth resolution. Images are acquired line by line while rotating the transmitting and receiving antennas about a vertical axis. Phase differences between successive images acquired from the same location are used to determine line-of-sight displacements. The instrument operates at 17.2 GHz and has measurement sensitivity better than 1 mm. The instrument uses two receiving antennas with a short baseline to form an interferometer. Phase differences between simultaneous acquisitions by these antennas are used to calculate the precise look angle relative to the baseline, permitting derivation of the surface topography. Expected statistical noise in the height measurements is on the order of 1 meter. In this contribution the design, measurement principles and characteristics of GAMMA's Portable Radar Interferometer (GPRI) are presented.

Information on snow coverage, structure, and liquid water content are important for many applications including avalanche warning, numerical weather prognosis (NWP), and snow pack water storage estimate. Active microwave remote sensing from space has an excellent potential to address these needs. However, the current generation of C-band SAR satellite systems are not well suited for snow related applications because of the small impact of the dry snow cover on the backscattering signal at 5.3 GHz. In order to be more sensitive to snow properties the envisaged ESA CoReH2O mission proposes a dual-frequency radar operating at 9 and 18 GHz (X- and Ku-band). In support of this mission, a ground based 9 to 18 GHz scatterometer is being developed to help to address the lack of simultaneous backscatter information of snow in this frequency range. We are also preparing a dedicated field campaign in the Swiss Alps to evaluate the performance of the system and acquire first data.

L-band (1-2 GHz) microwave radiometry is an important remote sensing technique to monitor soil moisture over land surfaces at the global scale. The ESA L-Band SMOS radiometer mission aims at providing asglobal maps of soil moisture, with accuracy better than 0.04 m3m-3every 3 days, with a spatial resolution better than 50 km. To improve the models used and for specifying the accuracy of the aimed global soil moisture data product, ground based radiometer campaigns before launch, during the commissioning phase and during the operative SMOS mission are important. Furthermore, the availability of ground-based L-band data measured simultaneously with the over-flying SMOS satellite are required for calibration and validation purposes. To address these needs three ELBARA II radiometers are being constructed for ESA. They rely on the proven architecture of the ELBARA radiometer [1] with improvements on the user interface, the mechanics, and the microwave electronics. Especially the development of an Active Cold Load (ACL) [2] as cold reference is expected to improve the radiometric accuracy.

The accuracy of SAR interferometric coherence estimates depends on the precision of several processing steps. In particular decorrelation can occur if range common-band filtering does not perform optimally. Typically a planar surface is adopted which introduces additional decorrelation in case of sloped terrain. To take into account topographic variations a slope-adaptive range common-band filtering method has been developed. A DEM is used to simulate an unwrapped interferogram and the fringe rate is used as driver for the filter size in the range common-band filtering step. Tests with several spaceborne interferometric SAR datasets confirmed the robustness of the method. The improvement of the coherence increased for increasing perpendicular baseline. As a consequence, the fringe visibility also greatly improved. To quantify the improvement of the coherence estimates with the slope-adaptive range common-band filtering, we considered the variation of classification in forest mapping, i.e. an application in which accurate coherence estimates are needed. With improved coherence the classified forest stem volume agreed better with forest maps derived from other remote sensing datasets.

The goal of this contribution is the assessment of the potential of a C-band SAR mission with repeat-pass interval of 12 days - as the intended European satellite system Sentinel-1 - to derive ice surface velocity using SAR interferometry (InSAR) and offset-tracking. For this purpose we investigated ERS-1 SAR data acquired during the ice missions in 1992 and 1994 in 3-day repeatorbits at Nordaustlandet in the northeast Svalbard archipelago. In 12-days winter InSAR pairs phase decorrellation is mainly observed in areas of high strain rates and, in certain cases, because of snow melting or redistribution through snowfall or wind. Velocity maps derived from these image pairs were found useful to enhance in regions of slow glacier flow the mapping of the surface ice-flow divides previously determined from optical imagery and topographic information. Range-azimuth offset-tracking investigations suggest that the expected error of this method is on the order of 50 m/year and that spatial coverage is generally satisfactory. Application of dual-azimuth offsettracking, use of HH polarization and enhanced spatial resolution, feasible with Sentinel-1, could enhance the expected error.

