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The work describes a system for converting VLBI observation data using the algorithms of coherent dedispersion and compensation of two-bit signal sampling. Coherent dedispersion is important for processing pulsar observations to obtain the best temporal resolution, while correction for signal sampling makes it possible to get rid of a number of parasitic effects that interfere with the analysis of the diffraction pattern of pulsars. A pipeline has been established that uses the developed converter and the ASC Software Correlator, which will allow reprocessing all archived data of Radioastron pulsar observations and to conduct a search for giant pulses, which requires the best temporal resolution., Comment: 16 pages, 4 figures

Scintillation of pulsar radio signals caused by the interstellar medium can in principle be used for interstellar interferometry. Changes of the dynamic spectra as a function of pulsar longitude were in the past interpreted as having spatially resolved the pulsar magnetosphere. Guided by this prospect we used VLBI observations of PSR B1237+25 with the Arecibo and Green Bank radio telescopes at 324 MHz and analyzed such scintillation at separate longitudes of the pulse profile. We found that the fringe phase characteristics of the visibility function changed quasi-sinusoidally as a function of longitude. Also, the dynamic spectra from each of the telescopes shifted in frequency as a function of longitude. Similar effects were found for PSR B1133+16. However, we show that these effects are not signatures of having resolved the pulsar magnetosphere. Instead the changes can be related to the effect of low-level digitizing of the pulsar signal. After correcting for these effects the frequency shifts largely disappeared. Residual effects may be partly due to feed polarization impurities. Upper limits for the pulse emission altitudes of PSR B1237+25 would likely be well below the pulsar light cylinder radius. In view of our analysis we think that observations with the intent of spatially resolving the pulsar magnetosphere need to be critically evaluated in terms of these constraints on interstellar interferometry., Comment: 19 pages, 11 figures, published in The Astrophysical Journal

We observed the scintillation pattern of nine bright pulsars at 324 MHz and three at 1.68 GHz and analyzed the wavenumber spectrum which is related to electron density variations of the plasma turbulence of the interstellar medium. For all pulsars the frequency section of the autocorrelation function of the dynamic spectra to at least 45\% of the maximum corresponds to predictions of scattering theories with a range of power-law exponents of the wavenumber spectrum of $3.56 \leq \alpha \leq 3.97$ with errors $\leq 0.05$ and a mean with standard deviation of $3.76\pm0.13$. The range includes $\alpha=3.67$ for the Kolmogorov spectrum. Similar results although with larger errors were found from the Fourier transform of the autocorrelation functions down to $\sim 10^{-3}$ of the maximum. No clear case of a distinction between thin-screen and extended-medium scattering models was found. The average frequency profile of the scintles can be characterized for steep wavenumber spectra with $\alpha\lesssim4$ by a cusp with a somewhat rounded peak. For flatter spectra, down to at least $\alpha\sim 3.56$ the cusp with its peak becomes more pronounced and its decay steepens. We discuss our findings in the context of scattering characteristics of the interstellar medium., Comment: 16 pages, 8 figures, accepted by The Astrophysical Journal. Changes in this revision: change of the paper title on the HTML page, correction of misprints

We report on slow phase variations of the response of the space-ground radio interferometer RadioAstron during observations of pulsar B0329+54. The phase variations are due to the ionosphere and clearly distinguishable from effects of interstellar scintillation. Observations were made in a frequency range of 316-332~MHz with the 110-m Green Bank Telescope and the 10-m RadioAstron telescope in 1-hour sessions on 2012 November 26, 27, 28, and 29 with progressively increasing baseline projections of about 60, 90, 180, and 240 thousand kilometres. Quasi-periodic phase variations of interferometric scintles were detected in two observing sessions with characteristic time-scales of 12 and 10 minutes and amplitudes of up to 6.9~radians. We attribute the variations to the influence of medium-scale Travelling Ionospheric Disturbances. The measured amplitude corresponds to variations in vertical total electron content in ionosphere of about $0.1\times10^{16}\, \mathrm{m}^{-2}$. Such variations would noticeably constrain the coherent integration time in VLBI studies of compact radio sources at low frequencies., Comment: 6 pages, 4 figures. Accepted for publication in MNRAS Author list is corrected

