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NASA NTRS1992

Embedding impedance approximations in the analysis of SIS mixers

Future millimeter-wave radio astronomy instruments will use arrays of many SIS receivers, either as focal plane arrays on individual radio telescopes, or as individual receivers on the many antennas of radio interferometers. Such applications will require broadband integrated mixers without mechanical tuners. To produce such mixers, it will be necessary to improve present mixer design techniques, most of which use the three-frequency approximation to Tucker's quantum mixer theory. This paper examines the adequacy of three approximations to Tucker's theory: (1) the usual three-frequency approximation which assumes a sinusoidal LO voltage at the junction, and a short-circuit at all frequencies above the upper sideband; (2) a five-frequency approximation which allows two LO voltage harmonics and five small-signal sidebands; and (3) a quasi five-frequency approximation in which five small-signal sidebands are allowed, but the LO voltage is assumed sinusoidal. These are compared with a full harmonic-Newton solution of Tucker's equations, including eight LO harmonics and their corresponding sidebands, for realistic SIS mixer circuits. It is shown that the accuracy of the three approximations depends strongly on the value of omega R(sub N)C for the SIS junctions used. For large omega R(sub N)C, all three approximations approach the eight-harmonic solution. For omega R(sub N)C values in the range 0.5 to 10, the range of most practical interest, the quasi five-frequency approximation is a considerable improvement over the three-frequency approximation, and should be suitable for much design work. For the realistic SIS mixers considered here, the five-frequency approximation gives results very close to those of the eight-harmonic solution. Use of these approximations, where appropriate, considerably reduces the computational effort needed to analyze an SIS mixer, and allows the design and optimization of mixers using a personal computer.

Kerr, A. R.↗
NASA NTRS1984

The Telecommunications and Data Acquisition Report

Developments in space communications, radio navigation, radio science, ground-base radio astronomy, reports on the Deep Space Network (DSN) and its Ground Communications Facility (GCF), and applications of radio interferometry at microwave frequencies are discussed.

Posner, E. C.↗
NASA NTRS1985

The peculiar X-ray and radio star AS431

Einstein observations are reported which reveal that the highly reddened emission-line star AS431 has a highly absorbed X-ray spectrum and a relatively strong intrinsic flux of 5 x 10 to the -12th erg/sq cm/s or more. Observations at 20 cm and 6 cm with the National Radio Astronomy Observatory Very Large Array are also reported which show that AS431 is also a moderately strong radio source (about 35 mJy). These data, together with optical and infrared observations, suggest a model in which both the radio and X-ray emissions arise in a chaotic stellar wind emerging from a single luminous Wolf-Rayet star.

Caillault, J.-P.↗
NASA NTRS1989

Neptune's wind speeds obtained by tracking clouds in Voyager images

Images of Neptune obtained by the narrow-angle camera of the Voyager 2 spacecraft reveal large-scale cloud features that persist for several months or longer. The features' periods of rotation about the planetary axis range from 15.8 to 18.4 hours. The atmosphere equatorward of -53 deg rotates with periods longer than the 16.05-hour period deduced from Voyager's planetary radio astronomy experiment (presumably the planet's internal rotation period). The wind speeds computed with respect to this radio period range from 20 meters per second eastward to 325 meters per second westward. Thus, the cloud-top wind speeds are roughly the same for all the planets ranging from Venus to Neptune, even though the solar energy inputs to the atmospheres vary by a factor of 1000.

Hammel, H. B.↗
NASA NTRS1993

Navigation of space VLBI missions: Radioastron and VSOP

In the mid-1990s, Russian and Japanese space agencies will each place into highly elliptic earth orbit a radio telescope consisting of a large antenna and radio astronomy receivers. Very long baseline interferometry (VLBI) techniques will be used to obtain high resolution images of radio sources observed by the space and ground based antennas. Stringent navigation accuracy requirements are imposed on the space VLBI missions by the need to transfer an ultra-stable ground reference frequency standard to the spacecraft and by the demands of the VLBI correlation process. Orbit determination for the mission will be the joint responsibility of navigation centers in the U.S., Russia, and Japan with orbit estimates based on combining tracking data from NASA, Russian, and Japanese sites. This paper describes the operational plans, the inter-agency coordination, and data exchange between the navigation centers required for space VLBI navigation.

