Contains reports on twelve research projects.National Science Foundation (Grant AST77-06052)Joint Services Electronics Program (Contract DAAB07-76-C-1400)National Aeronautics and Space Administration (Contract NAS5-21980)U.S. Air Force - Electronic Systems Division (Contract F19628-75-C-0122)U.S. Department of Commerce - National Oceanic Atmospheric Administration (Grant 04-8-M01-1)National Aeronautics and Space Administration (Contract NAS5-22929)National Aeronautics and Space Administration (Contract NAS5-23677)National Science Foundation (Grant AST73-05042-A03)National Science Foundation (Grant AST76-20376

Baan, Willem A.

Barrett, Alan H.

Burke, Bernard F.

Giuffrida, Thomas S.

Haschick, Aubrey D.

Ledsham, William H.

Melcher, James R.

Myers, Philip C.

Rosenkranz, Philip W.

Shao, Michael

Staelin, David H.

Walker, Robert C.

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XV. RADIO ASTRONOMYAcademic and Research StaffProf. Alan H. BarrettProf. Bernard F. BurkeProf. James R. MelcherProf. Philip C. MyersBarry R. AllenRichard B. BuxtonAlan L. CasselRowley L. ChengArthur D. FisherJ. Antonio Garcia-Barret,Thomas S. GiuffridaProf. David H. StaelinProf. Edward L. WrightDr. Alan ParrishGraduate StudentsPerry E. GreenfieldAubrey D. HaschickSteven T. KirschJeffrey H. LangWilliam H. Ledshamo David A. LevittTerence H. MarshallDr. Philip W. RosenkranzJohn W. BarrettJohn D. KiersteadD. Cosmo PapaDavid F. McDonoughMatthew H. SchnepsMichael ShaoDiane C. SimmonsDaniel D. StancilPaul ToldalagiRobert C. Walker1. LONG-BASELINE ASTROMETRIC INTERFEROMETERNational Science Foundation (Grant AST77-06052)David H. Staelin, Michael ShaoWork began this year on a new astrometric interferometer under NSF sponsorship.This is a continuation of a Sloan Fund-sponsored effort to develop a Michelson stellaroptical interferometer that can track fringe positions in two colors so as to removeatmospheric effects and permit stellar separations to be measured with up to 10 - 4 arc-1sec accuracy.The first system has one-inch optics and is designed to demonstrate the basic con-cept of fringe tracking. It is expected to be completed in the spring of 1978 and thenmoved to a western observatory site for field operations. If successful, it is hoped thatan operational instrument with 4- inch optics might be developed.References1. M. Shao and D. H. Staelin, "Long-Baseline Optical Interferometer for Astrometry,"J. Opt. Soc. Am. 67, 81-86 (1977).2. CONTROLLED THIN-FILM ANTENNAJoint Services Electronics Program (Contract DAAB07-76-C- 1400)David H. Staelin, James R. MelcherAnalysis of a distributed feedback control system for shaping thin-film reflectorantennas began in 1977. The concept being studied involves the use of a scanning electronPR No. 120(XV. RADIO ASTRONOMY)beam for depositing charge on a semirigid segmented surface parallel to a thin film ormesh that can serve as the reflecting surface for a very large microwave antenna inspace. Electrostatic forces between these two surfaces shape the reflector.Analysis of a particular 1-km diameter antenna showed that there were -50 unstablemodes in the reflector surface if it were suspended only by its rim. The growth timeof these modes is sufficiently long (>20 s) that a minicomputer appears to be adequatefor computing the commands needed by the electron gun assembly.A scale model is being assembled in the laboratory to test the distributed feedbackconcept. An assembly for measuring the secondary emission characteristics of thinfilms is also being constructed.3. SCANNING MICROWAVE SPECTROMETER EXPERIMENTNational Aeronautics and Space Administration (Contract NAS5-21980)David H. Staelin, Philip W. RosenkranzThe five-channel scanning microwave spectrometer (SCAMS) yielded 10 months ofalmost continuous data from the Nimbus-6 satellite, launched in 1975.1Analysis of Typhoon June has shown that passive microwave soundings can yield wind-1information in two ways: The surface wind was inferred for velocities up to -75 m s-1from the enhanced surface emissivity due to foam and rough seas, and the winds aloftwere deduced from the observed horizontal temperature gradients by means of the ther-mal wind equation.Kalman filtering was applied to the problem of temperature profile retrievals. Bysimultaneously operating on data vectors for adjacent image elements, improvementsin accuracy were obtained because of the systematic differences between the radiativetransfer functions for different viewing angles and the correlations between adjacentatmospheric cells.The propagation of electromagnetic waves in inhomogeneous media was analyzed forthe case where the inhomogeneities can be characterized by the autocorrelation functionsof the index of refraction with respect to depth and also radial separation. This modelhas been successfully applied to interpretation problems for snow and