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Abstract
Ground-based observation at frequencies below 30 MHz is hindered by the ionosphere of the Earth and radio frequency interference. To map the sky at these low frequencies, we have proposed the Discovering the Sky at the Longest wavelength mission (DSL, also known as the ‘Hongmeng’ mission, which means ‘Primordial Universe’ in Chinese) concept, which employs a linear array of micro-satellites orbiting the Moon. Such an array can make interferometric observations achieving good angular resolutions despite the small size of the antennas. However, it differs from the conventional ground-based interferometer array or even the previous orbital interferometers in many aspects, new data-processing methods need to be developed. In this work, we make a series of simulations to assess the imaging quality and sensitivity of such an array. We start with an input sky model and a simple orbit model, generate mock interferometric visibilities, and then reconstruct the sky map. We consider various observational effects and practical issues, such as the system noise, antenna response, and Moon blockage. Based on the quality of the recovered image, we quantify the imaging capability of the array for different satellite numbers and array configurations. For the first time, we make practical estimates of the point source sensitivity for such a lunar orbit array, and predict the expected number of detectable sources for the mission. Depending on the radio source number distribution which is still very uncertain at these frequencies, the proposed mission can detect 102 ∼ 104 sources during its operation.
... We shall present the outline of the DSL project, and then we show how the all-sky map can be synthesized from the interferometric data by an inversion of the linear relation between the sky pixels and the interferometric visibilities [2,3]. The pixelization scheme adopted is necessarily limited in its resolution, and noise on the subpixel scale may affect the image synthesis on such scales. ...
... Currently, a number of lunar radio astronomy mission concepts are under development, including lunar orbit single satellites such as The Dark Ages Polarimeter Pathfinder (DAPPER) [12] , or satellite arrays like Discovering Sky at the Longest Wavelength (DSL) [13][14][15][16] . It is also a consideration in lunar surface project concepts, such as the Lunar Surface Electromagnetics Experiment (LuSEE) [17] , the Lunar Crater Radio Telescope (LCRT) [18] , and the Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) [12] proposed by the USA, the Astronomical Lunar Observatory (ALO) project [6] , and the Large Array for Radio Astronomy on the Farside (LARAF) proposed by China [19] . ...
... A description of the DSL concept has been given in a paper [4] prepared for the 2020 meeting on 'Astronomy from the Moon: the next decades', and published in the earlier Philosophical Transactions collection on the same topic [5]. Some scientific and technological problems associated with the DSL are also studied in a number of other papers [6][7][8][9][10]. ...
At the Royal Society meeting in 2023, we have mainly presented our lunar orbit array concept called DSL, and also briefly introduced a concept of a lunar surface array, LARAF. As the DSL concept had been presented before, in this article, we introduce the LARAF. We propose to build an array in the far side of the Moon, with a master station which handles the data collection and processing, and 20 stations with maximum baseline of 10 km. Each station consists of 12 membrane antenna units, and the stations are connected to the master station by power line and optical fibre. The array will make interferometric observation in the 0.1–50 MHz band during the lunar night, powered by regenerated fuel cells. The whole array can be carried to the lunar surface with a heavy rocket mission, and deployed with a rover in eight months. Such an array would be an important step in the long-term development of lunar-based ultralong wavelength radio astronomy. It has a sufficiently high sensitivity to observe many radio sources in the sky, though still short of the dark age fluctuations. We discuss the possible options in the power supply, data communication, deployment etc.
This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades (part 2)'.
... We will present the basic design of the Discovering the Sky at the Longest wavelength (DSL) project (also known as the Hongmeng project) [1][2][3]. In this scheme, a group of satellites will be launched by a single rocket to the lunar orbit. ...
... While working on the synthesis imaging problem of the Discovering the Sky at the Longest Wavelength (DSL) project (Chen et al. 2019(Chen et al. , 2020, which is a lunar-orbit interferometer array, some of us have found that the full-sky map can be reconstructed very well from the interferometric data (Huang et al. 2018;Shi et al. 2022b). Although we were not particularly seeking to measure the global spectrum using interferometry there (in the DSL project, the global spectrum is to be measured with a single antenna; see Shi et al. 2022a), in one simulation where we assumed a uniform primary beam, the full-sky map was well recovered, suggesting that even without the modulation of the primary beam, the monopole is still recoverable from interferometric data. ...
We theoretically investigate the recovery of the global spectrum (monopole) from visibilities (cross-correlation only) measured by an interferometer array and the feasibility of extracting the 21 cm signal of the cosmic dawn. In our approach, the global spectrum is obtained by solving the monopole and higher-order components simultaneously from visibilities measured with up to thousands of baselines. Using this algorithm, the monopole of both the foreground and the 21 cm signal can be correctly recovered in a broad range of conditions. We find that a 3D baseline distribution can have much better performance than a 2D (planar) baseline distribution, particularly when there is a lack of shorter baselines. We simulate for ground-based 2D and 3D array configurations, and a cross-shaped space array located at the Sun–Earth L2 point that can form 3D baselines through orbital precession. In all simulations we obtain a good recovered global spectrum, and successfully extract the 21 cm signal from it, with a reasonable number of antennas and observation time.