Ellis, Jordan↗
NASA NTRS2013

Semi-Supervised Eigenbasis Novelty Detection

Recent discoveries in high-time-resolution radio astronomy data have focused attention on a new class of events. Fast transients are rare pulses of radio frequency energy lasting from microseconds to seconds that might be produced by a variety of exotic astrophysical phenomena. For example, X-ray bursts, neutron stars, and active galactic nuclei are all possible sources of short-duration, transient radio signals. It is difficult to anticipate where such signals might appear, and they are most commonly discovered through analysis of high-time- resolution data that had been collected for other purposes. Transients are often faint and difficult to detect, so improved detection algorithms can directly benefit the science yield of all such commensal monitoring. A new detection algorithm learns a low-dimensional linear manifold for describing the normal data. High reconstruction error indicates a novel signal that does not match the patterns of normal data. One unsupervised portion of the manifold model adapts its representation in response to recent data. A second supervised portion of the model is made of a basis trained in advance using labeled examples of RFI; this prevents false positives due to these events. For a linear model, an orthonormalization operation is used to combine these bases prior to the anomaly detection decision. Another novel aspect of the approach lies in combining basis vectors learned in an unsupervised, online fashion from the data stream with supervised basis vectors learned in advance from known examples of false alarms. Adaptive, data-driven detection is achieved that is also informed by existing domain knowledge about signals that may be statistically anomalous, but are not interesting and should therefore be ignored. The method was evaluated using data from the Parkes Multibeam Survey. This data set was originally collected to search for pulsars, which are astronomical sources that emit radio pulses at regular periods. However, several non-pulsar anomalies have recently been discovered in this dataset, making it a compelling test case. By explicitly filtering known false alarm patterns, the approach yields significantly better performance than current transient detection methods.

Wagstaff, Kiri L.↗
NASA NTRS1990

A comparison of the radial distribution of molecular gas and non-thermal radio continuum in spiral disks

The present study includes 65 spiral galaxies selected from the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey for which the major axis distributions of CO emission and 1.49 GHz radio continuum emission are well determined. The radial distribution of the CO emission has been measured with the FCRAO at positions along the major axis that are spaced by one half power beam width (HPBW) (45 seconds). The radial profile of the 1.49 GHz radio continuum emission was constructed by determining the radio emission at the location of the CO measurements from the 1.49 GHz maps of Condon (1987). Large, greater than a factor of ten, radially decreasing gradients in the star formation efficiency are observed for a small percentage, approx. 10 percent, of the spirals in this sample. The majority of spirals, however, are associated with small gradients in the star formation efficiency that do not systematically increase or decrease with radius. That the star formation efficiency does not systematically decrease with radius tends to argue against a global dynamical mechanism, such as a spiral density wave, for being the dominant mechanism triggering disk star formation for the majority of spirals in this sample. The results tend to support the view that the star formation in spiral disks is dominated by a local process that depends more on the molecular cloud properties than the dynamical structure of a galaxy.