In the alpine zone snow is a dominant factor for more than half of the year and has strong influence on the ecosystem and economy. The knowledge of snow coverage, structure, liquid water content etc. is important and useful for many applications ranging from flood management to avalanche warning. Remote sensing from space has good potential to address these needs. Within ASSIST, Alpine Safety, Security and Information services and Technologies, these topics are also of interest. Two snow related products were identified that can be produced on an operational base with the available satellite systems to be ingested into the ASSIST service. Avalanche maps, mapping the contours of avalanches, and snow cover maps, mapping the snow covered area. The produced products are in good agreement with validation data. Unfortunately the current available satellite systems (mainly c-band SAR that is applicable) are not very well suited for snow related applications due to the small influence of the dry snow on the microwave signal at C- band. To overcome this limitation the CoReH2O mission was designed. With its X- and Ku-band system and repeat rates of 3 and 15 days it has high potential for alpine snow applications. Additional microwave signature measurements at these frequencies with standardized and reproducible snow characterization information will be needed for model development and validation. Recent developments allow a more quantitative snow characterizations and will be considered in the SnowScat project in combination with traditional snow characterization methods.

European Remote Sensing satellite (ERS) ENVISAT coherence is a new repeat-pass interferometric synthetic aperture radar product characterized by a very short repeat-pass interval between acquisitions (28min). In this paper, we investigate the properties of two 28-mins coherence images acquired over the regions of Paris, France, and Prague, Czech Republic, as well as a 35-day coherence image from Prague. Bare soils showed higher coherence compared to forests and urban areas. The range of coherence was larger for 28-min pairs, perpendicular baseline closer to 2km, and stable weather conditions between acquisitions. Coherence modeling shows that because of the long baseline: 1) significant surface decorrelation occurs when slopes are not accounted for in the common-band filtering and for variation of topography in the resolution cell and 2) volume decorrelation is significant. Temporal decorrelation seems to be relevant for vegetation even at the 28-min interval time scale. Observations and modeled coherence were found to be in good agreement. Land cover classification using coherence and backscatter has been tested for the Paris scene for mapping the four major classes. The classification accuracy was 86% when an edge-eroded version of the reference land cover map was used. It decreases to 68% when the full land cover map was used as reference because of local mismatches between the coherence image and the land cover map, and the reduced common band in range. The high coherence difference between forests and bare fields suggests the possibility to use the ERS-ENVISAT coherence for forest/nonforest mapping and estimation of biophysical properties of short vegetation. Coherence, decorrelation, ENVISAT Advanced Synthetic Aperture Radar (ASAR), European Remote Sensing satellite (ERS), land cover, synthetic aperture radar interferometry (InSAR).

ERS-ENVISAT coherence is a new SAR interferometric product with interesting properties due to the very short repeat-pass interval between acquisitions. When the requirements set by ERS-ENVISAT interferometry on baseline length and Doppler centroids are fulfilled, the ERS-ENVISAT coherence can present a large span of values. For an ERS- 2/ENVISAT image pair acquired over the region surrounding the city of Paris, bare soils have much higher coherence compared to forests and urban areas. As a consequence the use of the ERS- ENVISAT coherence for detecting forest cover changes and estimation of biophysical properties of regrowing forest / crops seems feasible. An analysis of the decorrelation phenomena for the ERS-ENVISAT coherence shows that (i) decorrelation can increase from near to far range if the common band filtering is not perfect, (ii) volume decorrelation is relevant because of the long baseline, and (iii) temporal decorrelation seems to be relevant for vegetation even at the 30 minutes interval time scale.

