Polarimetric GNSS-R systems, equipped with an additional polarization channel, offer enhanced capabilities for separating vegetation and surface scattering effects, thereby improving GNSS-R land remote sensing applications such as soil moisture retrieval in vegetated and forested areas and biomass estimation. However, the effectiveness of these applications relies on accurate calibration of the polarimetric GNSS-R sensor. In the Rongowai mission, a newly developed Next Generation GNSS-R Receiver (NGRx) is installed on a domestic Air New Zealand airplane to collect data during its commercial flights. The NGRx processes multi-GNSS satellite signals simultaneously and utilizes a dual-channel (LHCP and RHCP) antenna, thereby improving spatial coverage and retrieval accuracy. The dual-polarized antenna also provides the possibility to examine the polarimetric GNSS-R system. In this article, a new methodology is developed to calibrate the Level-1 power measurement and the on-board antenna cross-pol gain by comparing measurements from inland lakes and ocean with modeled results. The calibration results in a 34% decrease in the uncertainty in co-pol reflectivity retrieval. The retrieved cross-pol and co-pol reflectivity after calibration are examined by their statistical distribution and spatial mapping with 1.5 km resolution, with multi-land surface types and incidence angles. These results validate the effectiveness of the calibration method and pave the way for future terrestrial science applications.

This paper presents the calibration process for an airborne polarimetric Global Navigation Satellite System Reflectometry (GNSS-R) sensor developed for the Rongowai project. We calibrate the sensor's dual-polarized antenna pattern and power measurement by comparing field data obtained from an inland lake surface with model predictions. The model evaluates the dual-polarized power of coherent polarimetric GPS signals reflected from inland water body surfaces with slight roughness caused by winds. The calibration results suggest a rotation of the polarimetric antenna gain pattern. A different scale factor is applied to the system’s dual-polarized power measurement for 1 and 2 millisecond coherent integration times. The absolute power calibration allows accurate surface reflectivity and normalized bistatic radar cross section (NBRCS) retrieval and enables future science applications.

We present here the development and initial results of a low latency GNSS-R flood detection and visualisation framework, the Flood Assessment Spatial Triage. This uses airborne GNSS-Reflectometry data which are routinely collected as part of the Rongowai mission, hosted on an Air New Zealand Q300 domestic aircraft. The framework is able to detect flooding with low latency after the aircraft lands, through rapid processing of the Level-0 data which combines reflected GNSS signals with terrain data. These may be useful for flood preparedness and response, and may additionally help to guide the tasking of imaging assets.

We developed a random forest model to predict soil moisture at the field scale (∼1 km), combining 6.5 years of CYGNSS Global Navigation Satellite System Reflectometry (GNSS-R) data with observations from 1049 gauges which are part of the International Soil Moisture Network. Several relevant predictor variables were developed, including geophysical parameters related to soil properties, topography, land cover and climatology, and dynamic variables including precipitation from the Global Precipitation Mission and evapotranspiration derived from MODIS temperature. The model achieved good accuracy for predicted soil moisture (R2= 0.68; RMSE = 0.064 mm/mm), which was further improved through postprocessing using a linear regression model of the residuals for bias correction (R2= 0.82; RMSE = 0.048 mm/mm). Our work demonstrates the potential for field-scale prediction of soil moisture from GNSS-R data, which may be further improved through enhanced co-variates, and is applicable to recent and near-future GNSS-R missions such as Rongowai and HydroGNSS.

This paper proposes a novel global-to-local (G2L) searching algorithm to find the locations of specular reflections over terrestrial surfaces for GNSS-R applications. In the proposed method, an initial coordinate of the specular reflection is first computed on a smooth WGS84 datum using the Fibonacci method. Second, a local coordinate frame that is centered at the initial WGS84 specular point (SP) is defined with respect to the local topography, and the G2L searching algorithm is applied to more precisely locate the specular reflection point. The effects of the complex land topography, including the local slope and incidence angles, are considered. This method is simulated for an airborne GNSS-R scenario where the receiver position is based on historical flight data from Gisborne to Wellington in New Zealand, where the results show that the finalized SP positions have been significantly shifted due to the complex topographical surfaces. This G2L method can be implemented to accurately track the location of reflected GNSS signal power from measured delay-Doppler maps (DDM) for land applications.

