Wendy N. Edelstein

The Soil Moisture Active Passive (SMAP) Mission

The Soil Moisture Active Passive (SMAP) mission is one of the first Earth observation satellites being developed by NASA in response to the National Research Councils Decadal Survey. SMAP will make global measurements of the soil moisture present at the Earths land surface and will distinguish frozen from thawed land surfaces. Direct observations of soil moisture and freeze/thaw state from space will allow significantly improved estimates of water, energy, and carbon transfers between the land and the atmosphere. The accuracy of numerical models of the atmosphere used in weather prediction and climate projections are critically dependent on the correct characterization of these transfers. Soil moisture measurements are also directly applicable to flood assessment and drought monitoring. SMAP observations can help monitor these natural hazards, resulting in potentially great economic and social benefits. SMAP observations of soil moisture and freeze/thaw timing will also reduce a major uncertainty in quantifying the global carbon balance by helping to resolve an apparent missing carbon sink on land over the boreal latitudes. The SMAP mission concept will utilize L-band radar and radiometer instruments sharing a rotating 6-m mesh reflector antenna to provide high-resolution and high-accuracy global maps of soil moisture and freeze/thaw state every two to three days. In addition, the SMAP project will use these observations with advanced modeling and data assimilation to provide deeper root-zone soil moisture and net ecosystem exchange of carbon. SMAP is scheduled for launch in the 2014-2015 time frame.

Radar options for global earthquake monitoring

Fine temporal sampling is essential for disaster management, e.g. of flooding, fires, landslides, hurricanes, and earthquakes. A powerful technique for mapping such natural hazards is synthetic aperture radar (SAR) interferometry, providing displacement measurements at the subwavelength scale and decorrelation estimates. Pre-seismic deformation, one of the most elusive aspects of earthquakes, will require much finer temporal sampling than present InSAR capabilities provide. Observations taken every few hours could provide time series data of rapidly evolving phenomena, such as pre-eruptive volcano dynamics, leading to major advances in predictive capability, improving the potential for modeling as well as for civil protection. Such radical performance improvements could be attained through large constellations of conventional low Earth orbit (LEO) satellites or small constellations of geosynchronous SARs. The unique capability of a geosynchronous SAR in terms of instantaneously accessible area is contrasted with the requirements for huge electronically steered array (ESA) antennas. The optimal approach is very much dependant on technological developments, in particular geosynchronous SAR depends on the development of affordable very large ESA antennas, but also other technological developments will be required.

System concepts and technologies for high orbit SAR

This paper discusses large aperture, high orbit radar concepts for measuring sub-centimeter-level surface displacements from space. These measurements will enable applications such as earthquake simulation, modeling and forecasting. We explain the need for large aperture, high orbit arrays and discuss the technologies required to achieve these missions.

The NASA DC-8 airborne cloud radar: design and preliminary results

The University of Massachusetts Microwave Remote Sensing Laboratory and NASA Jet Propulsion Laboratory have developed a 95 GHz airborne radar system for remote sensing of clouds. This instrument was recently operated aboard NASAs DC-8 Airborne Laboratory and participated in the Cloud Layer Experiment (CLEX) in the central U.S. and in the Southern Alps Experiment (SALPEX) in New Zealand. The development of this system was motivated by the need for a sensitive, well-calibrated millimeter-wave cloud radar that can probe clouds from above. This geometry avoids the attenuation suffered by ground based systems that look up through precipitation and also simulates the viewing geometry of a spaceborne sensor. This paper describes the design and operating characteristics of the instrument as well as preliminary results from the initial deployments. Engineering test flights were conducted during June 1996, concurrent with CLEX. The instrument also collected data during two flights from California to Hawaii and New Zealand and another flight near New Zealand. Data from the flights are used to demonstrate the instruments capabilities. The data from both experiments are classified into the four classes of cloud systems used by the GEWEX Cloud System Study (GCSS). Reflectivity statistics for each class of cloud system are presented.

SMAP L-Band Microwave Radiometer: Instrument Design and First Year on Orbit

The Soil Moisture Active–Passive (SMAP) L-band microwave radiometer is a conical scanning instrument designed to measure soil moisture with 4% volumetric accuracy at 40-km spatial resolution. SMAP is NASA’s first Earth Systematic Mission developed in response to its first Earth science decadal survey. Here, the design is reviewed and the results of its first year on orbit are presented. Unique features of the radiometer include a large 6-m rotating reflector, fully polarimetric radiometer receiver with internal calibration, and radio-frequency interference detection and filtering hardware. The radiometer electronics are thermally controlled to achieve good radiometric stability. Analyses of on-orbit results indicate that the electrical and thermal characteristics of the electronics and internal calibration sources are very stable and promote excellent gain stability. Radiometer NEDT < 1 K for 17-ms samples. The gain spectrum exhibits low noise at frequencies >1 MHz and 1/f noise rising at longer time scales fully captured by the internal calibration scheme. Results from sky observations and global swath imagery of all four Stokes antenna temperatures indicate that the instrument is operating as expected.

