Phase Self-calibration Research Articles

A Phase-Modulation Phase-Shifting Phased-Array Transmitter With Phase Self-Calibration and Deep PBOs Efficiency Enhancement

A Phase-Modulation Phase-Shifting Phased-Array Transmitter With Phase Self-Calibration and Deep PBOs Efficiency Enhancement

First GMVA observations with the upgraded NOEMA facility: VLBI imaging of BL Lacertae in a flaring state

We analyze a single-epoch Global mm-VLBI Array (GMVA) observation of the blazar BL\,Lacertae (BL\,Lac) at 86\,GHz from April 2021. The participation of the upgraded, phased Northern Extended Millimetre Array (NOEMA) adds additional sensitivity to the GMVA, which has facilitated the imaging of BL\,Lac during an unprecedentedly strong gamma -ray flare. We aim to explore the nature of the inner subparsec jet of BL\,Lac and the impact of the NOEMA participation in the observation. For the data reduction, we employed two advanced automatic pipelines: rPICARD for the flux density calibration as well as the model-agnostic signal stabilization and GPCAL for the antenna leakage calibration. The conventional hybrid imaging (CLEAN\,$+$\,amplitude and phase self-calibration) was applied to the calibrated visibilities to generate final VLBI images. We performed a ridge-line analysis and Gaussian model-fits on the final jet image to derive the jet parameters. In our data, the presence of NOEMA improves the image sensitivity by a factor of 2.5. The jet shows a clear wiggling structure within 0.4\,mas from the core. Our ridge-line analysis suggests the presence of a helical jet structure (i.e., a sinusoidal pattern). Six circular Gaussian components were fitted to the inner jet region. We estimated an apparent brightness temperature of sim 3\,times \,1012\,K in the two innermost components. They are likely to be highly boosted by relativistic beaming effect. We find four significant polarized knots in the jet. Interestingly, two of them are located in the core region. Finally, we suggest a number of physical scenarios to interpret our results.

Absolute Phase of the Radio Frequency Transfer over Optical Fiber with Phase Self-calibration

Absolute Phase of the Radio Frequency Transfer over Optical Fiber with Phase Self-calibration

Pragmatic Approach to Phase Self-Calibration for Planar Array Millimeter-Wave MIMO Imaging

To reduce the adverse effects of propagation delay uncertainty caused phase error, a pragmatic self-calibration approach for millimeter-wave multiple-in multiple-out (MIMO) imaging is proposed and demonstrated. Based on the analysis of the contributions of delay uncertainty and the assumption of identical channel design, the direct coupling between array elements is used to estimate the hardware-caused inherent channel delay, requiring no additional reference aid. Of the contributions to the delay, the common part and the part due to the interchannel difference are estimated separately. Combined with the array geometrical configuration, the direct coupling reflections are averaged to find the common channel delay, and the interchannel differences are estimated in a reduced geometric average scheme. The delay uncertainty is compensated by performing a phase shift operation to the received echoes for each channel. In addition, the proposed method also allows the accommodation of the slow change of the channel inherent delay automatically and constantly. Numerical analysis and experimental tests demonstrate the projected performance improvement of the proposed scheme as an alternative approach, which can also serve as a benchmark or booster for reference aided phase calibration methods.

Interferometric Imaging Directly with Closure Phases and Closure Amplitudes

Abstract Interferometric imaging now achieves angular resolutions as fine as ∼10 μas, probing scales that are inaccessible to single telescopes. Traditional synthesis imaging methods require calibrated visibilities; however, interferometric calibration is challenging, especially at high frequencies. Nevertheless, most studies present only a single image of their data after a process of “self-calibration,” an iterative procedure where the initial image and calibration assumptions can significantly influence the final image. We present a method for efficient interferometric imaging directly using only closure amplitudes and closure phases, which are immune to station-based calibration errors. Closure-only imaging provides results that are as noncommittal as possible and allows for reconstructing an image independently from separate amplitude and phase self-calibration. While closure-only imaging eliminates some image information (e.g., the total image flux density and the image centroid), this information can be recovered through a small number of additional constraints. We demonstrate that closure-only imaging can produce high-fidelity results, even for sparse arrays such as the Event Horizon Telescope, and that the resulting images are independent of the level of systematic amplitude error. We apply closure imaging to VLBA and ALMA data and show that it is capable of matching or exceeding the performance of traditional self-calibration and CLEAN for these data sets.

LCoS display phase self-calibration method based on diffractive lens schemes

An experimental method to calibrate Liquid Crystal on Silicon (LCoS) displays by self-generating lens configurations on the studied device is proposed in this paper. On the one hand, a split-lens is displayed in the LCoS to self-generate an interference pattern from which the phase-voltage curve of the modulator is calculated. On the other hand, a microlens array is displayed on the LCoS, within a same experimental set-up, to implement a Shack-Hartmann (S-H) wavefront sensor, from which the display surface profile is retrieved. Specifically, by means of a feasible set-up, the proposed method allows measuring the deviation from flatness of the LCoS displays as well as to determine the phase-voltage response of phase-only SLMs. Experimental results demonstrate a linear tendency phase-voltage curve that ranges from 0 rad up to ∼6.28 rad, for the used light wavelength. Moreover, by extracting the LCoS phase distribution measured with the S-H configuration, the LCoS surface inhomogeneity is corrected by 95%.

