Atmospheric Phase Noise and Aperture Synthesis Imaging at Millimeter Wavelengths

Wright, M. C. H.

doi:10.1086/133757

Cited by 35 publications

(20 citation statements)

References 23 publications

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“…At higher frequencies, studies with 80 − 230 GHz (1 − 3 mm) are available with baseline lengths up to only about 1 km (Wright 1996;Asaki et al 1998;Matsushita & Chen 2010). These previous cm-/mm-wave studies displayed similar results for cases where the atmospheric phase fluctuation is caused by water vapor in the atmosphere.…”

Section: Introductionmentioning

confidence: 67%

ALMA Long Baseline Campaigns: Phase Characteristics of Atmosphere at Long Baselines in the Millimeter and Submillimeter Wavelengths

Matsushita

Asaki

Fomalont

et al. 2017

PASP

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We present millimeter-and submillimeter-wave phase characteristics measured between 2012 and 2014 of Atacama Large Millimeter/submillimeter Array (ALMA) long baseline campaigns. This paper presents the first detailed investigation of the characteristics of phase fluctuation and phase correction methods obtained with baseline lengths up to ∼ 15 km. The basic phase fluctuation characteristics can be expressed with the spatial structure function (SSF). Most of the SSFs show that the phase fluctuation increases as a function of baseline length, with a power-law slope of ∼ 0.6. In many cases, we find that the slope becomes shallower (average of ∼ 0.2 − 0.3) at baseline lengths longer than ∼ 1 km, namely showing a turn-over in SSF. These power law slopes do not change with the amount of precipitable water vapor (PWV), but the fitted constants have a weak correlation with PWV, so that the phase fluctuation at a baseline length of 10 km also increases as a function of PWV. The phase correction method using water vapor radiometers (WVRs) works well, especially for the cases where PWV > 1 mm, which reduces the degree of phase fluctuations by a factor of two in many cases. However, phase fluctuations still remain after the WVR phase correction, suggesting the existence of other turbulent constituent that cause the phase fluctuation. This is supported by occasional SSFs that do not exhibit any turn-over; these are only seen when the PWV is low (i.e., when the WVR phase correction works less effectively) or after WVR phase correction. This means that the phase fluctuation caused by this turbulent constituent is inherently smaller than that caused by water vapor. Since in these rare cases there is no turn-over in the SSF up to the maximum baseline length of ∼ 15 km, this turbulent constituent must have scale height of 10 km or more, and thus cannot be water vapor, whose scale height is around 1 km. Based on the characteristics, this large scale height turbulent constituent is likely to be water ice or a dry component. Excess path length fluctuation after the WVR phase correction at a baseline length of 10 km is large ( 200 µm), which is significant for high frequency (> 450 GHz or < 700 µm) observations. These results suggest the need for an additional phase correction method to reduce the degree of phase fluctuation, such as fast switching, in addition to the WVR phase correction. We simulated the fast switching phase correction method using observations of single quasars, and the result suggests that it works well, with shorter cycle times linearly improving the coherence.

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Section: Introductionmentioning

confidence: 67%

ALMA Long Baseline Campaigns: Phase Characteristics of Atmosphere at Long Baselines in the Millimeter and Submillimeter Wavelengths

Matsushita

Asaki

Fomalont

et al. 2017

PASP

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“…Figure 11, which shows a clear correlation of log c θ 2 with the groundlevel temperature, also suggests that increased AR fluctuations are in fact associated with increased convective activity near the ground. Figures 10 and 11 indicate that an increase in the humidity does not by itself lead to an increase in AR activity, as also discussed by Wright (1996) in regards of interferometer phase fluctuations, but that both are associated with larger (δQ) 2 / Q 2 fluctuations as a result of an increased tropospheric turbulence.…”

Section: Correlationsmentioning

confidence: 61%

“…3.5 for a discussion on the parameter C 2 n L). The Allan variance is another useful method to describe the atmopsheric phase fluctuations (see, e.g., Armstrong & Sramek 1982;Thompson et al 1986;Olmi & Downes 1992;Wright 1996). The Allan variance of the AR fluctuations removes linear drifts from the data and is defined as:…”

Section: Power Spectra and Allan Variancementioning

confidence: 99%

“…Several studies of the problem of phase fluctuations with both centimeter (e.g., Armstrong & Sramek 1982) and millimeter (e.g., Bieging et al 1984;Olmi & Downes 1992;Wright 1996) interferometers have led to the development of a number of radiometric devices to compensate for these fluctuations and restore the uncorrupted phase off-line (e.g., Bremer 1995;Marvel & Woody 1998).…”

Section: Introductionmentioning

confidence: 99%

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Systematic observations of anomalous refraction at millimeter wavelengths