Aquarius instrument design for sea surface salinity measurements

Sea surface salinity is a key parameter for the study of ocean circulation, global water cycle and hence climate changes. In response to these measurement needs, Aquarius was selected recently for the third NASA Earth System Science Pathfinder (ESSP) Announcement of Opportunity for a planned launch date in September 2008. The characteristics of the Aquarius instrument are provided in this paper.

High-efficiency L-band transmit/receive module for synthetic aperture radar

Space-based radar places significant demands on the spacecraft resources (mass, power, data rate) and is therefore very expensive to implement. These systems typically require active phased-array antennas with hundreds or thousands of transmit/receive (T/R) modules distributed on the array. High-efficiency is a vitally important figure of merit for the radar T/R module because it reduces the power consumption and therefore makes best possible use of the limited power available. High efficiency also improves the thermal design and reliability. In this paper, we describe the design and preliminary results of a novel L-band (1250 MHz) T/R module technology to achieve ultra-high efficiencies. We will show that a dramatic improvement in overall T/R module efficiency is possible using high-efficiency class-E/F amplifiers. The T/R module performance goals are to achieve an overall module efficiency greater than 70% with a minimum of 30-W output power at L-band frequencies.

A new coherent radar for ice sounding in Greenland

This paper discusses a new multifrequency dual channel coherent radar depth sounder for sounding ice. This sounder is unique, since it is a fully coherent chirp radar designed to operate at low (1,500 ft) as well as high altitudes (30,000 ft). The dynamic range of the radar is sufficient for simultaneously imaging both the top and bottom returns from ice without the need for data blanking or sensitivity time control (STC) equalization of the return signal. The first deployment of this radar on board a P-3 aircraft took place in May of 1999 over Greenland with successful results.

An update on the NASA-ISRO dual-frequency DBF SAR (NISAR) mission

The National Aeronautics and Space Administration (NASA) in the United States and the Indian Space Research Organisation (ISRO) are developing a synthetic aperture radar (SAR) mission to map Earths surface every 12 days, known as the NASA-ISRO SAR (NISAR) Mission. NISAR has two radars sharing a mechanical structure and reflector, one operating at L-band (24 cm wavelength) and the other at S-band (10 cm wavelength). To achieve wide-swath observations at both wavelengths, NISAR is designed as a reflector-feed system where the feed aperture elements are individually sampled to allow a scan-on-receive capability. In the partnership, NASA provides the instrument structure for both L- and S-band electronics, the L-band electronics, the reflector and associated boom, and an avionics payload to interface with the radar including a solid-state data recorder, high-rate Ka-band telecommunication link, and a GPS receiver. ISRO provides the spacecraft and launch vehicle, and the S-band radar electronics, and an additional high-rate Ka-band telecom package. Hardware prototyping has matured designs for engineering models, which are currently under development.

NASA's Soil Moisture Active Passive (SMAP) observatory

The Soil Moisture Active Passive (SMAP) mission, one of the first-tier missions recommended by the 2007 U.S. National Research Council Committee on Earth Science and Applications from Space, was confirmed in May 2012 by NASA to proceed into Implementation Phase (Phase C) with a planned launch in October 2014. SMAP will produce high-resolution and accurate global maps of soil moisture and its freeze/thaw state using data from a non-imaging synthetic aperture radar and a radiometer, both operating at L-band. Major challenges addressed by the observatory design include: (1) achieving global coverage every 2-3 days with a single observatory; (2) producing both high resolution and high accuracy soil moisture data, including through moderate vegetation; (3) using a mesh reflector antenna for L-band radiometry; (4) minimizing science data loss from terrestrial L-band radio frequency interference; (5) designing fault protection that also minimizes science data loss; (6) adapting planetary heritage avionics to meet SMAPs unique application and data volume needs; (7) ensuring observatory electromagnetic compatibility to avoid degrading science; (8) controlling a large spinning instrument with a small spacecraft; and (9) accommodating launch vehicle selection late in the observatorys development lifecycle.