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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Control design for the SERC experimental testbeds

Viewgraphs on control design for the Space Engineering Research Center experimental testbeds are presented. Topics covered include: SISO control design and results; sensor and actuator location; model identification; control design; experimental results; preliminary LAC experimental results; active vibration isolation problem statement; base flexibility coupling into isolation feedback loop; cantilever beam testbed; and closed loop results.

Jacques, Robert↗
NASA NTRS1991

The application of controlled structures technology to adaptive optics

Viewgraphs on the application of controlled structures technology (CST) to adaptive optics are presented. Topics covered include: a typical large optical system (LDR); overview of current optical programs; typical adaptive optics system; actuation approaches for deforming a mirror; control approach comparison; control structure interaction (CSI) control bandwidth limitations; applications of CST concepts to optics; distributed control approaches; quasistatic error correction; low authority control (LAC); and high authority control (HAC) design methodology.

Anderson, Eric H.↗
NASA NTRS1992

Intensive optical monitoring of the BL Lacertae object PKS 2155-304

Ground-based optical polarimetry and photometry of the BL Lac object PKS 2155-304 conducted just before and during the IUE monitoring program from November 1 to 19, 1991 are presented. The optical observations reveal strong daily variability in both flux and linear polarization. Broadband UBVRI photometry shows that the slope of the optical continuum remained relatively constant even though the object brightened by nearly a magnitude. PKS 2155-304 was near its historical photometric maximum during the last two weeks of November. The degree of polarization varied between 2 and 8 percent, and fluctuations of 2-3 percent were observed in a day. Daily variations in the position angle of the polarization were observed to be as high as 25 percent, though the position angle was observed to lie between 105 and 140 deg throughout the monitoring program. Some of the implications that these optical data have on models of the source of the continuum emission in PKS 2155-304 are discussed.

Smith, Paul S.↗
NASA NTRS1992

Comparison of multi-temporal NOAA-AVHRR and SPOT-XS satellite data for mapping land-cover dynamics in the West African Sahel

Multi-resolution and multi-temporal remote sensing data (SPOT-XS and AVHRR) were evaluated for mapping local land-cover dynamics in the Sahel of West Africa. The aim of this research was to evaluate the agricultural information that could be derived from both high and low spatial resolution data in areas where there is very often limited ground information. A combination of raster-based image processing and vector-based geographical information system mapping was found to be effective for understanding both spatial and spectral land-cover dynamics. The SPOT data proved useful for mapping local land-cover classes in a dominantly recessive agricultural region. The AVHRR-LAC data could be used to map the dynamics of riparian vegetation, but not the changes associated with recession agriculture. In areas where there was a complex mixture of recession and irrigated agriculture, as well as riparian vegetation, the AVHRR data did not provide an accurate temporal assessment of vegetation dynamics.

Marsh, S. E.↗
NASA NTRS1993

The X-ray spectral variability of the BL Lacertae type object PKS 2155-304

We present a detailed study of the hard X-ray properties of the BL Lacertae object PKS 2155-304 based on measurements made in 1988 and 1989 with the Large Area Counter (LAC) on board the Ginga satellite. The source exhibited a high degree of variability with a dynamic range of a factor 7 in the 2-6 keV band. The fastest amplitude variation was a factor 2 decline in the intensity in this band within 4 hours. The spectrum is characterized by a break which occurs at about 4 keV. Spectral fits to the data integrated in 6400 s time bins reveal that, in common with previous observations of BL Lacertae objects, the spectral slope is generally anticorrelated with intensity in the sense that the spectrum hardens as the intensity increases. However, the tracks of sequential points in the index-intensity plane are occasionally seen to differ during the rise and decay stages of individual flares. Furthermore, during one, or possibly two, flaring episodes the spectral index is observed to correlate with intensity variations.

Sembay, S.↗
NASA NTRS1991

The cosmological evolution of BL Lacertae objects

Using a complete, X-ray selected sample of BL Lacertae Objects (XBLs), we find that V sub e/V sub a = 0.34 +/- 0.06 implying that the cosmological evolution of this type of AGN is 'negative'; i.e., fewer or less luminous BL Lacs in the past compared to the current epoch. In the process of deriving this result, the selection criteria and observational properties of XBLs are described.

