Showing papers on "Radio wave published in 2009"

Ionospheric Effects on GPS

Abstract: T HE ionosphere is an important source of range and range-rate errors for users of the global positioning system (GPS) satellites who require highaccuracy measurements. At times, the range errors of the troposphere and the ionosphere can be comparable, but the variability of the Earth's ionosphere is much larger than that of the troposphere, and it is more difficult to model. The ionospheric range error can vary from only a few meters, to many tens of meters at the zenith, whereas the tropospheric range error at the zenith is generally between two to three meters. Fortunately, the ionosphere is a dispersive medium; that is, the refractive index is a function of the operating frequency, and twofrequency GPS users can take advantage of this property of the ionosphere to measure and correct for the first-order ionospheric range and range-rate effects directly. Unlike the troposphere, the ionosphere can change rapidly in absolute value. Although the range error of the troposphere generally does not change by more than ±10%, even over long periods of time, the ionosphere frequently changes by at least one order of magnitude during the course of each day. The major effects the ionosphere can have on GPS are the following: 1) group delay of the signal modulation, or absolute range error; 2) carrier phase advance, or relative range error; 3) Doppler shift, or range-rate errors; 4) Faraday rotation of linearly polarized signals; 5) refraction or bending of the radio wave; 6) distortion of pulse waveforms; 7) signal amplitude fading or amplitude scintillation; and 8) phase scintillations. In order to understand the reasons for these potential effects on GPS performance, a brief description of the major characteristics of the ionosphere is necessary.

Ionospheric calibration of low frequency radio interferometric observations using the peeling scheme I. Method description and first results

Abstract: Calibration of radio interferometric observations becomes increasingly difficult towards lower frequencies. Below ?300 MHz, spatially variant refractions and propagation delays of radio waves traveling through the ionosphere cause phase rotations that can vary significantly with time, viewing direction and antenna location. In this article we present a description and first results of SPAM (Source Peeling and Atmospheric Modeling), a new calibration method that attempts to iteratively solve and correct for ionospheric phase errors. To model the ionosphere, we construct a time-variant, 2-dimensional phase screen at fixed height above the Earth’s surface. Spatial variations are described by a truncated set of discrete Karhunen-Loeve base functions, optimized for an assumed power-law spectral density of free electrons density fluctuations, and a given configuration of calibrator sources and antenna locations. The model is constrained using antenna-based gain phases from individual self-calibrations on the available bright sources in the field-of-view. Application of SPAM on three test cases, a simulated visibility data set and two selected 74 MHz VLA data sets, yields significant improvements in image background noise (5–75 percent reduction) and source peak fluxes (up to 25 percent increase) as compared to the existing self-calibration and field-based calibration methods, which indicates a significant improvement in ionospheric phase calibration accuracy.

Radio Imaging of the Very-High-Energy gamma-Ray Emission Region in the Central Engine of a Radio Galaxy

Abstract: The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.

Science with a lunar low-frequency array: from the dark ages of the Universe to nearby exoplanets

Abstract: 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 need for setting 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 and other 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. Whichever science is deemed most important, pathfinder arrays are needed to test the feasibility of these experiments in the not too distant future. Lunar low-frequency arrays are thus a timely option to consider, offering the potential for significant new insights into a wide range of today's crucial scientific topics. This would open up one of the last unexplored frequency domains in the electromagnetic spectrum.

Modeling and Characterization of Biotelemetric Radio Channel From Ingested Implants Considering Organ Contents

Abstract: The paper introduces numerical and experimental investigations of biotelemetry radio channels and wave attenuation in human subjects with ingested wireless implants. The study covers commonly used frequencies in telemedicine applications: ultrahigh frequencies at 402 MHz, 868 MHz and the industrial, scientific and medical (ISM) band frequency at 2.4 GHz. Numerical electromagnetic analysis is applied to model in/on-body radio propagation channels and the resulted parameters demonstrated the importance of digital phantom accuracy in ccharacterization of wave absorption and attenuation with regards to organ contents, specifically for the digestion system. Path gain variations of biotelemetry radio channels, in the close vicinity of the subject, with wireless implants were measured using a near field scanner. Simulation results were verified with measurement in good agreement.

