Contains reports on six research projects.National Aeronautics and Space Administration (Grant NsG-419)National Aeronautics and Space Administration (Contract NSR-22-009-120

Barrett, Alan H.

Henckels, L. P. A.

Kebabian, P. L.

Law, Sara E.

Rosenkranz, P. W.

Staelin, David H.

Waters, J. W.

Wilson, W. J.

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III. RADIO ASTRONOMYAcademic and Research StaffProf. A. H. Barrett Prof. D. H. Staelin J. W. BarrettProf. B. F. Burke Dr. S. H. Zisk D. C. PapaProf. R. M. Price C. A. ZapataGraduate StudentsR. M. Blotky J. M. Moran, Jr. L. W. Schaper, Jr.M. S. Ewing P. C. Myers P. R. SchwartzW. W. Gebhart G. D. Papadopoulos J. W. WatersL. P. A. Henckels E. C. Reifenstein III T. T. Wilheit, Jr.H. F. Hinteregger P. W. Rosenkranz T. L. WilsonSara E. Law W. J. WilsonA. OBSERVATIONS OF INTERSTELLAR 018HThis spring, we made a series of radio astronomical observations using the 140-ftradio telescope of the National Radio Astronomy Observatory in a search for the emis-sion and absorption lines of interstellar 0 18H molecules. (The majority of OH studieshave been on the most abundant isotopic species 0 16H.)Observations of the galactic center (Sagittarius A) were made in March and April 1968.In March a wideband 50-channel filter receiver (filter bandwidth, 100 kHz) was used toobserve both the 1639-MHz and the 1637-MHz 0 18H lines simultaneously. (These linescorrespond to the 1667-MHz and the 1665-MHz lines of 0 1 6 H, respectively.) Two nights(6 hours on the source) were spent with the 50-channel receiver. Also in March and thenin April, further observations of the galactic center were made with the 100-channelautocorrelation receiver, which had an effective resolution bandwidth of 30 kHz. The1639-MHz line was observed on 4 nights (14 hours on the source), and the 1637-MHz lineon 2 nights (7 hours on the source). Only one line could be observed at a time, since thecorrelator bandwidth was 2 MHz. In these observations both frequency switching and theOFF/ON technique were used to reduce baseline nonlinearities.Both of the 018H lines were detected in absorption in the galactic center with absorp-tion strengths of 0.33 0K at 1639 MHz, and 0.24'K at 1637 MHz. With rest frequencies11639.46 MHz and 1637.46 MHz, as calculated by Barrett and Rogers, the velocity ofabsorption (+40 km/s) agrees very well with the 0 1 6 H absorption velocity, which impliesthat the absorption is occurring in the same cloud. The ratio of the line strengths of018H to those of 0 16H is approximately 1/100 (measured directly with the 140-ft tele-scope in March and April). If an abundance ratio of 1/500 for 0 18H to 0 16H is assumed(the abundance ratio on Earth of 018/0 1 6 is 1/490), a large optical depth of the orderThis work was supported by the National Aeronautics and Space Administration(Grant NsG-419 and Contract NSR-22-009-120).QPR No. 90(III. RADIO ASTRONOMY)of T = 10 may be predicted for these OH clouds. This is a surprising result, since itwas believed previously that the optical depth was of the order of unity. Further obser-vations of the galactic center will be made this summer to better define the linestrengths and shapes.In March, at the National Radio Astronomy Observatory, a search for 0 18H emis-sion at 1637 MHz in the source W3 was performed. (W3 is one of the strongest 0 6Hemission sources at 1665 MHz.) W3 was observed for 26 hours with an effective resolu-tion bandwidth of 3 kHz. The rms noise was measured to be 0.04'K, with all spectrumpoints less than 0.08'K. (Linear polarization was used on this equatorially mountedradio telescope.) The peak temperature measured on W3 with linear polarization at1665 MHz was 65 0 K; therefore, this measurement sets an upper limit for 0 18H emis-sion from W3 of 1/800 as strong as the 0 1 6 H emission.W. J. Wilson, A. H. BarrettReferences1. A. H. Barrett and A. E. E. Rogers, Nature 204, 62 (1964).B. LONG BASELINE STELLAR INTERFEROMETERIn 1921, Michelson and Pease, using an optical interferometer developed byMichelson,I made the first measurement of a stellar diameter. This original system,with a 20-ft baseline, was later superseded by a 50-ft system described by Pease. 2More recently, Hanbury-Brown measured the diameters of several more bright stars,using an optical intensity correlation interferometer capable of baseline lengths up to600 ft. All of these stellar measurements were limited instrumentally to stars brighterthan magnitude 2.5. Furthermore, the intensity-correlation system requires 1 monthof observation per star. The limitations of stellar magnitude and baseline are signifi-cant because many stars are neither bright nor of large diameter.A different type of interferometer system incorporating photomultiplier tubes hasbeen analyzed theoretically, and found to enable observation of stars of magnitude 6with a signal-to-noise ratio of 10 for 100-sec integration. 