Contains reports on six research projects.National Aeronautics and Space Administration (Grant NGL-22-009-016)National Science Foundation (Grant GP-8415

Burke, Bernard F.

Hinteregger, H. F.

Papa, D. Cosmo

Papadopoulos, George D.

Price, R. M.

Schaper, L. W., Jr.

Shapiro, I. I.

Staelin, David H.

Waters, J. W.

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II. RADIO ASTRONOMYAcademic and Research StaffProf. A. H. Barrett Prof. R. M. Price D. C. PapaProf. B. F. Burke Prof. D. H. Staelin C. A. ZapataJ. W. BarrettGraduate StudentsR. M. Blotky Sara E. Law P. R. SchwartzM. S. Ewing P. C. Myers J. W. WatersW. W. Gebhart G. D. Papadopoulos T. T. Wilheit, Jr.L. P. A. Henckels P. W. Rosenkranz T. L. WilsonH. F. Hinteregger L. W. Schaper, Jr. W. J. WilsonA. PULSAR SEARCHSome long-drift scans have been carried out, using the Arecibo 1000-ft dish. Theprimary observing frequency was 430 MHz, with additional information taken at 195 andand 611 MHz in some regions. In all cases, the final time constant used was 0. 2 sec.Drifts were taken at intervals of 10 minutes of arc from declination 230 to 250 in theregion of the sky 15 h-0h-3 h . The scan spacing was slightly larger than one-half beam-width at 430 MHz.From visual inspection of the analog records, it is possible to state that no periodic-26 2pulses with peak amplitudes greater than approximately 2 X 10 W/m -Hz wereobserved. When observing at 430 MHz a given point in the sky was in the 16 minute ofarc half-power beam for approximately 1 minute. Because of the apparently randomslow fading or scintillations of the pulse amplitudes occurring with periods of severalminutes, pulses from some of the known pulsars are sometimes not above the level forvisual detection set in these observations for approximately 10-25T% of the time. It ispossible, therefore, that the amplitude of an unknown pulsar could have been below thelimit for detection while the source was in the 430-MHz beam. For regions whereobservations were simultaneously made at 195 MHz, the chance of missing a sourcebecause of fading is much lower. The source would have been in the 195-MHz half-power beam for nearly 2 1/2 minutes. It has also been noted that fading on widely sep-arated frequencies is often uncorrelated, and thus it is unlikely that both 195- and430-MHz signals should suffer fading over the same period of observation.This study is continuing.R. M. PriceThis work was supported in part by the National Aeronautics and Space Adminis-tration (Grant NGL-22-009-016) and in part by the National Science Foundation (GrantGP-8415).QPR No. 91(II. RADIO ASTRONOMY)B. PULSAR FACILITYThe announcement, in February 1968, of the discovery of a new class of radiosources by Cambridge University investigators 1 has caused considerable interest amongradio astronomers. These radio sources are characterized by short pulses of radiationwith a highly constant repetition frequency (of the order of one part in 108) and are thuscalled "pulsars." The time of arrival of individual pulses varies with observed radiofrequency. The pulses arrive later on lower frequencies than higher, because of therelative delays in propagation at different frequencies caused by electrons in the inter-stellar medium along the line of sight to the source. It has been postulated that thepulsars are associated with either white dwarfs or neutron stars.In order to study these peculiar radio sources (of which 9 are now known) and toconduct a systematic search for new pulsars, we have undertaken the building of alarge, low-frequency antenna. The best site readily available for the location of theantenna was an open field adjacent to the MIT-LNS LINAC project under constructionin Middleton township approximately 20 miles north of the M. I. T. campus. The antennaconsists of 16 50-ft spherical surface dishes in a 4 X 4 array with edges touching.Each dish consists of two circles of fourteen 4 X 4 timber posts, at radii of 25 ftand 15 ft. The posts are approximately 8 ft and 2 1/2 ft high in the outer and innerrings, respectively. One by four timber stringers are used to form a ring girderjoining the outer posts. One by four radials connect the inner and outer rings of poststo provide solid attachment points for the chicken wire that forms the surface of eachdish.The feeds for each dish, which are to be crossed 3-element Yagi's, are supportedon wooden tripods that are approximately 24 ft high. The f/D ratio for the dishes isapproximately 0.4. The pointing of each dish in the N-S direction will be controlledby the positioning of the feed. A coverage of ±200 from the zenith in the N-S directionis planned.The pulsars have been found to have steep spectra,2 and are, therefore, bestobserved at lower frequencies. At too low a frequency, the galactic backgroundbrightness temperature is high, and the frequency dispersion in the time of arrival ofthe observed pulses also increases. This calls for the use of smaller receiver band-widths (to obtain the desired time resolution <10 ms) and results in a correspondingloss in sensitivity. The array will initially operate at a frequency of approximately140 MHz.At present, the dishes and tripods are complete, with the exception of a smallamount of chicken wire surfacing to be finalized. Work is progressing on the feeds,receivers, and transmission lines.B. F. Burke, R. M. PriceQPR No. 91(II. RADIO ASTRONOMY)References1. A. Hewish, S. J. Bell, J. D. H. Pilkington, P. F. Scott, and R. A. Collins,Nature 217, 709 (1968).2. F. D. Drake, E. J. Gundermann, D. L. Jauncey, J. M. Comella, G. A. Zeissig, andH. D. Craft, Science 160, 503 (1968).C. 1. 