Devereux, Nicholas A.↗
NASA NTRS1990

The CO-12 and CO-13 J=2-1 and J=1-0 observations of hot and cold galaxies

Researchers observed the nuclear regions of the galaxies NGC 2146 and IC 342 in CO-12 and CO-13 J=1-0 and J=2-1 lines using the Five College Radio Astronomy Observatory (FCRAO) 14m telescope. NGC 2146 is a peculiar Sab spiral galaxy. Its complex optical morphology and strong nuclear radio continuum emission suggest that it is experiencing a phase of violent activity and could have a polar ring which may have resulted from an interaction. IC 342 is a nearby luminous Scd spiral galaxy. Strong CO, infrared and radio continuum emission from the nuclear region of IC 342 indicate enhanced star-forming activity, and interferometric CO-12 J=1-0 observations reveal a bar-like structure centered on the nucleus, along the dark lane in the NS direction. These two galaxies are selected based on their different dust temperatures and star formation efficiencies (SFE) as derived from the Infrared Astronomy Satellite (IRAS) S sub 60 mu/S sub 100 mu flux density ratio and L sub IR/M(H2), respectively, with a relatively high SFE and dust temperature of 45 K in NGC 2146 and a relatively low SFE and dust temperature of 35 K in IC 342. The data from the different CO-12 and CO-13 lines are used to study the physical conditions in the molecular clouds in the galaxies. Researchers also consider the radiative transfer to determine whether a warm and optically thin gas component exists in these galaxies, as has been suggested in the case of M82 (Knapp et al. 1980), and whether the warm gas is related to the dust properties. Since optically thin CO-12 gas is rarely detected in our own Galaxy (except in outflow sources), to confirm its existence in external galaxies is very important in understanding the molecular content of external galaxies and its relationship to star formation activity. The present CO-12 J=2-1 and CO-13 J=2-1 and J=1-0 data for NGC 2146 are the first detections of this galaxy to our knowledge. The CO-12 J=1-0 distribution in NGC 2146 has been measured as part of the FCRAO Extragalactic Survey. For the well-studied IC 342, the data are compared with 30m observations and other available data. Researchers present the observed results.

Xie, Shuding↗
NASA NTRS1974

Analysis of satellite measurements of terrestrial radio noise

Worldwide distributions of terrestrial radio noise as monitored by Radio Astronomy Explorer 1 (RAE 1) generated and compared with CCIR predictions. These contour maps show the global morphology of radio noise at 6.55 and 9.18 MHz for fall, winter, spring and summer during the local time blocks of 00-08 LT and 16-24 LT. These computer produced maps show general agreement with CCIR predictions over large land masses. The RAE and CCIR maps diverge at high latitudes over Asia and frequently over ocean regions. Higher noise levels observed by RAE at high latitudes are attributed to magnetospheric emission while higher noise levels observed by RAE over Asia are attributable to high power transmitters. Analysis of RAE noise observations in conjunction with various geophysical phenomena showed no obvious correlation.

Bakalyar, G.↗
NASA NTRS1981

The 2f/p/ circumterrestrial radio radiation as seen from ISEE 3

A narrowband highly spin-modulated radio source has been observed with the radio astronomy experiment on ISEE 3 from a position 0.01 AU upstream of the earth. The source is interpreted as radio line emission generated at twice the local plasma frequency in the vicinity of the earth's bow shock by energetic particles. The bandwidth of the 2f(p) emission varies from less than 3 kHz to more than 20 kHz and is probably broadened by solar wind density variations across the source region. The 2f(p) source is visible about 50% of the time, and the true position of its centroid is usually located within 30 earth radii of the subsolar point, but at times is observed at a distance of up to 60 earth radii. But most of these variations might result from changes in the orientation of the polarization vector in a partially polarized source or from refraction across an electron density front.

Hoang, S.↗
NASA NTRS1992

Remarks on the nature of gamma-ray bursts

We first recall that in radio astronomy, isotropy and source counts steeper than the Euclidean value of -1.5 provided early evidence for the cosmological nature of radio sources. We discuss briefly whether anything can be wrong with the evidence for isotropy and a nonuniform distribution of GRBs. We identify the selection of cosmic GRBs among recorded bursts and their isotropy as the weaker part of the evidence. We suggest a way to relate the various V/V(max) values to each other. We point out that the strong concentration of small V/V(max) values fits well with a uniform distribution of sources out to a redshift of about 2, but note that sources observed at these redshifts generally exhibit evolution, i.e., a nonuniform space distribution.

