Tropospheric phase calibration in millimeter interferometry

We review millimeter interferometric phase variations caused by variations in the precipitable water vapor content of the troposphere, and we discuss techniques proposed to correct for these variations. We present observations with the Very Large Array (VLA) at 22 and 43 GHz designed to test these techniques. We find that both the fast switching and paired array calibration techniques are effective at reducing tropospheric phase noise for radio interferometers. In both cases, the residual rms phase fluctuations after correction are independent of baseline length for b > beff. These techniques allow for diffraction-limited imaging of faint sources on arbitrarily long baselines at millimeter wavelengths. We consider the technique of tropospheric phase correction using a measurement of the precipitable water vapor content of the troposphere via a radiometric measurement of the brightness temperature of the atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz for the millimeter array (MMA) and to 120 mK for the VLA at 22 GHz. The minimum gain stability requirement is 200 at 185 GHz at the MMA, assuming that the astronomical receivers are used for radiometry. This increases to 2000 for an uncooled system. The stability requirement is 450 for the cooled system at the VLA at 22 GHz. To perform absolute radiometric phase corrections also requires knowledge of the tropospheric parameters and models to an accuracy of a few percent. It may be possible to perform an “empirically calibrated” radiometric phase correction, in which the relationship between fluctuations in brightness temperature differences and fluctuations in interferometric phases is calibrated by observing a strong celestial calibrator at regular intervals. A number of questions remain concerning this technique, including the following: (1) Over what timescale and distance will this technique allow for radiometric phase corrections when switching between the source and the calibrator? (2) How often will calibration of the TrmsB – Orms relationship be required?

[1]  V. I. Tatarskii.  Wave Propagation in Turbulent Medium , 1961 .

[2]  Martin Ryle,et al.  Atmospheric Limitations to the Angular Resolution of Aperture Synthesis Radio Telescopes , 1971 .

[3]  S. Keihm,et al.  A test of water vapor radiometer‐based troposphere calibration using very long baseline interferometry observations on a 21‐km baseline , 1996 .

[4]  D. C. Hogg,et al.  Measurement of excess radio transmission length on earth-space paths , 1981 .

[5]  J. Garratt.  The Atmospheric Boundary Layer , 1992 .

[6]  D. Staelin.  Measurements and interpretation of the microwave spectrum of the terrestrial atmosphere near 1‐centimeter wavelength , 1966 .

[7]  Yoshiharu Asaki,et al.  Phase compensation experiments with the paired antennas method , 1996 .

[8]  Duk-Gyoo Roh,et al.  Phase compensation experiments with the paired antennas method: 2. Millimeter‐wave fringe correction using centimeter‐wave reference , 1998 .

[9]  Gunnar Elgered,et al.  Geodesy by radio interferometry - Water vapor radiometry for estimation of the wet delay , 1991 .

[10]  Melvyn C. H. Wright.  ATMOSPHERIC PHASE NOISE AND APERTURE SYNTHESIS IMAGING AT MILLIMETER WAVELENGTHS , 1996 .

[11]  P. Napier,et al.  The Very Large Array. , 1980 .

[12]  David P. Woody,et al.  Phase correction at millimeter wavelengths using observations of water vapor at 22 GHz , 1998, Astronomical Telescopes and Instrumentation.

[13]  D. Wilner,et al.  Subarcsecond VLA Observations of HL Tauri: Imaging the Circumstellar Disk , 1996 .

[14]  Andrew I. Harris,et al.  Phase correction for the BIMA array: atmospherical model calculations for the design of a prototype correlation radiometer , 1998, Astronomical Telescopes and Instrumentation.

[15]  W. J. Welch,et al.  Astronomy with millimeter and submillimeter wave interferometry : IAU Colloquium no. 140, Meeting held 5-9 October 1992, Hakone, Japan , 1994 .

[16]  Henri Sauvageot.  Book Review: Atmospheric Remote Sensing by Microwave Radiometry. Edited by Michael A. Janssen. John Wiley, Chichester, 1993, 572 p. Price: £ 79.00. ISBN 0-47162891-3 , 1995 .

[17]  P. Rosenkranz.  Water vapor microwave continuum absorption: A comparison of measurements and models , 1998 .

[18]  B. L. Ulich,et al.  Recommendations for calibration of millimeter-wavelength spectral line data. , 1981 .

[19]  W. Moore.  Methods of Experimental Physics. , 1960 .

[20]  O. P. Lay.  Phase calibration and water vapor radiometry for millimeter-wave arrays , 1997 .

[21]  Kenneth J. Button,et al.  Editorial comment on the purpose of the newInternational journal of infrared and millimeter waves , 1980 .

[22]  A. M. Finkelstein,et al.  Tropospheric limitations in phase and frequency coordinate measurements in astronomy , 1979 .

[23]  C. Coulman,et al.  FUNDAMENTAL AND APPLIED ASPECTS OF ASTRONOMICAL "SEEING" , 1985 .

[24]  Robert N. Treuhaft,et al.  The effect of the dynamic wet troposphere on radio interferometric measurements , 1987 .

[25]  John E. Carlstrom,et al.  CSO-JCMT Interferometer and 183-GHz radiometric phase correction , 1998, Astronomical Telescopes and Instrumentation.

[26]  P. Napier,et al.  The very large array: Design and performance of a modern synthesis radio telescope , 1983, Proceedings of the IEEE.

[27]  Ralph G. Marson,et al.  Large convection cells as the source of Betelgeuse's extended atmosphere , 1998, Nature.

[28]  O. Lay.  The temporal power spectrum of atmospheric fluctuations due to water vapor , 1997 .

[29]  A. B. Vane,et al.  Atmospheric Absorption Measurements with a Microwave Radiometer , 1946 .

[30]  M. Janssen.  Atmospheric Remote Sensing by Microwave Radiometry , 1993 .

[31]  R. Hueckstaedt,et al.  Radiometric monitoring of atmospheric water vapor as it pertains to phase correction in millimeter interferometry , 1996 .

[32]  Hans J. Liebe,et al.  MPM—An atmospheric millimeter-wave propagation model , 1989 .