scispace - formally typeset
Search or ask a question

Showing papers in "Radio Science in 1997"


Journal ArticleDOI
TL;DR: In this article, an accurate upper bound to the dimension of such a space is evaluated in both the single incidence and multiview cases, and an optimal sampling strategy for the monostatic radar cross section is also provided.
Abstract: With reference to inverse scattering from an unknown object of limited extension embedded in a homogeneous background at a fixed frequency, we show that only a finite-dimensional representation of the unknown contrast can be hopefully retrieved. Exploiting the quasi-band-limitedness property of scattered fields, an accurate upper bound to the dimension of such a space is evaluated in both the single incidence and multiview cases. Moreover, effective schemes are given to collect all the information available from the scattering experiments in a nonredundant manner. As a by-product, an optimal (minimally redundant) sampling strategy for the monostatic radar cross section is also provided. Finally, we briefly discuss how the requirement for a globally effective and reliable solution scheme can lead to a reduction of the actually retrievable information.

336 citations


Journal ArticleDOI
TL;DR: In this paper, the GPS satellite and receiver instrumental biases from 19 months of data and the study of their variation during that time are presented. But the main conclusion of this work is that due to the stability of the GPS instrumental biases, only an estimation or calibration of them from time to time is required.
Abstract: The main source of error in the estimation of TEC (total electron content) from dual Global Positioning System (GPS) data is the effect of the differential satellite and receiver instrumental delay biases. These biases are normally estimated simultaneously with the TEC. However, the additional estimation of the instrumental biases may constitute an insurmountable burden in some practical applications like real-time estimation of TEC, or the estimation may be difficult or correlated to the ionospheric parameters, particularly in situations where the TEC behavior may be harder to model (equatorial or auroral zone, ionospheric storms, etc.). A priori values of the instrumental biases, estimated under good conditions or with global networks, could solve those problems if we could determine how stable those instrumental biases are in time and how often we need to check or reestimate their values. In this paper we will present our estimation of the GPS satellite and receiver instrumental biases from 19 months of data and the study of their variation during that time. We will also show some situations of changes in the instrumental biases and the possible influence of antispoofmg (AS). The main conclusion of this work is that the variation of the estimated differential GPS satellite biases during the 19 months is smaller than 1 ns (1 ns = 2.86 × 1016 e/m2) in most of the cases, with a mean RMS of 0.15 ns. For the GPS receivers used, that variation is greater than for the satellites, with the larger variations corresponding to physical changes in the receivers. The difference of the estimated differential instrumental biases between two consecutive days is in practically all cases smaller than 0.5 ns for the GPS satellites and smaller than 1 ns for the GPS receivers. Regarding the influence of AS, we have detected some significant changes in the instrumental biases of some satellites and some stations whether AS is activated or not. Our main conclusion is that due to the stability of the GPS instrumental biases, only an estimation or calibration of them (under optimal conditions) from time to time is required.

224 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the Scintillation network decision aid, which consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites.
Abstract: The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short-term forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.

206 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the opportunities to use the Global Positioning System (GPS) for ionospheric total electron content (TEC) research and showed that with appropriate smoothing, GPS TECs can be used to extend the existing database.
Abstract: Opportunities to use the Global Positioning System (GPS) for ionospheric total electron content (TEC) research are reviewed. The era of TEC measurements using very high frequency geosynchronous beacons is essentially over, and the new GPS TECs need to be treated in special ways if they are to augment the existing database of total electron content. Data taken at Boulder, Colorado, show that with appropriate smoothing, GPS TECs can be used to extend the existing database. The time delay data from the International GPS Geodynamics Service over central Europe are used to map the total electron content and reveal regional structures. Global electron-density profiles can now be measured using the GPS/MET (meteorology) system. The measurement of precipitable water vapor in the troposphere to an accuracy of 10% requires that 99.9% of the ionospheric delay be removed.

175 citations


Journal ArticleDOI
TL;DR: The ESR system design was adapted to make use of commercial off‐the‐shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum, and it is now being operated by EISCAT staff on a campaign basis, to provide ground‐based support data for a number of other magnetospheric satellites, notably Polar and FAST.
Abstract: The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off-the-shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State-of-the-art exciter and receiver hardware has been developed in-house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron-induced spurious Doppler effects from the data. Intermediate-frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground-based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.

