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Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a Chinese mega-science project to build the largest single dish radio telescope in the world. Its innovative engineering concept and design pave a new road to realize a huge single dish in the most effective way. FAST also represents Chinese contribution in the international efforts to build the square kilometer array (SKA). Being the most sensitive single dish radio telescope, FAST will enable astronomers to jump-start many science goals, such as surveying the neutral hydrogen in the Milky Way and other galaxies, detecting faint pulsars, looking for the first shining stars, hearing the possible signals from other civilizations, etc.

The idea of sitting a large spherical dish in a karst depression is rooted in Arecibo telescope. FAST is an Arecibo-type antenna with three outstanding aspects: the karst depression used as the site, which is large to host the 500-meter telescope and deep to allow a zenith angle of 40 degrees; the active main reflector correcting for spherical aberration on the ground to achieve a full polarization and a wide band without involving complex feed systems; and the light-weight feed cabin driven by cables and servomechanism plus a parallel robot as a secondary adjustable system to move with high precision. The feasibility studies for FAST have been carried out for 14 years, supported by Chinese and world astronomical communities. Funding for FAST has been approved by the National Development and Reform Commission in July of 2007 with a capital budget ~ 700 million RMB. The project time is 5.5 years from the commencement of work in March of 2011 and the first light is expected to be in 2016.

This review intends to introduce the project of FAST with emphasis on the recent progress since 2006. In this paper, the subsystems of FAST are described in modest details followed by discussions of the fundamental science goals and examples of early science projects.

Contemporary wideband radio telescope backends are generally developed on Field Programmable Gate Arrays (FPGA) or hybrid (FPGA+GPU) platforms. One of the challenges faced while developing such instruments is the functional verification of the signal processing backend at various stages of development. In the case of an interferometer or pulsar backend, the typical requirement is for one independent noise source per input, with provision for a common, correlated signal component across all the inputs, with controllable level of correlation. This paper describes the design of a FPGA-based variable correlation Digital Noise Source (DNS), and its applications to built-in testing and debugging of correlators and beamformers. This DNS uses the Central Limit Theorem-based approach for generation of Gaussian noise, and the architecture is optimized for resource requirements and ease of integration with existing signal processing blocks on FPGA.

In the period 2012 June–2013 October, the Sardinia Radio Telescope (SRT) went through the technical commissioning phase. The characterization involved three first-light receivers, ranging in frequency between 300MHz and 26GHz, connected to a Total Power back-end. It also tested and employed the telescope active surface installed in the main reflector of the antenna. The instrument status and performance proved to be in good agreement with the expectations in terms of surface panels alignment (at present 300$\mu$mrms to be improved with microwave holography), gain ($\sim$0.6K/Jy in the given frequency range), pointing accuracy (5 arcsec at 22GHz) and overall single-dish operational capabilities. Unresolved issues include the commissioning of the receiver centered at 350MHz, which was compromised by several radio frequency interferences, and a lower-than-expected aperture efficiency for the 22-GHz receiver when pointing at low elevations. Nevertheless, the SRT, at present completing its Astronomical Validation phase, is positively approaching its opening to the scientific community.

Traditionally, back-ends for radio telescopes have been built using a hardware-based approach with ASICs, FPGAs, etc. With advancements in processing power of CPUs, software-based systems have emerged as an alternative option, which has received additional impetus with the advent of GPU-based computing. We present here the design of a hybrid system combining the best of FPGAs, CPUs and GPUs, to implement a next generation back-end for the upgraded GMRT. This back-end can process 400 MHz bandwidth signals from 32 dual-polarized antennas, for both interferometry and beamformer applications, including narrowband spectral line modes for the interferometer, incoherent array and phased array mode of operations for the beamfomer, and also a voltage mode attached to a real-time coherent dedispersion system for the beamformer. We describe in detail the design and architecture of this system, including the novel features and capabilities. We also present sample results from the system that validate its performance in conjunction with the entire receiver chain of the upgraded GMRT.

Radio Frequency Interference (RFI) is a growing concern for contemporary radio telescopes. This paper describes techniques for real-time threshold-based detection and filtering of broadband and narrowband RFI for the correlator and beamformer chains of a telescope back-end, with specific applications to the upgraded Giant Meterwave Radio Telescope (uGMRT). The Median Absolute Deviation (MAD) estimator is used for robust estimation of dispersion of the received signal in temporal and spectral domains. Results from the tests carried out for the GMRT wide-band backend (GWB) using this technique show 10 dB improvement in the signal-to-noise ratio. MAD-based estimation and filtering was also found to be useful for filtering beamformer data. The RFI filtering technique demonstrated in this paper will find applications in other radio telescopes as well as receivers for digital communication and passive radiometry.