ice data. 2References1. D. H. Staelin, P. W. Rosenkranz, F. T. Barath, E. J. Johnston, and J. W. Waters,"Microwave Spectroscopic Imagery of the Earth," Science 197, 991-993 (1977).2. A. D. Fisher, "A Model for Microwave Intensity Propagation in an InhomogeneousMedium," IEEE Trans. on Antennas and Propagation, Vol. AP-25, No. 6, pp. 876-882, November 1977.PR No. 120(XV. RADIO ASTRONOMY)4. OBSERVATIONS OF CLOUDS AND ATMOSPHERIC TEMPERATUREPROFILES BY MEANS OF A 118-GHz SPECTROMETERU.S. Air Force - Electronic Systems Division (Contract F19628-75-C-0122)David H. Staelin, Philip W. RosenkranzThe 118-GHz spectral region contains a strong molecular resonance of 02 which canbe used for atmospheric temperature sounding and, by combination with 60-GHz chan-nels, for measurement of heavy clouds. A 7- channel 118-GHz spectrometer was devel-oped for studying this spectral region and flown in the NASA/Ames Convair 990 aircraftin March 1977. Measurements near zenith at altitudes from zero to 35, 000 ft generallyconfirmed the theoretical expressions for 02 absorption. Recent ground-based mea-surements of the 118-GHz and 60-GHz spectra of clouds passing overhead are still beinganalyzed. The ability of 118-GHz spectrometers to sound temperature profiles has beendetermined theoretically to be slightly superior to 60-GHz soundings over ocean andinferior over land. The ability of combined 60-GHz and 118-GHz spectrometers to soundcloud density profiles has been found to be only moderately better than 60-GHz data aloneif linear estimation methods are employed. Tsang et al. I analyzed theoretically themicrowave emission of clouds.References1. L. Tsang, J. A. Kong, E. Njoku, D. H. Staelin, and J. W. Waters, "Theory forMicrowave Thermal Emission from a Layer of Cloud or Rain," IEEE Trans. onAntennas and Propagation, Vol. AP-25, No. 5, September 1977, pp. 650-657.5. TIROS-N SATELLITE MICROWAVE SOUNDERU.S. Department of Commerce - National Oceanic Atmospheric Administration(Grant 04-8-M01-1)David H. Staelin, Philip WV. RosenkranzThe Tiros-N Operational Weather Satellite carries a microwave temperature-profile sounder (MSU) which has one window channel near 50 GHz, plus three chan-nels at 52-58 GHz; these respond to the average temperatures of atmospheric layerscentered near 3, 8, and 12 km altitudes, respectively. The first two such satelliteswill be launched in 1978. The performance of these instruments will be limited by pre-cipitation, clouds, surface effects, and the mathematical techniques employed toestimate the atmospheric temperature field. This research program focuses on develop-ment of an improved understanding of the limits to performance and the developmentof high-performance estimation procedures. The problem is nonlinear and involvesnonstationary, non-Gaussian statistics.PR No. 120(XV. RADIO ASTRONOMY)6. SC ANNING MULTICHANNEL MICROWAVE RADIOMETER (SMMR)EXPERIMENTNational Aeronautics and Space Administration (Contract NAS5-22929)David H. Staelin, Philip W. RosenkranzThe SMMR instrument is scheduled for launch in 1978 on the Nimbus-G and the Sea-sat satellites; they will scan the Earth at a constant incidence angle at frequencies of6. 6, 10. 7, 18, 21, and 37 GHz using dual linear polarization. These data will yieldinformation about the microwave characteristics of snow, sea ice, foam, precipitation,humidity, sea surface, vegetation, soil moisture, etc., and will permit their variousparameters to be mapped and monitored.The parameter estimation problem involves nonlinear phenomena and non-Gaussianand nonstationary statistics. Kalman filtering and pattern classification techniques arebeing explored for retrieval of these parameters.The estimation of snow and ice and ocean parameters also requires improved physi-cal models that permit efficient computation of microwave emission characteristics.Such models are being developed.7. METEOROLOGICAL REMOTE SENSING NEAR 2-mm WAVELENGTHNational Aeronautics and Space Administration (Contract NAS5-23677)David H. Staelin, Philip W. Rosenkranz, William H. LedshamThe remote sensing technology developed in the 10-60 GHz spectral region can beapplied in large measure to the 100-200 GHz bands because both contain H20 and strongO resonances, as well as spectral windows which are nearly transparent. The princi-pal difference is that the 100-200 GHz region is 2-20 times more responsive to cloudsand precipitation and that the 183-GHz HZO resonance is quite opaque below a few kilo-meters altitude. An important advantage of these short wavelengths is that an antenna offixed size in synchronous orbit can have spatial resolution several times greater thanthat obtainable at 10-60 GHz. This feature is important for contemplated future weathersatellites in geostationary orbit.The capabilities of meteorological satellites operating at frequencies near 100, 118,135, and 