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The next-generation space VLBI mission, VSOP-2, is expected to provide unprecedented spatial resolution at 8.4, 22, and 43 GHz. In this report, phase referencing with VSOP-2 is examined in detail based on a simulation tool called ARIS. The criterion for successful phase referencing was set to keep the phase errors below one radian. Simulations with ARIS reveal that phase referencing achieves good performance at 8.4 GHz, even under poor tropospheric conditions. At 22 and 43 GHz, it is recommended to conduct phase referencing observations under good or typical tropospheric conditions. The satellite is required to have an attitude-switching capability with a one-minute or shorter cycle, and an orbit determination accuracy higher than $\sim$10 cm at apogee; the phase referencing calibrators are required to have a signal-to-noise ratio larger than four for a single scan. The probability to find a suitable phase referencing calibrator was estimated by using VLBI surveys. From the viewpoint of calibrator availability, VSOP-2 phase referencing at 8.4 GHz is promising. However, the chance of finding suitable calibrators at 22 and 43 GHz is significantly reduced; it is important to conduct specific investigations for each target at those frequencies.

We present and discuss the results of 1.6 GHz and 5 GHz test polarization observations obtained with HALCA, the VLBA, and phased VLA, specifically to evaluate the feasibility of calibrating and imaging HALCA polarization data. Our analysis provided the first demonstration that centimeter-wavelength polarization imaging using HALCA is technically viable for sources with sufficiently high correlated polarized flux densities, despite a number of factors which conspire to complicate the calibration, including relatively low sensitivity, inability to observe calibrators, lack of varying parallactic angle, and absence of redundant measurements on space baselines. The test data described here are limited and cannot rule out instrumental variability, but indicate that the HALCA instrumental polarization leakage is ∼ 2−5% at 1.6 GHz and ∼ 9−11% at 5 GHz. The prospects for effective polarization calibration on future orbiting VLBI missions are encouraging, especially since improvements in sensitivity are expected.

The VSOP mission is a Japanese-led project to image radio sources with sub-milliarcsec resolution by correlating the signal from the orbiting 8-m telescope, HALCA, with a global array of telescopes. Twenty-five percent of the scientific time of this mission is devoted to a survey of 402 bright, small-diameter extra-galactic radio sources at 5 GHz. The major goals of the VSOP Survey are statistical in nature: to determine the brightness temperature and approximate structure; to provide a source list for use with future space VLBI missions; and to compare radio properties with other data throughout the EM spectrum. This paper describes: the compilation of a complete list of radio sources associated with active galactic nuclei (AGN); the selection of the subsample of sources to be observed with VSOP; the extensive ground resources used for the Survey; the status of the observations as of 2000 July; the data-analysis methods; and several examples of results from the VSOP Survey. More detailed results from the full sample will be given in future papers.

The radio astronomy satellite HALCA was launched by the Institute of Space and Astronautical Science in 1997 February to participate in Very Long Baseline Interferometry (VLBI) observations with arrays of ground radio telescopes. HALCA is the main element of the VLBI Space Observatory Programme (VSOP), a complex international endeavor involving over 25 ground radio telescopes, five tracking stations and three correlators. Simultaneous observations with HALCA's 8 meter diameter radio telescope and ground radio telescopes synthesize a radio telescope over twice the size of the Earth, enabling the highest resolution 1.6 GHz and 5 GHz images to be made.

A new technique has been developed to remove the ionosphere's distorting effects from low-frequency VLBI data. By fitting dispersive and nondispersive components to the phases of multifrequency data, the ionosphere can be effectively removed from the data without the use of a priori calibration information. This technique, along with the new gating capability of the VLBA correlator, was used to perform accurate astrometry on pulsar B0950+08, resulting in a much improved measurement of this pulsar's proper motion (ma = -1.6 +- 0.4 mas yr-1, md = 29.5 +- 0.5 mas yr-1) and parallax (p = 3.6 +- 0.3 mas). This puts the pulsar at a distance of 280 +- 25 pc, about twice as far as previous estimates, but in good agreement with models of the electron density in the Local Bubble.

We have used the Very Large Array (VLA) of the National Radio Astronomy Observatory at 8.44 GHz to image a ~40 arcmin2 field with an uprecedented rms sensitivity of 1.5 mJy. After correcting for the effects of discrete foreground radio sources, we examined this most sensitive microwave image of the sky for fluctuations in the cosmic microwave background radiation (CBR). At the 6'' resolution of our VLA map, DT/T [?] (0.7 +- 0.8) x 10-4, with an upper limit of 1.3 x 10-4 at 95% confidence. At 1' resolution, we measure a fluctuation amplitude of DT/T = (1.2 +- 1.4) x 10-5. We also report on our observations of the linear and circular polarization of the CBR for which we derive upper limits (at 95% confidence) of DT/T [?] 1.1 x 10-5 and DT/T [?] 1.6 x 10-5, respectively, on an angular scale of 1'.

We characterize the accuracy of linear-polarization mosaics made using the Atacama Large Millimeter/submillimeter Array (ALMA). First, we observed the bright, highly linearly polarized blazar 3C 279 at Bands 3, 5, 6, and 7 (3 mm, 1.6 mm, 1.3 mm, and 0.87 mm, respectively). At each band, we measured the blazar's polarization on an 11 × 11 grid of evenly spaced offset pointings covering the full-width at half-maximum (FWHM) area of the primary beam. After applying calibration solutions derived from the on-axis pointing of 3C 279 to all of the on- and off-axis data, we find that the residual polarization errors across the primary beam are similar at all frequencies: the residual errors in linear polarization fraction Pfracand polarization position angle ?are ?0.001 (?0.1% of Stokes I) and ? 1° near the center of the primary beam; the errors increase to ?0.003-0.005 (?0.3%-0.5% of Stokes I) and ?1°-5° near the FWHM as a result of the asymmetric beam patterns in the (linearly polarized) Qand Umaps. We see the expected double-lobed "beam squint" pattern in the circular polarization (Stokes V) maps. Second, to test the polarization accuracy in a typical ALMA project, we performed observations of continuum linear polarization toward the Kleinmann-Low nebula in Orion (Orion-KL) using several mosaic patterns at Bands 3 and 6. We show that after mosaicking, the residual off-axis errors decrease as a result of overlapping multiple pointings. Finally, we compare the ALMA mosaics with an archival 1.3 mm Combined Array for Research in Millimeter-wave Astronomy polarization mosaic of Orion-KL and find good consistency in the polarization patterns.

We present millimeter- and submillimeter-wave phase characteristics measured between 2012 and 2014 of Atacama Large Millimeter/submillimeter Array 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 , 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 (), which is significant for high frequency (or ) 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.