This paper provides an overview of the Copernicus L-band SAR mission referred to as ROSE-L (Radar Observing System for Europe at L-band).In particular, the paper provides an overview of the ROSE-L spacecraft and focuses on the ROSE-L SAR instrument architecture, performance and critical technology developments.The current status of the instrument activities (project is in Phase C) will be presented, highlighting the main achievements and challenges.

Radio Frequency Interference (RFI) is a growing problem for future Synthetic Aperture Radar (SAR) missions, resulting in data loss, image artifacts and undetected biases. A new approach for mitigating RFI is digital beamforming (DBF), which is possible with the next generation of multichannel SAR systems and allows for a spatial filtering of signals from different directions. While on-board RFI removal with DBF is challenging for spaceborne systems due to the computational load, past publications have shown that this problem can be overcome with DBF-based auxiliary beams by moving most of the processing to the ground without requiring the down-linking of all channels. A remaining problem of measuring RFI information with auxiliary beams is determining the interferer position. This paper shows the performance simulation of a series of digital square-law detectors which can be used to overcome this issue. It is shown that a series of digital square-law detectors provides the opportunity to simultaneously determine if RFI is present and under which direction, while maintaining a low system complexity. This is possible because the detectors provide a good probability of detection and false alarm rate even if the data rate is decimated. The simulated system performs well for a decimation factor of 110.

The paper details a SAR imaging mode named frequency SCan On Receive and Transmit (fSCORT) where, a narrow, frequency-scanning transmit antenna beam is formed illuminating the swath of interest from far to near range. Similarly, the frequency-scanning receive antenna beam collects the return echo signal reflected from the ground. It is shown that the imaging technique trades (sacrifices) range resolution for (improved) signal-to-noise ratio, azimuth resolution, and swath width. It is shown that the (transmit) pulse duty cycle may be significantly increased over what is common for pulsed SAR systems, allowing for a lower peak-to-average transmit power ratio. A comparison to classical stripmap operation mode shows an improvement of several decibels. The imaging mode is especially suitable for spaceborne SAR systems operating at high carrier frequencies where other advanced digital beam-forming techniques may not be feasible due to technology limitations, while, on the other hand, a large system bandwidth may be available. The final paper provides a simple mathematical model describing fSCORT operation modes. Closed expressions for the mode performance parameters, such as range resolution, echo window length, signal-to-noise ratio, etc.\ are derived. The performance of fSCORT is compared to a SAR operating in stripmap and SCORE mode.

Radio frequency interference (RFI) is becoming a major concern for future synthetic aperture radar (SAR) missions due to the increased user demand for frequency occupation in a number of applications. Each occurrence of interference introduces artifacts in the radar imagery, biasing the measurements and leading to erroneous results. In addition to conventional techniques, the use of multichannel SAR for RFI mitigation has been proposed, because its digital beamforming (DBF) capability allows for a spatial filtering of the received signals. Thereby, it becomes possible to remove RFI that arrives from a different direction than the SAR signal. Past publications on the topic presented highly flexible spatial filtering techniques. Those methods require either additional on-board processing or a substantial increase in the downlink capacity. This article shows that by slightly reducing the flexibility of the spatial filtering, DBF can be utilized for RFI mitigation without either drawbacks: the processing is performed on-ground after downlinking the data and the data volume remains manageable. This is achieved with auxiliary beams. Their concept and limitations are discussed in detail in this article and are supported with simulated RFI mitigation results. Furthermore, it is shown that the information collected with an auxiliary beam can also be used to filter the RFI signal when it is spatially nonorthogonal to the SAR signal.

The synthetic aperture radar (SAR) imaging mode described in this paper utilizes the available signal bandwidth to form a narrow frequency-scanning transmit antenna beam illuminating the swath of interest from far to near range. The imaging technique is named frequency Scan for Time-of-Echo Compression (f-STEC), because, for a proper choice of mode parameters, the radar echo duration is reduced, i.e., compressed. The paper provides a detailed analysis of the f-STEC imaging technique; derives the operational and performance parameters as well as — to the authors knowledge — for the first time, analytic expressions for the f-STEC timing constraints. Further, the performance and trade-space is reported and compared to both conventional and modern, i.e., digital beamforming imaging modes. The f-STEC imaging technique is shown to be specifically advantageous for SAR systems operating at higher carrier frequencies or an attractive add-on for state-of-the-art SAR instruments.

