Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Source Recovery”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

MULTIWAVELENGTH OBSERVATIONS OF A0620-00 IN QUIESCENCE

We present contemporaneous X-ray, ultraviolet, optical, near-infrared, and radio observations of the black hole binary system, A0620-00, acquired in 2010 March. Using the Cosmic Origins Spectrograph on the Hubble Space Telescope, we have obtained the first FUV spectrum of A0620-00 as well as NUV observations with the Space Telescope Imaging Spectrograph. The observed spectrum is flat in the FUV and very faint (with continuum fluxes {approx_equal} 1e - 17 erg cm{sup -2} s{sup -1} A{sup -1}). The UV spectra also show strong, broad (FWHM {approx} 2000 km s{sup -1}) emission lines of Si IV, C IV, He II, Fe II, and Mg II. The C IV doublet is anomalously weak compared to the other lines, which is consistent with the low carbon abundance seen in NIR spectra of the source. Comparison of these observations with previous NUV spectra of A0620-00 shows that the UV flux has varied by factors of 2-8 over several years. We compiled the dereddened, broadband spectral energy distribution (SED) of A0620-00 and compared it to previous SEDs as well as theoretical models. The SEDs show that the source varies at all wavelengths for which we have multiple samples. Contrary to previous observations, the optical-UV spectrum does not continue to drop to shorter wavelengths, but instead shows a recovery and an increasingly blue spectrum in the FUV. We created an optical-UV spectrum of A0620-00 with the donor star contribution removed. The non-stellar spectrum peaks at {approx_equal}3000 A. The peak can be fit with a T = 10,000 K blackbody with a small emitting area, probably originating in the hot spot where the accretion stream impacts the outer disk. However, one or more components in addition to the blackbody are needed to fit the FUV upturn and the red optical fluxes in the optical-UV spectrum. By comparing the mass accretion rate determined from the hot spot luminosity to the mean accretion rate inferred from the outburst history, we find that the latter is an order of magnitude smaller than the former, indicating that {approx}90% of the accreted mass must be lost from the system if the predictions of the disk instability model and the estimated interoutburst interval are correct. The mass accretion rate at the hot spot is 10{sup 5} the accretion rate at the black hole inferred from the X-ray luminosity. To reconcile these requires that outflows carry away virtually all of the accreted mass, a very low rate of mass transfer from the outer cold disk into the inner hot region, and/or radiatively inefficient accretion. We compared our broadband SED to two models of A0620-00 in quiescence: the advection-dominated accretion flow model and the maximally jet-dominated model. The comparison suggests that strong outflows may be present in the system, indicated by the discrepancies in accretion rates and the FUV upturn in flux in the SED.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Basic parameter estimation of binary neutron star systems by the advanced LIGO/Vigro network

Within the next five years, it is expected that the Advanced LIGO/Virgo network will have reached a sensitivity sufficient to enable the routine detection of gravitational waves. Beyond the initial detection, the scientific promise of these instruments relies on the effectiveness of our physical parameter estimation capabilities. A major part of this effort has been toward the detection and characterization of gravitational waves from compact binary coalescence, e.g., the coalescence of binary neutron stars. While several previous studies have investigated the accuracy of parameter estimation with advanced detectors, the majority have relied on approximation techniques such as the Fisher Matrix which are insensitive to the non-Gaussian nature of the gravitational wave posterior distribution function. Here we report average statistical uncertainties that will be achievable for strong detection candidates (S/N = 20) over a comprehensive sample of source parameters. We use the Markov Chain Monte Carlo based parameter estimation software developed by the LIGO/Virgo Collaboration with the goal of updating the previously quoted Fisher Matrix bounds. We find the recovery of the individual masses to be fractionally within 9% (15%) at the 68% (95%) credible intervals for equal-mass systems, and within 1.9% (3.7%) for unequal-mass systems. We also find that the Advanced LIGO/Virgo network will constrain the locations of binary neutron star mergers to a median uncertainty of 5.1 deg{sup 2} (13.5 deg{sup 2}) on the sky. This region is improved to 2.3 deg{sup 2} (6 deg{sup 2}) with the addition of the proposed LIGO India detector to the network. We also report the average uncertainties on the luminosity distances and orbital inclinations of strong detections that can be achieved by different network configurations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Technological Cost-Reduction Pathways for Axial-Flow Turbines in the Marine Hydrokinetic Environment.