We report on the substructure of visibility functions in the delay domain of PSRs B0329+54, B0823+26, B0834+06, B1933+16 and B0833-45 (Vela) observed with earth-earth and RadioAstron space-earth two-element interferometers at frequencies of 324 MHz and 1668 MHz. All visibility functions display unresolved spikes distributed over a range of delays. They are due to band-limited scintillation noise and related to the scattering time. The envelopes for each but the Vela pulsar are well fit by a single Lorentzian which we interpret as being indicative of isotropic scattering on the plane of the sky due to a thin scattering screen between the pulsar and us. In contrast, the envelope for the Vela pulsar needs to be mostly fit by at least two Lorentzians, a narrow and a broad one at the same zero delay. We interpret this characteristic as indicative of anisotropic scattering due to more complex structure of scattering screens in the supernova remnant. The possibility of describing the delay visibility functions by Lorentzians is likely a general property of pulsars and offers a new way of describing scattering parameters of the intervening interstellar medium. Furthermore, for all our pulsars, the unresolved spikes in visibility functions of similar projected baselines were well correlated indicating that the telescopes are located in the same diffraction spot. The correlation vanished for visibilities from largely different baselines, when some radio telescopes are not in the same spot., Comment: 13 pages, 3 figures, 4 tables. Accepted for publication to ApJ

The largest Key Science Program of the RadioAstron space VLBI mission is a survey of active galactic nuclei (AGN). The main goal of the survey is to measure and study the brightness of AGN cores in order to better understand the physics of their emission while taking interstellar scattering into consideration. In this paper we present detection statistics for observations on ground-space baselines of a complete sample of radio-strong AGN at the wavelengths of 18, 6, and 1.3 cm. Two-thirds of them are indeed detected by RadioAstron and are found to contain extremely compact, tens to hundreds of $\mu$as structures within their cores., Comment: Accepted to the Advances in Space Research special issue "High-resolution Space-Borne Radio Astronomy"

The aim of our work was to study the spatial structure of inhomogeneities of interstellar plasma in the directions of five pulsars: B0823+26, B0834+06, B1237+25, B1929+10, and B2016+28. Observations of these pulsars were made with RadioAstron space-ground radio interferometer at 324 MHz. We measured the angular size of the scattering disks to be in range between 0.63 and 3.2 mas. We determined the position of scattering screens on the line of sight. Independent estimates of the distances to the screens were made from the curvature of parabolic arcs revealed in the secondary spectra of four pulsars. The model of uniform distribution of inhomogeneities on the line of sight is not suitable. According to the results, we came to the conclusion that scattering is mainly produced by compact plasma layers and the uniform model of inhomogeneties distribution on the line of sight in not applicable., Comment: 15 pages, 12 figures

In this work, topological spaces are enriched by additional structures in order to give a more realistic representation of real life phenomena and computational processes and at the same time, to provide for utilization of the powerful technique developed in topology. The suggested approach is based on the concept of a discontinuity structure or scale Q in a topological space X. This structure serves as a scale for the initial topology on X. Problems of science and engineering need such a scale because all measurements and the majority of computations are performed in a definite scale and problems of scalability are important both for physical theories and computational algorithms. Taking a mapping of a topological space X with the discontinuity structure Q into a topological space Y with a discontinuity structure R, we define (Q,R)-continuity, weak (Q,R)-continuity, and R-continuity of this mapping. Fuzzy continuous functions, which are studied in neoclassical analysis, are examples of R-continuous mappings. Different properties of scales, Q-open, Q-closed sets, and (Q,R)-continuous mappings are obtained.