Continuous Profiles of Electromagnetic Wave Velocity and Water Content in Glaciers: An Example from Bench Glacier, Alaska, USA

Abstract: We conducted two-dimensional continuous multi-offset georadar surveys on Bench Glacier, south-central Alaska, USA, to measure the distribution of englacial water. We acquired data with a multichannel 25 MHz radar system using transmitter-receiver offsets ranging from 5 to 150 m. We towed the radar system at 5-10 km h -1 with a snow machine with transmitter/receiver positions established by geodetic-grade kinematic differentially corrected GPS (nominal 0.5 m trace spacing). For radar velocity analyses, we employed reflection tomography in the pre-stack depth-migrated domain to attain an estimated 2% velocity uncertainty when averaged over three to five wavelengths. We estimated water content from the velocity structure using the complex refractive index method equation and use a three-phase model (ice, water, air) that accounts for compression of air bubbles as a function of depth. Our analysis produced laterally continuous profiles of glacier water content over several kilometers. These profiles show a laterally variable, stratified velocity structure with a low-water- content (� 0-0.5%) shallow layer (� 20-30 m) underlain by high-water-content (1-2.5%) ice.

Progress in Measurements of the Gravitational Bending of Radio Waves Using the VLBA

Abstract: We have used the Very Long Baseline Array (VLBA) at 43, 23, and 15 GHz to measure the solar gravitational deflection of radio waves among four radio sources during an 18 day period in 2005 October Using phase-referenced radio interferometry to fit the measured phase delay to the propagation equation of the parameterized post-Newtonian formalism, we have determined the deflection parameter γ = 09998 ± 00003 (68% confidence level), in agreement with general relativity The results come mainly from 43 GHz observations where the refraction effects of the solar corona were negligible beyond 3 deg from the Sun The purpose of this experiment is three-fold: to improve on the previous results in the gravitational bending experiments near the solar limb; to examine and evaluate the accuracy limits of terrestrial VLBI techniques; and to determine the prospects and outcomes of future experiments Our conclusion is that a series of improved designed experiments with the VLBA could increase the presented accuracy by at least a factor of 4

Study of propagation loss prediction in forest environment

Abstract: A comprehensive review of radio wave attenuation in forest environments is presented in this paper. The classic analytical methods of propagation loss modeling and prediction are described flrst. This provides information on the physical processes that the radio waves undergo while propagating through a forest. The focus of this paper is on the review and summary of the experimental work done in this area and the development of empirical propagation loss prediction models. The propagation loss variation due to external factors such as antenna height-gain, depolarization, humidity efiect etc. are examined and discussed individually. In view of current research work done in this area, some possible future work is proposed to improve the performance of radio links in forest environment.

Incoherent Method of Optical Interference Cancellation for Radio-Frequency Communications

Abstract: In this paper, we describe a system for cancelling radio-frequency (RF) interference using optical techniques. Specifically, we attempt to receive a weak RF signal-which we assume to be of the order of microwatts-in the presence of high-power local RF interference. This local interference is a signal whose power is of the order of 100 W and is generated in close proximity to the receiver. We wish to emphasize that the nature of the interfering signal is completely known to us in practice, since we are generating it for communications purposes. This knowledge of the interfering signal will prove to be useful in our attempts to cancel it, as will be shown. We refer to this technique as optical interference cancellation, or opto-cancellation. We have demonstrated that this opto-cancellation system can cancel a simple sinusoid at 3 GHz, as well as broadband interference of approximately 100-MHz bandwidth centered at 3 GHz. We have also demonstrated cancellation of sinusoids and broadband signals at other center frequencies as well. In the case of sinusoidal signals, we have demonstrated optical cancellation over 70 dB; and in the case of the ~ 100-MHz signal, we have demonstrated optical cancellation over 30 dB.