148 citations


Journal ArticleDOI
TL;DR: FASoft as discussed by the authors is a system for discrete channel frequency assignment that incorporates state-of-the-art heuristics, sequential assignment algorithms, and a maximal clique algorithm to aid in the assignment process.
Abstract: This paper describes a system, FASoft, for discrete channel frequency assignment. In practice, the assignment of frequencies in a network of compatible equipment is often done manually or by the use of a single computational technique. FASoft incorporates state-of-the-art heuristics, sequential assignment algorithms, and a maximal clique algorithm to aid in the assignment process. Lower bounding procedures are included into the system to assess the performance of the assignment techniques and to provide an assessment of how close a particular assignment is to the optimal. The results show that FASoft produces optimal solutions to several practical examples.

112 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used 30-s samples of total electron content obtained from phase differences between 1.2 and 1.6 GHz signals as gathered by the International GPS Service for Geodynamics (IGS).
Abstract: GPS multisite and multisatellite observations of phase fluctuations are used to characterize irregularity development in the equatorial region. We use 30-s samples of total electron content obtained from phase differences between 1.2 and 1.6-GHz signals as gathered by the International GPS Service for Geodynamics (IGS). These observations from a number of ground receivers allowed for simultaneous studies over a range of latitudes and longitudes in the region near and distant from the magnetic equator. Several storm periods were analyzed in the time period from November 3,1993, to October 2,1995. By examining phase fluctuations at latitudes near the magnetic equator, in the anomaly region, and poleward of the anomaly region, it was found that very high altitudes were reached by plumes of irregularities during seven magnetic storms in solar minimum years. These also occurred during periods of magnetic quiet. The data set allowed the determination of effective altitudes of equatorial irregularities. During low solar flux years, there was a plethora of different altitude regimes from thin layers to altitudes over 2000 km.

111 citations


Journal ArticleDOI
TL;DR: In this paper, simultaneous and colocated comparisons of winds and tides by MF and meteor VHF radars have been made in the 85-94 km height range from July 1994 to June 1996 over London, Ontario, Canada.
Abstract: Simultaneous and colocated comparisons of winds and tides by MF and meteor VHF radars have been made in the 85–94 km height range from July 1994 to June 1996 over London, Ontario, Canada. Results have been obtained for every month of the year. From these comparisons it is concluded that, in general, the MF spaced antenna technique and the meteor method both provide reliable means for synoptic studies of neutral air motions in the height range 85–94 km, at timescales of greater than 12 hours, and therefore are valuable tools in middle atmospheric research. However, we do find that some parameters seem to be estimated with greater precision than others. In particular, the measurements of the zonal long-term wind variations, and the semidiurnal tide in both the zonal and meridional directions, seem particularly robust and reliable, while there is somewhat less consistency between measurements of the zonal diurnal tide and the meridional monthly mean winds. The former problem is very likely to be an artifact of the strong diurnal variation in meteor count rates. However, we cannot claim that our meridional monthly mean wind agreement is always good, and these discrepancies deserve further investigation.

109 citations


Journal ArticleDOI
TL;DR: In this article, the authors describe a method for radar remote sensing of the upper atmosphere which relies upon commercial FM broadcasts near 100 MHz, which have high average power and excellent radar ambiguity function.
Abstract: We describe a novel method for radar remote sensing of the upper atmosphere which relies upon commercial FM broadcasts near 100 MHz. These broadcasts have high average power and excellent radar ambiguity function. With proper processing we can study the spatial and temporal distribution and Doppler spectrum with excellent and completely unambiguous resolution. Since this passive system has no transmitter, there are enormous benefits in safety, expense, shielding, antenna and receiver design, and licensing issues. Some new problems are introduced, but these are solved with relatively little expense. After presenting the technical basis for such a radar, we describe an instrument that we are building at the University of Washington to study high-latitude plasma irregularities in the E region.