This report considers and prioritizes potential technical cost reduction pathways for axial flow turbines designed for tidal, river, and ocean current resources. This report focuses on technical research and development cost reduction pathways related to the device technology rather than environmental monitoring or permitting opportunities. Three sources of information were utilized to understand current cost drivers and develop a list of potential cost reduction pathways: a literature review of technical work related to axial flow turbines, the U.S. Department of Energy Reference Model effort, and informal webinars and other targeted interactions with industry developers. Data from these various information sources were aggregated and prioritized with respect to potential impact on the lifetime levelized cost of energy. The four most promising cost reduction pathways include structural design optimization; improved deployment, maintenance, and recovery; system simplicity and reliability; and array optimization.

Laird, Daniel L.↗
NASA NTRS1970

Regulated-current dc power supply for gaseous-discharge lamps

Controlled current source having a high output resistance feeds continuous-flow hydrogen lamps in vacuum-ultraviolet photometric equipment. The power supply, also used with low-pressure sealed lamps, has a short recovery time and smooth regulation without overshoot.

Freeman, W.↗
NASA NTRS2014

Ground Testing of the EMCS Seed Cassette for Biocompatibility with the Cellular Slime Mold, Dictyostelium Discoideum

The European Modular Cultivation System, EMCS, was developed by ESA for plant experiments. To expand the use of flight verified hardware for various model organisms, we performed ground experiments to determine whether ARC EMCS Seed Cassettes could be adapted for use with cellular slime mold for future space flight experiments. Dictyostelium is a cellular slime mold that can exist both as a single-celled independent organism and as a part of a multicellular colony which functions as a unit (pseudoplasmodium). Under certain stress conditions, individual amoebae will aggregate to form multicellular structures. Developmental pathways are very similar to those found in Eukaryotic organisms, making this a uniquely interesting organism for use in genetic studies. Dictyostelium has been used as a genetic model organism for prior space flight experiments. Due to the formation of spores that are resistant to unfavorable conditions such as desiccation, Dictyostelium is also a good candidate for use in the EMCS Seed Cassettes. The growth substratum in the cassettes is a gridded polyether sulfone (PES) membrane. A blotter beneath the PES membranes contains dried growth medium. The goals of this study were to (1) verify that Dictyostelium are capable of normal growth and development on PES membranes, (2) develop a method for dehydration of Dictyostelium spores with successful recovery and development after rehydration, and (3) successful mock rehydration experiments in cassettes. Our results show normal developmental progression in two strains of Dictyostelium discoideum on PES membranes with a bacterial food source. We have successfully performed a mock rehydration of spores with developmental progression from aggregation to slug formation, and production of morphologically normal spores within 9 days of rehydration. Our results indicate that experiments on the ISS using the slime mold, Dictyostelium discoideum could potentially be performed in the flight verified hardware of the EMCS ARC Seed Cassettes.

biocompatibility↗
NASA NTRS2007

Sensitive Quantitative Assessment of Balance Disorders

Computerized dynamic posturography (CDP) has become a standard technique for objectively quantifying balance control performance, diagnosing the nature of functional impairments underlying balance disorders, and monitoring clinical treatment outcomes. We have long used CDP protocols to assess recovery of sensory-motor function in astronauts following space flight. The most reliable indicators of post-flight crew performance are the sensory organization tests (SOTs), particularly SOTs 5 and 6, which are sensitive to changes in availability and/or utilization of vestibular cues. We have noted, however, that some astronauts exhibiting obvious signs of balance impairment after flight are able to score within clinical norms on these tests, perhaps as a result of adopting competitive strategies or by their natural skills at substituting alternate sensory information sources. This insensitivity of the CDP protocol could underestimate of the degree of impairment and, perhaps, lead to premature release of those crewmembers to normal duties. To improve the sensitivity of the CDP protocol we have introduced static and dynamic head tilt SOT trials into our protocol. The pattern of postflight recovery quantified by the enhanced CDP protocol appears to more aptly track the re-integration of sensory-motor function, with recovery time increasing as the complexity of sensory-motor/biomechanical task increases. The new CDP protocol therefore seems more suitable for monitoring post-flight sensory-motor recovery and for indicating to crewmembers and flight surgeons fitness for return to duty and/or activities of daily living. There may be classes of patients (e.g., athletes, pilots) having motivation and/or performance characteristics similar to astronauts whose sensory-motor treatment outcomes would also be more accurately monitored using the enhanced CDP protocol. Furthermore, the enhanced protocol may be useful in early detection of age-related balance disorders.