75-cm APERTURE SYNTHESISA Ku-band interferometer is being built in order to obtain the contour maps of cer-tain interesting discrete radio sources (Crab Nebula, Cas A, Cy A, Vir A) at this highfrequency. The building blocks of this interferometer are the following.a. The RF front end.b. The phase-lock system which will keep the frequency of the 2 klystrons stablewithin 1 part in 107c. The multiplier which will give the averaged product of the signals coming fromthe two dishes. This multiplier has a dynamic range of 40-50 dB and is flat overthe IF bandwidth which is 50-70 MHz.d. The concurrent calibration and the synchronous detector: A small amount ofphase-switched noise is injected in the signal path and is later on detected by means ofthe synchronous detector. Thus we have a continuous indication of the gain stability ofthe system.e. The antenna control units which enable us to control the dishes either manuallyor by means of the computer.f. The computer program and interface for the PDP-8 which will perform the fol-lowing functions:(i) Point the dishes to the source and track it for a programmed amount oftime after which they automatically switch to a calibration source which theytrack for a short time. Upon the completion of the tracking of the calibrationsource the dishes switch, again automatically, to the original source. This pro-cedure continues for approximately 12 hours, the time allowed by the hour anglespan of the dishes.(ii) Sample the outputs of the multiplier and the synchronous detector. Thesesamples are then converted into a 10-bit binary word which is fed into the computer.The output of the multiplier is sampled every 0. 2 sec. To these samples a least-squares fit is applied in real time.(iii) Compensate for the RF signal path difference by digitally inserting andextracting fixed amounts of delay placed in the IF strip. In this manner we are contin-uously looking at the white fringes.QPR No. 91(II. RADIO ASTRONOMY)All of these units have been built and 80 per cent of them have been tested. Obser-vations and system-checking will begin as soon as the cabling between the antennas andthe control house is completed.G. D. Papadopoulos, D. C. Papa, B. F. BurkeD. GALACTIC STRUCTURE STUDIESFurther studies of galactic structure have been carried out using the 1000-ft dish ofthe Arecibo Ionospheric Observatory (operated by Cornell University for AFOSR). Thesecurrent observations at 195, 430, and 611 MHz supplement earlier observations at450 MHz made at the Algonquin Radio Observatory (ARO) of the Canadian NationalResearch Council.The ARO observations were made with a beamwidth of slightly more than 10",whereas the Arecibo beamwidth at approximately the same frequency is 0. 250.The combination of the results of the two surveys will give information about thesize distribution of nearby continuum emission regions in the Galaxy. The reductionand interpretation of the Arecibo measurements is in progress.R. M. PriceE. VERY LONG BASELINE INTERFEROMETRYThe very long baseline interferometry technique is being extended to allow measure-ments of source positions, baseline vectors, and relative time to accuracies consistentwith the extremely high spatial resolutions that have been achieved. An interferometeroperating at X-band, from 7.3 to 8.2 GHz, between the Haystack 120-ft and the NRAO140-ft antennas is under construction. It uses phase coherent frequency switching oflocal oscillators to obtain an effectively large fractional bandwidth over which phase canbe extrapolated without ambiguity. It will first be used early in October 1968 to observethe gravitational "bending" of electromagnetic waves, which should produce an apparentshift in the position of the quasar 3C279 as the Sun passes within approximately 1 of it.The theoretical absolute mensuration capabilities of the instrument will be put to theirfirst empirical test.Extensive use is being made of the Haystack facility of the Lincoln Laboratory, M.I.T.H. F. Hinteregger, B. F. Burke, I. I. ShapiroF. ESTIMATION OF ATMOSPHERIC ELECTRICAL PATH LENGTHFROM SIMULATED MICROWAVE RADIOMETRIC DATAThe statistical data inversion technique described in a previous report1 has beenapplied to the problem of estimating the electrical path length of the terrestrial atmo-sphere. Knowledge of the atmospheric path length is of value in any experimentQPR No. 91(II. RADIO ASTRONOMY)requiring phase stability between two separated receiving stations, such as the recentlong baseline interferometer experiments.The increase in the electrical path length caused by the earth's atmosphere isL = L n-1) dl, (1)where n is the index of refraction, and L is the path of the electrical signal. The indexof refraction can be related to meteorological quantities. For frequencies belowapproximately 30 GHz, the relation is-57. 