The Atacama Large Millimeter/submillimeter Array (ALMA) is the world's largest millimeter/submillimeter telescope and provides unprecedented sensitivities and spatial resolutions. To achieve the highest imaging capabilities, interferometric phase calibration for the long baselines is one of the most important subjects: The longer the baselines, the worse the phase stability becomes because of turbulent motions of the Earth's atmosphere, especially, the water vapor in the troposphere. To overcome this subject, ALMA adopts a phase correction scheme using a Water Vapor Radiometer (WVR) to estimate the amount of water vapor content along the antenna line of sight. An additional technique is phase referencing, in which a science target and a nearby calibrator are observed by turn by quickly changing the antenna pointing. We conducted feasibility studies of the hybrid technique with the WVR phase correction and the antenna Fast Switching (FS) phase referencing (WVR+FS phase correction) for the ALMA 16 km longest baselines in cases that (1) the same observing frequency both for a target and calibrator is used, and (2) higher and lower frequencies for a target and calibrator, respectively, with a typical switching cycle time of 20 s. It was found that the phase correction performance of the hybrid technique is promising where a nearby calibrator is located within roughly 3◦ from a science target, and that the phase correction with 20 s switching cycle time significantly improves the performance with the above separation angle criterion comparing to the 120 s switching cycle time. The currently trial phase calibration method shows the same performance independent of the observing frequencies. This result is especially important for the higher frequency observations because it becomes difficult to find a bright calibrator close to an arbitrary sky position. In the series of our experiments, it is also found that phase errors affecting the image quality come from not only the water vapor content in the lower troposphere but also a large structure of the atmosphere with a typical cell scale of a few tens of kilometers.
Atacama Large Millimeter/submillimeter Array (ALMA) is the world’s largest millimeter/ submillimeter (mm / Submm) interferometer. Along with science observations, ALMA has performed several long baseline campaigns in the last 6 years to characterize and optimize its long baseline capabilities. To achieve full long baseline capability of ALMA, it is important to understand the characteristics of atmospheric phase fluctuation at long baselines, since it is believed to be the main cause of mm/submm image degradation. For the first time, we present detailed properties of atmospheric phase fluctuation at mm/submm wavelength from baselines up to 15 km in length. Atmospheric phase fluctuation increases as a function of baseline length with a power-law slope close to 0.6, and many of the data display a shallower slope (02.-03) at baseline length greater than about 15 km. Some of the data, on the other hand, show a single slope up to the maximum baseline length of around 15 km. The phase correction method based on water vapor radiometers (WVRs) works well, especially for cases with precipitable water vapor (PWV) greater than 1 mm, typically yielding a 50% decrease or more in the degree of phase fluctuation. However, signicant amount of atmospheric phase fluctuation still remains after the WVR phase correction: about 200 micron in rms excess path length (rms phase fluctuation in unit of length) even at PWV less than 1 mm. This result suggests the existence of other non-water-vapor sources of phase fluctuation. and emphasizes the need for additional phase correction methods, such as band-to-band and/or fast switching.
We present the phase characteristics study of the Atacama Large Millimeter / submillimeter Array (ALMA) long (up to 3 km) baseline, which is the longest baseline tested so far using ALMA. The data consist of long time-scale (10 20 minutes) measurements on a strong point source (i.e., bright quasar) at various frequency bands (bands 3, 6, and 7, which correspond to the frequencies of about 88 GHz, 232 GHz, and 336 GHz) Water vapor radiometer (WVR) phase correction works well even at long baselines, and the efficiency is better at higher PWV (< 1mm) condition, consistent with the past studies. We calculate the spatial structure function of phase fluctuation, and display that the phase fluctuation (i.e., rms phase) increases as a function of baseline length, and some data sets show turn-over around several hundred meters to km and being almost constant at longer baselines. This is the first millimeter / submillimeter structure function at this long baseline length, and to show the turn-over of the structure function. Furthermore, the observation of the turn-over indicates that even if the ALMA baseline length extends to the planned longest baseline of 15 km, fringes will be detected at a similar rms phase fluctuation as that at a few km baseline lengths. We also calculate the coherence time using the 3 km baseline data, and the results indicate that the coherence time for band 3 is longer than 400 seconds in most of the data (both in the raw and WVR-corrected data) For bands 6 and 7, WVR-corrected data have about twice longer coherence time, but it is better to use fast switching method to avoid the coherence loss.