Paloski, Willilam H.↗
DOE OSTI2020

Techno-Economic and Life Cycle Assessments for Sustainable Rare Earth Recovery from Coal Byproducts using Biosorption

Coal byproducts could be a promising feedstock to alleviate the supply risk of critical rare earth elements (REEs) due to their abundance and REE content. Herein, we investigated the economic and environmental potential of producing REEs from coal fly ash and lignite through an integrated process of leaching, biosorption, and oxalic precipitation on the basis of experimental data and modeling results. Two microbe immobilization systems (polyethylene glycol diacrylate (PEGDA) microbe beads and Si sol–gels) were examined for their efficiency in immobilizing Arthrobacter nicotianae to selectively recover REEs. Techno-economic analysis revealed that North Dakota lignite could be a profitable feedstock when Si sol–gel is used due to its high cell loading and REE adsorption capacity as well as high reuse cycles. Life cycle analysis revealed that Si sol–gel-based biosorption could be more environmental friendly than the prevailing REE production in China due to the use of less toxic chemicals. However, fly ash sourced from Powder River Basin coals was neither profitable nor environmentally sustainable, primarily due to low solubility of high-value scandium at an economical pulp density (100 g ash/L of acid solution). To further improve the proposed biotechnology, future research could focus on scandium recovery, leaching efficiency at a high pulp density, and reuse cycles of the immobilized microbes.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2016

Charged particle flows in the beam extraction region of a negative ion source for NBI

Experiments by a four-pin probe and photodetachment technique were carried out to investigate the charged particle flows in the beam extraction region of a negative hydrogen ion source for neutral beam injector. Electron and positive ion flows were obtained from the polar distribution of the probe saturation current. Negative hydrogen ion flow velocity and temperature were obtained by comparing the recovery times of the photodetachment signals at opposite probe tips. Electron and positive ions flows are dominated by crossed field drift and ambipolar diffusion. Negative hydrogen ion temperature is evaluated to be 0.12 eV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1975

Automatic liquid inventory collecting and dispensing unit

A means is provided for automatically collecting waste liquids and for simultaneously feeding these liquids into water recovery processes. The invention includes first and second tanks with first and second two-way solenoid valves associated with each of the tanks. The first solenoid valve is connected to the liquid source and its associated tank so as to allow liquid to flow into the tank when the valve is in its normal position and to allow the liquid to flow out of the tank when the valve is in its actuated position. The second valve is connected to its associated tank and a gas supply so as to allow gas inside the tank to flow out when the valve is in its normal position and to allow gas to flow from the gas supply into the tank when the valve is in its actuated position. Control circuits are included for actuating the two valves associated with the first tank and not actuating the valves associated with the second tank. The first tank is filled and the second tank is emptied.

Hall, J. B., Jr.↗
NASA NTRS2003

MODIS NDVI Response Following Fires in Siberia

The Siberian boreal forest is considered a carbon sink but may become an important source of carbon dioxide if climatic warming predictions are correct. The forest is continually changing through various disturbance mechanisms such as insects, logging, mineral exploitation, and especially fires. Patterns of disturbance and forest recovery processes are important factors regulating carbon flux in this area. NASA's Terra MODIS provides useful information for assessing location of fires and post fire changes in forests. MODIS fire (MOD14), and NDVI (MOD13) products were used to examine fire occurrence and post fire variability in vegetation cover as indicated by NDVI. Results were interpreted for various post fire outcomes, such as decreased NDVI after fire, no change in NDVI after fire and positive NDVI change after fire. The fire frequency data were also evaluated in terms of proximity to population centers, and transportation networks.