Correcting for Radio Frequency Interference (RFI) has become a critical aspect of current and future Synthetic Aperture Radar (SAR) missions. While most RFI mitigation methods suffer significant drawbacks (e.g., loss of resolution) or require individual filter parameters for each acquisition, spatial filtering methods only rely on system design parameters, such as antenna size. Further, combining spatial filtering with digital beamforming (DBF) allows for the removal of in-swath interference while the instantaneous SAR and RFI signal arrive from different directions. One option to apply a DBF-based RFI mitigation on the ground in post processing are auxiliary beams. Hereby, the RFI is measured with simultaneous auxiliary beams and this information is downlinked at the cost of a small increase of data volume. This paper presents an on-board implementation of RFI detection, which can then be used to select the best auxiliary beams.

The development of synthetic aperture radar (SAR) instruments at higher carrier frequency is motivated, among others, by the relative wavelength scaling, which provides an increased bandwidth and reduced physical dimensions of the RF hardware, for the same coverage. The later causes a reduced area and a high length-to-height aspect ratio of the antenna, which is unfavorable, since the resulting design compromise leads to systems of small swath width and low signal-to-noise ratio.

The important task of deciding and revising frequency allocations at international level falls under the responsibility of the World Radiocommunication Conference (WRC) which is organized by a specialized agency of the United Nations, the Radiocommunications Sector of the International Telecommunications Union (ITU-R). A WRC is held every approximately four years for the purpose of updating the international Radio Regulations (RR). The most recent of these conferences, WRC-19, held at the end of 2019, also defined the Agenda Items for the next WRC that is scheduled to occur in 2023. Here, we review and discuss the WRC-23 Agenda Items that are likely to have the most impact on future remote sensing operations.

Radio Frequency Interference (RFI) is a growing problem in Synthetic Aperture Radar (SAR). The choice of operational frequency of remote sensing instruments is dictated by the physics of the parameters that are to be observed. On the other hand, the frequency spectrum becomes more crowded with the increasing demand for wireless services. This requires cohabitation of multiple systems at the same or neighbouring frequency bands. As a result, RFI can introduce artefacts and degrade the derived SAR products. A mitigation of RFI is critical. New removal techniques can be implemented with Digital Beamforming (DBF) radars. In this paper, we present a Pulse and Range-Dependent Time Minimum Variance Distortionless Response (PRDTMVDR) Beamformer. The antenna pattern (AP) is adaptively changed for each pixel based on the inherent imaging geometry of SAR and thus the RFI suppression is improved compared to an AP that is fixed for the pulse duration.

An increasing challenge for P-band Synthetic Aperture Radar (SAR) is Radio Frequency Interference (RFI). RFI results in image distortions and degrades the derived science products. This makes it critical to apply RFI removal techniques to restore the image quality. New advanced techniques can be achieved with Digital Beamforming (DBF) radars such as EcoSAR. In this paper, we present a Range-Dependent Time Minimum Variance Distortionless Response (RDTMVDR) Beamformer and apply it to EcoSAR flight data during post-processing. The antenna pattern (AP) is adaptively changed for each range line which increases the RFI suppression compared to a fixed AP for each pulse. The interferometric image quality is assessed before and after RFI suppression.

Wideband radar systems operating at L- or P-band encounter an increasing amount of Radio Frequency Interference (RFI). If this RFI is not removed from the obtained radar signal, the quality of extracted science data can be decreased significantly. The removal of RFI can be improved with Digital Beamforming (DBF), a technique that is becoming more important in many radar applications. DBF enables to digitally steer the antenna beam into different directions and to place nulls into the antenna pattern. This technology is implemented in EcoSAR, a synthetic aperture radar (SAR) operating at P-band. Further, in data acquired by EcoSAR in Costa Rica in 2014, the impact of multiple RFI sources can be observed. This gives the opportunity to implement and test new RFI mitigation techniques based on DBF. In this paper, we estimate RFI with DBF and subtract it from EcoSAR data. The method is applied to several acquisitions and results are being presented.

EcoSAR is a single-pass (dual antenna) digital beamforming, P-band radar system that is designed for remote sensing of dense forest structure. Forest structure retrievals require the measurement related to the vertical dimension, for which several techniques have been developed over the years. These techniques use polarimetric and interferometric aspects of the SAR data, which can be collected using EcoSAR. In this paper we describe EcoSAR system in light of its interferometric capabilities and investigate forest structure retrieval techniques.

Next Generation Digital Beamforming (DBF) Synthetic Aperture Radar (SAR) is a technological area being pursued at the NASA Goddard Space Flight Center (GSFC). Two such systems - DBSAR-2 and EcoSAR-have been recently developed and tested. The new instruments employ advanced architectures characterized by multi-mode operation, software defined waveform generation, digital beamforming, and configurable radar parameters. The instruments have been developed to support several disciplines in Earth and Planetary sciences. This paper will describe EcoSAR and DBSAR-2 advanced features and report on the latest SAR processing and calibration efforts.