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NASA NTRS1992

A space-based microwave radar concept

A space-based microwave radar (SBR) concept is defined using a tether trans-receive antenna supported between two gravity gradient low earth-orbiting satellites. A cluster of four tether antennas each of 6 km maximum length and 1.5 km separation between tethers constitutes a radar. A system of eight to eleven such clusters constitutes the overall radar scheme which will cover approximately one third of the earth surface for detecting sea-based targets. Issues identified are the array structure, coherence of tethered arrays, grating lobe energy clamping, clutter effects, communications, system requirements and the overall radar system concept including stability considerations. This paper presents the base-line definition of an alternate space-based radar scheme.

Chakraborty, D.↗

NASA NTRS1992

High dimensional reflectance analysis of soil organic matter

Recent breakthroughs in remote-sensing technology have led to the development of high spectral resolution imaging sensors for observation of earth surface features. This research was conducted to evaluate the effects of organic matter content and composition on narrowband soil reflectance across the visible and reflective infrared spectral ranges. Organic matter from four Indiana agricultural soils, ranging in organic C content from 0.99 to 1.72 percent, was extracted, fractionated, and purified. Six components of each soil were isolated and prepared for spectral analysis. Reflectance was measured in 210 narrow bands in the 400- to 2500-nm wavelength range. Statistical analysis of reflectance values indicated the potential of high dimensional reflectance data in specific visible, near-infrared, and middle-infrared bands to provide information about soil organic C content, but not organic matter composition. These bands also responded significantly to Fe- and Mn-oxide content.

Henderson, T. L.↗

NASA NTRS1992

HIRIS, the instrument and its science

The High Resolution Imaging Spectrometer (HIRIS) is a facility instrument slated for flight on the second EOS series AM platforms. HIRIS is designed to acquire 24 km wide, 30 m pixel images in 192 spectral bands simultaneously in the 0.4-2.45 micron wavelength region. With pointing mirrors it can sample any place on Earth, except the poles, every 2 days. HIRIS operates at the intermediate scale between the human and the global and therefore links studies of Earth surface processes to global monitoring carried out by lower resolution instruments. So far, over 50 science data products from HIRIS images have been identified in the fields of atmospheric gases, clouds, snow and ice, water, vegetation, and rocks and soils. The key attribute of imaging spectrometry that makes it possible to derive quantitative information from the data is the large number of contiguous, spectral bands. Therefore, spectrum-matching techniques can be applied. Such techniques are not possible with present-day, multispectral scanner data.

Goetz, Alexander F. H.↗

NASA NTRS1990

Space vehicle deployment from Space Station

When launching a spacecraft from Earth parking orbit to deep space, it is highly desirable to have the hyperbolic excess velocity vector (v-infinity) contained in the parking orbit plane. Ground launches can force the parking orbit plane to contain the v-infinity vector by using launch azimuth and lift-off time as independent variables. When launching from the Space Station, a new set of variables comes into play. The Space Station orbit is of fixed inclination but precessing due to the Earth's oblateness. Its plane will seldom (and may never) contain the desired v-infinity vector. Consequently, the departure strategy will usually require multiple burns and include a plane change. Also, the concept of 'launch window' will be somewhat different from Earth surface launches. An analysis of the deployment of interplanetary spacecraft from the Space Station is described, with emphasis on the effect of the trajectory characteristics on station operations. Several planetary mission types are analyzed, including manned Mars missions and unmanned high declination departures. The constraint of Space Station orbit nodal position is quantified and the operational implications for Space Station reboost strategy are examined.

Henry, Paul K.↗

NASA NTRS1993

Polarity reversals and tilt of the Earth's magnetic dipole

There is evidence that the terrestrial magnetic field is connected with the Earth's mantle: (1) there are magnetic anomalies that do not take part in the westward drift of the main field, but are fixed with respect to the mantle; (2) the geomagnetic pole position flips in a particular way by preferred meridional paths during a reversal; and (3) magnetic polarity reversals are correlated with the activations of geological processes. These facts may be explained if we take into account that a significant horizontal temperature gradient can exist in the top levels of the liquid core because of the different thermoconductivity of the different areas of the core-mantle boundary. These temperature inhomogeneities can penetrate the core because fluxes along the core boundary (the thermal wind) can be strongly suppressed by a small redistribution of the chemical composition in the top of the core. The nonparallel gradients of the temperature, density, and composition on the top of the core create a curled electric field that produces a current and a magnetic field. This seed-field can be amplified by motions in the core. The resulting field does not forget the seed-field distribution and in this way the field on the Earth surface (that can be created only in regions with high conductivity, i.e. in the core) is connected with the core-mantle boundary. Contrary to the usual approach to the dynamo problem, we will take into account that the seed field of thermoelectric origin is acting not only at some initial moment of time but permanently.