Ranson, K. Jon↗
DOE OSTI2012

NEW MATERIALS DEVELOPED TO MEET REGULATORY AND TECHNICAL REQUIREMENTS ASSOCIATED WITH IN-SITU DECOMMISSIONING OF NUCLEAR REACTORS AND ASSOCIATED FACILITIES

For the 2010 ANS Embedded Topical Meeting on Decommissioning, Decontamination and Reutilization and Technology, Savannah River National Laboratory's Mike Serrato reported initial information on the newly developed specialty grout materials necessary to satisfy all requirements associated with in-situ decommissioning of P-Reactor and R-Reactor at the U.S. Department of Energy's Savannah River Site. Since that report, both projects have been successfully completed and extensive test data on both fresh properties and cured properties has been gathered and analyzed for a total of almost 191,150 m{sup 3} (250,000 yd{sup 3}) of new materials placed. The focus of this paper is to describe the (1) special grout mix for filling the P-Reactor vessel (RV) and (2) the new flowable structural fill materials used to fill the below grade portions of the facilities. With a wealth of data now in hand, this paper also captures the test results and reports on the performance of these new materials. Both reactors were constructed and entered service in the early 1950s, producing weapons grade materials for the nation's defense nuclear program. R-Reactor was shut down in 1964 and the P-Reactor in 1991. In-situ decommissioning (ISD) was selected for both facilities and performed as Comprehensive Environmental Response, Compensations and Liability Act actions (an early action for P-Reactor and a removal action for R-Reactor), beginning in October 2009. The U.S. Department of Energy concept for ISD is to physically stabilize and isolate intact, structurally robust facilities that are no longer needed for their original purpose of producing (reactor facilities), processing (isotope separation facilities), or storing radioactive materials. Funding for accelerated decommissioning was provided under the American Recovery and Reinvestment Act. Decommissioning of both facilities was completed in September 2011. ISD objectives for these CERCLA actions included: (1) Prevent industrial worker exposure to radioactive or hazardous contamination exceeding Principal Threat Source Material levels; (2) Minimize human and ecological exposure to unacceptable risk associated with radiological and hazardous constituents that are or may be present; (3) Prevent to the extent practicable the migration of radioactive or hazardous contaminants from the closed facility to the groundwater so that concentrations in groundwater do not exceed regulatory standards; (4) Eliminate or control all routes of human exposure to radiological and chemical contamination; and (5) Prevent animal intruder exposure to radioactive and hazardous contamination.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2012

Long-term patterns of fruit production in five forest types of the South Carolina upper coastal plain

Fleshy fruit is a key food resource for many vertebrates and may be particularly important energy source to birds during fall migration and winter. Hence, land managers should know how fruit availability varies among forest types, seasons, and years. We quantified fleshy fruit abundance monthly for 9 years (1995–2003) in 56 0.1-ha plots in 5 forest types of South Carolina's upper Coastal Plain, USA. Forest types were mature upland hardwood and bottomland hardwood forest, mature closed-canopy loblolly ( Pinus taeda ) and longleaf pine ( P. palustris ) plantation, and recent clearcut regeneration harvests planted with longleaf pine seedlings. Mean annual number of fruits and dry fruit pulp mass were highest in regeneration harvests (264,592 ± 37,444 fruits; 12,009 ± 2,392 g/ha), upland hardwoods (60,769 ± 7,667 fruits; 5,079 ± 529 g/ha), and bottomland hardwoods (65,614 ± 8,351 fruits; 4,621 ± 677 g/ha), and lowest in longleaf pine (44,104 ± 8,301 fruits; 4,102 ± 877 g/ha) and loblolly (39,532 ± 5,034 fruits; 3,261 ± 492 g/ha) plantations. Fruit production was initially high in regeneration harvests and declined with stand development and canopy closure (1995–2003). Fruit availability was highest June–September and lowest in April. More species of fruit-producing plants occurred in upland hardwoods, bottomland hardwoods, and regeneration harvests than in loblolly and longleaf pine plantations. Several species produced fruit only in 1 or 2 forest types. In sum, fruit availability varied temporally and spatially because of differences in species composition among forest types and age classes, patchy distributions of fruiting plants both within and among forest types, fruiting phenology, high inter-annual variation in fruit crop size by some dominant fruit-producing species, and the dynamic process of disturbance-adapted species colonization and decline, or recovery in recently harvested stands. As a result, land managers could enhance fruit availability for wildlife by creating and maintaining diverse forest types and age classes.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Improving and Assessing Planet Sensitivity of the GPI Exoplanet Survey with a Forward Model Matched Filter