... When working on the synthesis imaging problem of the Discovering the Sky at the Longest wavelength (DSL) project (Chen et al. 2019;Chen et al. 2020), which is a lunar orbit interferometer array, some of us found that the full sky map can be reconstructed very well from the interferometric data (Huang et al. 2018;Shi et al. 2022b). Although we were not particularly seeking to measure the global spectrum using interferometry there (in the DSL project, the global spectrum is to be measured with single antenna, see Shi et al. 2022a), in one simulation where we assumed uniform primary beam, the full sky map is well-recovered, suggesting that even without the modulation of the primary beam, the monopole is still recoverable from interferometric data. ...
We theoretically investigate the recovery of global spectrum (monopole) from visibilities (cross-correlation only) measured by the interferometer array and the feasibility of extracting 21 cm signal of cosmic dawn. In our approach, the global spectrum is obtained by solving the monopole and higher-order components simultaneously from the visibilities measured with up to thousands of baselines. Using this algorithm, the monopole of both foreground and the 21 cm signal can be correctly recovered in a broad range of conditions. We find that a 3D baseline distribution can have much better performance than a 2D (planar) baseline distribution, particularly when there is a lack of shorter baselines. We simulate for ground-based 2D and 3D array configurations, and a cross-shaped space array located at the Sun-Earth L2 point that can form 3D baselines through orbital precession. In all simulations we obtain good recovered global spectrum, and successfully extract the 21 cm signal from it, with reasonable number of antennas and observation time.
... However, reconstruction of the full 3D Galactic electron structures will be feasible when highresolution ultralong-wavelength sky maps are available. Recently, a number of ultralong-wavelength space missions have been proposed (Chen et al. 2019), such as the Discovering the Sky at the Longest Wavelength Lunar Orbit Array (Chen et al. 2021;Shi et al. 2022aShi et al. , 2022b, and the Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets , which have the capability of producing high-resolution maps, and will enable the full 3D reconstruction. Anticipating high-resolution multifrequency sky maps in the near future, we propose here to reconstruct the full 3D distribution of electrons using the ultralong-wavelength observation. ...
The free–free absorption of low-frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at ≲10 MHz (ultralong wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest Wavelength and Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets will produce high-resolution multifrequency sky maps at the ultralong wavelength, providing a new window to observe the universe. In this Paper we propose that from these ultralong-wavelength multifrequency maps, the 3D distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the H ii regions around massive stars.
... Regarding the noise level of the observations, at lowfrequency the system temperature is dominated by the sky temperature (Shi et al. 2022b ...
The free-free absorption of low frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at $\lesssim 10$ MHz (ultralong-wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest wavelength (DSL) and Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) will produce high-resolution multi-frequency sky maps at the ultralong-wavelength, providing a new window to observe the Universe. In this paper we propose that from these ultralong-wavelength multi-frequency maps, the three-dimensional distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the HII regions around massive stars.
... A number of Snowmass Cosmic Frontier whitepapers ( [18], [19], [20], [21]) develop the science case for a 21 cm Dark Ages experiment on the Moon. An alternative, equally viable approach is to observe from a satellite or satellite formation in a lunar orbit selected to provide periodic blocking of terrestrial RFI, wider and higher-cadence sky coverage, and interferometric imaging ( [22], [23]). RF electronics with the best possible balance of processing power for a highly constrained mass and power budget will be the key enabling technology for a lunar Dark Ages experiment, along with optimized antennas. ...
For many decades High Energy Physics (HEP) instrumentation has been concentrated on detectors of ionizing radiation -- where the energy of incident particles or photons is sufficient to create mobile charge in gas, liquid, or solid material, which can be processed by front end electronics (FEE) to provide information about the position, energy, and timing of the incident radiation. However, recently-proposed HEP experiments need to sense or control EM radiation in the radiofrequency (RF) range, where ionization detectors are unavailable. These experiments can take advantage of emerging microelectronics developments fostered by the explosive growth of wireless data communications in the commercial sector. Moore's Law advances in semiconductor technology have brought about the recent development of advanced microelectronic components with groundbreaking levels of analog-digital integration and processing speed. In particular, RF "System-on-Chip" (RFSoC) platforms offer multiple data converter interfaces to the analog world (ADCs and DACs) having bandwidths approaching 10GHz and abundant digital signal processing resources on the same silicon die. Such devices eliminate the complex PC board interfaces that have long been used to couple discrete ADCs and DACs to FPGA processors, thus radically reducing power consumption, impedance mismatch, and footprint area, while allowing analog preconditioning circuits to be eliminated in favor of digital processing. Costed for wide deployment, these devices are helping to accelerate the trend towards "software defined radio" in several high-volume commercial markets. In this whitepaper we highlight some HEP applications where leading-edge RF microelectronics can be a key enabler.
... During the mission, these satellites will make both interferometric imaging (Huang et al. 2018) and high precision global spectrum measurement on the part of orbit behind the Moon, and the data will be transmitted to the Earth at the near side part of the orbit. Shi et al. (2021) (hereafter referred as Paper I) have investigated the imaging quality and sensitivity of such an array in the presence of thermal noise for interferometric observation. ...
