David M. Le Vine

Large area mapping of soil moisture using the ESTAR passive microwave radiometer in Washita'92

Investigations designed to study land surface hydrologic-atmospheric interactions, showing the potential of L band passive microwave radiometry for measuring surface soil moisture over large areas, are discussed. Satisfying the data needs of these investigations requires the ability to map large areas rapidly. With aircraft systems this means a need for more beam positions over a wider swath on each flightline. For satellite systems the essential problem is resolution. Both of these needs are currently being addressed through the development and verification of Electronically Scanned Thinned Array Radiometer (ESTAR) technology. The ESTAR L band radiometer was evaluated for soil moisture mapping applications in two studies. The first was conducted over the semiarid rangeland Walnut Gulch watershed located in south eastern Arizona (U.S.). The second was performed in the subhumid Little Washita watershed in south west Oklahoma (U.S.). Both tests showed that the ESTAR is capable of providing soil moisture with the same level of accuracy as existing systems.

Aquarius and Remote Sensing of Sea Surface Salinity from Space

Aquarius is an L-band radiometer and scatterometer instrument combination designed to map the salinity field at the surface of the ocean from space. The instrument is designed to provide global salinity maps on a monthly basis with a spatial resolution of 150 km and an accuracy of 0.2 psu. The science objective is to monitor the seasonal and interannual variation of the large scale features of the surface salinity field in the open ocean. This data will promote understanding of ocean circulation and its role in the global water cycle and climate. Aquarius is the primary instrument on the Aquarius/SAC-D mission which is a partnership between the space agencies in the USA (NASA) and Argentina (CONAE). Launch is scheduled for late in 2010.

A microwave polarimetric scattering model for forest canopies based on vector radiative transfer theory

A microwave polarimetric scattering model for a forest canopy is developed based on the iterative solution of the vector radiative transfer equations up to the second order. The forest canopy constituents (branches, leaves, stems, and trunks) are embedded in a multi-layered medium over a rough interface. The branches, stems, and trunks are modeled as finite randomly oriented cylinders. Deciduous leaves are modeled as randomly oriented discs and coniferous leaves are modeled as randomly oriented needles. The vector radiative transfer equations contain non-diagonal extinction matrices that account for the difference in propagation constants and the attenuation rates between the vertical and horizontal polarizations. For a plane wave exciting the canopy, the average Mueller matrix is formulated, and then used to determine the linearly polarized backscattering coefficients including both the copolarized and cross-polarized power returns. Comparisons of the model with measurements from Les Landes Forest of France showed good agreements over a wide frequency band and gave a quantitative understanding of the relation between the backscattering coefficients and the age of the trees in the forest and forest biomass.

Aquarius RFI Detection and Mitigation Algorithm: Assessment and Examples

Aquarius is an L-band radiometer system designed to map sea surface salinity from space. This is a sensitive measurement, and protection from radio frequency interference (RFI) is important for success. An initial look at the performance of the Aquarius RFI detection and mitigation algorithm is reported together with examples of the global distribution of RFI at the L-band. To protect against RFI, Aquarius employs rapid sampling (10 ms) and a “glitch” detection algorithm that looks for outliers among the samples. Samples identified as RFI are removed, and the remainder is averaged to produce an RFI-free signal for the salinity retrieval algorithm. The RFI detection algorithm appears to work well over the ocean with modest rates for false alarms (5%) and missed detection. The global distribution of RFI coincides well with population centers and is consistent with observations reported by the Soil Moisture and Ocean Salinity mission.

Status of Aquarius/SAC-D and Aquarius Salinity Retrievals

Aquarius was launched in June 2011 to monitor the global salinity field in the open ocean. This radiometer/scatterometer (i.e., passive/active) instrument is part of the Aquarius/SAC-D mission, a partnership between the USA and Argentina. In general, the observatory and instruments have functioned well with mostly minor issues. Aquarius has been operating successfully since being turned on August 25, 2011. The quality of the salinity retrieval has improved continuously and is approaching the accuracy goal of 0.2 psu (monthly global RMS). The maps produced by Aquarius show the global structure and dynamic features of the salinity field and, now after 3 years of operations, data are available to give a first look at the interannual changes in sea surface salinity. An improved salinity product, Version 3.0, has been released to the public (June 2014) and includes significant refinements in the retrieval algorithm such as reduced ascending/descending differences and improved calibration. Additional data such as global maps of soil moisture and maps of RFI are also available.