107 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the vertical resolution that can be achieved in atmospheric profiles retrieved from radio occultation measurements using the multiple phase-screen method and found that profiles retrieved through Abel inversion, the standard algorithm derived from geometrical optics, have a vertical resolution which is diffraction-limited, as expected.
Abstract: We have investigated the vertical resolution that can be achieved in atmospheric profiles retrieved from radio occultation measurements. The results are based on forward simulations of radio wave propagation through model atmospheres using the multiple phase-screen method. We find that profiles retrieved through Abel inversion, the standard algorithm derived from geometrical optics, have a vertical resolution that is diffraction-limited, as expected. To overcome this limitation, we have developed an advanced retrieval algorithm, which is based on scalar diffraction theory and properly accounts for diffraction effects. We demonstrate that the method is capable of retrieving accurate refractivity profiles at sub-Fresnel-scale resolution for Mars- and Earth-like atmospheres. It also provides a natural means for deciphering multipath propagation effects. The method seems capable of enhancing resolution by a factor of 10 beyond the diffraction limit. In one simulation involving an Earth-like model atmosphere the algorithm successfully retrieved a profile in which the refractive index changed by 10−5 over a vertical scale of 250 m. The maximum vertical gradient of refractive index within this small-scale feature was about 0.8×10−7 m−1. For comparison, critical refraction occurs for a gradient of about 1.6×10−7 m−1. The feature was embedded in a smooth refractive index profile at the level where the pressure and mean refractive index were 42 kPa and 1.00013, respectively. Total refractive bending for a ray that grazed this level was 0.01–0.02 rad.

104 citations


Journal ArticleDOI
TL;DR: In this article, the authors compared the accuracy of GIM and TOPEX with respect to global ionospheric TEC (total electron content) measurements, and found that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model.
Abstract: Global ionospheric mapping (GIM) is a new and emerging technique for determining global ionospheric TEC (total electron content) based on measurements from a worldwide network of Global Positioning System (GPS) receivers. In this study, GIM accuracy in specifying TEC is investigated by comparison with direct ionospheric measurements from the TOPEX altimeter. A climatological model (Bent model) is also used to compare with the TOPEX altimeter data. We find that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model. The difference between GIM and TOPEX in TEC measurements is very small (less than 1.5 TEC units (TECU)) within a 1500-km range from a reference GPS station. The RMS gradually increases with increasing distance from the station, while the Bent model shows a constant large RMS, unrelated to any station location. Within a 1000-km distance of a GPS site (elevation angle > 25°), GIM has a good correlation (R > 0.93) to TOPEX with respect to TEC measurements. The slope of the linear fitting line to the data set from two TOPEX cycles is 44.5° (near the ideal 45°). In the northern hemispheric regions, ionospheric specification by GIM appears to be accurate to within 3-10 TECU up to 2000+ km away from nearest GPS station (corresponding to ∼1° elevation angle cutoff). Beyond 2000 km, GIM accuracy, on average, is reduced to the Bent model levels. In the equatorial region, the Bent model predictions are systematically lower (∼5.0 TECU) than TOPEX values and often show a saturation at large TEC values. During ionospheric disturbed periods, GIM sometimes shows differences from TOPEX values due to transient variations of the ionosphere. Such problems may be improved by the continuous addition of new GPS stations in data-sparse regions. Thus, over a GPS station's measurement realm (up to 2000 km in radius), GIM can produce generally accurate TEC values. Through a spatial and temporal extrapolation of GPS-derived TEC measurements, the GIM technique provides a powerful tool for monitoring global ionospheric features in near real time.

Journal ArticleDOI
TL;DR: The availability of real-time electron density profiles from a network of stations makes it possible to calculate the three-dimensional electron density distribution in the region of interest using Fourier transform techniques, which are the basis for the OTH radar coordinate registration.
Abstract: Precise coordinate registration for HF over-the-horizon (OTH) radar applications requires accurate knowledge of the ionospheric structure. In the mid-1980s Digisonde 256 systems were deployed in the American sector to provide this information from strategically located sites via telephone lines to the user. The mid-1990s saw the development of a new advanced system, the Digisonde portable sounder, or DPS, now being deployed in Australia in support of the Australian OTH radar system. A summary of the new features provided by the DPS is as follows: low radio frequency power (300 W); narrow transmission bandwidth; advanced automatic scaling; and control and data access via the Internet. The availability of real-time electron density profiles as function of time from a network of stations makes it possible to calculate the three-dimensional electron density distribution in the region of interest using Fourier transform techniques. The resulting density maps are the basis for the OTH radar coordinate registration. The DPS uses Doppler interferometry to determine the development of ionospheric irregularities.

Journal ArticleDOI
TL;DR: In this article, the adaptive classification of the rays received from a constellation of geodetic satellites (the Global Positioning System (GPS)) by a set of ground receivers is performed using neural networks.
Abstract: The adaptative classification of the rays received from a constellation of geodetic satellites (the Global Positioning System (GPS)) by a set of ground receivers is performed using neural networks. This strategy allows us to improve the reliability of reconstructing the ionospheric electron distribution from GPS data. As an example, we present the evolution at global scale of the radially integrated electron density (total electron content (TEC)) for October 18, 1995, coinciding with an important geomagnetic storm.