Beamforming devices and methods

Abstract: Devices and methods are provided for directionally receiving and/or transmitting acoustic waves and/or radio waves for use in applications such as wireless communications systems and/or radar. High directional gain and spatial selectivity are achieved while employing an array of receiving antennas that is small as measured in units of the wavelength of radio waves being received or transmitted, especially in the case of spatially oversampled arrays. Frequency/wavenumber, multi-dimensional spectrum analysis, as well as one-dimensional frequency spectrum analysis can be performed.

Progress in Measurements of the Gravitational Bending of Radio Waves Using the VLBA

Abstract: We have used the Very Long Baseline Array (VLBA) at 43, 23 and 15 GHz to measure the solar gravitational deflection of radio waves among four radio sources during an 18-day period in October 2005. Using phase-referenced radio interferometry to fit the measured phase delay to the propagation equation of the parameterized post-Newtonian (PPN) formalism, we have determined the deflection parameter gamma = 0.9998 +/- 0.0003$ (68% confidence level), in agreement with General Relativity. The results come mainly from 43 GHz observations where the refraction effects of the solar corona were negligible beyond 3 degrees from the sun. The purpose of this experiment is three-fold: to improve on the previous results in the gravitational bending experiments near the solar limb; to examine and evaluate the accuracy limits of terrestrial VLBI techniques; and to determine the prospects and outcomes of future experiments. Our conclusion is that a series of improved designed experiments with the VLBA could increase the presented accuracy by at least a factor of 4.

Boolean Chaos

Abstract: We observe deterministic chaos in a simple network of electronic logic gates that are not regulated by a clocking signal The resulting power spectrum is ultra-wide-band, extending from dc to beyond 2 GHz The observed behavior is reproduced qualitatively using an autonomously updating Boolean model with signal propagation times that depend on the recent history of the gates and filtering of pulses of short duration, whose presence is confirmed experimentally Electronic Boolean chaos may find application as an ultra-wide-band source of radio waves

Determining the relativistic parameter γ using very long baseline interferometry

Abstract: Aims. Relativistic bending in the vicinity of a massive body is characterized only by the post-Newtonian parameter γ within the standard parameterized post-Newtonian formalism, which is unity in General Relativity. To estimate this parameter, we use very long baseline interferometry (VLBI) to measure the gravitational deflection of radio waves by Solar System bodies emitted by distant compact radio sources. Methods. We analyze geodetic VLBI observations recorded since 1979. We compare estimates of γ and errors obtained with various analysis schemes, including global estimations over several time spans and with various Sun elongation cut-off angles, and with analysis of radio source coordinate time series. Results. We arrive at the conclusion that the relativistic parameter γ cannot be estimated at better than 2 × 10 −4 . The main factor of limitation is the uncertainty in determining of (global or session-wise) radio source coordinates. A sum of various instrumental and modeling errors and analysis strategy defects, which cannot be decorrelated and corrected yet, is at the origin of the limitating noise.

A Decade of SN 1993J: Discovery of Radio Wavelength Effects in the Expansion Rate

