The low-frequency radio telescope of the Square Kilometre Array (SKA-Low), currently under construction in the remote Murchison shire in the Western Australia's outback, will observe the sky between 50 MHz and 350 MHz with unprecedented sensitivity and stringent requirements for polarization accuracy. In this work, we investigate the instrumental polarization purity of a SKA-Low prototype station by means of the Intrinsic Cross-Polarization Ratio (IXR) figure of merit. We derive all-sky experimental IXR maps using data from the Aperture Array Verification System 2 (AAVS2). The results are presented at three frequencies within the SKA-Low bandwidth (110, 160, and 230 MHz) with a quantitative comparison between observed and simulated all-sky IXR maps. Our findings show good agreement in IXR map distributions and promising consistency in their radial profiles, meeting SKA-Low's IXR specification overall. This study offers an empirical approach to verifying SKA-Low's polarization performance using all-sky observations from individual stations and will potentially support the telescope's early science commissioning phase.

Radio over Fiber (RoF) constitutes the appropriate technology when the connections to the Remote Processing Facility of Large Radioastronomic Antenna Arrays have to be realized, particularly when the habitat exhibits extreme environmental conditions combined with a lack of infrastructures.Within such context, part of the electronics of the receiving chain must necessarily be located outdoors, and fluctuations of the link gain then are likely to appear, which in turn risk to impair the operations of beamforming referred to the weak sky signals.In the present work, some mitigation techniques aiming to minimize these fluctuations will be illustrated, which have been already adopted, or are under consideration for implementation, within the Low Frequency radiotelescope of the Square Kilometre Array (SKA-Low).

The Low-Frequency Array, or LOFAR, is the world's largest low-frequency radio telescope consisting of over 100 000 antenna elements spread across more than 50 stations throughout Europe. LOFAR2.0 is an upgrade of LOFAR which will significantly improve its sensitivity and overcome several limitations encountered during the last 10 years of operation. The digital beamformers form the core of each LOFAR station. They are called antenna processing sub-racks (APS), where all antenna signals are digitized and digitally processed to form beams on the sky. These beamformers have stringent performance requirements, such as high linearity due to strong radio-frequency interference, good timing for high beamformer efficiency, and very low common-noise and cross- talk to be sky-noise limited over long integration times. The designs of the new LOFAR2.0 beamformer are presented, showing how a balance was struck between performance and cost enabling the production of high volumes, easy installation and maintenance in the field. The antenna processing subrack consists of low-noise receiver units (RCU) which digitise about a hundred RF signals, a clock and control board (APSCT) to distribute the sampling clock and control the digitisers, a power generation board (APSPU), UniBoard2s where FPGAs perform the beamforming, and a midplane that connects all the boards together while also shielding the sensitive receivers. The APS therefore has boards ranging from high-speed, high-density digital processing devices and high-current power converters to low- noise RF electronics. It has hundreds of devices to power, cool, control and monitor and hundreds of gigabits of data which need to be transferred between boards. The first LOFAR station has been upgraded with new beamformers and the first results will be presented. This demonstrates the new capabilities LOFAR2.0 will have with the new beamformers.

The performance of a low-cost Fibre-Transfer-Delay Measurement System for radio astronomy applications is described. The system, which relies on a microwave-over-fiber interferometer, is realized with off-the-shelf, low-cost components, and is capable to measure at the same time the delays of multiple sections of optical fiber with lengths of a few kilometers, with precisions in the order of few picoseconds.

A fiber transfer delay monitoring system, which exploits components and measurement instrumentation of lowcost, is presented and tested. Few picoseconds precision is demonstrated up to a distance of 4.5 km, which allows its exploitation in various areas of telecommunications and sensors.

RoF technology has proven to be fundamental in enabling the construction of SKA-Low, a huge array of more than 130K antennas operating at low frequency in a particularly hostile region, from an environmental point of view, such as the outback of Western Australia. The motivations that led to its adoption and how it has been implemented in the receiving system of the radio telescope are here described.

A fiber delay measurement and tracking system, which consists of low-cost, off-the-shelf instruments and components, and finds application in several areas, such as Telecommunications, Radar and Radio Astronomy, is presented and tested in an on-the-field realistic scenario. Through a comparison performed with a correspondent high-cost version of the same system, and thanks to an accurate, physical-model-based postprocessing procedure, a precision of 0.1 ps is estimated for the fiber delay over a monitored distance of around 100 m.