4 The system should becapable of operating over long baselines without the great expense entailed with eventhe 50-ft system of Pease. At the ends of the baseline the analyzed system incorporatedtwo 4-in. mirrors each of which directed the incident starlight into a signal-combinersystem located between the mirrors. This system combined the two plane waves andfocused them upon the face of a photomultiplier tube. One of the two interferometerarms contained a mirror that oscillated at a frequency of a few kHz, and the photomul-tiplier tube output was filtered at this frequency.The primary difficulties encountered in such a system are the angle of arrival andQPR No. 90(III. RADIO ASTRONOMY)path-length fluctuations that affect the two light beams. The atmospheric effects havebeen modeled on the basis of stellar measurements and theoretical considerations. Theangular fluctuations were assumed to be 0.4 second of arc rms, and the differential path-length fluctuations to be 20X rms, which is believed appropriate for a 20-ft baseline.For longer baselines it might be necessary to integrate longer in order to obtain the20-ft baseline performance.P. L. Kebabian, D. H. StaelinReferences1. A. A. Michelson and F. G. Pease, "Measurement of the Diameter of a Orionis withthe Interferometer," Astrophys. J. 53, 249 (1921).2. F. G. Pease, "The New Fifty-ft Stellar Interferometer," Sci. American 143, 290(1930).3. R. Hanbury-Brown, "Stellar Interferometer at Narrabio Observatory," Nature 218,637 (May 18, 1968).4. P. L. Kebabian, "Analysis of a Long Baseline Stellar Interferometer System," S.M.Thesis, Department of Electrical Engineering, Massachusetts Institute of Technol-ogy, May 1968.C. RESULTS OF INFERRING ATMOSPHERIC TEMPERATUREFROM SIMULATED MICROWAVE MEASUREMENTSThis report describes results of inferring atmospheric temperature profiles fromsimulated microwave radiometric measurements. The measurements are simulationsof those that would be obtained by a satellite-borne microwave radiometer operating inthe 60-GHz molecular oxygen resonance band. The statistical inversion technique usedin obtaining the results presented here has been described in a previous report. 1Figure III-1 shows the results of inversions performed on simulated data computedfrom 100 U.S. Weather Bureau radiosonde records. The radiosonde records were madeat Huntington, West Virginia, and included 50 summer and 50 winter soundings. Thesimulated experiment was that of a radiometer looking down at nadir from an altitudeof 20 km. Brightness temperatures of the radiation received by the radiometer werecomputed at 3 frequencies in the oxygen band, and were used as data in the inversiontechnique. Inversion results are shown for noise-free data and for data to which 1'Knoise was added. The dashed lines in Fig. III-1 give the standard deviation in theerrors of the 100 inversions, and represent the accuracy of inferring the temperatureprofile from the radiometric measurements. The average error was zero for all alti-tudes. For comparison, the standard deviation of the actual temperature profile for the100 records is also shown. The results shown in Fig. III-1 supersede those presentedin Fig. III-5 of our previous report. l These results incorporate the method of invertingnearly singular matrices that was described, but not used, in our earlier report.QPR No. 90-A PRIORI STANDARD1 ~DEVIATION OF T(h)/ - STANDARD DEVIATION OF- ( INVERSION ERRORS\ \ INVERSIONS PERFORMED ON COMPU\\ \ BRIGHTNESS TEMPERATURES LOOKINC-\ DOWN AT NADIR FROM 20 km ATSFREQUENCIES:v1 = 53.60 GHzSv2 = 60.82 GHz/ 3 = 64.47 GHzSURFACE: FLAT WATER AT 283"K.I-ATrms = IKL--- Trms = 0K1 2 3 4 5TEDSTANDARD DEVIATION ('K)Inversions performed on computed brightnesstemperatures looking down at nadir from 20 km.DATA: BRIGHTNESS TEMPERATURESCOMPUTED LOOKING DOWN ATNADIR FROM 5mbFREQUENCIES USED (GHz):22235 - H2,0 LINE53 6058 39 02 BAND5930 (TEMPERATURE)6082 \ SOUNDING /62466447STANDARD DE VIATION, o (°K)Inversion accuracy; only summer statis-tics used.20 -Fig. III-1.Fig. III-2.QPR No. 90jI(III. RADIO ASTRONOMY)Figure III-2 shows inversion results based only on summer radiosonde records. Thesoundings used are from Peoria, Illinois, for the