Abstract: We studied the growth of the shell-like radio structure of supernova SN 1993J in M 81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18 cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN 1993J. The source structure of SN 1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J until approximately day 1500 after explosion with an expansion parameter m = 0.845 ± 0.005 (R ∝ t m ). However, from that day onwards the expansion differs when observed at 6 and 18 cm. Indeed, at 18 cm, the expansion can be well characterized by the same m as before day 1500, while at 6 cm the expansion appears more decelerated, and is characterized by another expansion parameter, m6 = 0.788 ± 0.015. Therefore, since about day 1500 onwards, the radio source size has been progressively smaller at 6 cm than at 18 cm. These findings differ significantly from those of other authors in the details of the expansion. In our interpretation, the supernova expands with a single expansion parameter, m = 0.845 ± 0.005, and the 6 cm results beyond day 1500 are caused by physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6 cm radiation; and (b) a radial decrease of the magnetic field in the emitting region. We also found that at 6 cm about 80% of the radio emission from the backside of the shell behind the ejecta is absorbed (our average estimate, since we cannot determine any possible evolution of the opacity), and the width of the radio shell is (31 ± 2)% of the outer radius. The shell width at 18 cm depends on the degree of assumed absorption. For 80% absorption, the width is (33.5 ± 1.7)%, and for 100% absorption, it is (37.8 ± 1.3)%. A comparison of our VLBI results with optical spectral line velocities shows that the deceleration is more pronounced in the radio than in the optical. This difference might be due to a progressive penetration of ejecta instabilities into the shocked circumstellar medium, as also suggested by other authors.

Determination of the relativistic parameter gamma using very long baseline interferometry

Abstract: Relativistic bending in the vicinity of a massive body is characterized only by the post-Newtonian parameter $\gamma$ within the standard parameterized post-Newtonian formalism, which is unity in General Relativity. Aiming at estimating this parameter, we use very long baseline interferometry (VLBI) to measure the gravitational deflection of radio waves emitted by distant compact radio sources, by Solar System bodies. We analyze geodetic VLBI observations recorded since 1979. We compare estimates of $\gamma$ and errors obtained using various analysis schemes including global estimations over several time spans and with various Sun elongation cut-off angles, and analysis of radio source coordinate time series. We arrive at the conclusion that the relativistic parameter $\gamma$ cannot be estimated at better than $2\times10^{-4}$. The main factor of limitation is the uncertainty in the determination of (global or session-wise) radio source coordinates. A sum of various instrumental and modeling errors and analysis strategy defects, that cannot be decorrelated and corrected yet, is at the origin of the limitating noise.

Determination of the electron temperature in the modified ionosphere over HAARP using the HF pumped Stimulated Brillouin Scatter (SBS) emission lines

Abstract: . An ordinary mode electromagnetic wave can decay into an ion acoustic wave and a scattered electromagnetic wave by a process called stimulated Brillouin scatter (SBS). The first detection of this process during ionospheric modification with high power radio waves was reported by Norin et al. (2009) using the HAARP transmitter in Alaska. Subsequent experiments have provided additional verification of this process and quantitative interpretation of the scattered wave frequency offsets to yield measurements of the electron temperatures in the heated ionosphere. Using the SBS technique, electron temperatures between 3000 and 4000 K were measured over the HAARP facility. The matching conditions for decay of the high frequency pump wave show that in addition to the production of an ion-acoustic wave, an electrostatic ion cyclotron wave may also be produced by the generalized SBS processes. Based on the matching condition theory, the first profiles of the scattered wave amplitude are produced using the stimulated Brillouin scatter (SBS) matching conditions. These profiles are consistent with maximum ionospheric interactions at the upper-hybrid resonance height and at a region just below the plasma resonance altitude where the pump wave electric fields reach their maximum values.

Unprecedentedly Strong and Narrow Electromagnetic Emissions Stimulated by High-Frequency Radio Waves in the Ionosphere

Abstract: Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in high-frequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.

In-situ measurement procedures for temporal RF electromagnetic field exposure of the general public.

Abstract: In this paper, the general public's exposure to FM, GSM, and UMTS over 7 day's time is investigated. The purpose of this paper is to investigate how short-period measurements can be representative for the actual maximal and average exposure during longer periods such as 1 week. Locations of public RF exposure have been categorized according to the type of environment, population density, and the amount of mobile phone traffic. Five different sites have been selected to perform measurements of the electric fields over time. In total 352,800 time samples of the electric field were obtained from the measurement campaign. A factor X is defined as the ratio between the actual maximal value of the temporal measurements and the estimated maximal value from short-period data. Three different methods to assess X are compared and an optimal method is proposed for an in-situ measurement procedure. Median values of X according to the proposed method are 1.05, 0.47, and 0.96, for FM, GSM, and UMTS, respectively. Moreover a factor R is defined as the ratio between the median and maximal value of the momentary temporal field measurements, indicating the level of variation of a certain signal over time. R enables to calculate maximal values from median values and vice versa. Median values of R are 0.92, 0.66, and 0.71 for FM, GSM, and UMTS, respectively. By combining X and R one can estimate the actual maximal and median exposure during longer periods from short-period measurements.