76 X 10 (P+4.8113e (2)(n-1) = T P+4. 81X 10 T), Wwhere T = temperature (degrees Kelvin)P = total pressure (millibars)e = partial pressure of water vapor (millibars).The index of refraction has separate contributions from dry air and from water vapor.Although most of the atmospheric electrical path length is due to dry air, the variabi-lity in the path length is dominated by water-vapor variability.Using Eqs. 1 and 2, we calculated the atmospheric electrical path length at variouselevation angles from an ensemble of U. S. Weather Bureau radiosonde records. Fiftysummer and fifty winter records from Huntington, West Virginia, were used. Therefractive bending attributable to the atmosphere was included in the calculations, butthe effects of clouds were not. Brightness temperatures at selected frequencies, aswould be observed by microwave radiometers looking at the respective elevationangles, were also calculated from this ensemble of records. Inversions were thenperformed on this computed data to infer atmospheric electrical path lengths. Surfacevalues of temperature, pressure, and relative humidity were also included in the data.The same ensemble of records was used for performing the inversions as for computingstatistical correlations required by the inversion algorithm. The inferred path lengthwas compared with the path length calculated directly from the radiosonde data to deter-mine errors in the inferred value.Figure II-1 gives inversion error statistics for the summer and winter ensemble ofrecords. The rms error in the inversion results is plotted as a function of elevationangle, or equivalently, the total electrical path length increase of the atmosphere. Alsoshown is the a priori standard deviation in the path length. Inversion results are shown,with the use of surface meteorological data alone and surface meteorological data plusbrightness temperatures at frequencies near the 22. 235 GHz water-vapor and 60-GHzoxygen resonances as indicated in the figure. Calculations were performed with datacontaining simulated noise.QPR No. 91A PRIORI STANDARD DEVIATION INATMOSPHERIC ELECTRICAL PATHLENGTHINVERSION ERRORS USINGSURFACE METEOROLOGICALDATAUSING SURFACEINVERSION ERRORS USING SURFACEMETEOROLOGICAL DATA PLUS BRIGHTNESSTEMPERATURES AT 22, 24, 52.65 AND 53.60 GHzI I I I I90 30' 100 5oELEVATION ANGLEI I I I I I I I 1 1 I I100 cm 300 1000 3000ELECTRICAL PATH LENGTH(a)A PRIORI STANDARD DEVIATION INATMOSPHERIC ELECTRICAL PATH LENGTHINVERSION ERRORS USING SURFACEMETEOROLOGICAL DATA0-INVERSION ERRORS USINGSURFACE METEOROLOGICAL3 DATA PLUS BRIGHTNESSTEMPERATURES AT 22 AND24 GHzINVERSION ERRORS USING SURFACEMETEOROLOGICAL DATA PLUSBRIGHTNESS TEMPERATURES AT22, 24, 52.65 AND 53.60 GHzI I I I90 30' 10° 50 ELEVATION ANGLEI I I I I 1 I I I I I I100 300 1000 3000 cmELECTRICAL PATH LENGTH(b)Fig. II-1. Results of estimating atmospheric electrical path length increase.(a) Summer. (b) Winter.Gaussian noise of following rms value added to data before per-forming inversions:i. 2% for surface relative humidityii. 0. 1 'K for surface temperatureiii. 1 mb. for surface pressureiv. 0. 20 K for brightness temperatures.QPR No. 91(II. RADIO ASTRONOMY)The results of these calculations indicate that, neglecting clouds, atmosphericzenith electrical path length increase can be inferred with an accuracy of 1 or 2 cmfrom measurements of a two-channel microwave radiometer operating near the22. 235 GHz water-vapor resonance and surface measurements of meteorological quan-tities. For summer, this is approximately a factor of three better than could be donefrom a priori statistical information alone, and a factor of two better than could beobtained by only measuring surface meteorological quantities. Preliminary calculationsindicate that a typical cloud will add approximately 0. 5 cm to the expected rms errorin inferring zenith electrical path length from the radiometric data.L. W. Schaper, Jr., D. H. Staelin, J. W. WatersReferences1. J. W. Waters and D. H. Staelin, "Statistical Inversion of Radiometric Data,"Quarterly Progress Report No. 89, Research Laboratory of Electronics, M. I. T.,April 15, 1968, pp. 25-28.2. B. R. Bean and E. J. Dutton, Radio Meteorology, U.S. National Bureau ofStandards Monograph 92, March 1966.QPR No. 91

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