Earth observations are pivotal for developing informed, evidence-based sustainable practices and are of direct consequence to four of the seventeen UN Sustainable Development Goals (12–15). Among these, Earth observations using signals of opportunity such as GNSS-R have significantly evolved in recent years including expanding efforts to consider geophysical retrievals over complex and heterogeneous surfaces, primarily from space-borne missions. Airborne GNSS-R however has the advantages of higher resolution and signal strength and as such can provide critical data to inform algorithm development for complex environments. This paper presents the upcoming airborne Rongowai mission whereby Air New Zealand will fly a NASA-developed next-generation GNSS-R receiver as a unique international collaboration. Rongowai promises to deliver rich environmental records for sustainable land-use and water management including coastal and open ocean over many years at unprecedented spatial and temporal resolutions. We present representative coverage, and methodology for product development with specific focus on soil-moisture as it relates to land-use and water management gather

This paper reviews the recent activities in the field of NewSpace synthetic aperture radar (SAR) with particular attention to the instrument aspects and serves as the introductory paper of the “NewSpace SAR Instruments” Invited Session of the 42ndIEEE International Geoscience and Remote Sensing Symposium (IGARSS).

NASA's Cyclone Global Navigation Satellite System (CYGNSS) mission has been a pathfinder in the field of GNSS reflectometry (GNSS-R), with a constellation of 8 microsatellites measuring windspeeds over the entire tropics every few hours. More recently, terrestrial GNSS-R is being exploited to reveal records of soil moisture and surface water inundation dynamics. In an exciting partnership between NASA and New Zealand, Air New Zealand will accommodate and host a recently developed next-generation GNSS-R receiver (NGRx) and commercial aviation GPS omnidirectional antennas on a domestic Q300 aircraft. The result will be unprecedented high-resolution, wide coverage long-term records of soil moisture and inundation dynamics over New Zealand's diverse landscape and ecosystems, independent of cloud cover. Such data will support terrestrial calibration and validation of CYGNSS while simultaneously advancing the technology readiness level of the NgRx and enabling new remote sensing science investigations in New Zealand.

Airborne GNSS-R instrument systems are typically only utilized for a limited number of aircraft flights to perform experiments, to test new instruments, and to collect data over specific targets (e.g. hurricanes). A new system is currently under development entailing the permanent installation of GNSS-R instruments on a commercial fleet of Air New Zealand Q300 regional aircraft. This novel and ambitious concept offers a fascinating and powerful system for the collection of science information over a large spatial region and short temporal scales. Here we present an exploratory analysis meant to quantify the merit of such a system as applied to these regional aircraft. We use simulations of realistic flight paths combined with GNSS orbit information to simulate GNSS-R measurement coverage. Results in this scenario confirm the exciting potential of a next-generation GNSS-R receiver.

Clean, fresh water is essential for life and forests. As land-use intensification and climate change place growing pressure on water resources, we need to understand the role that planted forests can play in managing water sustainably. However, our current knowledge on where and when forests use and release water is not sufficiently advanced to inform and support management practices. The 5-year Forest Flows programme has been funded by the New Zealand government and proposes to use advanced and novel remote-sensing and sensor-network technology in developing a model that will revolutionise the way forest hydrology is approached, both in New Zealand and internationally. This manuscript provides an overview of the approach of the Forest Flows programme and the unique fusion of terrestrial and remote-sensing technologies that will provide accurate predictions of hydrological fluxes in planted forests.