Journal ArticleDOI
TL;DR: In this article, the high-latitude sections of the Wideband ionospheric scintillation model (WBMOD) have been upgraded extensively, based on analysis of data collected at Sondre Stromfjord, Greenland; Tromso, Norway; Fort Churchill, Canada; and Bellevue, Washington ( United States) over a 4-year period.
Abstract: The high-latitude sections of the Wideband ionospheric scintillation model (WBMOD) have been upgraded extensively, based on analysis of scintillation data from the Defense Nuclear Agency Wideband, HiLat, and Polar BEAR satellite-beacon experiments. Data collected at Sondre Stromfjord, Greenland; Tromso, Norway; Fort Churchill, Canada; and Bellevue, Washington (United States) over a 4-year period were analyzed, and the results of these analyses were used to construct a completely new model for the behavior of the height-integrated irregularity-strength parameter (CkL) at high latitudes. The new high-latitude CkL model includes variations with sunspot number, geomagnetic activity (Kp), latitude, local time, longitude, and season. The new WBMOD CkL models (equatorial and high-latitude) have been implemented in a more versatile code, denoted SCINTMOD, which has the capability to generate a wide range of user-controlled maps of scintillation effects over large spatial areas. Examples of the types of graphical output that SCINTMOD can generate are presented.

Journal ArticleDOI
TL;DR: In this article, the prediction of long-term fade duration statistics in slant paths, obtained from rain attenuation time series simulated with the synthetic storm technique (with input from 1-min rain rate time series).
Abstract: The paper discusses the prediction of long-term fade duration statistics in slant paths, obtained from rain attenuation time series simulated with the synthetic storm technique (with input from 1-min rain rate time series). The results of the prediction were tested against the experimental fade duration statistics collected at three sites of the Sirio experiment in Italy at 11.6 GHz. Predicted and measured statistics are very similar for fade durations longer than about 64–128 s, which represent a fundamental lower limit to fade duration prediction using 1-min rain rate time series. The short durations have not been reproduced because they are mainly caused by tropospheric turbulence, not by the space-time structure of rain, well described statistically by the synthetic storm technique. The prediction is not sensitive to rainstorm speed.

Journal ArticleDOI
TL;DR: In this paper, the authors compared 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satellite System (NNSS) differential Doppler were compared with the corresponding TECs obtained from the Global Positioning System (GPS).
Abstract: About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satellite System (NNSS) differential Doppler were compared with the corresponding TECs obtained from the Global Positioning System (GPS). The method by which the comparison is effected is also described. The resulting statistics are very effective in showing a good agreement between the two techniques. The GPS TEC results are statistically larger than NNSS TEC, namely, 3±1 TEC units; the lower quartile of the differences is always greater than zero.

Journal ArticleDOI
TL;DR: In this article, the statistics of HF scattering occurrence were reported for 5.5 years (1988 to mid-1993) of routine observations with the Super Dual Auroral Radar Network (SuperDARN) located at Goose Bay, Labrador, Canada, and corresponds to the most recent period of solar cycle maximum.
Abstract: The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide continuous monitoring of the high-latitude ionosphere over large areas in both the northern and southern hemispheres. The HF technique generates estimates of the E×B convective drift of ionospheric plasma when suitable small-scale (∼10 m) irregularities are present and viewed under favorable magnetic aspect conditions (radar k vector ⊥ to B). In this paper we report on the statistics of HF scattering occurrence. The study period encompasses 5.5 years (1988 to mid-1993) of routine observations with the HF radar located at Goose Bay, Labrador, Canada, and corresponds to the most recent period of solar cycle maximum. The data were carefully filtered for those instances when high-confidence F region velocity measurements were obtained within 1°×3° magnetic latitude/longitude regions and 12-min UT intervals. For average conditions the rates at which scatter occurred at particular values of invariant latitude ranged from 40% with MLT. The probability of making velocity measurements somewhere in the radar field of view varied from ∼80% on the nightside to ∼45% on the noon meridian. When scatter was detected, the average latitudinal coverage varied from a minimum of 4° Λ on the dayside to over 6° Λ on the nightside. There were significant dependencies on Kp and season. The highest occurrence rates (>60%) were obtained on the nightside for quiet conditions and in the afternoon for disturbed conditions. Winter was the most active season. HF scattering was limited by the equatorward boundary of the Feldstein oval but extended poleward of the oval into the polar cap. Scattering in the noon sector was associated with the aggregate cusp/mantle/low-latitude boundary layer region. The scattering activity was notably suppressed in the morning sector under disturbed conditions and in the noon and afternoon sectors in summer. We discuss the geophysical factors that influence the likelihood of generating HF scatter. These results have value for the design of experiments involving the SuperDARN HF radars.