183 GHz have been analyzed and compared with systems operating below 60 GHz.In general the 100-200 GHz systems perform comparably with the lower frequency sys-tems in clear air or light clouds, but are more easily degraded by precipitation; theirperformance over land is also slightly degraded for water vapor or temperature profilesat altitudes below a few kilometers. These results were based upon the use of linearestimation procedures; nonlinear estimation should significantly improve the perfor-mance of 100-200 GHz sensors in certain situations.PR No. 120(XV. RADIO ASTRONOMY)8. MICROWAVE SPECTROSCOPY OF THE INTERSTELLARMEDIUMNational Science Foundation (Grant AST73-05042-A03)Alan H. Barrett, Philip C. MyersDuring the past year our studies of atomic and molecular interstellar gas cloudshave continued, with observations of microwave transitions of HI, CO, CS, NH 3 ' H2CO,and CH3 OH toward Bok globules, dark clouds, molecular clouds associated with HIIregions, and globular clusters. We have used radio telescopes of the National RadioAstronomy Observatory in Green Bank, West Virginia, and at Kitt Peak, Arizona; andof M. I. T. Haystack Observatory.A search for maser emission from methanol (CH 3 OH) in 132 sources confirmed thatOrion is a unique source of methanol line emission at kX 1 cm. A search for CO emis-sion from globular clusters indicates that if mass lost from cluster stars is still trappedin the clusters, no more than -5 M* is in molecular form at T z 100 K. A study of theNH3 inversion lines toward molecular clouds associated with Orion A and other H II re-gions indicates that the gas is highly clumped on a scale of -0. 04 pc. In the Taurusdark cloud, extremely narrow NH 3 linewidths were observed, indicating very cold gaswith T < 28 K. The molecular cloud CRL 437 has CO line shapes with remarkable sym-metry properties, suggesting differential rotation of the cloud. A study of the p Ophiuchidark cloud in several molecular lines shows scale lengths which vary from .1 pc to 6 pc.-1and linewidths increasing with scale length from -1 to -3 km s-1. This effect suggestscontracting radial motion. Observation of H I toward dark clouds with an interferometerwith resolutions -3' and -6' indicates very strong absorption at molecular line veloci-ties, suggesting very high H densities.9. VERY LONG BASELINE INTERFEROMETER APERTURESYNTHESIS OF MASERSNational Science Foundation (Grant AST76-20376)Robert C. Walker, Bernard F. Burkea. VLBI Aperture Synthesis of MasersVery Long Baseline Interferometer (VLBI) observations were made in an effort tostudy the structure of H 2 0 masers in greater detail than has been possible in the past.A system of programs for aperture synthesis mapping of spectral line VLBI data wasdeveloped and applied to data from a five-station experiment. Synthesis maps were pro-duced of the H20 emission in W51, CRI. 2591, and ON2. The emission in many velocityPR No. 120(XV. RADIO ASTRONOMY)channels in these sources is spatially complex, demonstrating the value of the synthesistechnique over the fringe-rate method of mapping used in most experiments. Fringe-ratemaps were made of W3(C)H 2 0 and W3(OH) which could not be synthesized due to extremecomplexity in the sources and instrumental difficulties. In a separate experiment, anew map of the emission from W49N, including much of the high-velocity emission, wasmade using data from two stations. Intercontinental baselines were used in a thirdexperiment to study the dependence of the apparent size of a maser spot on frequencyacross single, isolated lines. A dependence, in the sense that the source appears largerin the wings of the line, was found for an extraordinarily powerful line in W49N.The observations reveal several significant scale sizes in H20 sources. The appar-13ent size of individual maser spots is on the order of a few times 10 cm. The depen-dence of source size on frequency supports the "hot spot" idea that the apparent size ofmaser spots can be much smaller than their real size. The separation of maser spots,which is deduced from the synthesis maps, can be taken as an indication of the actualsize of masers: approximately 5 X 1014 cm. Results from the synthesis experimentand the W49N map show that tight clusters of emission lines exist with scale sizes on15the order of 4) x10 cm. W49N contains several of these tight clusters. The high-velocity emission in W49N was found to be associated with the tight clusters of low-velocity emission. The scale size of the high-velocity emission about a given cluster16 17is on the order of 10 cm. The total extent of many sources is 10 cm or more.The observations have been discussed in terms of a model for the early evolutionmassive stars. The important feature of the model is a dust shell and associated regionof shocked gas which forms about the star as it begins nuclear