The on-field validation of a fiber delay monitoring system (FDMS) based on a microwave Michelson’s interferometer operating over optical fiber is presented. The FDMS has been designed for monitoring the impact of optical cables installed in radioastronomy applications. The system was tested in the second version of the Aperture Array Verification System (AAVS), which represents the prototype for the upcoming low frequency radioastronomy plant Square Kilometre Array (SKA-Low), installed on site at the Murchison Radio Observatory, in Western Australia. The FDMS has been used to monitor two different typologies of cable installation: Surface Laid and Underground Buried. This work shows that the behavior of the delay measurement agrees with the measured average temperature of the cable, which depends on several aspects other than the air temperature. The FDMS can be used also to estimate the average temperature effects of the fiber optic cables.

In the context of Radioastronomic applications where the Analog Radio-over-Fiber technology is used for the antenna downlink, detrimental nonlinearity effects arise because of the interference between the forward signal generated by the laser and the Rayleigh backscattered one which is re-forwarded by the laser itself toward the photodetector. The adoption of the so called dithering technique, which involves the direct modulation of the laser with a sinusoidal tone and takes advantage of the laser chirping phenomenon, has been proved to reduce such Rayleigh Back Scattering - induced nonlinearities. The frequency and the amplitude of the dithering tone should both be as low as possible, in order to avoid undesired collateral effects on the received spectrum as well as keep at low levels the global energy consumption. Through a comprehensive analysis of dithered Radio over Fiber systems, it is demonstrated that a progressive reduction of the dithering tone frequency affects in a peculiar fashion both the chirping characteristics of the field emitted by the laser and the spectrum pattern of the received signal at the fiber end. Accounting for the concurrent effects caused by such phenomena, optimal operating conditions are identified for the implementation of the dithering tone technique in radioastronomic systems.

The Square Kilometer Array low (SKAlow) is an outstanding project aiming at building the world’s largest and most sensitive radio telescope. The Australian Murchison Shire desert has been chosen as location for the low-frequency antennas because of many reasons, such as the atmospheric above the site and the radio quietness due to one of the most remote location worldwide. This location is the optimal choice by a radio astronomy and RF engineering point of views. However, it brings several drawbacks in terms of system availability due to the high temperature conditions that deeply affect the electronics reliability, and the difficulties and the costs associated to maintenance tasks performed in a remote location. Thus, it is fundamental to study the reliability and functional performances of the auxiliary electronic devices in the antennas array using adequate aging tests under temperature stress conditions. The experimental activities carried out in this work allowed to investigate the outbreak of failure mechanisms due to aging and hot temperature conditions in the receiver unit for low-frequency antennas of the SKA project. The experimental results are intended to better determine the strengths and weaknesses of the device under test in order to optimize its installation, its maintenance operations and its functionalities.

The low frequency component of the Square Kilometre Array (SKA1-Low) will be an aperture phased array located at the Murchison Radio-astronomy Observatory (MRO) site in Western Australia. It will be composed of 512 stations, each consisting of 256 log-periodic dual-polarized antennas, and will operate in the low frequency range (50 to 350 MHz) of the SKA bandwidth. The Aperture Array Verification System 2 (AAVS2), operational since late 2019, is the last full-size engineering prototype station deployed at the MRO site before the start of the SKA1-Low construction phase. The aim of this paper is to characterize the station performance through commissioning observations at six different frequencies (55, 70, 110, 160, 230, and 320 MHz) collected during its first year of activities. We describe the calibration procedure, present the resulting all-sky images and their analysis, and discuss the station calibratability and system stability. Using the difference imaging method, we also derive estimates of the SKA1-Low sensitivity for the same frequencies and compare them with those obtained through electromagnetic simulations across the entire telescope bandwidth, finding good agreement (within 13%). Moreover, our estimates exceed the SKA1-Low requirements at all considered frequencies by up to a factor of similar to 2.3. Our results are very promising and allow for an initial validation of the AAVS2 prototype station performance, which is an important step toward the coming SKA1-Low telescope construction and science. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License.