Large differences are observed between radar backscatter measurements made by the ENVISAT and ERS-2 satellite synthetic aperture radars, within 30 min of each other over certain agricultural fields in Flevoland, The Netherlands. The differences appear to be caused by the presence of highly directive scattering combined with very small variations in the azimuth illumination angle of the two sensors at the degree or subdegree level. There is relatively little awareness of such sensitivity to illumination direction, and it is not predicted by models for microwave interaction usually applied to soils and vegetation. It is, however, to be expected if there are significant contributions from coherent scattering extending over patches of the agricultural fields in question. We present analysis that supports such a conclusion.

Multi-temporal, multi-incidence ASAR WSM data over Siberia were used to derive land cover information. In an initial step several hundred ASAR WSM scenes were integrated into a database. Then the backscattering characteristics (signatures) of different land cover types were investigated. Based on the results, methodologies to retrieve land cover classes, including different forest biomass classes, were developed and applied. Parameters considered include a temporal backscattering variability parameter and an incidence angle dependence parameter.

Interferometric point target analysis using Gamma Remote Sensing's IPTA module has been applied to two interferometric stacks of RADARSAT-1 FINE mode data over the Belridge/Lost Hills oil fields in California. The stacks (/spl sim/30 images each) are both from descending orbits and span the same 2 year time period (2002/02-2004/02) but incidence angles are different (F1 vs. F3F). An IPTA analysis is carried out on each stack separately and results are then compared in terms of achieved density of the scatterers and residual errors of the motion analysis. We verify the accuracy of our methods indirectly by studying the subtle difference in line-of-sight motion caused by the different incidence angles of the data stacks (38.4 vs. 43.1 degrees at the study site).

There exists a significant demand for the monitoring of mining induced surface deformation for legal obligation safety, and environmental reasons. In recent years land surface deformation monitoring with SAR data reached some maturity. Nevertheless, there remain important limitations to the availability of the EO based information. Reasons include incomplete spatial coverage with information gaps for low coherence areas, problems in resolving high phase gradients, and unavailability of the required SAR data. The objective of our contribution is to show that important improvements to these limitations are still possible. In vegetated areas and for fast deformations L-band INSAR was found to be more robust with a wider applicability than C-band INSAR. In some low coherence areas point-like scatterers can be identified which permit to fill important information gaps. Furthermore, pairs with long baselines can be included in the interferometric analysis, supporting a more complete use of the available SAR acquisitions and to improve the temporal sampling

The UNOSAT service is an ESA Earth Observation Market Development initiative whose objective is to encourage, facilitate, accelerate and expand the use of accurate geo-information derived from EO satellite imagery by professionals involved in the implementation of humanitarian aid and development assistance projects. We focus on the role of SAR within this context. Space-borne SAR has a particularly good potential to support disaster management and humanitarian relief projects thanks to its all-weather capability, its capabilities for change detection, the large existing data archives, and, of course, the geometric and thematic information content of the images. Basic processing techniques, including precision image co-registration, multi-temporal analysis, terrain corrected geocoding, interferometry, and radargrammetry are well developed and operational. Advanced techniques like differential interferometry and interferometric point target analysis (IPTA) became more and more operational during the last years and provide unique information about terrain stability and deformation.

The objective of this paper is to present an approach that combines the interferometric point target analysis (IPTA) and the extended target based interferometric techniques (InSAR). The improvements achieved are most obvious in cases where the individual techniques result in complementary coverage in the information retrieved.

Interferometric Point Target Analysis (IPTA) is a method to exploit the temporal and spatial characteristics of interferometric signatures collected from point targets to accurately map surface deformation histories, terrain heights, and relative atmospheric path delays. In this contribution the IPTA concept is introduced, including the point selection criteria, the phase model and the iterative improvement of the model parameters. Intermediate and final results of an IPTA example using a stack of ERS-1 and ERS-2 data, confirm the validity of the concept and indicate a high accuracy of the resulting products.