A redshifted 21 cm line absorption signature is commonly expected from the cosmic dawn era, when the first stars and galaxies formed. The detailed traits of this signal can provide important insight on the cosmic history. However, high precision measurement of this signal is hampered by the ionosphere refraction and absorption, as well as radio frequency interference (RFI). A space observation can solve the problem of the ionosphere, and the Moon can shield the RFI from the Earth. In this paper, we present simulations of the global spectrum measurement in the 30 -- 120 MHz frequency band on the lunar orbit, from the proposed Discovering the Sky at the Longest wavelength (DSL) project. In particular, we consider how the measured signal varies as the satellite moves along the orbit, take into account the blockage of different parts of the sky by the Moon and the antenna response. We estimate the sensitivity for such a 21 cm global spectrum experiment. An RMS noise level of $\le 0.05$ K is expected at 75 MHz after 10 orbits ($\sim$ 1 day) observation, for a frequency channel width of 0.4 MHz. We also study the influence of a frequency-dependent beam, which may generate complex spectral structures in the spectrum. Estimates of the uncertainties in the foreground and 21 cm model parameters are obtained.
Weak gravitational lensing is an invaluable tool for understanding fundamental cosmological physics. An unresolved issue in weak lensing cosmology is to accurately reconstruct the lensing convergence κ maps from discrete shear catalog with survey masks, which the seminal Kaiser-Squire (KS) method is not designed to address. We present the accurate kappa reconstruction algorithm for masked shear catalog (AKRA) to address the issue of mask. AKRA is built upon the prior-free maximum-likelihood map making method (or the unbiased minimum variance linear estimator). It is mathematically robust in dealing with mask, numerically stable to implement, and practically effective in improving the reconstruction accuracy. Using simulated maps with mask fractions ranging from 10% to 50% and various mask shapes, we demonstrate that AKRA outperforms KS at both the map level and summary statistics such as the autopower spectrum Cκ of the reconstructed map, its cross-correlation coefficient rℓ with the true κ map, the scatter plot and the localization measure. Unlike the Wiener filter method, it adopts no priors on the signal power spectrum, and therefore avoids the Wiener filter related biases at both the map level and cross-correlation statistics. If we only use the reconstructed map in the unmasked regions, the reconstructed Cκ is accurate to 1% or better and 1−rℓ≲1%, even for extreme cases of mask fraction and shape. As the first step, the current version of AKRA only addresses the mask issue and therefore ignores complexities such as curved sky and inhomogeneous shape measurement noise. AKRA is capable of dealing with these issues straightforwardly, and will be addressed in the next version.
The observation of the global 21 cm signal produced by neutral hydrogen gas in the intergalactic medium (IGM) during the Dark Ages, Cosmic Dawn, and Epoch of Reionization requires measurements with extremely well-calibrated wideband radiometers. We describe the design and characterization of the Mapper of the IGM Spin Temperature (MIST), which is a new ground-based, single-antenna, global 21 cm experiment. The design of MIST was guided by the objectives of avoiding systematics from an antenna ground plane and cables around the antenna, as well as maximizing the instrument’s on-sky efficiency and portability for operations at remote sites. We have built two MIST instruments, which observe in the range 25–105 MHz. For the 21 cm signal, this frequency range approximately corresponds to redshifts 55.5 > z > 12.5, encompassing the Dark Ages and Cosmic Dawn. The MIST antenna is a horizontal blade dipole of 2.42 m in length, 60 cm in width, and 52 cm in height above the ground. This antenna operates without a metal ground plane. The instruments run on 12 V batteries and have a maximum power consumption of 17 W. The batteries and electronics are contained in a single receiver box located under the antenna. We present the characterization of the instruments using electromagnetic simulations and lab measurements. We also show sample sky measurements from recent observations at remote sites in California, Nevada, and the Canadian High Arctic. These measurements indicate that the instruments perform as expected. Detailed analyses of the sky measurements are left for future work.
Ground-based radio observations below 30 MHz are susceptible to the ionosphere of the Earth and the radio frequency interference. Compared with other space mission concepts, making low frequency observations using an interferometer array on lunar orbit is one of the most feasible ones due to a number of technical and economic advantages. Different from traditional interferometer arrays, the interferometer array on lunar orbit faces some complications such as the three-dimensional distribution of baselines and the changing sky blockage by the Moon. Although the brute-force method based on the linear mapping relationship between the visibilities and the sky temperature can produce satisfactory results in general, there are still large residual errors on account of the loss of the edge information. To obtain the full-sky maps with higher accuracy, in this paper we propose a novel imaging method based on reweighted total variation (RTV) for a lunar orbit interferometer array. Meanwhile, a split Bregman iteration method is introduced to optimize the proposed RTV model so as to decrease the computation time. The simulation results show that, compared with the traditional brute-force method, the RTV regularization method can effectively reduce the reconstruction errors and obtain more accurate sky maps, which proves the effectiveness of the proposed method.
