Description: 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.

Description: The SWATH-D experiment is dense deployment of 154 seismic stations in the Central and Eastern Alps between Italy and Austria, complementing the larger-scale sparser AlpArray Seismic Network (AASN). SWATH-D will provide high resolution images from the surface into the upper mantle, and allow observations of local seismicity. SWATH-D focuses on a key area of the Alps where the hypothesized flip in subduction polarity has been suggested, and where an earlier seismic profile (TRANSALP) has imaged a jump in the Moho. Where mains power is available (at ca. 80 sites) stations are providing realtime data via the cellphone network and are equipped with Güralp CMG-3EPSC (60s) seismometers and Earth Data Recorders EDR-210. The rest of the stations are offline and consist mainly of Nanometrics Trillium Compact (120s) and Güralp CMG-3EPSC (60s) seismometers equipped with either Omnirecs CUBE3 or PR6-24 Earth Data Loggers. All stations are equipped with external GPS antennas and the sampling rate is 100 Hz (Heit, et al., 2018). The network will operate for 2 years starting in July 2017. The Swath-D data will be used directly by 20 individual proposals of the MB-4D Priority Program (Mountain Building Processes in Four Dimensions, 2017) of the German Research Foundation (DFG) and data products derived from it will contribute to additional 13 proposals. SWATH-D is thus an important link between the MB-4D Priority Program and the international AlpArray communities and a scientific service to many of the proposals within the DFG Priority Program. Waveform data are available from the GEOFON data centre, under network code ZS, and are embargoed until August 2023. After the end of embargo, data will be openly available under CC-BY 4.0 license according to GIPP-rules.

Description: The SWATH‐D experiment involved the deployment of a dense temporary broadband seismic network in the Eastern Alps. Its primary purpose was enhanced seismic imaging of the crust and crust–mantle transition, as well as improved constraints on local event locations and focal mechanisms in a complex part of the Alpine orogen. The study region is a key area of the Alps, where European crust in the north is juxtaposed and partially interwoven with Adriatic crust in the south, and a significant jump in the Moho depth was observed by the 2002 TRANSALP north–south profile. Here, a flip in subduction polarity has been suggested to occur. This dense network encompasses 163 stations and complements the larger‐scale sparser AlpArray seismic network. The nominal station spacing in SWATH‐D is 15 km in a high alpine, yet densely populated and industrialized region. We present here the challenges resulting from operating a large broadband network under these conditions and summarize how we addressed them, including the way we planned, deployed, maintained, and operated the stations in the field. Finally, we present some recommendations based on our experiences.

Description: Our focus is the way various scales of motion in the tropical ocean are linked through mixing and its modification by larger scales. Enhanced mixing caused by small vertical scale features (SVSs) in the equatorial thermocline is known to impact the state of the ocean and its interaction with the atmosphere, in particular the sea surface temperature of the Pacific cold tongue and ENSO variability. The SVSs are produced by wind variability and instabilities, with an equatorial enhancement caused by a combination of factors including the characteristics of the forcing and propagation of internal waves and near-equator inertial and sub-harmonic parametric instabilities. Numerous scale interactions are at play. For instance, an eastward extension of the warm(fresh) pool in the western tropical Pacific, typical under El Niño conditions, stratifies the upper ocean. This stratification can produce a dramatic decrease in the downward propagation of wind-generated inertia-gravity waves and a decrease in the mixing in the main thermocline. The associated changes to the thermocline are advected to the east and impact the eastern cold tongue and hence the coupling with the atmosphere. Using a combination of observations and models we investigate the properties of SVS activity, its impact on mixing, and interaction with larger scales. Of particular interest is the dependency on stratification, the spatial and temporal variability of wind forcing, the impact on larger scales, and the resolution of both observations and models.

Description: An exponentially increasing number of scientific and commercial Earth orbiting satellites are delivering global and timely sensing of the Earth from space, on its surface or from inside the Earth. The onset of climate change and its dire consequences has been exacerbating the adverse impacts on Earth’s environments and its inhabitants. Timely satellite-based Big Earth observations at adequate spatiotemporal resolution provide a means to monitor the evolutions of more frequent and abrupt climate induced and enhanced hazards. These observations could contribute towards the elucidation of their respective governing climatic processes, and enable improved hazards forecasting, water resources monitoring, and informed hazards management and response. Example satellite geodetic and other observations include satellite gravimetry, altimetry, GNSS, GNSS bistatic altimetry, SAR/InSAR, and Planet PBC's high spatiotemporal (subdaily and 3-5 m) resolution multispectral imageries. We illustrate that the use of deep machine learning analytics can effectively integrating hydrometeorological model and other data, and downscaling the satellite geodetic observations, towards enabling timely monitoring of abrupt weather episode evolutions, including floods, groundwater depletions, cyclone landfall, snowstorms, and meteotsunamis.

Description: The economic and population growth in India accompanied by increasingly frequent and severe disasters, such as floods and forest fires, raises concerns about the future. In recent decades, satellite technology has become an important tool for monitoring the Earth's surface, including natural hazards and land surface changes. Our USAID (United States Agency for International Development) project, REmote Sensing for Forest Renewal, Ecosystem Services, and Sustainable Hydrological Management (REFRESH), aims to improve forest sustainability in India with the help of satellite geodetic and remote sensing data. Here we report the progress for the physical science component of the REFRESH Project. Our team has developed a deep-learning model for classifying 10 types of Land Use/Land Cover (LULC) to detect long-term changes in forest areas over India. With collaboration from Indian partners, we validate our classification results using their in-situ land cover data. We used PlanetScope daily images at 3–5 m spatial resolution to monitor the evolutions of floods and forest fires over India. Other satellite data products include in-land water level virtual stations over rivers, dams, and lakes using three decades of satellite radar altimetry, daily-sampled GRACE/GRACE-FO Level 1B gravimetry data to monitor monsoonal floods and seasonal droughts, spaceborne lidar and GEDI for mapping canopy changes, and deep-learning downscaled satellite gravimetry measured total water storage anomalies over entire India. Finally, we plan to generate spaceborne GNSS-Reflectometry and NASA’s CYGNSS for water/land classification data products over India. The data products can be retrieved, visualized, and analyzed on an elaborate observation portal.

Description: Climate-induced natural hazards are thought to be happening more frequent and more intense under an increasingly warmer Earth. Earth-observing satellites including the ones operate in constellations and deliver accurate and sub-daily high-resolution/stereo multispectral satellite imagery, provide an opportunity to rapidly monitoring and quantifying the magnitude of disasters. Planet PBC’s Dove/SuperDove, SkySat, RapidEye (retired in March 2020) satellite constellations have at present over 150 CubeSats providing daily/sub-daily sampled 0.5-5-meter resolution images globally. Here, we use the available multispectral images from Planet CubeSat constellations, other geodetic and remote sensing data with the objective to conduct a feasibility study to monitor hazards. In particular, our goal is to identify, track, and quantify the scopes of active wildfires and rapid flood events even at relatively small scales and relatively short-durations worldwide, to study the feasibility of using timely and multi-platform satellite observations to complement informed disaster responses. In this study, we provide case studies and demonstrations of example brush fire and flood hazards to examine the feasibility of a satellite observation-based decision-support disaster response and management tool.