Radio pumping of ionospheric plasma with orbital angular momentum.

Abstract: Experimental results are presented of pumping ionospheric plasma with a radio wave carrying orbital angular momentum (OAM), using the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Optical emissions from the pumped plasma turbulence exhibit the characteristic ring-shaped morphology when the pump beam carries OAM. Features of stimulated electromagnetic emissions (SEE) that are attributed to cascading Langmuir turbulence are well developed for a regular beam but are significantly weaker for a ring-shaped OAM beam in which case upper hybrid turbulence dominates the SEE.

A Photon Modeling Method for the Characterization of Indoor Optical Wireless Communication

Abstract: In this paper, an analysis method for optical wave propagation based on photon model is presented for the characterization of optical wireless communication environment. In contrast to radio waves, optical waves have very short wavelengths, so that material properties become important and often cause diffuse reflections. Channel models including diffuse reflections and absorption effects due to material surface textures make conventional electromagnetic wave analysis methods based on ray tracing consume enormous time. To overcome these problems, an analysis method using photon model is presented that approximates light intensity by density of photons. The photon model also ensures that simulation time is within a predictable limit and the accuracy is proportional to the number of total photons used in the simulation.

Composition and Structure of the Ionosphere and Thermosphere

Abstract: Airglow emissions, radio and solar occultation data from the Voyager mission over a quarter of a century ago provided the main source of information on the composition and structure of Titan's upper atmosphere and ionosphere until October 2004, when the Cassini Orbiter first encountered Titan during the “Ta” fly-by. During this encounter, in situ measurements were made by many instruments onboard the Orbiter, including the Ion and Neutral Mass Spectrometer (INMS), the Radio Wave and Plasma Wave Spectrometer (RPWS), the Magnetometer (MAG), and the Cassini Plasma Spectrometer (CAPS). For example, INMS measurements confirmed that the major neutral species were molecular nitrogen and methane. Other species detected included mole cular hydrogen, acetylene, ethylene, benzene, and propane. The Langmuir probe part of the RPWS experiment observed substantial ionospheric electron densities and measured electron temperatures significantly exceeding the neutral temperature. A large set of data on the upper atmosphere and ionosphere has been collected during the many Titan encounters following Ta. The first composition measurements for the ionosphere were made by INMS during the outbound leg of the T5 pass in April 2005. A rich and complex ion-neutral chemistry scheme was predicted prior to the Cassini mission and the INMS composition data indeed revealed the presence of a very large number of ion species, both predicted and unpre-dicted. Stellar occultation measurements made by the Cassini Ultraviolet Spectrometer (UVIS) provided important information on the structure and composition of Titan's upper atmosphere, and radio occultation measurements made by the Radio Science Subsystem (RSS) revealed the existence of a substantial ionosphere even for altitudes below 1000 km. The discovery of negative ions in the ionosphere was also very exciting. A vigorous modeling effort aimed at explaining the structure and composition of the upper atmosphere and ionosphere is helping to put the data into a broader theoretical context. For example, solar extreme ultraviolet and x-ray radiation and energetic electrons from Saturn's magnetosphere interact with the upper atmosphere producing the ionosphere and initiating a complex neutral and ion chemistry that has important effects extending deep into the atmosphere.

