Satellite remote sensing of total herbaceous biomass production in the Senegalese Sahel - 1980-1984
Explore the source record for details and available documents.
Tucker, C. J.↗
SEARCH · Engineering Papers
Results for “Tucker”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Directional reflectance factor distributions for cover types of Northern Africa
Directional reflectance factors that spanned the entire exitance hemisphere were collected on the ground throughout the morning period for common cover types in Tunisia, Africa. NOAA 7/8 AVHRR bands 1(0.58-0.68 micron) and 2 (0.7301.1 micron) were used in data collection. The cover types reported were a plowed field, annual grassland, steppe grassland, hard wheat, salt plain, and irrigated wheat. Several of these cover types had geometric structures that are extreme as compared to those reported in the literature. Comparisons were made between the dynamics of the observed reflectance distributions and those reported in the literature. It was found that the dynamics of the measured data could be explained by a combination of soil and vegetation scattering components. The data and analysis further validated physical principles that cause the reflectance distribution dynamics as proposed by field and simulation studies in the literature. Finally, the normalized difference transformation (Band 2 - Band 1)/(Band 1 + Band 2), which is useful in monitoring vegetation cover, generally decreased the variation in signal with changing view angle. However, several exceptions were noted.
Kimes, D. S.↗
Boolean integral calculus for digital systems
The concept of Boolean integration is introduced and developed. When the changes in a desired function are specified in terms of changes in its arguments, then ways of 'integrating' (i.e., realizing) the function, if it exists, are presented. Boolean integral calculus has applications in design of logic circuits.
Tucker, J. H.↗
Application of AVHRR vegetation index to study atmosphere-biosphere exchange of CO2
The Advanced Very High Resolution Radiometer (AVHRR) satellite-borne sensor is presently used, together with field data, to determine the geographic distributions of the seasonal exchange of CO2 between the earth's atmosphere and the terrestrial biota. The exchange functions thus obtained are validated in virtue of the ability to reproduce the observed annual cycle of atmospheric CO2 in a three-dimensional tracer transport model. The AVHRR is carried by the TIROS-N series of polar-orbiting satellites.
Fung, I. Y.↗
Continental land cover classification using satellite data
Four different approaches to the classification of land cover for whole continents using multitemporal images of the normalized difference vegetation index derived from the Advanced Very High Resolution Radiometer of the NOAA series of satellites are discussed. The first approach uses only two dates from different seasons and classification dependent upon subdivision of the resultant two-dimensional feature space by an analyst using a track ball. The second approach involves a similar method of partitioning the feature space, but with the two dimensions being the first and second principal components derived from 13 four-week composite images. The third approach uses the maximum likelihood rule to derive the classified map. In the fourth approach, the amount of deviation from characteristic curves is used as a basis for classification.
Townshend, J. R. G.↗
Multitemporal dimensionality of images of normalized difference vegetation index at continental scales
Two sets of multitemporal data derived from NOAA world data product are analyzed by means of principal components analysis in order to examine their underlying multitemporal dimensionality. Specifically, images of the normalized difference vegetation index (NDVI) were analyzed for eight 3-week periods for Africa and ten 3-week periods for North America sampled from throughout the year extending from April 1982 to March 1983. The two multitemporal sets of images displayed remarkable similarities in terms of their first two components, the first corresponding very closely to the annualized integrated NDVI and the second to the seasonality of the NDVI. In particular, for the African data set the feature space defined by the first two components allows separation of the main cover types.
Townshend, J. R. G.↗
Relationship between atmospheric CO2 variations and a satellite-derived vegetation index
Weekly data on the normalized difference vegetation index (NDVI), obtained from the advanced very-high-resolution radiometer sensors flown on the NOAA's polar-orbiting meteorological satellites, were correlated with monthly changes in atmospheric CO2 concentration measured at the surface stations in Alaska, Hawaii, and the South Pole in the 1982-1984 period. While the South Pole data showed no relationship between CO2 concentration and the NDVI, the CO2 data taken at Alaska and Hawaii showed inverse correlation with the NDVI, demonstrating a measurable link between the CO2 drawdowns and the dynamics of NDVI, which in turn reflect the dynamics of photosynthesis. The data suggest that satellite data can be used for estimations of terrestrial photosynthetic activity.
