| 1. |
- Flygare, Jonas, 1988, et al.
(författare)
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Sensitivity simulation and measurement of the SKA Band 1 wideband feed package on MeerKAT
- 2019
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Ingår i: 13th European Conference on Antennas and Propagation, EuCAP 2019. ; EuCAP 2019
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Konferensbidrag (refereegranskat)abstract
- Advances in wideband feed technology for radio telescopes enable high sensitivity observations over large bandwidths. The wideband quad-ridge flared horn (QRFH) feed package for Band 1 of the Square Kilometre Array (SKA) was optimized for high sensitivity. The 3:1 feed package covers 350-1050 MHz and is a complete room temperature system with low-noise amplifiers integrated inside the ridges of the horn. The QRFH is dual-linear polarized and designed with spline-defined profiles for the horn and ridge shape. Measured feed s-parameters show input reflection less than -11 dB across the band with good port isolation. In this paper we present the first measured sensitivity levels of the Band 1 feed package, which was tested on the SKA precursor reflector MeerKAT. We also present measured aperture efficiency and intrinsic cross-polarization (IXR). The measured results show good agreement with simulations.
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| 2. |
- Bergman, Jan, et al.
(författare)
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DARTS - Distributed Aperture Radio Telescope in Space - First Starlight Explorer
- 2018
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Ingår i: 2018 2nd URSI Atlantic Radio Science Meeting, AT-RASC 2018. - : IEEE. - 9789082598735
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Konferensbidrag (refereegranskat)abstract
- SBERGMANURSIATDARTS1. However, despite the strong science case for the opening up of a brand-new window on the Universe at a low radio frequency that is largely unexplored, mission proposals have been significantly challenged with the lack of flight proven technologies, to implement a space telescope using low cost small spacecraft formations. In recent years, the advent of CubeSat technologies, interplanetary missions and radio frequency inter-satellite link (ISL) technology, have improved to the point that developing a mission where a space radio telescope is formed using a constellation of small satellites is now viable, to bring unique science offerings.
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| 3. |
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| 4. |
- Carozzi, Tobia, 1966, et al.
(författare)
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Enhancing SKA Band 1 Polarimetry by Using Two Different Feed Rotations
- 2018
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Ingår i: 2018 2nd URSI Atlantic Radio Science Meeting, AT-RASC 2018. - 9789082598735
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Konferensbidrag (refereegranskat)abstract
- We investigate the possibility of enhancing the polarimetric performance of the Square Kilometre Array (SKA) telescope's band 1 by mounting the feeds with two different rotations relative the dish. One rotation aligns the two polarizations along the horizontal and vertical directions and the other rotation is 45 degrees to the horizontal, vertical directions. Taken together, this configuration amounts to introducing a simple and cheap type of polarization diversity. We find that such a configuration can improve polarimetry. This is important since the offset gregorian dish design employed for SKA1-mid tends to produce beam-shapes that are not rotationally symmetric, and thus polarimetry tends to vary azimuthally.
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| 5. |
- Carozzi, Tobia, 1966
(författare)
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Imaging on a sphere with interferometers: The spherical wave harmonic transform
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society: Letters. - : Oxford University Press (OUP). - 1745-3925 .- 1745-3933. ; 451:1, s. L6-L10
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Tidskriftsartikel (refereegranskat)abstract
- I present an exact and explicit solution to the scalar (Stokes flux intensity) radio interferometer imaging equation on a spherical surface which is valid also for non-coplanar interferometer configurations. This imaging equation is comparable to ω-term imaging algorithms, but by using a spherical rather than a Cartesian formulation this term has no special significance. The solution presented also allows direct identification of the scalar (spin 0 weighted) spherical harmonics on the sky. The method should be of interest for future multispacecraft interferometers, wide-field imaging with non-coplanar arrays, and cosmic microwave background spherical harmonic measurements using interferometers.
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| 6. |
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| 7. |
- Carozzi, Tobia, 1966
(författare)
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Intrinsic cross-polarization ratio (IXR) for antenna arrays and improving polarimetry via polarization diversity
- 2015
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Ingår i: Proceedings of the 2015 International Conference on Electromagnetics in Advanced Applications, ICEAA 2015. ; , s. 630-633
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Konferensbidrag (refereegranskat)abstract
- I extend the concept of the intrinsic cross-polarization ratio (IXR) to an array of an arbitrary number of antenna elements. IXR is a figure-of-merit for the polarimetric performance of dual-polarized antenna elements, but it is easily generalized to more than two antennas by considering the effective dual-polarized response of polarimetrically beam-formed array. I also show that for a sparse array (i.e. arrays in which mutual-coupling can be ignored) of identical dual-polarized elements all with the same alignment, it is not possible to make the array IXR better than the IXR of dual-polarized element through beam-forming. However, such arrays can be improved by physically rotating the dual-polarized elements such that their alignments are not all the same. An array in which not all the dual-polarized elements do not all have the same alignment is a form of polarization diversity. I argue that polarization diversity is way to improve the polarimetric performance of arrays.