Interferometric Point Target Analysis (IPTA) is a method that exploits the temporal and spatial characteristics of interferometric signatures collected from point targets that exhibit long-term coherence to map surface deformation. This paper demonstrates the viability of this technique for L-band data collected by the JERS-1 sensor during the time period 1992-1998. A data set covering Koga, Japan is used for demonstration and indicates regions of substantial subsidence.

In order to provide the best knowledge of the subsidence process around the Lagoon of Venice to the authorities that manage the area, SAR-based monitoring techniques (differential SAR interferometry and the interferometric point target analysis) will be integrated with levelling and GPS surveys into an overall database and information system. In this contribution the different monitoring techniques are briefly introduced, the results presented and compared, and the integration concept explained.

The X-SAR SRTM DEM of the alpine region in Switzerland was validated for SAR geocoding and 2-pass differential interferometry using ERS and JERS data. The absolute horizontal and vertical accuracies of the X-SAR SRTM DEM are in general in line with the nominal indications, but in very rugged areas we found horizontal offsets of up to 50 m in both northing and easting directions and some extreme absolute height errors up to even 1000 m. Also visible in the X-SAR SRTM DEM is an important noise for the slopes of SAR illumination. The ERS and JERS SAR images geocoded with the X-SAR SRTM DEM do not contain significant geometric artifacts, but, in parallel to what was discussed before, show in comparison to the map offsets in both easting and northing directions. The matching of geocoded products of descending and ascending modes is very good, i.e. on the order of 1 to 2 pixels. Considering that in our geocoding approach the geocoded SAR images are accurately co-registered to the DEM, the horizontal offsets have no influence for 2-pass differential interferometry. We found that the X-SAR SRTM DEM is well suited for ERS and JERS interferograms of the descending mode of baselines up to 100 m. For the differential interferometric analysis of ERS SAR data in ascending mode, however, attention has to be paid to the noise of the slopes facing away from the X-SAR.

In 2002 ENVISAT with the advanced SAR (ASAR) as one important instrument was launched. As compared to the preceding SAR sensors on ERS-1 and ERS-2 the ASAR has beam-steering capability permitting the acquisition of data over the same area under the different incidence angles as required by radargrammetry. In this paper a concept of combined use of radargrammetry and space triangulation to derive digital elevation models (DEMs) is presented. Space triangulation with opposite side acquisitions is used to determine a number of control points. Radargrammetry is then used to derive a DEM using these control points as reference. Preliminary test on the different steps of the combined method are presented.

In 2002 ENVISAT with the advanced SAR (ASAR) as one important instrument was launched. As compared to the preceding SARs on ERS-1 and ERS-2 the ASAR has additional functionality with many different modes. Important aspects of the ASAR include the alternating polarization modes for quasi simultaneous acquisition of data at two different polarizations, and beam-steering capability, which permits to acquire data at different incidence angles. In this paper the use of the ENVISAT ASAR for land cover information retrieval is discussed. For multi-incidence angle data a coefficient to characterize the incidence angle dependence is proposed.

In 2002 ENVISAT with the advanced SAR (ASAR) as one important instrument was launched. As compared to the preceding SARs on ERS-1 and ERS-2 the ASAR has additional functionality with many different modes. Important aspects of the ASAR include the alternating polarization modes for quasi simultaneous acquisition of data at two different polarizations, and beam-steering capability, which permits to acquire data at different incidence angles. To optimize the use of the new functionality adequate processing techniques are required. This is particularly important for data acquired in different swaths, i.e. with different image geometries. In this paper ASAR multi-swath techniques are discussed, including image co-registration, multi-channel filtering, and multi-swath multi-temporal techniques.

The objective of our investigation was to validate the potential of L-band SAR for the mapping of rapid subsidence and to demonstrate its monitoring capability for the specific case, Belridge in California USA. The results confirm the good potential of L-band repeat-pass interferometry to map and monitor subsidence. Subsidence rates of more than 46 cm per year were observed and mapped.