The Baryon acoustic oscillations from Integrated Neutral Gas Observations (BINGO) telescope is a new 40m class radio telescope to measure the large-angular-scale intensity of H i emission at 980–1260MHz to constrain dark energy parameters. As it needs to measure faint cosmological signals at the milliKelvin level, it requires a site that has very low radio frequency interference (RFI) at frequencies around 1GHz. We report on measurement campaigns across Uruguay and Brazil to find a suitable site, which looked at the strength of the mobile phone signals and other radio transmissions, the location of wind turbines, and also included mapping airplane flight paths. The site chosen for the BINGO telescope is a valley at Serra do Urubu, a remote part of Paraíba in North-East Brazil, which has sheltering terrain. During our measurements with a portable receiver, we did not detect any RFI in or near the BINGO band, given the sensitivity of the equipment. A radio quiet zone around the selected site has been requested from the Brazilian authorities ahead of the telescope construction.

Radio Frequency Interference (RFI) excision in wideband radio telescope receivers is gaining significance due to increasing levels of manmade RFI and operation outside the protected radio astronomy bands. The effect of RFI on astronomical data can be significantly reduced through real-time excision. In this paper, Median Absolute Deviation (MAD) is used for excising signals corrupted by strong impulsive interference. MAD estimation requires recursive median calculation which is a computationally challenging problem for real-time excision. This challenge is addressed by implementation of a histogram-based technique for MAD computation. The architecture is developed and optimized for Field Programmable Gate Array (FPGA) implementation. The design of a more robust variant of MAD called Median-of-MAD (MoM) is described. The architecture of MAD and MoM techniques and subsequent optimization allows for four RFI excision blocks on a single Xilinx Virtex-5 FPGA. These techniques have been tested on the GMRT wideband backend (GWB) processing a maximum of 400MHz bandwidth and the results show significant improvement in the signal-to-noise ratio (SNR).

A number of countries have identified redundant large telecommunications antennas (TA) and indicated their intention to convert them into radio telescopes (RT). As the efficiency of a parabolic dish radio telescope depends on its surface quality and optical alignment, a careful assessment of these properties should be undertaken before conversion. Here, as a case study, we describe a laser scanning (LS) procedure we developed and used for the Warkworth 30m Cassegrain antenna. To investigate gravity-induced mechanical deformation of the antenna surfaces and structure, we conducted measurements at elevation angles ranging from 6 to 90 degrees. The ability of a laser scanner to survey its nominal $4\pi$ steradian surroundings allows for simultaneous study of the main and subreflectors, readily permitting a dynamic investigation of variation of the telescope optics as elevation changes occur. In particular, the method we present here allows determination of the surface quality of both main and subreflectors, the displacement between centers of the reflectors, their relative rotations and focal length variation as a function of elevation angle. We discuss details of settings, measurements, data processing and analysis focusing on possible difficulties and pitfalls. In our case study, no significant elevation-dependent surface deformation of the reflectors was observed, with the overall standard deviation of the postfit residuals varying between 1.0 and 1.7mm as elevation angle changes from 90^∘ to 6^∘, respectively. We, therefore, conclude that in our case both the main reflector and the subreflector, as well as the telescope optics, remain unaffected by gravitational deformation within the accuracy of the measurements, a conclusion that can possibly be extended to the similar class of TA currently considered for conversion.

This paper presents the technique of numerical estimation of ground parameters impact on the performances of an active antenna used as an element of a phased array antenna of a modern low-frequency radio telescope. Three ground conditions were considered, two of them wet and dry, which correspond to the extreme values of seasonal deviations of its parameters, as well as its median state (“normal ground”). The results of computer simulation are given for the active antenna of the GURT radio telescope with a ground screen in the form of a square wire grid of $3\times 3$m and without it. It is shown that the addition of the ground screen markedly reduces the influence of the ground conditions on some parameters of the antenna, in particular, the dipole impedance and radiation efficiency. At the same time, the ground screen does not protect the most important antenna parameters for radio astronomy, such as sensitivity and absorption area, from the impact of the ground conditions changes. Variations of these parameters for an active antenna with a screen remain approximately the same as without it.

Electromagnetic radiation from human activities, known as man-made Radio Frequency Interference (RFI), adversely affects radio astronomy observations. In the vicinity of the Upgraded Giant Metrewave Radio Telescope (uGMRT) array, the sparking on power lines is the major cause of interference at observing frequencies less than 800MHz. A real-time broadband RFI detection and filtering system is implemented as part of the uGMRT wideband signal processing backend to mitigate the effect of broadband RFI. Performance analysis techniques used for testing and commissioning the system for observations in the beamformer and correlator modes of the uGMRT are presented. The concept and implementation of recording simultaneous unfiltered and filtered data along with data analysis and interpretation is illustrated using an example. For the beamformer mode, spectrogram, single spectral channel, and its Fourier transform is used for performance analysis whereas, in the correlator mode, the cross-correlation function, closure phase, and visibilities from the simultaneously recorded unfiltered and filtered is carried out. These techniques are used for testing the performance of the broadband RFI filter and releasing it for uGMRT users.