Dolginov, A. Z.↗

NASA NTRS1993

Global coverage Mobile Satellite Systems: System availability versus channel propagation impairments

Mobile Satellite Systems (MSS) in Highly Elliptical (HEO) and circular Earth orbits at Medium (MEO) and Low (LEO) altitudes have been intensively studied in the last few years as an effective means of providing global communication services. Such global coverage MSS networks are also expected to mitigate typical channel impairments usually encountered in geostationary Land Mobile Satellite (LMS) systems. In the design stages of these satellite networks, information regarding the mobile propagation channel is needed to assess the overall link availability versus elevation angle and environmental scenarios. For multisatellite LMS configurations, the mobile user on the Earth surface sees, at any given time, more than one satellite of the constellation. In our paper, it is shown that, under certain working assumptions regarding the statistics of the propagation channel, an improvement of the link availability may be achieved through the use of a multisatellite constellation. The analyses have been carried out using the European Space Agency (ESA) LMS propagation data base which presently covers a wide range of elevation angles and environmental scenarios.

Sforza, M.↗

NASA NTRS1993

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a multispectral imaging radiometer scheduled to fly in Earth orbit in 1998 on NASA's Earth Observation System platform. The instrument will have 14 spectral bands from the visible to thermal infrared wavelength regions with high spectral and spatial resolution and with along-track stereoscopic capability. ASTER imagery will be used to study such phenomena as Earth surface properties, elements of the surface heat balance, cloud cover characteristics, glacier and sea ice extent, patterns of vegetation and land use, volcanoes, coral reefs and coastal processes, geology and topography, and hydrology. ASTER will have three separate radiometer subsystems, each with a swath width of 60 km. Any point on the globe will be accessible at least once every 16 days for the short wavelength infrared and thermal infrared subsystems, and once every five days for the visible and near infrared subsystem. Instrument and spacecraft resources are allocated to support an 8 percent average duty cycle, corresponding to over 700 60 by 60-km scenes per day. ASTER data will be acquired and processed according to specific user requirements over its five-year mission.

Kahle, Anne B.↗

NASA NTRS1994

Directional radiance measurements: Challenges in the sampling of landscapes

Most earth surfaces, particularly those supporting natural vegetation ecosystems, constitute structurally and spectrally complex surfaces that are distinctly non-Lambertian reflectors. Obtaining meaningful measurements of the directional radiances of landscapes and obtaining estimates of the complete bidirectional reflectance distribution functions of ground targets with complex and variable landscape and radiometric features are challenging tasks. Reasons for the increased interest in directional radiance measurements are presented, and the issues that must be addressed when trying to acquire directional radiances for vegetated land surfaces from different types of remote sensing platforms are discussed. Priority research emphases are suggested, concerning field measurements of directional surface radiances and reflectances for future research. Primarily, emphasis must be given to the acquisition of more complete and directly associated radiometric and biometric parameter data sets that will empower the exploitation of the 'angular dimension' in remote sensing of vegetation through enabling the further development and rigorous validation of state of the art plant canopy models.