The observation of the global 21 cm signal produced by neutral hydrogen gas in the intergalactic medium (IGM) during the Dark Ages, Cosmic Dawn, and Epoch of Reionization requires measurements with extremely well-calibrated wideband radiometers. We describe the design and characterization of the Mapper of the IGM Spin Temperature (MIST), which is a new ground-based, single-antenna, global 21 cm experiment. The design of MIST was guided by the objectives of avoiding systematics from an antenna ground plane and cables around the antenna, as well as maximizing the instrument's on-sky efficiency and portability for operations at remote sites. We have built two MIST instruments, which observe in the range 25-105 MHz. For the 21 cm signal, this frequency range approximately corresponds to redshifts 55.5 > z > 12.5, encompassing the Dark Ages and Cosmic Dawn. The MIST antenna is a horizontal blade dipole of 2.42 m in length, 60 cm in width, and 52 cm in height above the ground. This antenna operates without a metal ground plane. The instruments run on 12 V batteries and have a maximum power consumption of 17 W. The batteries and electronics are contained in a single receiver box located under the antenna. We present the characterization of the instruments using electromagnetic simulations and lab measurements. We also show sample sky measurements from recent observations at remote sites in California, Nevada, and the Canadian High Arctic. These measurements indicate that the instruments perform as expected. Detailed analyses of the sky measurements are left for future work.
A redshifted 21 cm line absorption signature is commonly expected from the cosmic dawn era, when the first stars and galaxies formed. The detailed traits of this signal can provide important insight on the cosmic history. However, high-precision measurement of this signal is hampered by ionosphere refraction and absorption, as well as radio frequency interference (RFI). Space observation can solve the problem of the ionosphere, and the Moon can shield the RFI from Earth. In this paper, we present simulations of the global spectrum measurement in the 30–120 MHz frequency band on the lunar orbit from the proposed Discovering the Sky at the Longest wavelength project. In particular, we consider how the measured signal varies as the satellite moves along the orbit and take into account the blockage of different parts of the sky by the Moon and the antenna response. We estimate the sensitivity for such a 21 cm global spectrum experiment. An rms noise level of ≤0.05 K is expected at 75 MHz after 10 orbits (∼1 day) observation, for a frequency channel width of 0.4 MHz. We also study the influence of a frequency-dependent beam, which may generate complex structures in the spectrum. Estimates of the uncertainties in the foreground and 21 cm model parameters are obtained.
Due to ionosphere absorption and the interference of natural and artificial radio emissions, astronomical observation from the ground becomes very difficult at the wavelengths of decametre or longer, which we shall refer to as the ultralong wavelengths. This unexplored part of the electromagnetic spectrum has the potential for great discoveries, notably in the study of cosmic dark ages and dawn, but also in heliophysics and space weather, planets and exoplanets, cosmic ray and neutrinos, pulsar and interstellar medium (ISM), extragalactic radio sources, and so on. The difficulty of the ionosphere can be overcome by space observation, and the Moon can shield the radio frequency interferences (RFIs) from the Earth. A lunar orbit array can be a practical first step to opening up the ultralong wave band. Compared with a lunar surface observatory on the far side, the lunar orbit array is simpler and more economical, as it does not need to make the risky and expensive landing, can be easily powered with solar energy, and the data can be transmitted back to the Earth when it is on the near-side part of the orbit. Here, I describe the discovering sky at the longest wavelength (DSL) project, which will consist of a mother satellite and 6–9 daughter satellites, flying on the same circular orbit around the Moon, and forming a linear interferometer array. The data are collected by the mother satellite which computes the interferometric cross-correlations (visibilities) and transmits the data back to the Earth. The whole array can be deployed on the lunar orbit with a single rocket launch. The project is under intensive study in China.
This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades’.
The reduced chi-squared statistic is a commonly used goodness-of-fit measure, but it cannot easily detect features near the noise level, even when a large amount of data is available. In this paper, we introduce a new goodness-of-fit measure that we name the reduced psi-squared statistic. It probes the two-point correlations in the residuals of a fit, whereas chi-squared accounts for only the absolute values of each residual point, not considering the relationship between these points. The new statistic maintains sensitivity to individual outliers, but is superior to chi-squared in detecting wide, low level features in the presence of a large number of noisy data points. After presenting this new statistic, we show an instance of its use in the context of analyzing radio spectroscopic data for 21-cm cosmology experiments. We perform fits to simulated data with four components: foreground emission, the global 21-cm signal, an idealized instrument systematic, and noise. This example is particularly timely given the ongoing efforts to confirm the first observational result for this signal, where this work found its original motivation. In addition, we release a Python script dubbed \texttt{psipy} which allows for quick, efficient calculation of the reduced psi-squared statistic on arbitrary data arrays, to be applied in any field of study.
The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.
The NRAO VLA Sky Survey (NVSS) covers the sky north of J2000.0 delta = -40 deg (82% of the celestial sphere) at 1.4 GHz. The principal data products are (1) a set of 2326 4 deg x 4 deg continuum "cubes'' with three planes containing Stokes I, Q, and U images plus (2) a catalog of almost 2 x 10^6 discrete sources stronger than S ~ 2.5 mJy. The images all have theta = 45" FWHM resolution and nearly uniform sensitivity. Their rms brightness fluctuations are sigma ~ 0.45 mJy beam^-1 ~ 0.14 K (Stokes I) and sigma ~ 0.29 mJy beam^-1 ~ 0.09 K (Stokes Q and U). The rms uncertainties in right ascension and declination vary from <~1" for the N ~ 4 x 10^5 sources stronger than 15 mJy to 7" at the survey limit. The NVSS was made as a service to the astronomical community. All data products, user software, and updates are being released via the World Wide Web as soon as they are produced and verified.