Olmi

2001

A&A

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Abstract.It is well known that the water vapor in the troposphere plays a fundamental role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in near-infrared or optical regimes. As a consequence, phase fluctuations at frequencies higher than about 1 GHz are predominantly caused by fluctuations in the distribution of water vapor, and thus radio seeing at these frequencies is predominantly caused by tropospheric turbulence. Radio seeing shows up on filled-aperture telescopes as an anomalous refraction (AR), i.e. an apparent displacement of a radio source from its nominal position, corrected for large-scale refractive effects. The magnitude of this effect, as a fraction of the beam width, is bigger on larger telescopes and thus its impact on the pointing is likely to become critically important in the next generation of electrically large filledaperture radio telescopes (D/λ > 10 4 ) and in particular on the Large Millimeter Telescope. AR effects are expected to reduce the total effective observing time at the highest frequencies and will affect on-the-fly mapping. Here we present the results of systematic AR measurements carried out with the 13.7-m telescope of the Five College Radio Astronomy Observatory. The measured AR pointing errors range from 1 −3 (winter) to about 20 (summer) and most of the events last less than about 4 s. We analysed the structure function, power spectrum and Allan variance of the data and we have carried out a statistical analysis to identify correlations of the statistical functions with selected observing parameters such as precipitable water vapor, time of day, season and elevation angle. Our results suggest that uncompensated AR may be the most important dynamic environmental source of pointing errors on the new large radio telescopes (ALMA, GBT, LMT, SRT) and may guide the design of active AR-compensation devices and help allocating suitable observing time through dynamic scheduling.

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“…Table 13.4 shows a compilation of the measurements of the structure function referred to a baseline of 100 m. The range of values reported for a fiducial baseline The rms phase deviation range referred to a baseline of 100 m usually represents the span from the median condition at nighttime during winter to daytime during summer. (2013); (5) Wright (1996), see also Wright and Welch (1990), Bieging et al (1984); (6) Millenaar (2011b); (7) Kasuga et al (1986), see also Ishiguro et al (1990); (8) Sramek (1990), see also Sramek (1983), Carilli and Holdaway (1999), Armstrong and Sramek (1982); (9) Butler and Desai (1999); (10) Olmi and Downes (1992); (11) Kimberk et al (2012); (12) Masson (1994a); (13) Butler et al (2001).…”

Section: Site Testing By Direct Measurement Of Phase Stabilitymentioning

confidence: 99%

Propagation Effects: Neutral Medium

Thompson

Moran

Swenson

2017

Astronomy and Astrophysics Library

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The neutral gas in the atmosphere has a significant effect on signals passing through it. We are concerned with three types of effects. First, the large-scale structures in the media give rise to refractive effects. These effects, which can be analyzed in terms of geometrical optics and Fermat's principle, are the deflection of the radio waves and the change of the propagation velocity. Second, radiation can be absorbed. Finally, radiation can be scattered by the turbulent structure of the media. The phenomenon of scattering results in scintillation, or seeing.In the troposphere, water vapor plays a particularly important role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in the near-infrared or optical regimes. The phase fluctuations in radio interferometers at centimeter, millimeter, and submillimeter wavelengths are caused predominantly by fluctuations in the distribution of water vapor. Water vapor is poorly mixed in the troposphere, and the total column density of water vapor cannot be accurately sensed from surface meteorological measurements. Uncertainties in the water vapor content are a serious limitation to the accuracy of VLBI measurements. Small-scale (< 1 km) fluctuations in water vapor distribution limit the angular resolution of connected-element interferometers in the absence of wavefront correction techniques. Furthermore, spectral lines of water vapor cause substantial absorption at frequencies above 100 GHz and usually render the troposphere highly opaque at frequencies between 1 and 10 THz (300 and 30 m). Thus, any discussion of the neutral atmosphere must be primarily concerned with the effects of water vapor. Propagation in the neutral atmosphere from the point of view of radio communications is discussed by Crane (1981) and Bohlander et al. (1985).Our interest in the propagation media arises because the media degrade interferometric measurements of radio sources. Alternately, observations of radio sources can be used to probe the characteristics of the propagation media. Radio interferometric measurements have been used widely for this purpose.

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Atmospheric Phase Noise and Aperture Synthesis Imaging at Millimeter Wavelengths

Cited by 35 publications

References 23 publications

ALMA Long Baseline Campaigns: Phase Characteristics of Atmosphere at Long Baselines in the Millimeter and Submillimeter Wavelengths

ALMA Long Baseline Campaigns: Phase Characteristics of Atmosphere at Long Baselines in the Millimeter and Submillimeter Wavelengths

Systematic observations of anomalous refraction at millimeter wavelengths

Propagation Effects: Neutral Medium

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