Deering, D. W.↗

NASA NTRS1993

A discussion of plausible solar irradiance variations, 1700-1992

From satellite observations the solar total irradiance is known to vary. Sunspot blocking, facular emission, and network emission are three identified causes for the variations. In this paper we examine several different solar indices measured over the past century that are potential proxy measures for the Sun's irradiance. These indices are (1) the equatorial solar rotation rate, (2) the sunspot structure, the decay rate of individual sunspots, and the number of sunspots without umbrae, and (3) the length and decay rate of the sunspot cycle. Each index can be used to develop a model for the Sun's total irradiance as seen at the Earth. Three solar indices allow the irradiance to be modeled back to the mid-1700s. The indices are (1) the length of the solar cycle, (2) the normalized decay rate of the solar cycle, and (3) the mean level of solar activity. All the indices are well correlated, and one possible explanation for their nearly simultaneous variations is changes in the Sun's convective energy transport. Although changes in the Sun's convective energy transport are outside the realm of normal stellar structure theory (e.g., mixing length theory), one can imagine variations arising from even the simplest view of sunspots as vertical tubes of magnetic flux, which would serve as rigid pillas affecting the energy flow patterns by ensuring larger-scale eddies. A composite solar irradiance model, based upon these proxies, is compared to the northern hemisphere temperature depatures for 1700-1992. Approximately 71% of the decadal variance in the last century can be modeled with these solar indices, although this analysis does not include anthropogenic or other variations which would affect the results. Over the entire three centuries, approx. 50% of the variance is modeled. Both this analysis and previous similar analyses have correlations of model solar irradiances and measured Earth surface temperatures that are significant at better than the 95% confidence level. To understand our present climate variations, we must place the anthropogenic variations in the context of natural variability from solar, volcanic, oceanic, and other sources.

Hoyt, Douglas V.↗

NASA NTRS1997

A Near Perfect Spin Balance (Measurement in Chaos)

The stringent spin balance requirements arise from the predecessor of SSMIS, the SSMI. The SSMI sensor spinning portion weighed only 85 pounds and contained 7 channels of radiometric data. The Aerospace Corporation recommended to pass on the same requirements from the smaller SSMI to our larger SSMIS (with slight change for increased weight). The SSMIS spinning portion will weigh about 155 pounds and contain 24 channels of radiometeric data. The SSMIS, on orbit, spins a CCW direction at 31.6 RPM its own drive motor. The packaging of this SSMIS is unique, as it combines three sensor into one unit. This combination allows for concurrently reading data in one beam. The unit will have a polar orbit about 500 miles above the earths surface. One of the primary influences for our receipt of the follow-on contract for the next generation sensor, was the ability to package 24 channels of radiometeric data into about the some volume as its predecessor. The data from SSMIS will be used to measure the following: (1) Ocean surface wind speed, (2) Rain over land an ocean, (3) Cloud water over Ocean, (4) Soil moisture, (5) Ice Concentration, (6) Ice age, (7) Ice Edge and snow edge, (8) Water vapor over Ocean, (9) Surface type, (10) Snow water content, (11) Land surface Temperature, (12) Cloud amount over ocean.

Luntz, R. A.↗

NASA NTRS1997

Investigation of Cloud Properties and Atmospheric Profiles with Modis

A major milestone was accomplished with the delivery of all five University of Wisconsin MODIS Level 2 science production software packages to the Science Data Support Team (SDST) for integration. These deliveries were the culmination of months of design and testing, with most of the work focused on tasks peripheral to the actual science contained in the code. LTW hosted a MODIS infrared calibration workshop in September. Considerable progress has been made by MCST, with help from LTW, in refining the calibration algorithm, and in identifying and characterization outstanding problems. Work continues on characterizing the effects of non-blackbody earth surfaces on atmospheric profile retrievals and modeling radiative transfer through cirrus clouds.

Menzel, Paul↗

NASA NTRS1998

ADEOS Total Ozone Mapping Spectrometer (TOMS) Data Products User's Guide

Two data products from the Total Ozone Mapping Spectrometer (ADEOS/TOMS) have been archived at the Distributed Active Archive Center, in the form of Hierarchical Data Format files. The ADEOS/ TOMS began taking measurements on September 11, 1996, and ended on June 29, 1997. The instrument measured backscattered Earth radiance and incoming solar irradiance; their ratio was used in ozone retrievals. Changes in the reflectivity of the solar diffuser used for the irradiance measurement were monitored using a carousel of three diffusers, each exposed to the degrading effects of solar irradiation at different rates. The algorithm to retrieve total column ozone compares measured Earth radiances at sets of three wavelengths with radiances calculated for different total ozone values, solar zenith angles, and optical paths. The initial error in the absolute scale for TOMS total ozone is 3 percent, the one standard deviation random error is 2 percent, and the drift is less than 0.5 percent over the 9-month data record. The Level 2 product contains the measured radiances, the derived total ozone amount, and reflectivity information for each scan position. The Level 3 product contains daily total ozone and reflectivity in a 1-degree latitude by 1.25 degrees longitude grid. The Level 3 files containing estimates of UVB at the Earth surface and tropospheric aerosol information will also be available. Detailed descriptions of both HDF data files and the CDROM product are provided.