Aquarius/SAC-D mission overview

Aquarius/SAC-D is a cooperative international mission developed between the National Aeronautics and Space Administration (NASA) of United States of America (USA) and the Comisión Nacional de Actividades Espaciales (CONAE) of Argentina. The overall mission objective is to contribute to the understanding of the total Earth system and the consequences of the natural and man-made changes in the environment of the planet. Major themes are: ocean surface salinity, carbon, water cycle, geo-hazards, and cryosphere.

Accurate Measurements of the Dielectric Constant of Seawater at L Band

This report describes measurements of the dielectric constant of seawater at a frequency of 1.413 GHz that is at the center of the L-Sand radiometric protected frequency spectrum. Aquarius will be sensing the sea surface salinity from space in this band. The objective of the project is to refine the model function for the dielectric constant as a function of salinity and temperature so that remote sensing measurements can be made with the accuracy needed to meet the measurement goals (0.2 psu) of the Aquarius mission. The measurements were made, using a microwave cavity operated in the transmission configuration. The cavitys temperature was accurately regulated to 0.02 C by immersing it in a temperature controlled bath of distilled water and ethanol glycol. Seawater had been purchased from Ocean Scientific International Limited (OS1L) at salinities of 30, 35 and 38 psu. Measurements of these seawater samples were then made over a range of temperatures, from l0 C to 35 C in 5 C intervals. Repeated measurements were made at each temperature and salinity, Mean values and standard deviations were then computed. Total error budgets indicated that the real and imaginary parts of the dielectric constant had a relative accuracy of about l%.

Soil Moisture Retrieval Using a Two-Dimensional L-Band Synthetic Aperture Radiometer in a Semiarid Environment

Surface soil moisture was retrieved from the L-band radiometer data collected in semiarid regions during the Soil Moisture Experiment in 2004. The 2-D synthetic aperture radiometer (2D-STAR) was flown over regional-scale study sites located in AZ, USA, and Sonora, Mexico (SO). The study sites are characterized by a range of topographic relief with a land cover that varies from bare soil to grass and scrubland and includes areas with high rock fraction near the soil surface. The 2D-STAR retrieval of soil moisture was in good agreement with the ground-based estimates of surface soil moisture in both AZ (raise = 0.012 m3 m-3) and SO (rmse = 0.011 m3 m-3). The 2D-STAR also showed a good performance in the Walnut Gulch Experimental Watershed (rmse = 0.014 m3 m-3) where the surface soil featured high rock fraction was as high as 60%. Comparison of the results with the Polarimetric Scanning Radiometer at the Cand X-band data indicates the superior soil moisture retrieval performance of the L-band data over the regions with high rock fraction and moderate vegetation density.

The influence of channel geometry on the fine scale structure of radiation from lightning return strokes

Evidence is presented indicating that channel geometry is a factor in determining the shape of the electric field waveforms radiated by subsequent return strokes in cloud-to-ground lightning. The data consist of 61 subsequent return strokes in both triggered flashes and natural lightning recorded at the NASA Kennedy Space Center during the summer of 1987. In the case of the triggered flashes, the data include the electric field change, the current at the channel base, and video images of the channel. Video images were obtained for about half of the natural flashes. The fine scale structure of the electric field changes radiated by these strokes has been compared during the first 10 μs. In cases where unambiguous identification of the channel could be made from the video recordings, this fine scale structure is similar for return strokes in the same channel and differs significantly for return strokes in different channels. This is true for both natural and triggered flashes. On the other hand, the current waveforms measured at the channel base (available for the triggered flashes) are similar in shape for all strokes, regardless of the channel. Similar results were also obtained for the natural flashes without video images, in which case strokes from the same flash exhibit a greater similarity than do strokes from different flashes (and therefore different channels).

Aquarius L-band Radiometers Calibration Using Cold Sky Observations

An important element in the calibration plan for the Aquarius radiometers is to look at the cold sky. This involves rotating the satellite 180° from its nominal Earth viewing configuration to point the main beams at the celestial sky. At L-band, the cold sky provides a stable, well-characterized scene to be used as a calibration reference. This paper describes the cold sky calibration for Aquarius and how it is used as part of the absolute calibration. Cold sky observations helped establish the radiometer bias, by correcting for an error in the spillover lobe of the antenna pattern, and monitor the long-term radiometer drift.