Monitoring the Petermann Ice Island with TanDEM-X

This paper presents the processing of TanDEM-X acquisitions for the monitoring of the topography of the Petermann ice island. In this particular case the area under study is continuously moving and the acquisition geometry is changing, so the processing of the iceberg’s DEMs is challenging and additional effects are to be considered. The SAR processing chain used is presented and the results obtained summarized, showing the effects and limitations observed during the process

Phase inconsistencies and water effects in SAR interferometric stacks

SAR Interferometry with stacks has already shown its potential in identifying permanent scatterers, in processing decorrelating targets, mitigating atmospheric delays, etc., but we believe that there is still potential for retrieving information on the scattering environment which has not been extensively studied yet. In particular interferometric stacks can reveal systematic phase inconsistencies which are not detectable in single interferograms, challenging any simple interpretation of the interferometric phase and associated coherence. The explanation of such inconsistencies requires more complex propagation models than the one based on a simple delay

SEASTAR: a mission to study ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas

High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere, e.g., freshwater, pollutants. As numerical models continue to evolve toward finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed toward spaceborne implementation within Europe and beyond

SKIM, a candidate satellite mission exploring global ocean currents and waves

The Sea surface KInematics Multiscale monitoring (SKIM) satellite mission is designed to explore ocean surface current and waves. This includes tropical currents, notably the poorly known patterns of divergence and their impact on the ocean heat budget, and monitoring of the emerging Arctic up to 82.5°N. SKIM will also make unprecedented direct measurements of strong currents, from boundary currents to the Antarctic circumpolar current, and their interaction with ocean waves with expected impacts on air-sea fluxes and extreme waves. For the first time, SKIM will directly measure the ocean surface current vector from space. The main instrument on SKIM is a Ka-band conically scanning, multi-beam Doppler radar altimeter/wave scatterometer that includes a state-of-the-art nadir beam comparable to the Poseidon-4 instrument on Sentinel 6. The well proven Doppler pulse-pair technique will give a surface drift velocity representative of the top meter of the ocean, after subtracting a large wave-induced contribution. Horizontal velocity components will be obtained with an accuracy better than 7 cm/s for horizontal wavelengths larger than 80 km and time resolutions larger than 15 days, with a mean revisit time of 4 days for of 99% of the global oceans. This will provide unique and innovative measurements that will further our understanding of the transports in the upper ocean layer, permanently distributing heat, carbon, plankton, and plastics. SKIM will also benefit from co-located measurements of water vapor, rain rate, sea ice concentration, and wind vectors provided by the European operational satellite MetOp-SG(B), allowing many joint analyses. SKIM is one of the two candidate satellite missions under development for ESA Earth Explorer 9. The other candidate is the Far infrared Radiation Understanding and Monitoring (FORUM). The final selection will be announced by September 2019, for a launch in the coming decade

Single-Pass Bistatic SAR Interferometry Using Fixed-Receiver Configurations: Theory and Experimental Validation

In this paper, bistatic interferometry using fixed-receiver configurations is addressed both theoretically and experimentally. The analytical expressions for interferometric phase and height sensitivity are derived, and a full interferometric processing chain for digital elevation model (DEM) generation is presented. The derived expressions are general, and they can be applied to two possible acquisition geometries: backscattering and forward scattering. The theoretical developments are complemented with experimental results done with the bistatic receiver Synthetic Aperture radar Bistatic Receiver for INterferometric Applications. The obtained DEMs are compared with a DEM from the Shuttle Radar Topography Mission and a digital terrain model from the Institut Cartografic de Catalunya. The comparison allows one to validate the results and demonstrate to which particular features of the scene that the bistatic radar is sensitive

Multi-Transmit Operation Scheme for a Reflector-Based SAR System

A transmit scheme that uses several timed and directed sub-pulses to cover one common swath is proposed. Timing and ambiguity suppression is presented. The advantages of the proposed scheme are an increased sensitivity compared to one transmit pulse per swath and a simpler activation scheme of the receivers in combination with lower data compression effort on-board a satellite

General Processing Approach for Bistatic SAR Systems: Description and Performance Analysis

For its intrinsic properties, bistatic SAR processing cannot be approached in the same manner monostatic SAR process- ing is approached. In general, a bistatic SAR processor is more than a powerful focussing algorithm. Because of the lack of precise time (range) and phase (Doppler) references in non-cooperative (and even cooperative) bistatic systems, a prior software synchronisation step is used to correct the residual errors caused by the use of different clocks. Moreover, depending on the bistatic configuration, different kinds of SAR focussing algorithms can be used. Azimuth-invariant configurations may benefit from the convolution property of the Fourier transform, whereas more general configurations are better suited for efficient time-domain techniques. A discussion on the performance of three Fourier-domain focussing algorithms, range-Doppler, chirp scaling, and range migration, is included in the paper

Lack of triangularity in SAR Interferometric phases

If three SAR images are available, it is possible to form three interferograms. In some cases the phases of the three averaged interferograms will not agree among each other and indicate a sort of phase excess or deficit (which we call "lack of triangularity"). In this paper we illustrate theoretically which models can explain such phenomenon and show some real-data examples. The observation of lack of triangularity might be useful to derive information on the target and also as a warning that the scatterer presents a temporal covariance matrix which is not intrinsically real

Bidirectional SAR Imaging Mode

The paper introduces the Bi-Directional (BiDi) SAR imaging mode, i.e. the simultaneous imaging of two directions by one antenna into one receiving channel, and presents short-term time series of images and interferograms acquired by the TerraSAR-X and TanDEM-X satellites. A comparison to alternative approaches for the acquisition of short-term time series is provid-ed. The BiDi acquisition geometry is defined and a TerraSAR-X bi-directional antenna pattern is analyzed. BiDi raw data are simulated, sampled with different PRF values and compared to real BiDi raw data. The spectral separation of simultaneously acquired forward and backward looking images is explained. The paper presents the image results of BiDi acquisitions with Ter-raSAR-X and TanDEM-X satellites flying with 20 km along-track separation. This pursuit configuration allowed for the acquisition of up to six short-term repeated images and up to three interferograms in a single pass. An overview of potential applications for the new bi-directional SAR imaging mode concludes the paper