Fully Focused SAR Altimetry: Theory and Applications

Egido, Alejandro; Smith, Walter H. F.

doi:10.1109/tgrs.2016.2607122

Cited by 89 publications

(123 citation statements)

References 22 publications

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“…Egido and Smith [16] discussed the SAR focused on point targets. Here we extend the idea to a generic water body of any shape and size by defining the following cost function (CF1):…”

Section: Phase-focused Methodsmentioning

confidence: 99%

“…It is not uncommon to see such hyperbolic patterns in radar altimeter data, especially from small vessels near coast. Egido and Smith [16] give a further example of the CryoSat-2 radar pass over the Svalbard transponder, which was designed as a point scatterer calibration for radar altimeters. Figure 3 in the Egido and Smith paper is the Z for a point specular target.…”

Section: Very Small Water Bodiesmentioning

confidence: 99%

See 1 more Smart Citation

Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water Altimetry Potentialities

Abileah¹,

Scozzari

Vignudelli

2017

Remote Sensing

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Abstract:The exploitation of synthetic aperture properties in nadir-looking radars is opening new scenarios in the framework of satellite radar altimetry. Both recent and upcoming missions including Cryosat-2, Sentinel-3, Sentinel-6 and SWOT take benefit from the coherent processing of radar data, aimed at improving range measurements in particular contexts, such as ice, open ocean, coastal zone, and even inland waters. This work investigates the possibilities offered by current and future satellite radar altimetry missions for the study of inland water bodies, probing into the peculiarities of the expected radar returns and their potential usage. In this regard, signals collected by the RA-2 instrument (Radar Altimeter 2) onboard the Envisat mission offer an unprecedented possibility, even with a relatively low pulse repetition frequency, to analyze the peculiarities of actual signals for detecting and ranging small water surfaces. In particular, the RA-2 instrument offers a global archive of Individual Echoes (IEs), collected at the native sampling rate of 1795 Hz, in addition to the 18 Hz data obtained by incoherent averaging, which are typically delivered to the users as standard products. RA-2 shares with future radar platforms such as Sentinel-6 a continuous and interleaved working modality, as was recommended by the scientific community in designing next missions' requirements. This is a further reason to consider the usage of RA-2 IEs as particularly attractive. Whilst only available for a small percentage of the earth's surface, sufficient IE data exist to study the height retrieval capability of these echoes, in particular for what concerns small water bodies, where we show that enough coherence is exhibited for focusing relatively narrow surfaces and range them correctly. A peculiar aspect of this work lies in the assumption that most of the returned echoes (in RA-2 IEs) are specular. A theoretical framework is developed according to this assumption, which is validated by investigating real RA-2 data and observing their related specular features. In particular, we discuss how specular echoes are expected to be very common in inland altimetry, and are most often associated with small to medium size lakes and rivers. This paper illustrates the expected electromagnetic behavior of specular water targets by exploiting the classical radar cross-section (RCS) theory for specular surfaces. Results from the model are compared with real IE data in three selected case studies, regarding two rivers of variable width and one flood plain, in order to check different hydrological regimes. The model very closely matches the data in all cases, making the results of this validation activity very promising. In particular, we demonstrate the feasibility of using satellite radar altimetry in rivers much smaller than what was considered possible until now.

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“…Egido and Smith [16] discussed the SAR focused on point targets. Here we extend the idea to a generic water body of any shape and size by defining the following cost function (CF1):…”

Section: Phase-focused Methodsmentioning

confidence: 99%

Section: Very Small Water Bodiesmentioning

confidence: 99%

Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water Altimetry Potentialities

Abileah¹,

Scozzari

Vignudelli

2017

Remote Sensing

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“…As for conventional and DDA altimetry, dedicated processing has been developed to improve the data in the coastal zone. This includes both fully focused SAR (Egido and Smith, 2017) and enhanced unfocused SAR processing. The standard unfocused SAR processing baseline consists of ground cell gridding at 20 Hz, Doppler Beam Forming, Doppler Beam Stacking, Slant Range Compensation, Range Compression and Multilooking (Raney, 1998;Cullen and Wingham, 2002) and of the open ocean SAMOSA SAR waveform model (Ray et al, 2015) in its formulation with zero-and first-order term (SAMOSA2).…”