Krueger, A.↗

NASA NTRS1998

Earth Probe Total Ozone Mapping Spectrometer (TOMS) Data Product User's Guide

Two data products from the Earth Probe Total Ozone Mapping Spectrometer (EP/TOMS) have been archived at the Distributed Active Archive Center, in the form of Hierarchical Data Format files. The EP/ TOMS began taking measurements on July 15, 1996. The instrument measures backscattered Earth radiance and incoming solar irradiance; their ratio is used in ozone retrievals. Changes in the reflectivity of the solar diffuser used for the irradiance measurement are monitored using a carousel of three diffusers, each exposed to the degrading effects of solar irradiation at different rates. The algorithm to retrieve total column ozone compares measured Earth radiances at sets of three wavelengths with radiances calculated for different total ozone values. The initial error in the absolute scale for TOMS total ozone is 3 percent, the one standard deviation random error is 2 percent, and the drift is less than 0.5 percent over the first year of data. The Level-2 product contains the measured radiances, the derived total ozone amount, and reflectivity information for each scan position. The Level-3 product contains daily total ozone and reflectivity in a 1-degree latitude by 1.25 degrees longitude grid. Level-3 files containing estimates of LTVB at the Earth surface and tropospheric aerosol information are also available, Detailed descriptions of both HDF data-files and the CD-ROM product are provided.

McPeters, R.↗

NASA NTRS1979

Biological and Organic Chemical Decomposition of Silicates

The weathering of silicate rocks and minerals, an important concern of geologists and geochemists for many years, traditionally has been approached from strictly physical and chemical points of view. Biological effects were either unrecognized, ignored, or were mentioned in passing to account for such phenomena as the accumulation of organic matter in sediments or the generation of reducing environments. A major exception occurred in soil science where agricultural scientists, studying the factors important in the development of soils and their ability to nourish and sustain various crops, laid the foundation for much of what is known of the biological breakdown of silicate rocks and minerals. The advent of the space age accelerated the realization that many environmental problems and geo- chemical processes on Earth can only be understood in terms of ecosystems. This in turn, spurred renewed interest and activity among modem biologists, geologists and soil scientists attempting to unravel the intimate relations between biology and the weathering of silicate rocks and minerals of the earth surface.

Silverman, M. P.↗

NASA NTRS1999

Assessed Using Satellite Data and Ground Based Sun/Sky Radiometers

The short aerosol lifetime and the variety of sources and atmospheric processes that affect the aerosol concentration and properties, generate a very heterogeneous aerosol field. Satellite data are needed to assess the daily global distribution of aerosol loading, properties and radiative forcing of climate. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument launched in July 1999 measures the aerosol properties and backscattered radiation in 7 spectral channels across the solar channel with spatial resolution of 500 m. Novel inversion techniques are used to derive the aerosol forcing at the top of the atmosphere in these 7 spectral bands with even higher accuracy than the derivation of the aerosol loading or microphysical properties. But satellites cannot sense the aerosol attenuation of radiation reaching the Earth surface. A network of 100 sun/sky radiometers, members of the AErosol RObotic NEtwork (AERONET) is used to assess, simultaneously, the radiative forcing of aerosol at the surface level. The relevance of MODIS and AERONET to assess the global aerosol forcing, and the inversion techniques will be presented and discussed.

