Initial Observations of Lunar Impact Melts and Ejecta Flows with the Mini-RF Radar

The Mini-RF radar on the Lunar Reconnaissance Orbiter's spacecraft has revealed a great variety of crater ejecta flow and impact melt deposits, some of which were not observed in prior radar imaging. The craters Tycho and Glushko have long melt flows that exhibit variations in radar backscatter and circular polarization ratio along the flow. Comparison with optical imaging reveals that these changes are caused by features commonly seen in terrestrial lava flows, such as rafted plates, pressure ridges, and ponding. Small (less than 20 km) sized craters also show a large variety of features, including melt flows and ponds. Two craters have flow features that may be ejecta flows caused by entrained debris flowing across the surface rather than by melted rock. The circular polarization ratios (CPRs) of the impact melt flows are typically very high; even ponded areas have CPR values between 0.7-1.0. This high CPR suggests that deposits that appear smooth in optical imagery may be rough at centimeter- and decimeter- scales. In some places, ponds and flows are visible with no easily discernable source crater. These melt deposits may have come from oblique impacts that are capable of ejecting melted material farther downrange. They may also be associated with older, nearby craters that no longer have a radar-bright proximal ejecta blanket. The observed morphology of the lunar crater flows has implications for similar features observed on Venus. In particular, changes in backscatter along many of the ejecta flows are probably caused by features typical of lava flows

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Hybrid Dual-Polarization Synthetic Aperture Radar

Compact polarimetry for a synthetic aperture radar (SAR) system is reviewed. Compact polarimetry (CP) is intended to provide useful polarimetric image classifications while avoiding the disadvantages of space-based quadrature-polarimetric (quad-pol) SARs. Two CP approaches are briefly described, π/4 and circular. A third form, hybrid compact polarimetry (HCP) has emerged as the preferred embodiment of compact polarimetry. HCP transmits circular polarization and receives on two orthogonal linear polarizations. When seen through its associated data processing and image classification algorithms, HPC’s heritage dates back to the Stokes parameters (1852), which are summarized and explained in plain language. Hybrid dual-polarimetric imaging radars were in the payloads of two lunar-orbiting satellites, India’s Earth-observing RISAT-1, and Japan’s ALOS-2. In lunar or planetary orbit, a satellite equipped with an HCP imaging radar delivers the same class of polarimetric information as Earth-based radar astronomy. In stark contrast to quad-pol, compact polarimetry is compatible with wide swath modes of a SAR, including ScanSAR. All operational modes of the SARs aboard Canada’s three-satellite Radarsat Constellation Mission (RCM) are hybrid dual-polarimetric. Image classification methodologies for HCP data are reviewed, two of which introduce errors for reasons explained. Their use is discouraged. An alternative and recommended group of methodologies yields reliable results, illustrated by polarimetrically classified images. A survey over numerous quantitative studies demonstrates HCP polarimetric classification effectiveness. The results verify that the performance accuracy of the HCP architecture is comparable to the accuracy delivered by a quadrature-polarized SAR. Four appendices are included covering related topics, including comments on inflight calibration of an HCP radar

RADARSAT Constellation Mission’s Operational Polarimetric Modes: A User-Driven Radar Architecture

Canada’s Earth-observing RADARSAT Constellation Mission (RCM) is intended to serve operational users. The users’ main objectives were to have routinely available high-quality quantitative information about their applications, with large area coverage potential. That two-part requirement was sufficient to establish an innovative synthetic aperture radar (SAR) polarimeter’s end-to-end system profile, the hybrid compact polarimetric (HCP) architecture. HCP’s essential and defining characteristic is circularly polarized transmission. This is sufficient to evaluate the backscatterer Stokes vector, but only half of the scattering matrix elements are measured. Hence image classification methodologies for linearly polarized full- (or quad-) pol (FP) radars that depend on knowledge of all four of the scattering matrix elements if applied to HCP-derived data lead to erroneous results. HCP-appropriate classifications are based on the Stokes vector. Related methods traditionally used for radar astronomy—for which circularly polarized transmission is the norm—are reviewed. Those known methods are extended, bringing to light fundamental characteristics of a polarimetric electromagnetic field. Analysis tools appropriate for HCP’s polarimetric data are introduced. The resulting polarimetric portraits—defined as the Stokes vector of the backscattered field in response to balanced illumination of the scene—from FP and HCP polarimeters are shown to be equivalent

CryoSat-2: From SAR to LRM (FBR) for quantitative precision comparison over identical sea state

