Contains reports on seven research projects and research objectives.National Science Foundation (Grant AST77-06052)Joint Services Electronics Program (Contract DAAG29-78-C-0020)National Aeronautics and Space Administration (Contract NAS5-21980)U.S. Department of Commerce - National Oceanic and Atmospheric Administration (Grant 04-8-M01-1)National Aeronautics and Space Administration (Contract NAS5-22929)National Aeronautics and Space Administration (Contract NAS5-25091)National Science Foundation (Grant AST77-12960)National Science Foundation (Grant AST77-26896

Barrett, Alan H.

Baumann, William T.

Bigelow, Timothy S.

Burke, Bernard F.

Johnson, Timothy L.

Lang, Jeffrey H.

Melcher, James R.

Myers, Philip C.

Rosenkranz, Philip W.

Shao, Michael

Staelin, David H.

Toldalagi, Paul

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XV. RADIO ASTRONOMYAcademic and Research StaffProf. A.H. BarrettProf. B.F. BurkeProf. J.R. MelcherProf. P.C. MyersA.D. AliB.R. AllenH.T. ArmstrongY. Bar-YamW.T. BaumannC.L. BennettP.J. BensonT.S. BigelowR.B. BuxtonProf. D.H. StaelinProf. E.L. WrightDr. D.H. RobertsDr. P.W. RosenkranzGraduate StudentsR.L. ChengJ,J, DalyS,D, DagaaE.E. FalcoJ.A. Garcia-BarretoJ.R. GershP.E. GreenfieldJ-s. JinJ.H, LangC.R. LawrenceDr. M. ShaoJ.W. BarrettJ.D. KiersteadD.C. PapaS, LeibovicT,H. MarshallB,R. RosenM.H. SchnepsD.D. StancilJ.C. SzczepanskiP. ToldalagiJ.V. VallergaY. Yam1. LONG-BASELINE ASTROMETRIC INTERFEROMETERNational Science Foundation (Grant AST77-06052)Michael Shao, David H. StaelinA one-inch aperture prototype stellar interferometer was shipped to Mt. WilsonObservatory last year. The interferometer first tracked the white-light fringe inMarch 1979. The servo maintained equal path lengths in the two arms of the inter-ferometer to 0.1 im with a three-stage optical delay line. The fringe position wasmeasured to 0.03 pm precision with 220 detected photons every 4 milliseconds. Theoptical bandwidth of the interferometer was 0.4 pm to 0.9 vm. Extrapolating fromthe fringe tracking data, the current tracking algorithm will track the fringes from8.7 mag stars using 12-cm aperture optics.lTwo-color fringe measurements were made in June of 1979. For a 1-second inte-gration time, two-color measurements were more accurate by approximately a factor of3 over one-color measurements.We have started work on phase two of the interferometer program, where we hopeto demonstrate the elimination of atmospheric, thermal, and seismic errors at themilli-arc-second level.PR No. 122(XV. RADIO ASTRONOMY)References1. M. Shao and D.H. Staelin, "First Fringe Measurement with a Phase Tracking StellarInterferometer," to appear in Appl. Opt. (May 1980).2. CONTROLLED THIN-FILM ANTENNAJoint Services Electronics Program (Contract DAAG29-78-C-0020)David H. Staelin, Timothy L. Johnson, Jeffrey H. Lang, James R. MelcherThe use of electrostatic charges for rapidly manipulating a thin membrane isbeing studied as a method for precisely controlling the shape of reflector antennasof %10-300 meter diameters in space; ground-based versions n2-10 meter diametermay also be of interest. This year the control experiments on the 1-meter squaremembrane were completed and rms surface tolerances of 710 pm rms were obtained inthe presence of an electric pressure twice that necessary to destabilize the sys-tem; %30-100 pm rms were obtained at four times the basic destabilization pressureand three unstable modes were then controlled.It was shown this year that the minimum diffraction-limited 3-dB beamwidth BBof such an antenna is approximately limited by1eB(radians) 3AN(f/D) 'where N is the number of controlled modes, A is a factor near 10 which representsthe intrinsic curvature of the reflector surface, and f/D is the reflector's ratioof focal length to diameter. Because the number of independent electrostatic con-trol elements is limited by the antenna geometry and by Laplace's equation, it canbe shown that one limit to resolution eB iseB (arc sec) % 900SAM2 (f/D) 3where M is the number of controlled instabilities. For example, an antenna withf/D = 1 might achieve 0B = 10 arc sec with 690 control elements and 3 controlledinstabilities.PR No. 122(XV, RADIO ASTRONOMY)These experimental and theoretical results favor the development of this tech-nology for space applications, and perhaps for terrestrial systems designed for sub-millimeter and millimeter wavelengths.