We present a new matched-filter algorithm for direct detection of point sources in the immediate vicinity of bright stars. The stellar point-spread function (PSF) is first subtracted using a Karhunen-Loéve image processing (KLIP) algorithm with angular and spectral differential imaging (ADI and SDI). The KLIP-induced distortion of the astrophysical signal is included in the matched-filter template by computing a forward model of the PSF at every position in the image. To optimize the performance of the algorithm, we conduct extensive planet injection and recovery tests and tune the exoplanet spectra template and KLIP reduction aggressiveness to maximize the signal-to-noise ratio (S/N) of the recovered planets. We show that only two spectral templates are necessary to recover any young Jovian exoplanets with minimal S/N loss. We also developed a complete pipeline for the automated detection of point-source candidates, the calculation of receiver operating characteristics (ROC), contrast curves based on false positives, and completeness contours. We process in a uniform manner more than 330 data sets from the Gemini Planet Imager Exoplanet Survey and assess GPI typical sensitivity as a function of the star and the hypothetical companion spectral type. This work allows for the first time a comparison of different detection algorithms at a survey scale accounting for both planet completeness and false-positive rate. We show that the new forward model matched filter allows the detection of 50% fainter objects than a conventional cross-correlation technique with a Gaussian PSF template for the same false-positive rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Controlling RPV embrittlement through wet annealing in support of life attainment and life extension decisions

As a main barrier against radioactivity outlet reactor pressure vessel (RPV) is a key component in terms of Nuclear Power Plant (NPP) safety. Therefore present-day demands in RPV reliability enhance have to be met by all possible actions for RPV in-service embrittlement mitigation. Annealing treatment is known to be the effective measure to restore the RPV metal properties deteriorated by neutron irradiation. Low temperature 'wet' annealing at a maximum coolant temperature which can be obtained using the reactor core or primary circuit pumps, although it cannot be expected to produce complete recovery, is more attractive from the practical point of view especially in cases when the removal of the internals is impossible. As a rule there is no recovery effect up to annealing and irradiation temperature difference of 70 deg. C. It is known, however, that along with radiation embrittlement neutron irradiation may mitigate the radiation damage in metals. Therefore we have tried to test the possibility to use the effect of radiation-induced ductilization in 'wet' annealing technology by means of nuclear heat utilization as heat and neutron irradiation sources at once. In support of the above-mentioned conception the 3-year duration reactor experiment on 15Cr3NiMoV type steel with preliminary irradiation at operating Pressurized Water Reactor (PWR) at 270 deg. C and following extra irradiation (87 h at 330 deg. C) at IR-8 test reactor was fulfilled. In fact, embrittlement was partly suppressed up to value equivalent to 1,5 fold neutron fluence decrease. The degree of recovery in case of radiation enhanced annealing is equal to 27% whereas furnace annealing results in zero effect under existing conditions. Mechanism of the radiation-induced damage mitigation is proposed. It is hoped that 'wet' annealing technology will help provide a better management of the RPV degradation as a factor affecting the lifetime of nuclear power plants which, together with associated management methods, will help facilitate safe and economic long-term operation of PWRs. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2012

