One relevant application of synthetic aperture radar (SAR) is ship monitoring. Conventional SAR modes, however, are limited in their ability to detect small ships over wide swaths (> 100 km) due to their low azimuth resolution. A novel SAR mode is presented in which the pulse repetition interval (PRI) is continuously varied and is much smaller than the minimum PRI imposed by the swath width, producing images that are corrupted by strong range ambiguities. For the same observed swath, this mode has significantly better detection performance than ScanSAR due to higher azimuth resolution and ambiguity blurring effect due to the PRI variation. Waveform alternation can be moreover explored to further blur ambiguities and increase the mapped swath. Spaceborne demonstrations using experimental TerraSAR-X acquisitions are presented. The idea of SAR systems affected by SAR ambiguities is then extended to the case of a cluster of small satellites, whose acquired data are jointly processed to create an accurate, high-resolution digital elevation model (DEM) of the observed scene in a single pass. This technique is demonstrated through a dedicated multi-baseline airborne campaign, carried out using DLR’s F-SAR airborne sensor.

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swaths and high resolution. This enables monitoring of wide areas with high detection probabilities and low false alarm rates at short time intervals. Ambiguous SAR modes have proven effective for ship monitoring in remote offshore regions. They deliver high-resolution SAR images over wide swaths that can be effectively exploited for ship detection, even though they are corrupted by significant ambiguities of ships and sea clutter. Building upon the successful demonstration of the staggered ambiguous mode using TerraSAR-X, this letter presents the results of a further experimental acquisition, carried out near the Dutch coast, where an ultrawide ground swath of 160 km is imaged at an azimuth resolution of 2.2 m by introducing in the staggered ambiguous mode alternation of up- and down-chirp waveforms; 79 small ships were detected in the dataset, some of which even near the coast and despite the presence of first-order range ambiguities caused by land scatterers, as these ambiguities are very blurred and manifest as noise like disturbances. These results are of fundamental importance for incorporating the ambiguous modes into the existing and future SAR systems as an efficient additional mode for ship monitoring, suitable for both open sea and coastal surveillance.

PERTEO is a mission proposal intended to provide real-time EO services to reduce Natural Disaster impact by proposing a heterogeneous small satellite constellation and performing in-orbit processing. For that aim, this mission takes advantage of the enhanced capabilities of AI edge processing by enabling on-demand user services through the "satellite as a service" concept. The design of the constellation includes three types of sensors: SAR, Hyperspectral and Multispectral imager VHR combining their capabilities to achieve almost real-time due to their in-orbit distances. Six spacecrafts are considered in each orbit organized in pairs (180° phased platforms) mounting the same instrument. The proposed solution is responsive achieving almost real-time latencies ( Ex: ≲ 1 min, down to seconds is achieved in the first observation product). Persistence is achieved with a low revisit time (≲ 1 hour any payload; ≲ 3 hour s any specific payload choice).

Spaceborne synthetic aperture radar (SAR) is an essential tool for Earth observation. The combination of multiple SAR images taken from different viewing angles allows forming accurate digital elevation models and high-resolution tomograms that unveil the three-dimensional structure of vegetation, ice, and dry soil. Whereas nowadays such images are mostly acquired sequentially, compromising product quality and hindering the monitoring of fast dynamics, multi-static SAR systems enable the simultaneous acquisition of all required data, paving the way for effective and powerful monitoring of our planet. One fundamental challenge is the conception of multi-static concepts that allow the generation of high-quality digital elevation models and tomograms from large sets of noisy and undersampled radar data acquired by clusters of smallsats with small antenna apertures. Swarms of drones, equipped with radars and localization systems, represent an affordable option to test multi-static concepts and acquire multi-static data with sub-hour temporal resolution.

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swath and high resolution. This allows frequent monitoring of large areas with high detection probability and low false alarm rate. Conventional SAR modes are, however, limited in that a wide swath can only be imaged at the expense of a reduced azimuth resolution. Ambiguous SAR modes, based on low pulse repetition frequency or continuous variation of short pulse repetition intervals (staggered ambiguous mode), overcome this limitation and allow imaging a wide swath with high resolution for specific ship monitoring applications without the need for digital beamforming or multiple receive apertures. This paper reports on the demonstration of the staggered ambiguous mode via an experimental acquisition with the TerraSAR-X satellite over the North Sea. In spite of technical limitations in the SAR instrument, a ground range swath of 110 km was imaged with an azimuth resolution of 2.2 m, i.e., with a resolution improvement of a factor of eight with respect to TerraSAR-X ScanSAR mode. Despite the higher disturbance level resulting from the presence of range ambiguities of the sea clutter a detection probability higher than 0.8 was achieved for small ships of 21 m × 6 m size. Range ambiguities of the ships were furthermore identified based on their position and signature. The detected ships were validated using maritime positioning data from their automatic identification system. These results motivate the adoption of ambiguous SAR modes in existing and future SAR systems and missions.

Synthetic aperture radar (SAR) interferometry is a well-established technique for producing high-resolution digital elevation models (DEMs) of the Earth’s surface and measuring displacements on different time scales. Observations of SAR interferograms, however, show that azimuth ambiguities can be coherently imaged and may lead to phase biases and coherence losses that significantly degrade the interferometric performance. Whereas imposing very low ambiguity levels may represent a severe design constraint for a spaceborne SAR system, a slight variation of the pulse repetition interval (PRI) is a new, simple, yet effective technique to decorrelate ambiguities, which in turn reduces the phase biases and coherence losses without substantially affecting the imaged swath width. An additional benefit of the PRI variation is that range ambiguities also become decorrelated. This paper addresses two cases: For the repeat-pass case, slightly different pulse repetition frequencies (PRFs) can be used for the two acquisitions and the minimum required PRF difference can be analytically derived resorting to the power spectral density of the ambiguous signals; For the single-pass case, a slight variation of the PRI during the common acquisition is an effective solution, in case an along-track baseline is present. In particular, a square wave PRI variation scheme outperforms sinusoidal or random ones. Finally, simulations using TanDEM-X data are presented to show the improvement in interferogram and DEM quality resulting from ambiguity decorrelation. This work is relevant for the design of future spaceborne interferometric SAR systems and for the enhanced exploitation of current ones.

Synthetic aperture radar (SAR) systems have unique imaging capabilities that are suitable for several Earth observation applications. This paper proposes a new concept for the design of a low-cost SAR for a single dedicated application, namely illegal vessel detection, that can be implemented using a small satellite and is characterized by reduced transmit power and high resolution. Minimum requirements in terms of noise-equivalent sigma zero (NESZ) and resolution that ensure acceptable detection performance are derived, by using TerraSAR-X data to retrieve the intensity distribution of typical ships. One peculiarity of the design is that a pulse repetition frequency (PRF) much smaller than the nominal Doppler bandwidth is selected to ensure a wider swath but still guaranteeing a high azimuth resolution. A design example of a system in X band imaging a 50-km ground swath and characterized by an average transmit power of only 15 W is presented.