We improved the repeat-pass InSAR processing capability for the L-band UAVSAR airborne synthetic aperture radar in order to support time-series analysis of repeat zero-baseline observations as well as multiple baseline observations for TomoSAR imaging. This new capability enabled us to conduct tomographic experiments in Gabon during the AfriSAR deployment in support of vegetation structure studies. For the GLISTIN-A Ka-band radar, we streamlined the radar operations and implemented a robust production processor that will routinely generate topographic data products in order to support large-scale science campaigns. The new capabilities were put to test in support of the Oceans Melting Glacier Greenland campaign in March 2016.

This paper discusses the evaluation of a state-of-the-art Ka-band (35.7GHz) single-pass interferometric synthetic aperture radar (InSAR) for snow-depth mapping during the NASA SnowEx experiment. The use of InSAR for this application presents a new approach and potential alternate technology to lidar with the advantage of wide-swath operation that is not hampered by cloud cover. We discuss the plans to leverage SnowEx for calibration, in particular characterizing potential biases due to interferometric penetration into the snow-cover by comparison with lidar and in situ measurements. In addition, lidar from the Airborne Snow Observatory will be used for validation of tree classifications and the derived digital surface models and snow-depth maps.

This letter presents the first demonstration of millimeter-wave single-pass interferometric synthetic aperture radar (InSAR) for snow-depth mapping. Maps are presented over the Tuolumne River Basin region of the Sierra Nevada, CA, USA, and compared with those collected by a scanning lidar onboard the NASA Airborne Snow Observatory for the same region on the same snow day. For this observation, the snow surface was wet and melting and as such penetration of the electromagnetic wave into the snow volume can be effectively neglected. Despite the rugged terrain, heavy tree-cover, and very low snow-volume, depth maps had a standard deviation <;1 m with the largest differences occurring on slopes exceeding 40°. While additional evaluation is needed with demonstration of the InSAR capability over a greater range of conditions and terrain, these results are promising. InSAR for snow-depth mapping holds significant advantages for a spaceborne mission if proven viable as it can operate through cloud cover, day or night, and measure snowpack when wet or melting.

The accumulation and impacts in the ocean of marine debris generated by anthropogenic activities and aggravated by natural disasters are of growing concern, yet our existing data collection systems are rudimentary and cannot answer even fundamental questions. This is partly due to the diversity of the debris and the vastness of the ocean that complicate observations. The magnitude and complexity of the problem prompted NASA to sponsor a workshop to review existing and emerging technologies that could be capable to remotely survey the state of marine debris in the ocean. This workshop, held January 2016, brought together oceanographers, technologists, and experts in marine debris. In this paper we summarize the state-of-the-art of science and observations, and the stated goals of this workshop.

This paper introduces the use of an airborne interferometric synthetic aperture radar (InSAR) to estimate water surface decorrelation times at Ka-Band. Such an assessment is directly relevant to the upcoming Surface Water and Ocean Topography mission, especially for surface water bodies such as lakes and rivers since the surface decorrelation may limit the spatial resolution achievable by the mission to delineate water spatial boundaries. Initial assessments indicate decorrelation times consistent with limited published observations for the ocean and fresh water bodies (several milliseconds). However, there are challenges both in terms of the phenomenology and in the instrument sensitivity to longer decorrelations.

River bathymetry plays a key role in estimating river discharge as well as improving our modeling capabilities of fluvial geomorphology. Genetic algorithm techniques can be used to derive river bed topography using only water surface elevations and the corresponding temporal and spatial rates of change. Each river bathymetric estimate, also referred to as a solution, is modeled as a successive collection of cross-sections, also referred to as genes. An initial population of potential solutions is created from solutions comprised of random cross-sections with depths ranging as deep as the surface height to no depth at all. The population is successively evolved by randomly mutating the depths at a random number of cross-sections. The solutions are selected for the next generation by evaluating their individual fitness. Three varieties of fitness tests are applied to each potential solution: Saint-Venant's 1-D flow equation, flow continuity, and the statistical power-law relationships suggested by Leapold & Maddock[1]. In this manner, an ideal solution is derived.