The use of Synthetic Aperture Radar (SAR) techniques in conventional altimetry-i.e., Delay Doppler Altimetry (DDA)-was first introduced by R.K. Raney in 1998 [1]. This technique provides an improved solution for water surface altimetry observations due to two major innovations: the addition of along track processing for increased resolution, and multi-look processing for improved SNR. Cryosat-2 (scheduled for launch 2010) will be the first satellite to operate a SAR altimetry mode. Although its main focus will be the cryosphere, this instrument will also be sporadically operative over water surfaces, thus provide an opportunity to test and refine the improved capabilities of DDA. Moreover, the work presented here is of interest to the ESA's Sentinel-3 mission. This mission will be devoted to the provision of operational oceanographic services within Global Monitoring for the Environment and Security (GMES), and will include a DDA altimeter on board. SAMOSA, an ESA funded project, has studied along the last two years the potentialities of advanced DDA over water surfaces. Its extension aims to better quantify the improvement of DDA over conventional altimetry for the characterization of water surfaces. Cryosat-2´s altimeter (SIRAL) has three operating modes: the Low Resolution Mode (LRM), the SAR mode and the inSAR mode. The first two are of interest for the work to be done. In LRM the altimeter performs as a conventional pulse limited altimeter (PRF of 1970 Hz); in SAR mode the pulses are transmitted in bursts (64 pulses per burst). In the last, correlation between echoes is desired [1], thus the PRF within a burst is higher than in LRM (PRF of 17.8 KHz). After transmission the altimeter waits for the returns, and transmits the next burst (burst repetition frequency of 85.7 Hz). The previous acquisition modes will provide different data products: level 1 or full bit rate data (FBR), level 1b or multi-looked waveform data, and level 2 for evaluation or geophysical products. This paper is only addressing FBR data for LRM and SAR mode. In LRM the FBR data corresponds to echoes incoherently multi-looked on-board the satellite at a rate of 2 0Hz, while in SAR mode FBR corresponds to individual complex echoes (I and Q), telemetered before the IFFT block In working to this aim, three methodologies were implemented in the SAMOSA contract, the results achieved and detailed discussions with JHU/APL identified a revised approach (to be implemented in the SAMOSA extension), which should allow the team to meet the task goal. The different approaches will be presented in this paper. ACKNOWLEDGEMEN

Review of state of knowledge for SAR altimetry over ocean. Report of the EUMETSAT JASON-CS SAR mode error budget study.

SAR altimetry over the ocean has attracted considerable attention in the past three years and remarkable progress has been made in a short space of time. Cryosat-2 is the first satellite to provide SAR altimeter data over the ocean, and the datahelped to demonstrate the significant benefits of SAR mode for ocean altimetry compared to conventional low-resolution mode(LRM). This document provides an overview of the state of knowledge for SAR altimetry over the ocean based on research reported between 2010 and 2013. There is increasing consensus between various independent investigation teams that SAR altimetry over the ocean leads to significant performance improvements when compared to even the best available conventional radar altimetry. The results are evident in reduced measurement noise, improved performance in coastal regions and improved spectral information content for Sea Level Anomaly at the ocean mesoscale. The convergence of results from different groups using different SAR waveform retrackers indicates that there is now a high level of confidence in the ability to retrieve geophysical data from SAR mode altimetry over ocean. Several issues particular to SAR altimetry remain open, specifically the sensitivity to platform mispointing, the lack of a sea state bias model in SAR mode, and the effects of swell and swell direction on SAR waveforms. It is noted that these issues disappear with SAR interleaved mode since the resulting SAR mode data can be transformed seamlessly into LRM data for self-calibration. The document discusses differences between SAR closed- burst and SAR interleaved mode and the transformation of SAR data into pseudo‐LRM waveforms. Closed-burst SAR used on CryoSat-2 and Sentinel-3 can be transformed into pseudo-LRM waveforms that look similar to LRM but are not statistically equivalent to real LRM data. Lack of equivalent P-LRM waveforms from closed burst SAR mode data precludes direct SAR/LRM cross-calibration. This fact jeopardizes the self-calibration potential of a closed burst SAR mode altimeter, and compromises attempts to relate with sufficient confidence and precision the closed-burst SAR sea level measurements to the existing sea level record. Adopting closed-burst SAR on Jason-CS would compromise the continuity of the high-precision sea level 20-year time series. In contrast, SAR interleaved would realize the theoretically optimal performance expected from a SAR mode altimeter, while ensuring continuity with conventional altimeters. This report explains that the SAR interleaved mode is essential since it is the only method that would assure continuity between the SAR mode aboard Jason-CS, and contemporaneous and prior conventional altimetric missions