Tucker, C. J.↗
Cavity-Modulation Autotuner For Hydrogen Maser
Cavity-modulation tuning system maintains resonance of hydrogen-maser cavity automatically, stabilizing maser frequency, without introducing undesirable frequency modulation. System tunes maser cavity rapidly by modulating one of physical characteristics of cavity affecting resonant frequency. Frequency-controlling components of tuning system placed at maser cavity, within maser vacuum. Thermal environment well controlled, and device insensitive to driving signal. Features contribute to stability of maser frequency.
Dick, G. J.↗
Radiometric calibration of the airborne imaging spectrometer
Several laboratory calibration tests were developed for the Airborne Imaging Spectrometer (AIS). The goals of these tests are to adjust the physical alignment of the detector within the optical system and set the spectral endpoints, and produce a calibration file of multipliers which equalizes the relative responses of each of the detector elements in the entire two-dimensional array. Tests are also performed as part of an ongoing effort to provide absolute radiometric calibration. The tests are described and the merits and limitations of each are discussed.
Tucker, D.↗
Space vehicle concepts
This paper presents several concepts of chemical-propulsion Space Vehicles (SVs) for manned Mars landing missions. For vehicle sizing purposes, several specific missions were chosen from opportunities in the late 1990's and early 2000's, and a vehicle "system" concept is then described which is applicable to the full range of missions and opportunities available. In general, missions utilizing planetary opposition alignments can he done with smaller vehicles than those utilizing planetary opposition alignments (reference I) The conjunction missions have a total mission time of about 3 years, including a required stay-time of about 60 days. Both types of missions might be desirable during a Mars program, the opposition type for early low-risk missions and/or for later unmanned cargo missions, and the conjunction type for more extensive science/exploration missions and/or for Mars base activities. Since the opposition missions appeared to drive the SV size more severely, there were probably more cases examined for them. Some of the concepts presented utilize all-propulsive braking, some utilize an all aerobraking approach, and some are hybrids. Weight statements are provided for various cases. The aerobraking cases have significant advantages in size and weight. Cryogenic propellants were used for the main propulsive elements in all cases, due to their significant weight advantage over storable propellants (reference 1). Extensive use is made of existing propulsive elements and other systems. Most of the work was done on O-g vehicle concepts, but partial-g and 1-g concepts are also provided and discussed. A recommendation is added that efforts be made to find ways to offset the long-term O-g effects on the crew, other than providing a g-field for the total SV or spacecraft, since this causes significant design and operations impacts. Several options for habitable elements are shown, such as large-diameter modules and Space Station (SS) types of modules. The latter were used as a reference because of their cost advantage as existing elements Several options are shown for the Mars landing vehicle, and a landing "system" is recommended which makes use of a large aeroshell to allow landing of payloads of various sizes and shapes over the course of a multi-year program. Because of the large size and weight of the SV it will be necessary to launch individual elements and assemble them in low Earth orbit (LEO). A configuration of one potential assembly concept is provided.
Michael Tucker↗
Aeolian abrasion on Venus: Preliminary results from the Venus simulator
The role of atmospheric pressure on aeolian abrasion was examined in the Venus Simulator with a constant temperature of 737 K. Both the rock target and the impactor were fine-grained basalt. The impactor was a 3 mm diameter angular particle chosen to represent a size of material that is entrainable by the dense Venusian atmosphere and potentially abrasive by virtue of its mass. It was projected at the target 10 to the 5 power times at a velocity of 0.7 m/s. The impactor showed a weight loss of approximately 1.2 x 10 to the -9 power gm per impact with the attrition occurring only at the edges. Results from scanning electron microscope analysis, profilometry, and weight measurement are summarized. It is concluded that particles can incur abrasion at Venusian temperatures even with low impact velocities expected for Venus.
Marshall, J. R.↗
The X-ray background and the evolution of quasars
The contribution of QSOs and active galaxies to the 2-10-keV X-ray background is calculated theoretically, assuming that the X-ray luminosity of each GSO has a power-law time evolution (dL/dt = AL exp alpha) and applying the continuity equation to derive the X-ray luminosity function at arbitrary redshift. The observed X-ray background is shown to require alpha greater than 1.2, ruling out pure luminosity evolution (alpha = 1).