Stocke, John T.↗
NASA NTRS1993

Simultaneous ROSAT/Ginga observations of 4U 1820-30

We have made simultaneous Ginga LAC and ROSAT PSPC observations of 4U 1820-30. The 685-s orbital light curves obtained with the two instruments are very similar, indicating that the energy dependence of the orbital modulation is small. Our measurements extend the baseline over which the period variations can be measured to 15 yr. The previous possibility, that the changes in the period are themselves periodic, with a period of about 8 yr, is no longer preferred over a constant P(dot). Over the interval 1976-91 the period has decreased, rather than increasing as predicted by the standard model for the orbital evolution of the binary. The average period derivative P(dot)/P was (-0.88 +/- 0.16) x 10 exp -7/yr, different by 11 sigma from the predicted value. Under the assumption that there are no intrinsic changes in the light curve that mimic a period change, we discuss three possible explanations.

Van Der Klis, M.↗
NASA NTRS1993

The Compton Gamma Ray Observatory

The Arthur Holly Compton Gamma Ray Observatory (Compton) was launched by the Space Shuttle Atlantis on 5 April 1991. The spacecraft and instruments are in good health and returning exciting results. The mission provides nearly six orders of magnitude in spectral coverage, from 30 keV to 30 GeV, with sensitivity over the entire range an order of magnitude better than that of previous observations. The 16,000 kilogram observatory contains four instruments on a stabilized platform. The mission began normal operations on 16 May 1991 and is now over half-way through a full-sky survey. The mission duration is expected to be from six to ten years. A Science Support Center has been established at Goddard Space Flight Center for the purpose of supporting a vigorous Guest Investigator Program. New scientific results to date include: (1) the establishment of the isotropy, combined with spatial inhomogeneity, of the distribution of gamma-ray bursts in the sky; (2) the discovery of intense high energy (100 MeV) gamma-ray emission from 3C 279 and other quasars and BL Lac objects, making these the most distant and luminous gamma-ray sources ever detected; (3) one of the first images of a gamma-ray burst; (4) the observation of intense nuclear and position-annihilation gamma-ray lines and neutrons from several large solar flares; and (5) the detection of a third gamma-ray pulsar, plus several other transient and pulsing hard X-ray sources.

Gehrels, N.↗
NASA NTRS1993

Compton scattering and the gamma-ray power-law spectrum in Markarian 421

The nearest BL Lac object, Mrk 421, has a gamma-ray spectrum which is approximately flat in EF-sub E from E less than about 50 MeV to E greater than about 1 TeV. Inverse Compton scattering can explain this smooth spectrum, despite the structure in the Klein-Nishina cross section, if the injected electron distribution function is proportional to gamma exp -2, where gamma is the electron Lorentz factor. When this is the case, the structure imprinted on the steady state electron distribution function by the structure in the Klein-Nishina cross section is almost exactly compensated in the radiated spectrum. Because particle acceleration in strong shocks injects particles with this distribution function, this shape injection function is in fact quite plausible. Other blazars may be explained by the same model if the cutoff below TeV energies observed in other objects is due to pair production on the cosmological IR background, as suggested by Stecker et al. (1992).

Zdziarski, Andrzej A.↗
NASA NTRS1993

Helical jets and the misalignment distribution for core-dominated radio sources

The type of a model which would explain the large population of orthogonally misaligned sources is considered. Two of the best examples of misaligned sources are investigated, namely, the quasar 3C 309.1 (Wilkinson et al., 1986; Kus et al., 1990, 1991) and the BL Lac object 1823 + 568 (Foley, 1982; O'Dea et al., 1988). It is shown that simple bend geometries cannot explain the misaligned sources, and the geometries required to give orthogonal misalignments are explained. A generally applicable model which might explain misaligned sources is proposed.

Conway, J. E.↗
NASA NTRS1993

On the relationship between thermal emissivity and the Normalized Difference Vegetation Index for natural surfaces

The spatial variation of both the thermal emissivity (8-14 microns) and Normalized Difference Vegetation Index (NDVI) was measured for a series of natural surfaces within a savanna environment in Botswana. The measurements were performed with an emissivity-box and with a combined red and near-IR radiometer, with spectral bands corresponding to NOAA/AVHRR. It was found that thermal emissivity was highly correlated with NDVI after logarithmic transformation, with a correlation coefficient of R = 0.94. This empirical relationship is of potential use for energy balance studies using thermal IR remote sensing. The relationship was used in combination with AVHRR (GAC), AVHRR (LAC), and Landsat (TM) data to demonstrate and compare the spatial variability of various spatial scales.