Kaufman, Yoram J.↗

NASA NTRS1999

Global Modeling of Mineral Dust and its Optical Thickness

Global distribution of dust in the atmosphere has been simulated using the NASA Goddard chemical transport model (GEOS-CTM) to help retrieve the aerosol optical thickness from TOMS absorbing aerosol index. The model contains a dust module which accounts for sources and removal processes. The transport is driven by the assimilated meteorological fields generated by the NASA Goddard Earth Observing System Data Assimilation System (GEOS DAS). One of the key parameters, in the retrieval algorithm of optical thickness from Total Ozone Mapping Spectrometer (TOMS) data, is the vertical profile of aerosols. During the period 10- 19 September 1994, Lidar on-space Technology Experiment (LITE) was flown on space shuttle Discovery. The 53 hours of data collected cover the lower atmosphere from the earth surface to 20 kilometers altitude and from 57 N to 57 S with a high resolution of about 15 meters. The model results are compared with LITE data over the source regions of dust (Africa, Middle East, Asia, Australia) and in the remote troposphere (Atlantic and Pacific). The simulated horizontal distribution is compared with TOMS absorbing aerosol index. Finally the calculated optical thickness will be assessed with ground based sun-photometers (AERONET).

Ginoux, Paul A.↗

NASA NTRS1999

Low-Boom SR-71 Modified Signature Demonstration Program

A flight program using the SR-71 airplane to validate sonic boom technologies for High-Speed Commercial Transport (HSCT) operation and potentially for low- or softened-boom design configurations is described. This program employs a shaped signature modification to the SR-71 airplane which is designed to demonstrate computational fluid dynamics (CFD) design technology at a full-scale HSCT operating condition of Mach 1.8 at 48,000 feet altitude. Test plans call for measurements in the near-field, at intermediate propagation altitudes, and through the more turbulent boundary layer near the Earth surface. The shaped signature modification to the airplane is comprised of added cross-section areas on the underside of the airplane forward of the wing and engine nacelles. Because the flight demonstration does not approach maximum SR-71 altitude or Mach number, the airplane provides more than adequate performance and maneuver margins for safe operation of the modified airplane. Probe airplane measurements in the near-field will use fast response pressure sensors. Far-field and ground-based boom measurements will use high response microphones or conventional sonic boom field recorders. Scope of the planned demonstration flights also includes ground level measurements during conditions which cause minimal signature distortion and conditions which cause high distortion of the signature.

David Lux↗

NASA NTRS2000

Fusion of AIRSAR and TM Data for Parameter Classification and Estimation in Dense and Hilly Forests

The expanded remotely sensed data space consisting of coincident radar backscatter and optical reflectance data provides for a more complete description of the Earth surface. This is especially useful where many parameters are needed to describe a certain scene, such as in the presence of dense and complex-structured vegetation or where there is considerable underlying topography. The goal of this paper is to use a combination of radar and optical data to develop a methodology for parameter classification for dense and hilly forests, and further, class-specific parameter estimation. The area to be used in this study is the H. J. Andrews Forest in Oregon, one of the Long-Term Ecological Research (LTER) sites in the US. This area consists of various dense old-growth conifer stands, and contains significant topographic relief. The Andrews forest has been the subject of many ecological studies over several decades, resulting in an abundance of ground measurements. Recently, biomass and leaf-area index (LAI) values for approximately 30 reference stands have also become available which span a large range of those parameters. The remote sensing data types to be used are the C-, L-, and P-band polarimetric radar data from the JPL airborne SAR (AIRSAR), the C-band single-polarization data from the JPL topographic SAR (TOPSAR), and the Thematic Mapper (TM) data from Landsat, all acquired in late April 1998. The total number of useful independent data channels from the AIRSAR is 15 (three frequencies, each with three unique polarizations and amplitude and phase of the like-polarized correlation), from the TOPSAR is 2 (amplitude and phase of the interferometric correlation), and from the TM is 6 (the thermal band is not used). The range pixel spacing of the AIRSAR is 3.3m for C- and L-bands and 6.6m for P-band. The TOPSAR pixel spacing is 10m, and the TM pixel size is 30m. To achieve parameter classification, first a number of parameters are defined which are of interest to ecologists for forest process modeling. These parameters include total biomass, leaf biomass, LAI, and tree height. The remote sensing data from radar and TM are used to formulate a multivariate analysis problem given the ground measurements of the parameters. Each class of each parameter is defined by a probability density function (pdf), the spread of which defines the range of that class. High classification accuracy results from situations in which little overlap occurs between pdfs. Classification results provide the basis for the future work of class-specific parameter estimation using radar and optical data. This work was performed in part by the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, and in part by the NASA Ames Research Center, Moffett Field, CA, both under contract from the National Aeronautics and Space Administration.

Moghaddam, Mahta↗