When planning new facilities, one is interested to know whether and by how much the new technique is superior to already existing ones. We describe a general approach permitting to evaluate the relative merits of various techniques used in astrophysical observations, following the theory of model parameter estimation. It is applied to compare two-aperture optical long baseline interferometry to classical spectroscopy, both used to define the model parameters of the P Cyg wind. The wind modeling was done using an efficient approximation for computation of the line source function; it allowed to analyse about 10e5 points in the parametric space of P Cyg envelope models. It is shown that interferometry offers no real advantage if the object can be described by stationary spherically symmetric models with a priori fixed thermal structure. However, if the object must be described by a model with a large number of free parameters, e.g. when the thermal structure of the envelope is not fixed a priori, then the interferometric measurements can reduce the error in model parameters determination by an order of magnitude. In the case of P Cyg, the reduction of errors provided by interferometry is highest for the baseline lengths in the range 45--90 m. This illustrates the capacity of the proposed method to be used for optimization of interferometric configurations. The simplifications adopted for this first trial application are critically analyzed, and future improvements are indicated., Comment: LaTeX2.09e + aa.cls + graphics.sty, 14 pages, 7 figures. Accepted for publication in Astron. Astrophys. Suppl

Spatial and temporal variations in near-surface soil moisture are important to measure for climate studies, numerical weather forecasts, and drought monitoring. Several previous studies have shown success in using ground-reflected Global Navigation Satellite System (GNSS) signals as a form of bistatic radar to sense soil moisture. However, the ability of this type of data to sense soil moisture variations from space is still a nascent field of study. In the past two years, three satellites have been launched that were either designed to capture ground-reflected GNSS signals or have been modified to record these signals. The data provided by these satellites are giving scientists an unprecedented opportunity to investigate their ability to detect changes in Earth’s land surface, including but certainly not limited to near-surface soil moisture. This paper will present spaceborne observations of ground-reflected GNSS signals and evaluate their sensitivity to near-surface soil moisture. This sensitivity will be compared to empirical and theoretical sensitivities of monostatic L-band radar measurements to soil moisture. We will also comment on possibilities for retrieval algorithm development, using techniques employed for monostatic radar as a guide.

NASA's Soil Moisture Active Passive (SMAP) mission launched on January 31, 2015 into a sun-synchronous 6 am/6 pm orbit with an objective to produce global mapping of high-resolution soil moisture and freeze-thaw state every 2-3 days using an L-band (active) radar and an L-band (passive) radiometer. The SMAP radiometer began acquiring routine science data on March 31, 2015 and continues to operate nominally. SMAP's radiometer-derived soil moisture product (L2_SM_P) provides soil moisture estimates posted on a 36 km fixed Earth grid using brightness temperature observations from descending (6 am) passes and ancillary data. A beta quality version of L2_SM_P was released to the public in September, 2015, with the fully validated L2_SM_P soil moisture data expected to be released in May, 2016. Additional improvements (including optimization of retrieval algorithm parameters and upscaling approaches) and methodology expansions (including increasing the number of core sites, model-based intercomparisons, and results from several intensive field campaigns) are anticipated in moving from accuracy assessment of the beta quality data to an evaluation of the fully validated L2_SM_P data product.

The Canadian Experiment for Soil Moisture in 2010 (CanEx-SM10) was carried out in Saskatchewan, Canada from 31 May to 16 June, 2010. Its main objective was to contribute to Soil Moisture and Ocean salinity (SMOS) mission validation and the pre-launch assessment of Soil Moisture and Active and Passive (SMAP) mission. During CanEx-SM10, SMOS data as well as other passive and active microwave measurements were collected by both airborne and satellite platforms. Ground-based measurements of soil (moisture, temperature, roughness, bulk density) and vegetation characteristics (Leaf Area Index, biomass, vegetation height) were conducted close in time to the airborne and satellite acquisitions. Besides, two ground-based in situ networks provided continuous measurements of meteorological conditions and soil moisture and soil temperature profiles. Two sites, each covering 33 km x 71 km (about two SMOS pixels) were selected in agricultural and boreal forested areas in order to provide contrasting soil and vegetation conditions. This paper describes the measurement strategy, provides an overview of the data sets and presents preliminary results. Over the agricultural area, the airborne L-band brightness temperatures matched up well with the SMOS data. The Radio frequency interference (RFI) observed in both SMOS and the airborne L-band radiometer data exhibited spatial and temporal variability and polarization dependency. The temporal evolution of SMOS soil moisture product matched that observed with the ground data, but the absolute soil moisture estimates did not meet the accuracy requirements (0.04 m3/m3) of the SMOS mission. AMSR-E soil moisture estimates are more closely correlated with measured soil moisture.