Effect of electric potential structures on Jovian S‐burst morphology

Abstract: [1] Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io-Jupiter interaction. A significant part of these emissions consists of short radio bursts (so-called S-bursts) drifting in time and frequency. Previous analyses suggest that these emissions are cyclotron-maser emissions in the flux tube connecting Io or Io's wake to Jupiter. We present simulations of these electrons under the assumption of acceleration by Alfven waves in the Io flux tube. Near Jupiter, a loss cone and a ring distribution appear in the magnetically mirrored electron population, which can then amplify extraordinary (X) mode radio waves. The X-mode growth rate is computed, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions. Additional potential structures are assumed in the Jovian auroral region. We reconstruct their impact on the morphology of the emission. They match some of the time-frequency patterns observed with Jovian S-bursts. This provides the first evidence of bipolar electrostatic structures in the Jovian auroral region.

Powerful electromagnetic waves for active environmental research in geospace

Abstract: Powerful electromagnetic (EM) waves can exert well-defined influence on the atmosphere, ionosphere, and magnetosphere. These active EM interactions can provide spatiotemporal information on the nea ...

Effect of TEC Variation on GPS Precise Point at Low Latitude

Abstract: The ionosphere is a dispersive medium of charged particles between the satellite and the user on Earth. These dispersive ionized media play a vital role in the various applications of GPS (Global Positioning Systems) because the ionosphere directly influences transionospheric radio waves propagating from the satellite to the receiver. Solar flares af- fect the ionization state of the ionosphere with their high intensity. Sometimes the intensity is so severe that it accelerates the rate of ionization, resulting in ionospheric storms; during the ionospheric storms the concentration of charged particles varies. Among the various phenomena in the ionosphere, TEC (Total Electron Content) is responsible for range error which produces a time delay in the radio signal. The rate of change of TEC with respect to time is abbreviated as ROT. It is one of the parameters that express the ionospheric irregularities with respect to time. This work investigates the effect of ROT fluctuation on the precise positioning of GPS receivers during low solar activity periods in the equatorial anomaly region. Good geometry and a sufficient number of locked satellites provide more accuracy within the centimeter level, but the case may be different when there are any ionospheric storms. Even a few satellite signals passing through the iono- spheric irregularities can cause a significant error in positioning. Thus, it is important to understand the ionospheric ir- regularities observed by GPS receivers in order to correct them. The ROT (TEC/Minute) parameter is used here to study the occurrence of TEC fluctuation and its potential effect on GPS, such as a horizontal positional error or the satellite ge- ometry of the GPS receiver. This investigation is based on the analysis of a one-year observation of a fixed GPS receiver installed at Bhopal (23.202 0 N, 77.452 0 E), India during low solar active period in 2005. The GPS receiver used here is a GISTM-based dual frequency NovAtel OEM4 GPS receiver.

Ionospheric Calibration of Low Frequency Radio Interferometric Observations using the Peeling Scheme: I. Method Description and First Results

Abstract: Calibration of radio interferometric observations becomes increasingly difficult towards lower frequencies. Below ~300 MHz, spatially variant refractions and propagation delays of radio waves traveling through the ionosphere cause phase rotations that can vary significantly with time, viewing direction and antenna location. In this article we present a description and first results of SPAM (Source Peeling and Atmospheric Modeling), a new calibration method that attempts to iteratively solve and correct for ionospheric phase errors. To model the ionosphere, we construct a time-variant, 2-dimensional phase screen at fixed height above the Earth's surface. Spatial variations are described by a truncated set of discrete Karhunen-Loeve base functions, optimized for an assumed power-law spectral density of free electrons density fluctuations, and a given configuration of calibrator sources and antenna locations. The model is constrained using antenna-based gain phases from individual self-calibrations on the available bright sources in the field-of-view. Application of SPAM on three test cases, a simulated visibility data set and two selected 74 MHz VLA data sets, yields significant improvements in image background noise (5-75 percent reduction) and source peak fluxes (up to 25 percent increase) as compared to the existing self-calibration and field-based calibration methods, which indicates a significant improvement in ionospheric phase calibration accuracy.