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. } \kword{ space vehicles: instruments
-- techniques: interferometric -- radio continuum: galaxies
Results are presented for measurements of the galactic radio background
at frequencies of 5.2, 9.0, 15.6, and 23.0 MHz, which were made by using
half-wave dipoles directed toward + and - 43 deg declination. It is
found that the temperatures of the regions closest to the galactic poles
for these declinations agree to within 10% and that the data are
compatible with recent satellite spectra and southern measurements. A
straight spectrum from 6 to 100 MHz is shown to yield a total background
spectral index of 0.55 + or - 0.03. It is concluded that the observed
polar spectra can be explained in terms of a simple two-component model
consisting of a disk of uniform emission and absorption as well as an
extragalactic component responsible for about 18% of the total emission
at 10 MHz.
A methodological framework for spatial prediction based on regression-kriging is described and compared with ordinary kriging and plain regression. The data are first transformed using logit transformation for target variables and factor analysis for continuous predictors (auxiliary maps). The target variables are then fitted using step-wise regression and residuals interpolated using kriging. A generic visualisation method is used to simultaneously display predictions and associated uncertainty. The framework was tested using 135 profile observations from the national survey in Croatia, divided into interpolation (100) and validation sets (35). Three target variables: organic matter, pH in topsoil and topsoil thickness were predicted from six relief parameters and nine soil mapping units. Prediction efficiency was evaluated using the mean error and root mean square error (RMSE) of prediction at validation points. The results show that the proposed framework improves efficiency of predictions. Moreover, it ensured normality of residuals and enforced prediction values to be within the physical range of a variable. For organic matter, it achieved lower relative RMSE than ordinary kriging (53.3% versus 66.5%). For topsoil thickness, it achieved a lower relative RMSE (66.5% versus 83.3%) and a lower bias than ordinary kriging (0.15 versus 0.69 cm). The prediction of pH in topsoil was difficult with all three methods. This framework can adopt both continuous and categorical soil variables in a semi-automated or automated manner. It opens a possibility to develop a bundle algorithm that can be implemented in a GIS to interpolate soil profile data from existing datasets.
HEALPix -- the Hierarchical Equal Area iso-Latitude Pixelization -- is a
versatile data structure with an associated library of computational algorithms
and visualization software that supports fast scientific applications
executable directly on very large volumes of astronomical data and large area
surveys in the form of discretized spherical maps. Originally developed to
address the data processing and analysis needs of the present generation of
cosmic microwave background (CMB) experiments (e.g. BOOMERanG, WMAP), HEALPix
can be expanded to meet many of the profound challenges that will arise in
confrontation with the observational output of future missions and experiments,
including e.g. Planck, Herschel, SAFIR, and the Beyond Einstein CMB
polarization probe. In this paper we consider the requirements and constraints
to be met in order to implement a sufficient framework for the efficient
discretization and fast analysis/synthesis of functions defined on the sphere,
and summarise how they are satisfied by HEALPix.
The China Square Kilometre Array (SKA) team recently completed the first SKA regional centre prototype, marking an important leap forward towards a future large-scale deployment, explain Tao An, Xiang-Ping Wu and Xiaoyu Hong.
The neutral hydrogen 21 cm line is potentially a very powerful probe of the observable universe, and a number of on-going experiments are trying to detect it at cosmological distances. However, the presence of strong foreground radiations such as the galactic synchrotron radiation, galactic free-free emission and extragalactic radio sources make it a very challenging task. For the design of 21 cm experiments and analysis of their data, simulation is an essential tool, and good sky foreground model is needed. With existing data the whole sky maps are available only in low angular resolutions or for limited patches of sky, which is inadequate in the simulation of these new 21 cm experiments. In this paper, we present the method of constructing a high resolution self-consistent sky model at low frequencies, which incorporates both diffuse foreground and point sources. Our diffuse map is constructed by generating physical foreground components including the galactic synchrotron emission and galactic free-free emission. The point source sample is generated using the actual data from the NRAO VLA Sky Survey (NVSS) and the Sydney University Molonglo Sky Survey (SUMSS) where they are available and complete in flux limit, and mock point sources according to statistical distributions. The entire model is made self-consistent by removing the integrated flux of the point sources from the diffuse map so that this part of radiation is not double counted. We show that with the point sources added, a significant angular power is introduced in the mock sky map, which may be important for foreground subtraction simulations. Our sky maps and point source catalogues are available to download.