months of June through September.Only soundings that reached a pressure altitude of 5 mb (approximately 37 km) wereincluded. Data consisted of computed brightness temperatures looking at nadir fromthe 5-mb level. Six frequencies in the oxygen band were used. These frequencies gavetemperature weighting functions at 6 separate atmospheric levels below 30 km. Alsoshown in Fig. III-2 is the effect of including a measurement at the 22.235-GHz water-vapor resonance.By considering only summer records, the a priori standard deviation is greatlyreduced from that of Fig. III-1, which included both summer and winter data. In thiscase, the simulated microwave measurements increase the accuracy of inferring thetemperature profile by one or two degrees over that which could be obtained froma priori statistical information alone. This is compared with the 5-10* improvementshown in Fig. III-1. The value of the microwave measurement is thus strongly depen-dent on the statistical information that we have concerning the atmosphere.For the results given in Figs. III-1 and III-2, the atmosphere was stratified intolayers of approximately 0.5-km thickness. In other words, a height resolution of 0.5 kmATrms = O K ATrms = IKOerror Oerror OTsmoothed DATA: BRIGHTNESS TEMPERATURES35 - COMPUTED LOOKING DOWN ATNADIR FROM 5mb1030 FREQUENCIES USED (GHz)22 235 - H2 0 LINET FULL WIDTH (Ic-)25 OF GAUSSIAN 53 60 0 BANDSMOOTHING E 58 39FUNCTION 59 30 (TEMPERATURE20 r 60 82 SOUNDINGco 62.461 - 0 64 46_j \ \ ~ 100 W10D 300SLI 1000i 2 3 4STANDARD DEVIATION, o (OK)Fig. 111-3. Inversion accuracy for 8-km height resolutionand summer statistics.was asked of the inversion scheme. If one is satisfied with the temperature averagedover a thicker vertical layer, the accuracy will be improved. Figure III-3 showsinversion results for the temperature profile which has been smoothed by a Gaus-sian function with a full width of 8 km. For 8-km smoothing, the atmosphericQPR No. 90(III. RADIO ASTRONOMY)temperature can be inferred to an accuracy of better than 2 0 K, if the brightness tem-perature of the upwelling radiation is measured to an accuracy of 1°K at each of thefrequencies. Such accuracy of measurement is within the limits of present radiometricsystems that could be carried on a satellite.J. W. Waters, D. H. StaelinReferences1. J. W. Waters and D. H. Staelin, "Statistical Inversion of Radiometric Data," Quar-terly Progress Report No. 89, Research Laboratory of Electronics, M.I.T., Janu-ary 15, 1968, pp. 25-28.D. DIGITAL SYNCHRONOUS DETECTORAn improved 20-channel version of the 5-channel digital synchronous detector anddata control systeml has been designed, and a 1-channel prototype has been tested.2Each digital synchronous detector consists of a voltage-to-pulse frequency converterand an 18-bit up-down counter, of which the 12 high-order bits are read out at periodicintervals. The system permits integration times as long as 15 minutes, with almostnegligible rms error, and a DC offset that is less than 1% of the dynamic range. Forintegration times less than 1 minute the 12-bit output is essentially error-free. Theuse of integrated circuits and printed circuits has reduced the approximate cost peradditional channel to $100.The wired program unit permits the radiometer to operate automatically in nonre-peating sequences as long as 9 hours.L. P. A. Henckels, D. H. StaelinReferences1. D. H. Staelin, "Microwave Spectrum of Venus," Sc.D. Thesis, Department of Elec-trical Engineering, M.I.T., January 1965.2. L. P. A. Henckels, "Digital Output Unit for a Multichannel Radiometer," S.M. The-sis, Department of Electrical Engineering, M.I.T., May 1968.E. RADIOMETER WITH A 23.8-GHz PARAMETRICAMPLIFIERA degenerate parametric amplifier, designed by D. H. Steinbrecher, has been con-structed, tested in several ways, and installed in front of the K-band 20-channel radi-ometer.1, 2 Noise figure and loss measurements of the components and the system haveyielded the noise and loss budget shown in Table III-1.QPR No. 90(III. RADIO ASTRONOMY)Table III-I. System losses and noise temperature.Component Loss Noise Figure Noise Temperature(dB) (dB) (0 K)Input Isolator 0.6Directional Coupler 0.5Dicke Switch 0.8Second Isolator 0.6Circulator 0.6Parametric Amplifier 1.6System 4.7 560Contribution from radiom-eter back end with 15-dBparametric amplifier gain 120Total System Temperature 680The system performance was evaluated by means of a noise tube and a noise figuremeter. The 3-dB system bandwidth during these tests was approximately 100 MHz, andthe system gain