PSR J1838-0537: DISCOVERY OF A YOUNG, ENERGETIC GAMMA-RAY PULSAR

We report the discovery of PSR J1838-0537, a gamma-ray pulsar found through a blind search of data from the Fermi Large Area Telescope (LAT). The pulsar has a spin frequency of 6.9 Hz and a frequency derivative of -2.2 Multiplication-Sign 10{sup -11} Hz s{sup -1}, implying a young characteristic age of 4970 yr and a large spin-down power of 5.9 Multiplication-Sign 10{sup 36} erg s{sup -1}. Follow-up observations with radio telescopes detected no pulsations; thus PSR J1838-0537 appears radio-quiet as viewed from Earth. In 2009 September the pulsar suffered the largest glitch so far seen in any gamma-ray-only pulsar, causing a relative increase in spin frequency of about 5.5 Multiplication-Sign 10{sup -6}. After the glitch, during a putative recovery period, the timing analysis is complicated by the sparsity of the LAT photon data, the weakness of the pulsations, and the reduction in average exposure from a coincidental, contemporaneous change in LAT's sky-survey observing pattern. The pulsar's sky position is coincident with the spatially extended TeV source HESS J1841-055 detected by the High Energy Stereoscopic System (H.E.S.S.). The inferred energetics suggest that HESS J1841-055 contains a pulsar wind nebula powered by the pulsar.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Cost curve of large-scale deployment of CO2-enhanced water recovery technology in modern coal chemical industries in China

China has emerged as a world leader in the coal chemical industry, which requires large amount of water and results in considerable CO2 emissions. This situation has led to the challenge of the CO2-Water nexus for China and particularly for the sustainable development of its coal chemical industry. CO2-enhanced water recovery (CO2-EWR) technology can provide large-scale CO2 mitigation and additional water supply in an integrated manner, especially in arid areas. Meanwhile, CO2 streams from industrial separation processes in the coal chemical industries are amenable to separation and can dramatically simplify or even dispense with the capture process. This study presents the first systematic assessment of a cost curve for onshore CO2-EWR potential using CO2 streams from industrial separation processes by an evaluation framework encompassing CO2 emission inventory, site suitability evaluation, and source–sink matching with techno-economic models. Preliminary results focused on the full capacity of several coal chemical processes as of 2015 suggest that CO2-EWR technology can mitigate 269 million tons of CO2 from industrial separation processes at relatively low cost ranging from 12 to 30 USD/t CO2 in China. Furthermore, 404 million tons of underground water could be produced for further desalination and utilization. When additional capacity under development could become fully operational, the emissions of 878 million tons of CO2 could be mitigated and provide 1318 million tons of vital water resources. Therefore, CO2-EWR technology can be essential to clean and sustainable development of the coal chemical industry and may provide low-cost opportunities to accelerate the deployment of large-scale CCUS projects in China.

CO2-enhanced water recovery, Modern coal chemical ↗
DOE OSTI2014

Subsurface Sources of CO 2 in the United States. Volume II: Exploration of CO 2 Systems

A study of the genesis and tectonic setting of subsurface CO 2 systems in the United States indicates that undiscovered CO 2 reservoirs could contribute materially to CO 2 supply for enhanced oil recovery. Five geographic areas are estimated to contain 42 Tcf of risked technically recoverable CO 2 resource (TRR). Two lead areas near the Permian Basin, Val Verde and San Juan, contain 34 Tcf CO 2 risked TRR, an amount roughly equivalent to the remaining TRR in discovered reservoirs that are currently supplying the region. The number of lead areas studied was limited and the aggregate TRR estimates are not comprehensive.

03 NATURAL GAS↗
DOE OSTI2019

21 cm Signal Recovery via Robust Principal Component Analysis

The redshifted 21 cm signal from neutral hydrogen (H i) is potentially a very powerful probe for cosmology, but a difficulty in its observation is that it is much weaker than foreground radiation from the Milky Way as well as extragalactic radio sources. The foreground radiation at different frequencies, however, is coherent along one line of sight, and various methods of foreground subtraction based on this property have been proposed. In this paper, we present a new method based on robust principal component analysis (RPCA) to subtract the foreground and extract the 21 cm signal, which explicitly uses both the low-rank property of the frequency covariance matrix (i.e., frequency coherence) of the foreground and the sparsity of the frequency covariance matrix of the 21 cm signal. The low-rank property of the foreground’s frequency covariance has been exploited in many previous works on foreground subtraction, but to our knowledge the sparsity of the frequency covariance of the 21 cm signal is first explored here. By exploiting both properties in the RPCA method, the frequency covariance matrix of the foreground and the 21 cm signal can be separated accurately. We then use the generalized internal linear combination method to recover the 21 cm signal from the data. Our method is applicable both to a small patch of sky with the flat-sky approximation and to a large area of sky where the sphericity has to be considered. It is also easy to extend to deal with more complex conditions such as a sky map with defects.

79 ASTRONOMY AND ASTROPHYSICS↗