The GaLactic and Extragalactic All-sky Murchison Widefield Array survey is a radio continuum survey at 72–231 MHz of the whole sky south of declination +30º, carried out with the Murchison Widefield Array. In this paper, we derive source counts from the GaLactic and Extragalactic All-sky Murchison data at 200, 154, 118, and 88 MHz, to a flux density limit of 50, 80, 120, and 290 mJy respectively, correcting for ionospheric smearing, incompleteness and source blending. These counts are more accurate than other counts in the literature at similar frequencies as a result of the large area of sky covered and this survey’s sensitivity to extended emission missed by other surveys. At S154 MHz > 0.5 Jy, there is no evidence of flattening in the average spectral index (α ≈ −0.8 where S ∝ vα ) towards the lower frequencies. We demonstrate that the Square Kilometre Array Design Study model by Wilman et al. significantly underpredicts the observed 154-MHz GaLactic and Extragalactic All-sky Murchison counts, particularly at the bright end. Using deeper Low-Frequency Array counts and the Square Kilometre Array Design Study model, we find that sidelobe confusion dominates the thermal noise and classical confusion at v ≳ 100 MHz due to both the limited CLEANing depth and the undeconvolved sources outside the field-of-view. We show that we can approach the theoretical noise limit using a more efficient and automated CLEAN algorithm.
Radio astronomical observations below 30 MHz are hampered by the refraction and absorption of the ionosphere as well as the radio frequency interference (RFI), and thus, high angular resolution sky intensity map is not yet available. An interferometer array on lunar orbit provides a perfect observatory in this frequency band: it is out of the ionosphere, and the Moon helps to block the RFIs from the Earth. The satellites can make observations on the far side of the Moon and then send back the data on the near-side part of the orbit. However, for such arrays, the traditional imaging algorithm is not applicable: the field of view is very wide (almost whole-sky), and for baselines distributed on a plane, there is a mirror symmetry between the two sides of the plane. A further complication is that for each baseline, the Moon blocks part of the sky, but as the satellites orbit the Moon, both the direction of the baseline and the blocked sky change, so even imaging algorithms that can deal with a noncoplanar baseline may not work in this case. Here, we present an imaging algorithm based on solving the linear mapping equations relating the sky intensity to the visibilities. We show that the mirror symmetry can be broken by the three-dimensional baseline distribution generated naturally by the precession of the orbital plane of the satellites. The algorithm is applicable and good maps can be reconstructed, even though the sky blocking by the Moon is different for each baseline. We also investigate how the map-making is affected by inhomogeneous baseline distributions. © 2018. The American Astronomical Society. All rights reserved.
We present an improved Global Sky Model (GSM) of diffuse galactic radio emission from 10 MHz to 5 THz, whose uses include foreground modeling for CMB and 21 cm cosmology. Our model improves on past work both algorithmically and by adding new data sets such as the Planck maps and the enhanced Haslam map. Our method generalizes the Principal Component Analysis approach to handle non-overlapping regions, enabling the inclusion of 29 sky maps with no region of the sky common to all. We also perform a blind separation of our GSM into physical components with a method that makes no assumptions about physical emission mechanisms (synchrotron, free-free, dust, etc). Remarkably, this blind method automatically finds five components that have previously only been found "by hand", which we identify with synchrotron, free-free, cold dust, warm dust, and the CMB anisotropy, with maps and spectra agreeing with previous work but in many cases with smaller error bars. The improved GSM is available online at github.com/jeffzhen/gsm2016.
Realizing the potential of 21 cm tomography to statistically probe the intergalactic medium before and during the Epoch of Reionization requires large telescopes and precise control of systematics. Next-generation telescopes are now being designed and built to meet these challenges, drawing lessons from first-generation experiments that showed the benefits of densely packed, highly redundant arrays--in which the same mode on the sky is sampled by many antenna pairs--for achieving high sensitivity, precise calibration, and robust foreground mitigation. In this work, we focus on the Hydrogen Epoch of Reionization Array (HERA) as an interferometer with a dense, redundant core designed following these lessons to be optimized for 21 cm cosmology. We show how modestly supplementing or modifying a compact design like HERA's can still deliver high sensitivity while enhancing strategies for calibration and foreground mitigation. In particular, we compare the imaging capability of several array configurations, both instantaneously (to address instrumental and ionospheric effects) and with rotation synthesis (for foreground removal). We also examine the effects that configuration has on calibratability using instantaneous redundancy. We find that improved imaging with sub-aperture sampling via "off-grid" antennas and increased angular resolution via far-flung "outrigger" antennas is possible with a redundantly calibratable array configuration.
In order to study the "Cosmic Dawn" and the Epoch of Reionization with 21 cm
tomography, we need to statistically separate the cosmological signal from
foregrounds known to be orders of magnitude brighter. Over the last few years,
we have learned much about the role our telescopes play in creating a
putatively foreground-free region called the "EoR window." In this work, we
examine how an interferometer's effects can be taken into account in a way that
allows for the rigorous estimation of 21 cm power spectra from interferometric
maps while mitigating foreground contamination and thus increasing sensitivity.
This requires a precise understanding of the statistical relationship between
the maps we make and the underlying true sky. While some of these calculations
would be computationally infeasible if performed exactly, we explore several
well-controlled approximations that make mapmaking and the calculation of map
statistics much faster, especially for compact and highly-redundant
interferometers designed specifically for 21 cm cosmology. We demonstrate the
utility of these methods and the parametrized trade-offs between accuracy and
speed using one such telescope, the upcoming Hydrogen Epoch of Reionization
Array, as a case study.