In this paper, the potential of L-band repeat-pass differential synthetic aperture radar (SAR) interferometry for land subsidence monitoring is evaluated using Japanese Earth Resources Satellite (JERS) SAR data. Bologna, Mexico City, and the Ruhrgebiet are selected as application sites representing slow to fast deformation velocities. The investigation includes feasibility aspects such as data availability, the temporal decorrelation over different landcover classes and the range of useful spatial baselines, an analysis of the achieved deformation accuracy, and considerations on the complementarity to European Remote Sensing satellite (ERS) SAR interferometry and leveling surveys. In spite of the rather limited data availability, land subsidence maps could be generated for the three selected application sites. In contrast to ERS C-band SAR data, JERS L-band interferometry permitted the retrieval of subsidence values over vegetated areas and forest when using interferograms of less than one year acquisition time interval and short baseline. In addition, the longer L-band wavelength was found to be superior in the case of large deformation gradients that lead to phase-unwrapping problems in C-band interferometry.

Potential and limitations of differential SAR interferometry for the periodical survey of Alpine displacements are investigated. For the Alpine territory, which is characterized by low vegetation, C-band differential SAR interferometry shows a relatively high coherence during the snow-free season permitting the detection and monitoring of unstable slopes on a regular basis. Limitations to the spatial coverage arise from decorrelation over forested and snow covered areas and from layover and shadowing caused by the very rugged topography. Significant results in the Swiss Alps using ERS-1/2 SAR data are used as support to our investigations.

Space-borne SAR has a good potential to support disaster management and humanitarian relief projects thanks to its all-weather capability, its capabilities for change detection, the large existing data archives, and, of course, the geometric and thematic information content of the images. Basic processing techniques, including precision image co-registration, multi-temporal analysis, terrain corrected geocoding, interferometry, and radargrammetry, are well developed and operational. This contribution is an assessment of possible roles of space-borne SAR and in particular ENVISAT ASAR in disaster management and humanitarian relief projects. Relevant additional functionality of ASAR as compared to ERS-1/2 comes from the beam-steering, multipolarization, larger incidence angle, wide-angle, and scan-SAR capability.

Quantitative analyses of remote sensing data acquired with variable geometries and different sensors are greatly facilitated by geocoding the data to a common geographical reference. Errors in geocoded SAR images are due to inaccuracies in the orbit data, errors in the processing parameters, and DEM errors. Automated terrain geocoding of SAR data is possible using only a single resampling of the native slant-range to the final map geometry after measurement and correction for residual geolocation errors. An accurate adaptive resampling algorithm for the interpolation from the SAR geometry to the map coordinate system is presented. This processing paradigm has been extended to permit combining data from different SAR instruments.

Two image-to-image patch offset techniques for estimating feature motion between satellite synthetic aperture radar (SAR) images are discussed. Intensity tracking, based on patch intensity cross-correlation optimization, and coherence tracking, based on patch coherence optimization, are used to estimate the movement of glacier surfaces between two SAR images in both slant-range and azimuth direction. The accuracy and application range of the two methods are examined in the case of the surge of Monacobreen in Northern Svalbard between 1992 and 1996. Offset-tracking procedures of SAR images are an alternative to differential SAR interferometry for the estimation of glacier motion when differential SAR interferometry is limited by loss of coherence, i.e. in the case of rapid and incoherent flow and of large acquisition time intervals between the two SAR images. In addition, an offset-tracking procedure in the azimuth direction may be combined with differential SAR interferometry in the slant-range direction in order to retrieve a two-dimensional displacement map when SAR data of only one orbit configuration are available.

Repeat pass ERS SAR interferometry was used to map different surface types of the Death Valley Salt Pan. At this arid site, the surface roughness varies between extremely smooth (salt lakes, clay pan) and very rough (alluvial fans, eroded salt formations). The moisture varies from very dry (alluvial fans) to wet (wet clay, salt crust with water ponds) with seasonal changes In certain areas. While the surface geometry is stable for certain surface types (alluvial fans, thick salt crusts), it changes for others as a result of the changing water level and erosion by wind and rain. The changing surface roughness, moisture, and vegetation conditions cause large variations of the backscattering and coherence. This information is used to distinguish different surface types. Furthermore, interferometry was used to map the topographic height. In a second part of the study, the potential of the degree of coherence to retrieve information on the vegetation density and the surface erosion was evaluated.