Van De Griend, A. A.↗
NASA NTRS1993

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. I - The IUE campaign

Daily monitoring of PKS 2155-304 with the IUE satellite throughout November 1991 revealed dramatic large-amplitude rapid variations in the UV flux of this BL Lac object. Many smaller, rapid flares are superposed on a general doubling of the intensity. During the five-day period when sampling was roughly continuous, the rapid flaring had an apparent quasi-periodic nature, with peaks repeating every 0.7 day. The short- and long-wavelength UV light curves are well correlated with each other, and with the optical light curve deduced from the Fine Error Sensor on IUE. The formal lag is zero, but the cross-correlation is asymmetric in the sense that the shorter wavelength emission leads the longer. The UV spectral shape varies a small but significant amount. The correlation between spectral shape and intensity is complex. The sign of the correlation is consistent with the nonthermal acceleration processes expected in relativistic plasmas, so that the present results are consistent with relativistic jet models, which can also account for quasi-periodic flaring.

Urry, C. M.↗
NASA NTRS1993

Observations of PKS 2155-304 with the Extreme Ultraviolet Explorer

We present a high-accuracy light curve for the BL Lac object PKS 2155-304, observed with the Extreme Ultraviolet Explorer. The light curve shows variations of about 10 percent over a 1.3 d observing period. The observed EUV flux is a factor of 1.7 fainter than when PKS 2155-304 was observed by the ROSAT Wide Field Camera (WFC) during the EUV survey. The optical flux during our observations is about a factor of 2 fainter than during the WFC observations, so we conclude that the overall spectral shape is invariant as the flux varies. These observations support the model in which the spectrum from the optical to the X-ray band is given by a synchrotron spectrum. Extrapolating an X-ray power-law spectrum with an energy index of 1.66 into the EUV only slightly overpredicts the observed EUV count rates so that changing any one of several spectral parameters within the uncertainties gives an acceptable model.

Marshall, H. L.↗
NASA NTRS1994

CATLAC - Calibration and Validation Analysis Tool of Local Area Coverage for the SeaWiFS Mission

Calibration and validation Analysis Tool of Local Area Coverage (CATLAC) is an analysis package for selecting and graphically displaying Earth and space targets for calibration and validation activities on a polar orbiting satellite. The package is written in the Interactive Data Language (IDL) and includes a graphical user interface. Although it is designed specifically for the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission, the package can be used for analysis on other Earth-viewing missions. An individual can use text or graphical methods in CATLAC to select Earth targets to be scanned by a satellite. Additional onboard calibration activities (such as observations of the moon, or solar irradiance from a solar diffuser), which use data recorder time, can also be specified. All information pertinent to the creation of a command schedule can be written to a file which is read by a command scheduler. The scheduler can be invoked and the Local Area Coverage (LAC) recording periods can be visually verified using CATLAC. The schedule can also be verified by examining record and error files written by the scheduler.

Woodward, Robert H.↗
NASA NTRS1994

SeaWiFS technical report series. Volume 15: The simulated SesWiFS data set, version 2

This document describes the second version of the simulated SeaWiFS data set. A realistic simulated data set is essential for mission readiness preparations and can potentially assist in all phases of ground support for a future mission. The second version improves on the first version primarily through additional realism and complexity. This version incorporates a representation of virtually every aspect of the flight mission. Thus, it provides a high-fidelity data set for testing several aspects of the ground system, including data acquisition, data processing, data transfers, calibration and validation, quality control, and mission operations. The data set is constructed for a seven-day period, 25-31 March 1994. Specific features of the data set include Global Area coverage (GAC), recorded Local Area Coverage (LAC), and realtime High Resolution Picture Transmission (HRPT) data for the seven-day period. A realistic orbit, which is propagated using a Brouwer-Lyddane model with drag, is used to simulate orbit positions. The simulated data corresponds to the command schedule based on the orbit for this seven-day period. It includes total (at-satellite) radiances not only for ocean, but for land, clouds, and ice. The simulation also utilizes a high-resolution land-sea mask. It includes the April 1993 SeaWiFS spectral responses and sensor saturation responses. The simulation is formatted according to July 1993 onboard data structures, which include corresponding telemetry (instrument and spacecraft) data. The methods are described and some examples of the output are given. The instrument response functions made available in April 1993 have been used to produce the Version 2 simulated data. These response functions will change as part of the sensor improvements initiated in July-August 1993.