In this paper we continue to develop the m-mode formalism, a technique for
efficient and optimal analysis of wide-field transit radio telescopes, targeted
at 21 cm cosmology. We extend this formalism to give an accurate treatment of
the polarised sky, fully accounting for the effects of polarisation leakage and
cross-polarisation. We use the geometry of the measured set of visibilities to
project down to pure temperature modes on the sky, serving as a significant
compression, and an effective first filter of polarised contaminants. We use
the m-mode formalism with the Karhunen-Loeve transform to give a highly
efficient method for foreground cleaning, and demonstrate its success in
cleaning realistic polarised skies observed with an instrument suffering from
substantial off axis polarisation leakage. We develop an optimal quadratic
estimator in the m-mode formalism, which can be efficiently calculated using a
Monte-Carlo technique. This is used to assess the implications of foreground
removal for power spectrum constraints where we find that our method can clean
foregrounds well below the foreground wedge, rendering only scales $k_\parallel
< 0.02 h \,\mathrm{Mpc}^{-1}$ inaccessible. As this approach assumes perfect
knowledge of the telescope, we perform a conservative test of how essential
this is by simulating and analysing datasets with deviations about our assumed
telescope. Assuming no other techniques to mitigate bias are applied, we
recover unbiased power spectra when the per-feed beam width to be measured to
0.1%, and amplifier gains to be known to 1% within each minute. Finally, as an
example application, we extend our forecasts to a wideband 400-800 MHz
cosmological observation and consider the implications for probing dark energy,
finding a medium-sized cylinder telescope improves the DETF Figure of Merit by
around 70% over Planck and Stage II experiments alone.
Recent ISO data have allowed, for the first time, observationally based estimates for source confusion in mid-infrared surveys. We use the extragalactic source counts from ISOCAM in conjunction with K-band counts to predict the confusion resulting from galaxies in deep mid-infrared observations. We specifically concentrate on the near-future Space Infrared Telescope Facility (SIRTF) mission, and calculate expected confusion for the Infrared Array Camera (IRAC) on board SIRTF. A defining scientific goal of the IRAC instrument will be the study of high-redshift galaxies using a deep, confusion-limited wide-field survey at 3-10mum. A deep survey can reach 3-muJy sources with reasonable confidence in the shorter wavelength IRAC bands. Truly confusion-limited images with the 8mum will be difficult to obtain because of practical time constraints, unless infrared galaxies exhibit very strong evolution beyond the deepest current observations. We find L* galaxies to be detectable to z=3-3.5 at 8mum, which is slightly more pessimistic than found in 1999 by Simpson & Eisenhardt.
In this paper we describe the spherical harmonic transit telescope, a novel
formalism for the analysis of transit radio telescopes. This all-sky approach
bypasses the curved sky complications of traditional interferometry and so is
particularly well suited to the analysis of wide-field radio interferometers.
It enables compact and computationally efficient representations of the data
and its statistics that allow new ways of approaching important problems like
map-making and foreground removal. In particular, we show how it enables the
use of the Karhunen-Loeve transform as a highly effective foreground filter,
suppressing realistic foreground residuals for our fiducial example by at least
a factor twenty below the 21cm signal even in highly contaminated regions of
the sky. This is despite the presence of the angle-frequency mode mixing
inherent in real-world instruments with frequency-dependent beams. We show,
using Fisher forecasting, that foreground cleaning has little effect on power
spectrum constraints compared to hypothetical foreground-free measurements.
Beyond providing a natural real-world data analysis framework for 21cm
telescopes now under construction and future experiments, this formalism allows
accurate power spectrum forecasts to be made that include the interplay of
design constraints and realistic experimental systematics with twenty-first
century 21cm science.
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We used the Karl G. Jansky Very Large Array to image one primary beam area at 3 GHz with 8'' FWHM resolution and 1.0 μJy beam–1 rms noise near the pointing center. The P(D) distribution from the central 10 arcmin of this confusion-limited image constrains the count of discrete sources in the 1 < S(μJy) < 10 range. At this level, the brightness-weighted differential count S
2n(S) is converging rapidly, as predicted by evolutionary models in which the faintest radio sources are star-forming galaxies; and ≈96% of the background originating in galaxies has been resolved into discrete sources. About 63% of the radio background is produced by active galactic nuclei (AGNs), and the remaining 37% comes from star-forming galaxies that obey the far-infrared (FIR)/radio correlation and account for most of the FIR background at λ ≈ 160 μm. Our new data confirm that radio sources powered by AGNs and star formation evolve at about the same rate, a result consistent with AGN feedback and the rough correlation of black hole and stellar masses. The confusion at centimeter wavelengths is low enough that neither the planned Square Kilometre Array nor its pathfinder ASKAP EMU survey should be confusion limited, and the ultimate source detection limit imposed by "natural" confusion is ≤0.01 μJy at ν = 1.4 GHz. If discrete sources dominate the bright extragalactic background reported by ARCADE 2 at 3.3 GHz, they cannot be located in or near galaxies and most are ≤0.03 μJy at 1.4 GHz.