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 ofdata from the Nimbus-6 satellite, launched in 1975.1Analysis of Typhoon June (1975) demonstrated that the radial derivative ofmicrowave brightness temperature is related to the vertically weighted tangentialwind through a wind weighting function; this relation follows from the thermal windequation. Agreement with simultaneous 700-meter aircraft reconnaissance winds wasgood except near the eye of the typhoon, where the 150-cm resolution of SCAMS lim-ited the performance. 2Snow and ice observations by SCAMS as a function of view angle and frequencyhave provided evidence that the microwave behavior of firn (long-term accumulatedsnow in Antarctica and Greenland) can be explained largely by the existence of ran-dom layering in the bulk of the firn with a vertical correlation length on theorder of a millimeter; the variance in dielectric constant for these layers appearsto grow as the ice grains grow with time and depth.3 Similar studies of sea icesuggest that more isotropic bulk scattering dominates its radiometric signature.3References1. 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. N.C. Grody, C.M. Hayden, W.C.C. Shen, P.W. Rosenkranz, and D.H. Staelin,"Typhoon June Winds Estimated from Scanning Microwave Spectrometer Measure-ments at 55.45 GHz," J. Geophys. Res. 84, 3689-3695 (1979).3. S.R. Rotman, A.D. Fisher, and D.H. Staelin, "Angular Microwave RadiometricSignatures of Firn and Sea Ice," in preparation.PR No. 122(XV. RADIO ASTRONOMY)4. TIROS-N SATELLITE MICROWAVE SOUNDERU.S. Department of Commerce - National Oceanic and Atmospheric Administration(Grant 04-8-M01-1)David H. Staelin, Philip W. Rosenkranz, Paul ToldalagiThe first two satellites of the operational Tiros-N series have been launched;each incorporates a four-channel passive microwave spectrometer operating in the50-58 GHz band.Linear statistical estimators and Kalman filters were developed to retrievetemperature profiles from this data, the latter providing improvements in rms er-rors of %20 percent, comparable to earlier results with microwave data from theNimbus-6 satellite. The most interesting result concerning Kalman filters was thatincorporation of second-order statistics based on the actual local three-dimensionaltemperature field did not significantly improve the retrieval performance for sum-mer data; it generally suffices to incorporate only the average latitudinal depen-dence and simple correlation constants between adjacent parcels of air. Significantimprovement was prevented in part by the conflict between adequacy of representa-tion and adequacy of statistics characterizing that representation.A statistical view of linear retrieval techniques has led to a novel and use-ful procedure for updating statistical measures used in the retrieval of tempera-ture profiles from Tiros-N data. The method has particular value over ocean, andshould be incorporated in operational data interpretation procedures. In essence,the technique uses the variable scan angle of the microwave sounder to improveknowledge of the mean temperature profile, including certain components that maybe invisible to the instrument at certain angles.5. SCANNING MULTICHANNEL MICROWAVE RADIOMETER (SMMR)National Aeronautics and Space Administration (Contract NAS5-22929)Philip W. Rosenkranz, Timothy S. Bigelow, William T. Baumann, David H. StaelinThe SMMR is an instrument on the Nimbus-7 satellite. 1 The objective of ourresearch is to use the measurements to infer parameters which characterize thestate of the atmosphere-ocean surface system. The parameters which we have foundPR No. 122(XV. RADIO ASTRONOMY)possible to retrieve using this instrument are sea surface temperature, near-surfacewind speed, integrated water-vapor content, integrated liquid-water content, and acharacteristic radius of the drop-size distribution function, The inversion methodwhich we have developed is one which operates on each spatial Fourier coefficientof the antenna temperature images (at five frequencies and two polari7ations) toestimate the corresponding spatial Fourier coefficients of the geophysical param-eters.2,3 Efforts are also being made to improve models for the calculation ofbrightness temperatures given a specified state of the atmosphere-ocean system, par-ticularly when the surface emissivity is increased by wind-generated white water androughness.References1. P. Gloersen and F.T. Barath, "A Scanning Multichannel Microwave Radiometer forNimbus-G and SeaSat-A," IEEE J. Oceanic Eng. OE-2, 172-178 (1977).2. P.W. Rosenkranz, "Inversion of Data from Diffraction-Limited MultiwavelengthRemote Sensors. 