Abstract. The signal reception chain is an essential element for achieving the square kilometer array-low (SKA-low) system requirements in terms of high sensitivity and dynamic range. The balance between gain, linearity, and low power consumption, as well as the cost, are fundamental parameters that influence the selection of the most suitable technology for SKA-low. Further factors, such as low self-generated radio frequency (RF) interference, high reliability, robustness under extreme environment, and last but not least, the distance between the antennas and the acquisition systems, have impacts on the selection for both architecture and receiver system design. The selected technology for the SKA-low RF signal transportation is RF-over-fiber systems, where the preamplified RF signal picked up by the antennas is carried via analogue modulation over optical fiber. The rationales behind the selection are reported, along with descriptions on the development of the receiver prototypes. The prototypes were deployed and installed on the demonstrator arrays at the selected SKA-low site in Western Australian. Particular attention has been put on the thermal characterization of the receiver system under the actual operating temperature on site, especially when both transmitting part and the optical medium are subjected to external ambient temperature variations. Performance issues encountered in the demonstrator arrays are also discussed along with some proposals for future activities.

Abstract. Square Kilometer Array (SKA)-Low is the radio telescope operating in the lowest frequency band of the SKA, from 50 up to 350 MHz. It consists of 512 stations, each composed of 256 dual-polarization log-periodic antennas for a total of 262,144 independent signal paths. The low-frequency aperture array (LFAA) is the portion of the SKA-Low telescope including the antennas and the related electronics. Signal processing is hosted in a temperature controlled and shielded facility: the central processing facility (CPF), for all the core stations, or remote processing facilities (RPF), for stations in the array arms, to limit the maximum fiber length. Such a geographically distributed and interconnected radio telescope, spanning ∼65  km in diameter, requires that frequency and timing reference signals are distributed to the processing facilities with high stability and precision to ensure the required system performances. We present the realization of the clock and pulse per second distribution network inside the LFAA signal processing cabinet where subracks containing signal acquisition boards are housed. We describe the different parts of the chain, and we report on the total jitter introduced by this structure.

Within important applicative scenarios (e.g. metrology, basic physics, radioastronomy) the optical fiber is utilized as a transmitting medium and is at the same time required to distribute and/or transmit signals with an almost constant value of the phase shift (or delay) induced in each one of them.In presence of variations of external environmental agents which can change in real time the electromagnetic properties of the fiber optic cable, it is necessary to perform a continuous monitoring of the fiber-induced delay in order to eventually adopt appropriate countermeasures.This paper proposes an innovative, accurate and low-cost technique based on a microwave interferometer-over-fiber which allows to monitor in real time the mentioned delay, without increasing the complexity of the global system, nor reducing the level of its original performance.

In large radioastronomic facilities, such as the Square Kilometre Array (SKA), where arrays of a great many antennas are connected to the processing station through optical fiber connections, the monitoring of the phase shift (or, equivalently, of the delay) induced to the transmitted signal by each optical link is crucial for the array calibration process. Indeed, these connections typically lie (partially or totally) in environments where climate/environmental agents can change the electromagnetic properties of the cable in a significant measure and in a different fashion between one fiber and the other, and this can therefore have a different impact on the value of phase shift (or delay) of each travelling signal. This paper proposes a simple and low-cost technique based on a microwave interferometer realized on optical fiber to monitor the delay introduced by the optical connections and its variations due to external climate agents.

A new receiver unit (RCU2) has been developed for the LOFAR 2.0 upgrade to simultaneously digitise all antennas. RCU2 has improved linearity performance and has a larger dynamic range, but it has a similar power consumption and noise performance. Monitoring points were also added for improved self testing.

We present the Engineering Development Array 2, which is one of two instruments built as a second generation prototype station for the future Square Kilometre Array Low Frequency Array. The array is comprised of 256 dual-polarization dipole antennas that can work as a phased array or as a standalone interferometer. We describe the design of the array and the details of design changes from previous generation instruments, as well as the motivation for the changes. Using the array as an imaging interferometer, we measure the sensitivity of the array at five frequencies ranging from 70 to 320 MHz.

The design and development process for the Square Kilometre Array (SKA) radio telescope, the Low Frequency Aperture Array component, was progressed during the SKA pre-construction phase by an international consortium, with the goal of meeting requirements for a critical design review. As part of the development process a full-sized prototype SKA Low station was deployed, the Aperture Array Verification System 1 (AAVS1). We provide a system overview and describe the commissioning results of AAVS1, which is a low frequency radio telescope with 256 dual-polarisation log-periodic dipole antennas working as a phased array. A detailed system description is provided, including an in-depth overview of relevant sub-systems, ranging from hardware, firmware, software, calibration,and control sub-systems. Early commissioning results cover initial bootstrapping, array calibration, stability testing, beam-forming,and on-sky sensitivity validation. Lessons learned are presented, along with future developments.