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| 8. |
- Carozzi, Tobia, 1966
(författare)
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New definition for dual-polarized antenna sensitivity
- 2019
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Ingår i: Proceedings of the 2019 21st International Conference on Electromagnetics in Advanced Applications, ICEAA 2019. ; , s. 979-981
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Konferensbidrag (refereegranskat)abstract
- Sensitivity is arguably the single most important defining characteristic of a radio telescope. This is because it sets the threshold for what sources can reasonably be detected. Intuitively, sensitivity is the noise level of an antenna referenced to measurements on the sky. So the lower it is, the easier one would expect to make detections of weak sources, and from this it would make sense to call it loosely the 'detection power' of telescopes. But this sensitivity concept does not take into account the polarization of the source or the polarization of the antenna itself, and could thus lead to problems. The traditionally used definition of sensitivity is namely based on single antennas. This antenna based sensitivity is derived assuming that the two polarized antennas that make up radio polarimeters are orthogonal and have identical amplitude gains. In practice these dual-polarized antennas will have different gains and exhibit polarization leakage due to nonorthogonality. To address these more realistic dual-polarized antennas, I introduce a new definition of dual-polarized (or full-polarization) antenna sensitivity. This new definition of sensitivity is a polarimetric generalization of the scalar, system equivalent flux density (SEFD). The new definition generalizes the total sensitivity, and also provides a quantity that represents sensitivity to purely polarized flux. Based on the new SEFD definition, I find that the intuitive notion that identical and orthogonal antennas should typically have better sensitivity than dissimilar, leaky antennas.
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| 9. |
- Carozzi, Tobia, 1966
(författare)
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Simple estimation of all-sky, direction-dependent Jones matrix of primary beams of radio interferometers
- 2016
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Ingår i: Astronomy and Computing. - : Elsevier BV. - 2213-1337. ; 16, s. 185-188
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Tidskriftsartikel (refereegranskat)abstract
- The dual-polarized primary beams of imaging radio telescopes are generally not perfectly orthogonal nor have the same gain, the resulting polarimetric images are distorted, in other words, they exhibit instrumental polarization. This is often modeled in radio astronomy using a Jones matrix formalism, and the standard practice is to calibrate (i.e. determine the Jones matrices) using known point source, polarized calibrators. Using point source calibrators on the other hand can be difficult and is ineffective for wide fields-of-view (FoV). Often however, a large portion of the FoV of imaging radio telescopes consists of unpolarized background. In this paper I estimate Jones matrices over the FoV by assuming that most of the background image is unpolarized and then taking the square-root of the brightness matrices. Results from LOFAR LBA data are shown and are consistent with expectation. The usefulness of this particular method, compared to using polarized point source calibrators, is its simplicity and the fact that it can cover most of the FoV.
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| 10. |
- Creaner, O., et al.
(författare)
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beamModelTester: Software framework for testing radio telescope beams
- 2019
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Ingår i: Astronomy and Computing. - : Elsevier BV. - 2213-1337. ; 28
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Tidskriftsartikel (refereegranskat)abstract
- The flux, polarimetric and spectral response of phased array radio telescopes with no moving parts such as LOFAR is known to vary considerably with orientation of the source to the receivers. Calibration models exist for this dependency such as those that are used in the LOFAR pipeline. Presented here is a system for comparing the predicted outputs from any given model with the results of an observation. In this paper, a sample observation of a bright source, Cassiopeia A, is used to demonstrate the software in operation, by providing an observation and a model of that observation which can be compared with one another. The package presented here is flexible to allow it to be used with other models and sources. The system operates by first calculating the predictions of the model and the results of an observation of linear fluxes and Stokes parameters separately. The model and observed values are then joined using the variables common to both, time and frequency. Normalisation and RFI excision are carried out and the differences between the prediction and the observation are calculated. A wide selection of 2-, 3- and 4-dimensional plots is generated to illustrate the dependence of the model and the observation as well as the difference between them on independent parameters time, frequency, altitude and azimuth. Thus, beamModelTester provides a framework by which it is possible to calibrate and propose refinements to models and to compare models with one another.
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