Schmidt, Maarten↗
NASA NTRS1981

Voyager measurement of the rotation period of Saturn's magnetic field

Saturn's radio rotation period is determined using measurements made by the Planetary Radio Astronomy experiment onboard the Voyager spacecraft. The sidereal period deduced is 10 hr 39 min 24 sec + or 7 sec. The radio rotation period is presumably that of the planet's magnetic field. A provisional Saturn longitude convention is proposed, and equations are provided to compute a longitude emphemeris and to transform between the proposed system and the (10 hr 14 min) system used for the Pioneer 11/Saturn encounter. In addition, the degree of longitude smearing which could result over the long term from the merging of data sets organized in this system is evaluated. Finally, no evidence of control of the radio emission by any of Saturn's satellites is found.

Desch, M. D.↗
NASA NTRS1988

MERI: An ultra-long-baseline Moon-Earth radio interferometer

An extension of the very long baseline technique currently used with ground based telescopes which will produce a resolution of 0.4 micro-arcsec at 300 GHz is described. The MERI array will open an entirely new regime of wavelengths, resolutions, and sensitivities for radio astronomy. As with any such leap in astronomical instrumentation, this advancement will result in the observation of a variety of known radio sources and the exploration of their structures in unprecedented detail. Even so, it may be the serendipitous discoveries, which are by definition impossible to predict, that ultimately justify the construction of MERI.

Burns, Jack O.↗
NASA NTRS1978

Discovering astronomy

An overview of basic astronomical knowledge is presented with attention to the structure and dynamics of the stars and planets. Also dealt with are techniques of astronomical measurement, e.g., stellar spectrometry, radio astronomy, star catalogs, etc. Basic physical principles as they pertain to astronomy are reviewed, including the nature of light, gravitation, and electromagnetism. Finally, stellar evolution and cosmology are discussed with reference to the possibility of life elsewhere in the universe.

Chapman, R. D.↗
NASA NTRS1979

Voyager spacecraft radio observations of Jupiter: Initial cruise results

Jupiter's low-frequency radio emission were detected by the planetary radio astronomy instruments onboard the two Voyager spacecraft. The emission is surprisingly similar in morphology but opposite in polarization to the high-frequency Jovian radio noise that were observed with ground-based telescopes for more than two decades. Several possible explanations for the behavior of the low-frequency emission are examined, but none of them is completely satisfactory.

Kaiser, M. L.↗
NASA NTRS2001

Dips in X-Ray Flux Associated with Superluminal Ejections in the Radio Galaxy 3C 120

We compare the RXTE X-ray light curve of 3C 120 from our late 1997 to early 2000 observations, plus early 1997 archival data, with both the cm-wave light curves (University of Michigan Radio Astronomy Observatory) and the times of ejections of apparent superluminal components (from VLBA observations). There is no correlation between the X-ray and radio light curves. However, the epochs of all four observed superluminal ejections correspond, to within the errors, with pronounced dips in the X-ray light curves (photon energies in the range 2-20 keV). This behavior is similar to that of the binary-system 'microquasar' GRS1915+105 in the Milky Way galaxy, although in 3C 120 there is no evidence for rapid, quasi-periodic fluctuations in X-ray flux. These observations therefore support the notion that in active galactic nuclei eruptions of energetic outflow are related to excess accretion onto a supermassive black hole.

Marscher, A. P.↗
NASA NTRS1974

Astronomy research at the Aerospace Corporation

This report reviews the astronomy research carried out at The Aerospace Corporation during 1974. The report describes the activities of the San Fernando Observatory, the research in millimeter wave radio astronomy as well as the space astronomy research.

Paulikas, G. A.↗
NASA NTRS1981

Radio evidence for shock acceleration of electrons in the solar corona

It is pointed out that the new class of kilometer-wavelength solar radio bursts observed with the ISEE-3 Radio Astronomy Experiment occurs at the reported times of type II events, which are indicative of a shock wave. An examination of records from the Culgoora Radio Observatory reveals that the associated type II bursts have fast drift elements emanating from them; that is, a herringbone structure is formed. It is proposed that this new class of bursts is a long-wavelength continuation of the herringbone structure, and it is thought probable that the electrons producing the radio emission are accelerated by shocks. These new events are referred to as shock-accelerated events, and their characteristics are discussed.

Cane, H. V.↗