Tucker, W. H.↗
Flight evaluation of height response characteristics for the hover bob-up task and comparison with proposed criteria
The CH-47B variable-stability helicopter was used to evaluate a range of height-response configurations while performing a precision hover bob-up task. The purpose of the study was to assess the validity of results obtained in previous ground-based and in-flight simulations in the context of a precision bob-up task and to provide additional flight data for inclusion in revisions to specifications for helicopter handling qualities. Height response characteristics were implemented using explicit model-following techniques, and the resulting CH-47B dynamics were validated using time-domain and frequency-domain data-analysis methods. The tests complemented the previous investigations by providing detailed pilot comments and ratings, and performance and control-utilization data that relate exclusively to the hover bob-up task. The results are compared with those from previous investigations and with new criteria that have been proposed for handling qualities requirements for helicopters.
Hindson, W. S.↗
New capabilities and recent research programs of the NASA/Army CH-47B variable-stability helicopter
The CH-47B is the third in a series of variable-stability helicopters developed and operated by NASA since 1952 to investigate helicopter and VTOL handling qualities. Recently, several new capabilities were added to this helicopter to enable it to better support new and evolving research requirements. The installation of a programmable force-feel system for the evaluation pilot's conventional cyclic stick, and a four-axis side-stick controller permit a range of in-flight investigations concerning manipulator characteristics and augmentation system features that had not been possible with earlier NASA research helicopters. A recently installed color electronic display system with a programmable symbol generator will permit the investigation of display formats for a variety of VTOL and helicopter missions. Finally, a powerful new general-purpose flight computer is now in operation. It is programmable in high-level languages and will provide more efficient support of research programs. In addition to these new hardware capabilities, flight-control software has been developed to improve the in-flight simulation capability of the aircraft. A brief description of the CH-47B's variable-stability research equipment is provided, recent research programs are summarized, and some remarks concerning the potential of the helicopter are presented.
Hindson, W. S.↗
Maximum normalized difference vegetation index images for sub-Saharan Africa for 1983-1985
Explore the source record for details and available documents.
Tucker, C. J.↗
Satellite remote sensing of primary production
Leaf structure and function are shown to result in distinctive variations in the absorption and reflection of solar radiation from plant canopies. The leaf properties that determine the radiation-interception characteristics of plant canopies are directly linked to photosynthesis, stomatal resistance and evapotranspiration and can be inferred from measurements of reflected solar energy. The effects of off-nadir viewing and atmospheric constituents, coupled with the need to measure changing surface conditions, emphasize the need for multitemporal measurements of reflected radiation if primary production is to be estimated.
Tucker, C. J.↗
Satellite remote sensing of rangelands in Botswana. II - NOAA AVHRR and herbaceous vegetation
The relation between the normalized difference vegetation index (NDVI) and the herbaceous vegetation in Tamasane, Shakwe, and Masama in eastern Botswana is studied using 1983-1984 AVHRR data. The procedures for Landsat MSS interpolation of ground measurements and the data processing of the AVHRR data are described. The temporal sequence AVHRR global-area coverage (GAC) composite NDVI is examined. The AVHRR GAC composite NDVI and biomass and Landsat MSS interpolations of field measurements are analyzed and compared.
Prince, S. D.↗
Monitoring the grasslands of the Sahel 1984-1985
Normalized difference vegetation index data obtained from polar-orbiting meteorological satellites were used to compare the growing or rainy seasons of 1984 and 1985 for the Sahelian zone of Africa. A substantial difference was found between these two years, with 1985 generally having higher normalized difference vegetation index values indicating higher levels of primary production in 1985 than in 1984. 1 km data were compared for Senegal, Mali, Niger and Sudan, and 7 km data were compared for sub-Saharan Africa. The qualitative comparison of these data suggests the use of similar data to assist in centralized monitoring of rangeland conditions, to identify areas of deficiencies in primary production and provide synoptic information in support of regional drought monitoring.
Tucker, C. J.↗