Section: Retracking Conventional and Sar Mode Altimetrymentioning

confidence: 99%

“…The fully focused SAR (FF-SAR) processing technique promises an along-track resolution of 0.5 m for typical SAR altimeters (Egido and Smith, 2017), instead of the 300-m resolution of the unfocused DDA processing. Also relevant for coastal zone, but scarcely investigated in this area is the DDA interferometry (SARin), which permits to extract spatial and temporal derivatives of water levels, providing an entirely new observable area.…”

Section: Sarin Mode and Fully Focused Sar Altimetrymentioning

confidence: 99%

Requirements for a Coastal Hazards Observing System

et al. 2019

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Coastal zones are highly dynamical systems affected by a variety of natural and anthropogenic forcing factors that include sea level rise, extreme events, local oceanic and atmospheric processes, ground subsidence, etc. However, so far, they remain poorly monitored on a global scale. To better understand changes affecting world coastal zones and to provide crucial information to decision-makers involved in adaptation to and mitigation of environmental risks, coastal observations of various types need to be collected and analyzed. In this white paper, we first discuss the main forcing agents acting on coastal regions (e.g., sea level, winds, waves and currents, river runoff, sediment supply and transport, vertical land motions, land use) and the induced coastal response (e.g., shoreline position, estuaries morphology, land topography at Frontiers in Marine Science | www.frontiersin.org 1 July 2019 | Volume 6 | Article 348Benveniste et al.Requirements for a Coastal Zone Observing System the land-sea interface and coastal bathymetry). We identify a number of space-based observational needs that have to be addressed in the near future to understand coastal zone evolution. Among these, improved monitoring of coastal sea level by satellite altimetry techniques is recognized as high priority. Classical altimeter data in the coastal zone are adversely affected by land contamination with degraded range and geophysical corrections. However, recent progress in coastal altimetry data processing and multisensor data synergy, offers new perspective to measure sea level change very close to the coast. This issue is discussed in much detail in this paper, including the development of a global coastal sea-level and sea state climate record with mission consistent coastal processing and products dedicated to coastal regimes. Finally, we present a new promising technology based on the use of Signals of Opportunity (SoOp), i.e., communication satellite transmissions that are reutilized as illumination sources in a bistatic radar configuration, for measuring coastal sea level. Since SoOp technology requires only receiver technology to be placed in orbit, small satellite platforms could be used, enabling a constellation to achieve high spatio-temporal resolutions of sea level in coastal zones.

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“…Unfocused indicates that the resolution is still larger than the Fresnel zone (Raney 1998), which is about 170 m for Cryosat-2. In a recent study, Egido and Smith (2017) demonstrated that the altimeter system onboard Cryosat-2 is stable enough to allow for interburst coherent processing. If the full synthetic aperture length of about 2 s is exploited, the processing is known as fully focused SAR (FF-SAR).…”

Section: Introductionmentioning

confidence: 99%

The second-order effect of Earth’s rotation on Cryosat-2 fully focused SAR processing

Kleinherenbrink

Smith

Naeije

et al. 2020

J Geod

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We investigated the sensitivity of fully focused SAR (FF-SAR) processing of Cryosat-2 altimeter data to Earth rotation. Earth's rotation causes scatterers at varying cross-track locations to have a different relative velocity with respect to the satellite. This second-order effect of Earth rotation on the phase is currently not corrected for in FF-SAR processing of altimetry data. The difference is largest near the poles, where the satellite flies parallel to the equator. Not correcting for the second-order effect yields a parabolic shape in the counter-rotated phase, which increases with the cross-track distance. Its effect is, however, limited by the time-in-view of the scatterer, which is shorter at the edge of the altimeter footprint, and therefore destructive interference will not occur when using Cryosat-2 data. For Cryosat-2, the only expected effect is a reduction in power and along-track resolution in the waveform tail and in the grating lobes. If the FF-SAR processor focuses on one point, and there is a bright scatterer at another, then there is a residual parabolic phase, whose sign and shape depend on the cross-track distance and whether the signal is left or right of the chosen focal point. In theory, if the viewed scene only has few bright coherent scatterers, then it might be possible to determine the cross-track position of each. In practice, however, natural targets are rarely coherent over the integration time.

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Fully Focused SAR Altimetry: Theory and Applications

Cited by 89 publications

References 22 publications

Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water Altimetry Potentialities

Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water Altimetry Potentialities

Requirements for a Coastal Hazards Observing System

The second-order effect of Earth’s rotation on Cryosat-2 fully focused SAR processing

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