fluctuation was observed over a period of 5 minutes to be approximately0.5% rms for an average gain of 15 dB.P. W. Rosenkranz, D. H. StaelinReferences1. P. W. Rosenkranz, "A Low-Noise K-Band Radiometer for the Measurement of Nar-row Spectral Lines," S.M. Thesis, Department of Electrical Engineering, M. I.T.,May 1968.2. D. H. Staelin, R. M. Weigand, and J. W. Waters, "20-Channel Microwave Spec-trometer," Quarterly Progress Report No. 86, Research Laboratory of Electronics,M.I.T., July 15, 1967, pp. 25-26.F. POSSIBLE MICROWAVE EXPERIMENT FORA PLANETARY PROBE TO VENUSThe recent Mariner V and Venera IV planetary probes to Venus1 indicate that theplanetary surface is hotter than 500 0 K, and the surface pressure is many atmospheres.The microwave spectrum between 0.3-cm and 30-cm wavelengths is therefore presum-ably dominated by high surface brightness temperatures attenuated at wavelengthsQPR No. 90(III. RADIO ASTRONOMY)shorter than -3 cm by nonresonant atmospheric absorption. Nonresonant absorptioncould occur in CO 2 , other gases at high pressures, clouds, or some combination ofthese sources. If the absorption is dominated by atmospheric gases with a nearly con-stant mixing ratio, then the temperature structure of the atmosphere might reasonablybe monitored by a planetary orbiter similar to those proposed for terrestrial studies(see Sec. III-C). That is, the brightness temperature observed from space as a func-tion of frequency, TB(v), would beTB 1 0T(h) W(h, v) dh,where T(h) is the desired temperature profile, and W(h, v) is the weighting function.Typical weighting functions are shown in Fig. III-4 for a 90% CO 2 atmosphere. Thus40 -30 -35 GHz- 2020 GHz1010 GHz02 3 4WEIGHTING FUNCTION (x 10- 7 cm - 1)Fig. 111-4. Venus temperature functions at nadir.QPR No. 90(III. RADIO ASTRONOMY)the temperature profile may be determined with a resolution of 25 km.Whether such an experiment might ever enable monitoring meteorological phenom-ena on Venus with a space probe in orbit about Venus depends upon the stability of theatmospheric absorption coefficient. If clouds are not negligible absorbers near thefrequencies of interest, then such observations could be severely hampered by varia-tions in cloud cover.An instrument package that could land on the planetary surface would establish thevalidity of such an orbiting meteorological experiment. Not only could such an instru-ment measure composition, temperature, and pressure profiles on the way to the sur-face, but it could also measure the microwave properties of the clouds. Cloud opacitiesat various frequencies could be determined by a microwave radiometer viewing spaceas it descends through the atmosphere. A thin cloud would be evident because of theresulting discontinuity in the apparent brightness temperature as a function of altitude,TB(h), and a thick cloud would be evident because of the resulting discrepancy betweentheoretical and experimental TB(h). The sensitivity of such an attempt to detect cloudabsorption would be limited by receiver sensitivity in the upper regions of the atmo-sphere, and by the accuracy of the a priori expression for T(V) in the lower regions.If it is assumed that a zenith-looking radiometer mounted on a landing vehicle wouldyield brightness temperatures with an accuracy (3a-) of 6 0 K, and that the simultaneouscomposition measurements would permit opacity calculations accurate to -10% (30-), thenthe presence of clouds or other constituents could be detected with an accuracy (3-) ofAT, where AT is presented in Table III-2 for 2 frequencies and 3 altitudes. The modelatmosphere used in the preparation of this table is consistent with the recent planetaryprobes. l The altitudes are presented in terms of equivalent pressures and temperaturesbecause pressure and temperatures are more relevant to cloud composition than altitude.Table 111-2. Minimum detectable cloud opacity AT.Pressure (mm Hg) Temperature (*K) 10 GHz 40 GHz266 245 0.01 0.014667 300 0.01 0.0142494 400 0.01 0.0406944 500 0.02 0.085Table III-2 indicates that such an instrument landing on the surace would be quitesensitive to clouds, and certainly should be capable of validating a temperature-sounding experiment mounted in a planetary orbiter.Sara E. Law, D. H. StaelinQPR No. 90(III. RADIO ASTRONOMY)References1. A. Kliore, G. S. Levy, D. L. Cain, G. Fjeldbo, and S. I. Rasool, "Atmosphere andIonosphere of Venus from the Mariner V S-band Radio Occultation Measurement,"Science 158, 1683-1688 (1967).QPR No. 90

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