The potential of SAR interferometric techniques for the retrieval of vegetation parameters was investigated using ERS-1 data over agricultural and forested test sites. In a first experiment an interferometrically derived forest map was generated. The classification was based on the interferometric correlation and the backscatter intensities. The result was geocoded, using the interferometrically derived height map generated from the same ERS SAR data pair, and validated with a conventional digital forest map. Forest mapping accuracies of around 90% and better were achieved. In a second experiment, multitemporal data over an agricultural site were used to investigate the potential of repeat-pass interferometry to monitor farming activity, crop development, and soil moisture variations. The interferometric correlation was used as an indicator of dense vegetation and geometric change. It was possible, for example, to identify harvesting by the high correlation of the post-harvest bare or stubble field. Decreasing interferometric correlation was observed as a consequence of crop growth.

The potential of SAR interferometry for forest mapping and monitoring is discussed. It is shown that forest can clearly be discriminated from other land categories. Furthermore it is possible to distinguish a number of forest types. The presented approach is based on the SAR interferometric correlation and the backscatter intensities using ERS-1 SAR repeat-pass data. Baseline, time interval, and seasonal dependences were analyzed, substantiating a wide applicability of the approach. Data over an Alaskan test site were used to extend the results found over temperate forest to boreal forest and to demonstrate the potential of the described technique over remote areas. In addition, repeat-pass SAR interferometry was found to be particularly sensitive to change. Examples for the recognition of freezing, mechanical cultivation of agricultural fields, and canopy growth are shown.>

An interferometric radar technique for topographic mapping of surfaces promises a high-resolution approach to the generation of digital elevation models. The authors present analyses of data collected by the synthetic aperture radar instrument on-board the ERS-1 satellite on successive orbits. Use of a single satellite in a nearly repeating orbit is attractive for reducing cost and spaceborne hardware complexity; also it permits inference of changes in the surface from the correlation properties of the radar echoes. The data have been reduced to correlation maps and digital elevation models. The correlation maps show that temporal correlation decreases significantly with time, but not necessarily at a constant well-defined rate, likely depending on environmental factors. When correlation among passes remains high, however, it is possible to form digital elevation models. Analyses of noise expected in ERS-1 interferometric data collected over Alaska and the southwestern United States indicate that maps with relative errors less than 5 m rms are possible in some regions. However, orbit uncertainties imply that tie points are required in order to reduce absolute height errors to a similar magnitude. The authors find that about 6 tie points per 40/spl times/40 km scene with 5 m rms or better height accuracy are needed to keep systematic map height errors below 5 m rms. The performance of the ERS-1 radar system for topographic applications, though useful for a variety of regional and local discipline studies, may be improved with respect to temporal decorrelation errors and absolute height acuity by modifying the orbit repeat period and incorporating precise orbit determination techniques. The resulting implementation will meet many, but not all, objectives of a global mapping mission.>

The Cassini Titan Radar Mapper is a multimode radar instrument designed to probe the optically inaccessible surface of Titan, Saturn's largest moon. The instrument is to be included in the payload of the Cassini Saturn Mission, scheduled for launch in 1995. The individual modes of Cassini Radar Mapper will allow topographic mapping and surface imaging at few hundred meters resolution. The requirements that lay behind the design are briefly discussed, and the configuration and capability of the instrument are described. The present limited knowledge of Titan's surface and the measurement requirements imposed on the radar instrument are addressed. Also discussed are the Cassini mission and the projected orbits, which imposed another set of design constraints that led to the multitude of modes and to an unconventional antenna configuration. The antenna configuration and the different radar modes are described.>