We present results of feasibility studies of Atacama Large Millimeter/submillimeter Array (ALMA) interferom-
eter phase calibration scheme combined with the Fast Switching (FS) phase referencing and the Water Vapor
Radiometer (WVR) phase correction (FS+WVR phase correction). With FS scheme, ALMA antennas observe
a scientific target source and a nearby calibrator by turn very quickly. Because interferometer phase errors of the
target due to the water vapor contents commonly exist in those of the calibrator, the target phase is corrected
with the calibrator phase. We have demonstrated the FS+WVR phase correction for ALMA with baselines up to
2.7 km for various switching cycle times and separations between sources. For instance, in the case of sources with
the 1° separation, root-mean-square phases of the target were reduced from 300 to 40 microns in path length for
1 km baselines, and the target interferometer phases could be stabilized to an ALMA specification requirement
level for the interferometer phase stability. We also analytically evaluated the root-mean-square phase corrected
with the FS+WVR phase correction to predict the performance as a function of the separation and switching
In Atacama Large Millimeter/submillimeter Array (ALMA) commissioning and science verification we have
conducted a series of experiments of a novel phase calibration scheme for Atacama Compact Array (ACA). In
this scheme water vapor radiometers (WVRs) devoted to measurements of tropospheric water vapor content
are attached to ACA’s four total-power array (TP Array) antennas surrounding the 7 m dish interferometer
array (7 m Array). The excess path length (EPL) due to the water vapor variations aloft is fitted to a simple
two-dimensional slope using WVR measurements. Interferometric phase fluctuations for each baseline of the
7 m Array are obtained from differences of EPL inferred from the two-dimensional slope and subtracted from
the interferometric phases. In the experiments we used nine ALMA 12-m antennas. Eight of them were closely
located in a 70-m square region, forming a compact array like ACA. We supposed the most four outsiders to be
the TP Array while the inner 4 antennas were supposed to be the 7 m Array, so that this phase correction scheme
(planar-fit) was tested and compared with the WVR phase correction. We estimated residual root-mean-square
(RMS) phases for 17- to 41-m baselines after the planar-fit phase correction, and found that this scheme reduces
the RMS phase to a 70 – 90 % level. The planar-fit phase correction was proved to be promising for ACA, and
how high or low PWV this scheme effectively works in ACA is an important item to be clarified.
The first Space-VLBI mission, VSOP, started successfully with the launch of the dedicated space-VLBI satellite HALCA in 1997. The
mission has been in scientific operation in the 1.6 GHz and 5 GHz bands, and studies have been done mainly of the jet phenomena related to active galactic nuclei. Observing at higher frequencies has the advantage of less absorption through the ambient plasma and less
contribution from scattering, and also has the merit of resulting in higher angular resolution observations. A second generation space-VLBI mission, VSOP-2, has been planned by the working group formed at ISAS/JAXA with many collaborators. The spacecraft is planned to observe in the 8, 22 and 43 GHz bands with cooled receivers for the two higher bands, and with a maximum angular resolution at 43 GHz
(7 mm) of about 40 micro-arcseconds. The system design, including the spacecraft and ground facilities, will be introduced, and the impact for sub-mm space-VLBI further into the future will be discussed.
Tropospheric phase fluctuation due to the water vapor content is one of difficult problems which degrades imaging performances of radio interferometry. One of the potential solutions is differential radiometry observations to measure the differential water vapor content along the line of sights. We developed a 22-GHz-line radiometer to be mounted on a ground data-link antenna which supplies the timing reference signal for a space VLBI satellite, HALCA. This system will allow us to compare directly the atmospheric phase fluctuation with the water vapor content along a single line of sight measured by the radiometer.
The Institute of Space and Astronautical Science (ISAS) launched the first space VLBI (Very Long Baseline Interferometry) satellite, HALCA, in February 1997. After completing a series of engineering experiments to verify space-VLBI observations, the first VLBI fringes and images were obtained in May and in June, respectively. HALCA has now been operated for science observations at 1.6 and 5 GHz for the VSOP (VLBI Space Observatory Programme) project in cooperation with many organizations and radio telescopes around the world. In this paper the current science activities of the mission are reviewed and results presented.