Hooker, Stanford B.↗
NASA NTRS1994

A comparison of CO(J = 1 yields 0) and CO(J = 2 yields 1) emission in the Milky Way molecular ring

We have carried out a CO (J = 2 right arrow 1) survey of the Scutum Arm region of the Milky Way molecular ring. Our goals are to compare CO (J = 2 right arrow 1) maps of individual Galactic clouds with the large-scale CO (J = 2 right arrow 1) emission from the Galactic plane, and to predict the CO (J = 2 right arrow 1) appearance of a Galactic cloud ensemble in an external galaxy. The angular resolution and spatial coverage of our survey are compatible with the existing CO (J = 1 right arrow 0) survey of this region by Sanders et al., which we use for comparison. We identify 34 molecular clouds in our map region; their relationships between size and line width and between virial mass and luminosity are consistent with the relationships seen in CO (J = 1 right arrow 0) emission. However, we note that previous studies have shown considerable variation in these relationships; we attribute much of this variation to differences in their cloud definition algorithm and statistical method. We find that the median ratio of integrated emission in the two lowest CO transitions for the clouds in our sample is I(2 right arrow 1)/ I(1 right arrow 0) = 0.69, implying that the typical emitting region in the line of sight contains cold gas that is not actively star-forming. Our conclusion that the molecular ring emission is not dominated by star-forming regions is consistent with other large-scale studies of the Milky Way. Our distributions of cloud size and temperature also imply that relatively massive molecular clouds that lac k star formation, such as Maddalena's cloud, are not rare in the inner Galaxy.

Chiar, Jean E.↗
NASA NTRS1994

The complex X-ray spectra of two high redshift quasars observed with ASCA

We report preliminary results of Advanced Satellite for Cosmology and Astrophysics (ASCA) observations of two high-redshift quasars (zeta approximately 3) PKS 0438-436 and PKS 2126-158. We find spectral flattening towards low energies in both sources, first found with Roentgen Satellite (ROSAT) and interpreted as being due to excess X-ray absorption. However, the ROSAT data lacked the bandpass and sensitivity to unambiguously support this interpretation. We find that for PKS 2126-158, absorption in intervening low-redshift material along the line-of-sight provides a statistically better description of the data than absorption in material intrinsic to the quasar. The redshift of the putative absorber is required to be less than a few tenths. Given the redshift constraints and the fact that absorption is not common in lower redshift quasars, this result is not easily explained by a simple model. Either the absorber may be complex or else the intrinsic X-ray spectrum may be complex, or both. Compton-reflection dominated models are ruled out but a broken power-law model provides good fits to the data without requiring excess absorption. The break energy is at approximately 6-8 keV in the quasar frame and the energy index of the soft component is required to be extremely flat. This may mean that the X-ray emission in these objects is dominated by synchrotron losses as in BL Lac objects.

Serlemitsos, Peter↗
NASA NTRS1994

Extreme ultraviolet spectroscopy of PKS 2155-304

We present the extreme ultraviolet (75-110 A) spectrum of the BL Lac object PKS 2155-304, the first spectrum of an extragalactic source obtained with the Extreme Ultraviolet Explorer (EUVE). The spectrum shows a generally smooth continuum, which can be modeled by a single power law plus interstellar absorption, and possibly an absorption feature at approximately 80 A. The best fit to the data suggests that the EUV spectrum can be interpreted as a simple extrapolation of the X-ray continuum, with an energy index alpha is approximately = 1.6; however, shallower or steeper power laws with indices between -0.4 and 2.7 cannot be ruled out by the existing EUV data alone. The data provide strong constraints on the interstellar neutral H and He along the line of sight. Using a column density of 1.36 x 10(exp 20)/sq cm for the Galactic neutral hydrogen along the PKS 2155-304 line of sight, the neutral helium column density is constrained to be 9%-10% of the hydrogen amount.

Fruscione, Antonella↗