To better prepare data processing system of the Ka-band Radar Interferometer (KaRIn) for the Surface Water and Ocean Topography (SWOT) [1] mission, we opportunistically collected data over several diverse fresh-water targets in the Van Hook Arm areas of North Dakota, USA, using a Ka-band interferometric radar [2] developed at JPL. To make the collection relevant to SWOT, the aircraft was rolled to direct the antenna boresight toward nadir to mimic the SWOT geometry. Using a modified airborne interferometric SAR processor [3] developed at JPL, we were able to process the collected Ka-band airborne data and produced the height and magnitude image products. These results are the first of the kind for Ka-band interferometric synthethic aperture radar over water surfaces with near nadir looking geometry. These initial results will help to characterize the power returns from water surface and land, and provide guidance for the design of post-processing algorithms including methods for water and land classification. The height accuracy we get from the water surface height images can be used to project the water surface height accuracy we would get from SWOT mission. This airborne experiment will help us verify the SWOT data processing chain and make us better prepared for SWOT data processing task.

As part of the NASA International Polar Year activities, a Ka-band cross-track interferometric synthetic aperture radar (SAR) recently demonstrated high-precision elevation swath mapping capability. This proof-of-concept instrument was achieved by interfacing two Ka-band slotted-waveguide antennas in a cross-track geometry and Ka-band electronics with the Jet Propulsion Laboratory's L-band uninhabited aerial vehicle SAR. Deployed on the NASA Gulfstream III, initial engineering flights in March and April 2009 marked the first airborne demonstration of single-pass cross-track interferometry at Ka-band. Results of a preliminary interferometric assessment indicate height precisions that, for a 3 m × 3 m posting, range from 30 cm in the near range to 3 m in the far range and greater than 5 km of swath over the urban areas imaged. The engineering flights were followed by a comprehensive campaign to Greenland in May 2009 for ice-surface topography mapping assessment. Toward that end, coordinated flights with the NASA Wallops Airborne Topographic Mapper lidar were conducted in addition to establishing ground calibration sites at both the Summit Station of the National Science Foundation and the Swiss Camp of the Cooperative Institute for Research in the Environmental Sciences. Comparisons of the radar-derived elevation measurements with both in situ and lidar data are planned for a subsequent paper; however, at this stage, a single data example over rugged ice cover produced a swath up to 7 km with the desired height precision as estimated from interferometric correlation data. While a systematic calibration, including assessment and modeling of biases, due to penetration of the electromagnetic waves into the snow cover has not yet been addressed, these initial results indicate that we will exceed our system requirements.

The Microwave Observatory of Subcanopy and Subsurface (MOSS) is a mission concept for a spaceborne synthetic aperture radar (SAR) system that provides global observations of soil moisture under substantial vegetation cover (exceeding 20 kg/m2) and at useful depths (1-5 m). The concept was developed and a number of new required technologies were demonstrated through a National Aeronautics and Space Administration Earth Science Technology Office Instrument Incubator Program project. This very high frequency (VHF)/ultrahigh frequency (UHF) polarimetric SAR is designed to provide 7-10-day observations of soil moisture at 1-km resolution. The rapid repeat cycle mandates swath widths in the range of 300-400 km, which must be realized by a 30-m-long antenna. Conventional array implementations would result in a mass of more than 4000 kg, whereas with the technology proposed and demonstrated in this project, the total antenna mass is less than 500 kg. The antenna concept is a dual-stacked patch array feed illuminating a 30-m mesh reflector to synthesize the long apertures and achieve the wide swath. The feed system prototype was fabricated and its performance demonstrated. Other major project components were: (1) system-level SAR and mission design; (2) demonstration of science data and products, using a tower-based VHF/UHF radar; (3) spacecraft and mesh reflector antenna mechanical design; (4) developing mitigation strategies for ionospheric effects; and (5) assessing frequency interference effects. Experimental science data were generated from the tower radar for soil moisture profiling in Arizona and for forest penetration in Oregon. The soil moisture products were demonstrated through an integrated inversion-processing algorithm. This paper summarizes the results from the MOSS project and demonstrates the feasibility of the spaceborne mission.