Identifiability Analysis of Local Oscillator Phase Self-Calibration Based on Hybrid Cramér–Rao Bound in MIMO Radar

The identifiability of self-calibration of local oscillator phase perturbation in multiple-input multiple-output (MIMO) radar is analyzed in this paper. Four cases are considered: the absolute phase perturbation (APP) and phase perturbation differences (PPDs) in MIMO radar with widely separated antennas (SMIMOR), and the APP and PPDs in co-located MIMO radar (CMIMOR). The tightness of the hybrid Cramér-Rao bound (HCRB) is proved in all four cases. It is interesting that the problem in this paper is a special case in which HCRB is asymptotically tight for nonlinear observation models containing unknown deterministic and random parameters. For each case, a signal model for multiple stationary targets is constructed. The closed-form HCRB of phase perturbation is derived and used to analyze the identifiability of phase perturbation. It is proven that only the PPDs in CMIMOR in the above-mentioned four cases are identifiable. Numerical simulations verify the conclusion.

Spurious source generation in mapping from noisy phase-self-calibrated data

Phase self-calibration (or selfcal) is an algorithm often used in the calibration of interferometric observations in astronomy. Although a powerful tool, this algorithm presents strong limitations when applied to data with a low signal-to-noise ratio. We analyze the artifacts that the phase selfcal algorithm produces when applied to extremely noisy data. We show how the phase selfcal may generate a spurious source in the sky from a distribution of completely random visibilities. This spurious source is indistinguishable from a real one. We numerically and analytically compute the relationship between the maximum spurious flux density generated by selfcal from noise and the particulars of the interferometric observations. Finally, we present two simple tests that can be applied to interferometric data for checking whether a source detection is real or whether the source is an artifact of the phase self-calibration algorithm.

Phase-Reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138

Abstract We describe a phase-reference VLBI observation that was conducted at 15.4 GHz by fast-switching VLBA antennas between the compact steep-spectrum radio source 3C 138 and the quasar PKS 0528$+$134, which are about $4^\circ$ away on the sky. By comparing the phase-reference mapping with the conventional hybrid mapping, we demonstrate the feasibility of high-precision astrometric measurements for sources separated by $4^\circ$. VLBI phase-reference mapping preserves the relative phase information, and thus provides an accurate relative position between 3C 138 and PKS 0528$+$134 of $\Delta\alpha =- {9{}^{\mathrm {m}}46\rlap {.}{}^{\mathrm {s}}531000} \pm {0\rlap {.}{}^{\mathrm {s}}000003}$ and $\Delta\delta = {3{}^{\mathrm {\circ }}6{}^{\mathrm {\prime }}26\rlap {.}{}^{mathrm {\prime }}90311} \pm {0\rlap {.}{}^{\mathrm {\prime\prime}}00007}$ (J2000.0) in right ascension and declination, respectively. This gives an improved position of the nucleus (component A) of 3C 138 in J2000.0 to be $\mathrm{RA} = {05{}^{\mathrm {h}}21{}^{\mathrm {m}}09\rlap {.}{}^{\mathrm {s}}885748}$ and $\mathrm{Dec} = {16{}^{\mathrm {\circ }}38{}^{\mathrm {\prime }}22\rlap {.}{}^{\mathrm {\prime \prime }}05261}$ under the assumption that the position of calibrator PKS 0528$+$134 is correct. We further made a hybrid map by performing several iterations of CLEAN and self-calibration on the phase-referenced data with the phase-reference map as an input model for the first phase self-calibration. Compared with the hybrid map from the limited visibility data directly obtained from fringe fitting 3C 138 data, this map has a similar dynamic range, but a higher angular resolution. Therefore, the phase-reference technique is not only a means of phase connection, but also a means of increasing the phase coherence time, and thus allowing the self-calibration technique to be applied to much weaker sources.

The cores of lobe-dominant quasars

view Abstract Citations (18) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Cores of Lobe-dominant Quasars Antonucci, Robert R. J. ; Barvainis, Richard Abstract Lobe-dominant radio quasars have quiescent infrared/optical spectra similar to those of radio-quiet quasars. The radio cores are thought to emit by the synchrotron mechanism, but the emission mechanisms in the infrared and optical are unknown. The IR could be the same component as the radio core, or it could be a separate component entirely. In the latter case the IR must arise from a different mechanism or from a separate location. We have tested this by observing the cores of all six lobe-dominant quasars detected by IRAS, at 6 cm, 2 cm, and 1.3 cm. We have used a compact VLA array, and phase self-calibration, to ensure that phase incoherence has not reduced our flux measurements. In three cases the radio core spectrum underpredicts the IR, showing that they are separate components. This disfavors the synchrotron hypothesis for the IR unless it can be argued that the radio emission arises in a different location. Beaming models do in fact suggest that the radio may arise slightly off-nucleus, in the base of a relativistic jet directed away from the observer. Publication: The Astrophysical Journal Pub Date: February 1988 DOI: 10.1086/185101 Bibcode: 1988ApJ...325L..21A Keywords: Extragalactic Radio Sources; Infrared Sources (Astronomy); Quasars; Radio Jets (Astronomy); Synchrotron Radiation; Infrared Astronomy Satellite; Infrared Spectra; Very Large Array (Vla); Visible Spectrum; Astrophysics; GALAXIES: JETS; QUASARS; RADIO SOURCES: GALAXIES full text sources ADS | data products SIMBAD (6) NED (6)