Fast radio bursts (FRBs) are extraordinary astrophysical phenomena characterized by short radio pulses that last only a few milliseconds, yet their power can surpass that of 500 million suns. To date, most detected FRBs originate from beyond our galaxy. However, if an FRB were to originate within the Milky Way, it could be detected using small antennas. In this paper, we propose a compact and ad-hoc antenna array designed for the efficient detection and localization of FRBs within the Milky Way. The antenna operates within the 1200–1800MHz range and consists of three sub-arrays placed in an L-shape for source localization, occupying a total volume of $80\times 25\times 6{\mathrm{\text{cm}}}^3$. Each sub-array consists of 4 miniaturized, dual-polarized, half-space radiation antenna elements, forming a one-dimensional array that allows shaping the radiation pattern to match the form of the Milky Way without exhibiting grating lobes. A prototype was constructed and characterized to validate the design. The measured results exhibit good agreement with the simulations. In addition to having a custom elongated radiation pattern, the array has attractive merits, such as low reflections at the input ports, high radiation efficiency, and a distribution that inhibits the existence of phase ambiguities, thus facilitating source localization.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a Chinese mega-science project aimed to build the largest single dish and most sensitive radio telescope in the world. The illuminated aperture of the FAST is 300 m, and its overall performance and sensitivity are several times higher than those of other existing radio telescopes. On September 25, 2016, the FAST was completed with the main structure of the telescope installed. It is expected to remain the global leading radio telescope for the next 20 years. After several years of operation, FAST has acquired abundant scientific achievements, and more than one hundred high-quality papers have been published based on the FAST data, which have produced far-reaching scientific achievements in astronomy.

Shanghai Tianma Radio telescope (TMRT) is a newly-built full-steerable 65-m antenna which can work from 1.3 to 50 GHz with observing efficiencies better than 50% using eight sets of low-noise receiver and active surface systems. Pulsars are not only important space laboratory of testing physical laws for extreme conditions (such as ultra-strong gravitational, magnetic and electronic fields) and detecting gravitational waves, but also can be used for deep-space autonomous navigation, time keeping, etc. So, pulsars were chosen as important scientific research objects at the TMRT. Besides baseband backends for Very Long Baseline Interferometry (VLBI) observations, a digital backend system (DIBAS) supporting commonly-used observation modes for pulsars and a real-time Fast Radio Burst monitoring backend were equipped at the TMRT to meet demands of pulsar-related observations. Taking advantages of its high sensitivity and multi-frequency working ability, it has supported a series of pulsar studies, such as magnetar monitoring, high frequency pulsar observations, pulsar timing, pulsar VLBI, etc. More and more interesting research results will be obtained as we will go on devoting more time and efforts at the TMRT.

The QiTai radio Telescope (QTT) is a fully steerable Gregorian-type telescope with the main reflector aperture 110 m in diameter. QTT adopts an umbrella support, homology-symmetric lightweight design, and the main reflector is active adjustable through actuators. QTT will operate from 150 MHz to 115 GHz frequency and ultra-wideband receivers and large field-of-view multi-beam receivers will be equipped. A multi-function signal processing system based on RFSoC and GPU processor will be developed. QTT will allow high sensitivity observations on pulsars, spectral line, continuum and Very Long Baseline Interferometer (VLBI) observing modes, and form a world-class observational platform in these areas. This chapter briefly introduces the engineering design and the science goals of QTT.

In Jingdong County (101°E, 24.5°N), Pu’er City, Yunnan Province, the Yunnan Observatories Chinese Academy of Science will construct the 120-meter Jingdong Radio Telescope (JRT) dedicated to pulsar-related science. This chapter introduces the scientific objectives and key technologies of the JRT. The telescope covers the frequency range from 100 MHz to 10 GHz (i.e., from the wavelength 3 m to 3 cm). Once finished, it will be the world’s largest fully steerable single-dish decimetre-wavelength radio telescope. The JRT will be driven by three major scientific goals, which include (1) long-term high-precision pulsar timing for time-frequency metrology and nanohertz gravitational wave detection; (2) pulsar astrophysics, fast radio bursts, gravity theory tests, and black hole physics; (3) deep space exploration, geodesy, and celestial reference system. We will also cover the engineering design aspects in this chapter, and present the current plan and design for the telescope mechanical structure, front-end, digital backend, and control system.