Measurements of deep and subcanopy soil moisture are critical in understanding the global water and energy cycle, as well as the interaction of the carbon and water cycles, but are presently not available on a synoptic basis. In this paper, a low-frequency UHF/VHF radar mission concept is presented and technology challenges to implement it are discussed. This mission concept is currently being studies under a NASA/ESTO instrument incubator program (IIP) project. The progresses of several aspects of the project are discussed.

Along-track interferometric synthetic aperture radar (ATI-SAR) is an active coherent imaging system, utilizing two antennas separated along the platform flight direction. The phase information of ATI-SAR from the Doppler shift of the backscattered signal represents the line-of-sight velocity of the water scatterers. While the advent of ATI-SAR provided us with a potentially powerful technique for ocean surface current and wave mapping, the surface current has not been measured exactly from the ATI-SAR velocity because the Doppler shift is not simply proportional to the component of the mean surface current. It also includes other types of contributions associated with the phase velocity of the Bragg waves and orbital motions of all ocean waves that are longer than Bragg waves. In this paper, we review how the phase difference measured by ATI-SAR is related to the mean Doppler frequency, and we develop a new and practically useful method to extract the surface current component utilizing simultaneously measured L- and C-band ATI-SAR data. Since the measured ATI-SAR velocity shows a different value at a different radar-frequency, we investigate the influence of Bragg-resonant waves and long ocean wave motions on the ATI-SAR velocity according to the radar frequency. The Bragg-wave phase velocity component, which is a significant source of error for extracting the surface current, can be effectively eliminated by using L- and C-band ATI-SAR. The method is applied to L- and C-band ATI-SAR measurements acquired at the Ulsan coast in the southeastern part of the Korean peninsula. The resulting ocean surface current vectors are compared with in situ measurements collected by recording current meter. We furthermore extract ocean surface wave information from the ATI-SAR phase image using a quasi-linear transform.

The along-track interferometry (ATI) SAR measures the Doppler shift of the backscattered signal and thus the line-of-sight velocity of the scatterers. This interferometric velocity is the sum of the orbital motion of water particles from the swell, phase velocities of the Bragg waves, and ocean surface currents. While the advent of ATI SAR provided us with a potentially powerful technique for ocean current mapping, the surface currents cannot yet measured exactly from interferometric velocity measurements. In this paper, we will apply a new method of extracting the surface current velocity from multiple-frequency (L- & C-band) ATI SAR data. We have tested ATI SAR data that were collected during the PACRIM-II AIRSAR experiment over the Ulsan coast on the southeastern part of the Korean peninsula. We have investigated the ocean waves and current features and have retrieved dominant ocean wave information. Furthermore, we could differentiate the ocean current and the Bragg wave phase velocities using multiple-frequency (C & L-band) ATI data.

A unique focused array imaging Doppler radar was used to measure directional spectra of Ocean surface waves in a nearshore experiment performed on the North Carolina Outer Banks. Radar images of the ocean surface’s Doppler velocity were used to generate two dimensional spectra of the radial component of the ocean surface velocity field. These are compared to simultaneous in-situ measurements made by a nearby array of submerged pressure sensors. Analysis of the resulting two-dimensional spectra include comparisons of dominant wave lengths, wave directions, and wave energy accounting for relative differences in water depth at the measurement locations. Limited estimates of the two-dimensional surface displacement spectrum are derived from the radar data. The radar measurements are analagous to those of interferometric synthetic aperture radars (INSAR), and the equivalent INSAR parameters are shown. The agreement between the remote and in-situ measurements suggests that an imaging Doppler radar is effective for these wave measurements at near grazing incidence angles.