1. Linear Case," Radio Sci. 13, 1003-1010 (1978).3, P.W. Rosenkranz and W.T. Baumann, "Inversion of Multiwavelength Radiometer Mea-surements by Three-Dimensional Filtering," in A. Deepak (Ed.), Remote Sensingof Oceans and Atmospheres (in press).6. COMMUNICATION SATELLITESNational Aeronautics and Space Administration (Contract NAS5-25091)David H. StaelinIn the late 1980's it will become practical to transfer a large fraction (per-haps 30 percent) of all long-distance telecommunications traffic to satellites, a1-3step which will require significant switching capacity in space.-3 Analysis ofmarket and technology issues has resulted in definition of a minimum-cost TDMA sat-ellite-communications system architecture that could serve the entire United Statesby handling 30 GHz traffic. More than 1000 ground stations operating at 20,30GHz would couple existing local toll centers or other nodes to a complex of verysimilar satellites sharing a single synchronous orbital slot. One novel featureof this architecture is the simultaneous use of more than one cooperating switchedsatellite with little intersatellite communication.PR No. 122(XV. RADIO ASTRONOMY)The projected costs for such a system appear to be very attractive and couldfacilitate establishment of new services such as video conferencing.References1. D.H. Staelin, "Expanding Broadband Switched Communications Networks," SatelliteCommunications, January 1979, pp. 26-30.2. D.H. Staelin and R.L. Harvey, "Architectures and Economics for Pervasive Broad-band Satellite Networks," Proc. IEEE 1979 International Conference on Communi-cations, pp. 35.4.1-35.4.7 (June 1979).3. D.H. Staelin and R.L. Harvey, "Future Large Broadband Switched Satellite Com-munications Networks," NASA Contract Report, December 1979. 270 p.7. MICROWAVE SPECTROSCOPY OF THE INTERSTELLAR MEDIUMNational Science Foundation (Grant AST77-12960)Alan H. Barrett, Philip C. MyersIn 1979 Professors Barrett and Myers and their students carried out the fol-lowing radio astronomical investigations of dense molecular clouds: (1) a studyof the dense neutral globules in the Rosette Nebula; (2) a study of NH3 emissionfrom the galactic center; (3) a survey of nearby reflection nebulae in spectrallines of CO and NH3; (4) first detection of the J = 3 doublet line of H2CO inOrion; (5) searches for new regions of emission from the rare molecule HC5N (sixsuch regions were found); (6) large-scale mapping (full coverage of several squaredegrees) of CO emission from the nearest star-forming complexes in Taurus andOphiuchus; and (7) construction of radiative transfer models permitting analysisof asymmetrical spectral lines in terms of cloud motions. These observational pro-grams are part of our attempt to understand the formation and evolution of darkclouds in the interstellar medium and how they may relate to star formation. Theseprograms, and others like them, will be continued in the coming year.PR No. 122(XV. RADIO ASTRONOMY)8. RESEARCH OBJECTIVESNational Science Foundation (Grant AST77-26896)Bernard F. BurkeThe radio astronomy program covers a broad range of galactic and extragalacticproblems. Some of these require equipment built in our laboratory, and some arecarried out at large national facilities. The major research thrusts are as fol-lows:1. Performance of VLBI studies of celestial radio sources at a variety ofwavelengths, to understand the physical processes in quasars and active galaxies.Long-wavelength (0.5-1 m) systems are being built and tested; the mm-band mixersunder development will be used at the Haystack Observatory and elsewhere. Observa-tions at standard VLBI frequencies will use existing equipment.2. Study of time variations of interstellar masers, using VLBI methods to ob-tain maps with milli-arc-second accuracy. Time variations of adjacent maser spotswould provide evidence for shock motions in maser complexes.3. Completion of a large-scale survey of equatorial radio sources, using the300-ft transit telescope of NRAO at Green Bank. Initial wavelength is 6 cm; ob-served sources will be reobserved at other wavelengths to determine spectra.PR No. 122

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