burning. The shell isdriven away from the star by radiation pressure, and eventually fragments. The H20masers occur in the region of shocked gas outside the shell. High-velocity features andextended regions of emission are manifestations of the fragmentation of the shell. Thestage of evolution of the dust shell may be determined from the appearance of VLBI maps.This material, in a thesis, entitled " Observations of the Detailed Structure of Inter-stellar Hz 0 Masers," was submitted by R. C. Walker to the Department of Physics onAugust 12, 1977, in partial fulfillment of the requirements for the degree of Doctor ofPhilosophy.10. MASER TIME VARIATIONSNational Science Foundation (Grant AST76-20376)Aubrey D. Haschick, Thomas S. Giuffrida, Bernard F. BurkeThe W3(OH)H 20 maser exhibited a well-defined variation in the intensity of one com-ponent during May-June 1977. The line rose to maximum intensity over a one-weekperiod, and declined during the following four weeks. A model for the variation hasPR No. 120(XV. RADIO ASTRONOMY)been constructed that fits the observations accurately. The calculation assumes thatthe maser is saturated, the pumping mechanism is collisional, the initial source ofenergy occurs as a pulse localized in space and time, and that the energy is quicklythermalized. A possible geometrical model can be constructed, involving the releaseof energy through a brief break in the dust cocoon of a young star, which provides suf-ficient energy for such an event. The model suggests that there is a lack of infraredradiation from the W3(OH) maser because the phenomena are being observed at such anearly stage that the infrared radiation has not yet diffused out.11. FRINGE-RATE MAPPING OF W3(OH)National Science Foundation (Grant AST76-20376)Thomas S. Giuffrida, Aubrey D. Haschick, Bernard F. BurkeOne of the principal methods of reducing spectral line Very Long Baseline Interfer-ometer (VLBI) data is the fringe-rate map. In the current implementation, this methodassumes only one source per spectral channel. This assumption is clearly incorrectfor many H20 maser sources, including W3(OH). A series of programs were writtenavoiding this assumption to reduce VLBI data taken on W3(OH) over one year's time.The data included three experiments run during the rapid turn-on of one feature duringMay 1977, referred to in Section XV-10.The resulting maps show that spot positions are repeated surprisingly during theyear. Even though some spots disappear and others emerge, most remain. Thosethat do remain gain complexity in velocity space, something not accounted for in masermodels. The maps have been discussed and the possible pump sources enumerated ina thesis, "Interferometric Observations of Astronomical Sources Emitting 1.35 cm Radiation," by T. S. Giuffrida, submitted to the Department of Physics on December 20,1977, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.The rapid turn-on implies a pump communication speed of . 05 times the speed of light.This is difficult to comprehend, except in the hot-dust cold-gas pump model.12. STUDIES OF HI ABSORPTION IN GALAXIESNational Science Foundation (Grant AST76-20376)Aubrey D. Haschick, Willem A. Baan, Bernard F. BurkeNeutral hydrogen absorption-line studies of external galaxies have continued, usingthe 300-ft transit telescope of the NRAO.Neutral hydrogen absorption at 21-cm wavelengths has been detected in the radioPR No. 120(XV. RADIO ASTRONOMY)galaxy 1506+34. The absorption feature has a halfwidth of 110 km s-1 and midpoint-1radial velocity (cAX/ ) of 13, 500 km s which is coincident with the optical emission-1line velocity of the galaxy of 13, 550 km s . The high optical depth of the absorbing19HI cloud of =T 0. 4 implies a neutral hydrogen column density of N H = 8. 5 x 10 Ts-2 sHI atoms cm The absorption feature in 1506+34 is comparable both in optical depthand width to features found in the spiral galaxies NGC 253, M82, and NGC 4945.The 300-ft transit telescope of the NRAO was used for the successful detection ofneutral hydrogen absorption at 21-centimeter wavelength arising from within the radiogalaxy 3C178. The radial velocity of the most prominent absorption feature of 2460 km-1s-1 lies close to the median velocity of the neutral hydrogen emission profile. An esti-mate of the optical depth -r of 0. 06 for this feature leads to a column density for the18 -2absorbing material of 3. 6 X 10 T cm . Three weak features appear at -34, +12,-1and +40 km s with respect to the central feature. The effect of the continuum radiosource on the spin temperature is important, and it appears to be very difficult to locatethe absorbing gas close to the nucleus. The low velocity is hard to explain dynamically,and it is possible that the main absorption arises in the galactic halo of 3C178.PR No. 120

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