Understanding diffuse Galactic radio emission is interesting both in its own right and for minimizing foreground contamination of cosmological measurements. cosmic microwave background experiments have focused on frequencies ≳10 GHz, whereas 21-cm tomography of the high-redshift universe will mainly focus on ≲0.2 GHz, for which less is currently known about Galactic emission. Motivated by this, we present a global sky model derived from all publicly available total power large-area radio surveys, digitized with optical character recognition when necessary and compiled into a uniform format, as well as the new Villa Elisa data extending the 1.42-GHz map to the entire sky. We quantify statistical and systematic uncertainties in these surveys by comparing them with various global multifrequency model fits. We find that a principal component based model with only three components can fit the 11 most accurate data sets (at 10, 22, 45 and 408 MHz and 1.42, 2.326, 23, 33, 41, 61, 94 GHz) to an accuracy around 1–10 per cent depending on frequency and sky region. Both our data compilation and our software returning a predicted all-sky map at any frequency from 10 MHz to 100 GHz are publicly available at http://space.mit.edu/home/angelica/gsm.
Low-frequency radio astronomy is limited by severe ionospheric distortions below 50 MHz and complete reflection of radio waves below 10–30 MHz. Shielding of man-made interference from long-range radio broadcasts, strong natural radio emission from the Earth’s aurora, and the opportunity to set up a large distributed antenna array make the lunar far side a supreme location for a low-frequency radio array. A number of new scientific drivers for such an array, such as the study of the dark ages and epoch of reionization, exoplanets, and ultra-high energy cosmic rays, have emerged and need to be studied in greater detail. Here we review the scientific potential and requirements of these new scientific drivers and discuss the constraints for various lunar surface arrays. In particular, we describe observability constraints imposed by the interstellar and interplanetary medium, calculate the achievable resolution, sensitivity, and confusion limit of a dipole array using general scaling laws, and apply them to various scientific questions.
The Radio Astronomy Explorer 2 (RAE 2) lunar-orbiting satellite has provided measurements of the nonthermal galactic radio emission at frequencies below 10 MHz. Measurements of the emission spectra are presented for the center, anticenter, north polar, and south polar directions at 22 frequencies between 0.25 and 9.18 MHz. Survey maps of the spatial distribution of the observed low-frequency Galactic emission at 1.31, 2.20, 3.93, 4.70, 6.55, and 9.18 MHz are presented. The observations were obtained with the 229-m traveling-wave V antenna on this lunar-orbiting spacecraft. The improved frequency coverage offers additional insights into the structure of the local Galactic neighborhood.
This paper is the second in a series describing the Sydney University
Molonglo Sky Survey (SUMSS) being carried out at 843MHz with the Molonglo
Observatory Synthesis Telescope (MOST). The survey will consist of ~590 4.3deg.
x 4.3deg. mosaic images with 45"x45"cosec|dec.| resolution, and a source
catalogue. In this paper we describe the initial release (version 1.0) of the
source catalogue consisting of 107,765 radio sources made by fitting elliptical
gaussians in 271 SUMSS mosaics to a limiting peak brightness of 6mJy/beam at
dec.<=-50deg. and 10mJy/beam at dec.>-50deg.. The catalogue covers
approximately 3500deg^2 of the southern sky with dec.<=-30deg., about 43 per
cent of the total survey area. Positions in the catalogue are accurate to
within 1"-2" for sources with peak brightness A>=20mJy/beam and are always
better than 10". The internal flux density scale is accurate to within 3 per
cent. Image artefacts have been classified using a decision tree, which
correctly identifies and rejects spurious sources in over 96 per cent of cases.
Analysis of the catalogue shows that it is highly uniform and is complete to
8mJy at dec.<=-50^deg. and 18mJy at dec.>-50deg.. In this release of the
catalogue about 7000 sources are found in the overlap region with the NRAO VLA
Sky Survey (NVSS) at 1.4GHz. We calculate a median spectral index of
alpha=-0.83 between 1.4GHz and 843MHz. This version of the catalogue will be
released via the World Wide Web with future updates as new mosaics are
released.
We present maps of the 22MHz radio emission between declinations -28d and +80d, covering ~73% of the sky, derived from observations with the 22MHz radiotelescope at the Dominion Radio Astrophysical Observatory (DRAO). The resolution of the telescopt (EWxNS) is 1.1d x 1.7d secant(zenith angle). The maps show the large scale features of the emission from the Galaxy including the thick non-thermal disk, the North Polar Spur (NPS) and absorption due to discrete HII regions and to an extended band of thermal electrons within 40d of the Galactic centre. We give the flux densities of nine extended supernova remnants shown on the maps.
The Molonglo Observatory Synthesis Telescope, operating at 843 MHz with a 5 square degree field of view, is carrying out a radio imaging survey of the sky south of declination -30 deg. This survey (the Sydney University Molonglo Sky Survey, or SUMSS) produces images with a resolution of 43" x 43" cosec(Dec.) and an rms noise level of about 1 mJy/beam. SUMSS is therefore similar in sensitivity and resolution to the northern NRAO VLA Sky Survey (NVSS; Condon et al. 1998). The survey is progressing at a rate of about 1000 square degrees per year, yielding individual and statistical data for many thousands of weak radio sources. This paper describes the main characteristics of the survey, and presents sample images from the first year of observation.
- Zheng
- Thompson