Journal ArticleDOI
TL;DR: In this paper, the use of the standard finite difference time domain (FDTD) algorithm is extended to quasi-static electromagnetic field problems, by combining two plane waves in opposite directions, a uniform electric or magnetic field can be created so that the electric and magnetic field solutions are decoupled.
Abstract: In this paper, the use of the standard finite difference time domain (FDTD) algorithm is extended to quasi-static electromagnetic field problems. While straightforward application of the standard FDTD algorithm at very low frequencies leads to excessively long simulation times, we show that for linear structures this problem can be circumvented by using a ramp excitation function. The use of appropriate absorbing boundary conditions such as Berenger's perfectly matched layer is also shown to be necessary. By combining two plane waves in opposite directions, a uniform electric or magnetic field can be created so that the electric and magnetic field solutions are decoupled, as required in quasi-static analysis. Calculations of the induced fields and currents in a human model exposed to power line frequency fields provide a realistic example of an application of this novel FDTD technique.

Journal ArticleDOI
TL;DR: In this article, hardware, software, and design features of a new VHF atmospheric radar situated in Canada are described, with particular emphasis being placed on the flexibility which has been implemented at quite low cost.
Abstract: Hardware, software, and design features of a new VHF atmospheric radar situated in Canada are described, with particular emphasis being placed on the flexibility which has been implemented at quite low cost. Called CLOVAR (Canadian (London, Ontario) VHF atmospheric radar), the instrument has now been operational since November 1993. It is located at 43°04.44′N, 81°20.20′W, operates at a frequency of 40.68 MHz, and is owned and operated by the nearby University of Western Ontario in London, Ontario, Canada. There are some unique features about this system, including its low-cost design, flexible beam-steering, and on-line software analysis procedures. In this paper we elaborate on these new developments and especially demonstrate the new signal processing algorithms currently in use. These new algorithms include procedures for rejection of signals due to aircraft, removal of instrumental drift, and full on-line spectral fitting of Gaussian functions. Typical data from the system are presented, including experimental data acquired with multibeam experiments, monthly mean vertical velocities, and some interesting results obtained during a solar eclipse. The radar can also function as an efficient meteor radar for determination of high-level winds, and this capability will also be briefly described. A special program of comparisons with colocated radiosonde flights is also discussed.

Journal ArticleDOI
TL;DR: In this article, the relationship between the statistical covariance of these samples and that of the radiating object field to be imaged is discussed in a self-contained and comprehensive way.
Abstract: The recently developed technique for imaging radar scattering irregularities has opened a great scientific potential for ionospheric and atmospheric coherent radars. These images are obtained by processing the diffraction pattern of the backscattered electromagnetic field at a finite number of sampling points on the ground. In this paper, we review the mathematical relationship between the statistical covariance of these samples, ( †), and that of the radiating object field to be imaged, ( †), in a self-contained and comprehensive way. It is shown that these matrices are related in a linear way by ( †) = aM(FF†)M†a*, where M is a discrete Fourier transform operator and a is a matrix operator representing the discrete and limited sampling of the field. The image, or brightness distribution, is the diagonal of (FF†). The equation can be linearly inverted only in special cases. In most cases, inversion algorithms which make use of a priori information or maximum entropy constraints must be used. A naive (biased) “image” can be estimated in a manner analogous to an optical camera by simply applying an inverse DFT operator to the sampled field and evaluating the average power of the elements of the resulting vector . Such a transformation can be obtained either digitally or in an analog way. For the latter we can use a Butler matrix consisting of properly interconnected transmission lines. The case of radar targets in the near field is included as a new contribution. This case involves an additional matrix operator b, which is an analog of an optical lens used to compensate for the curvature of the phase fronts of the backscattered field. This “focusing” can be done after the statistics have been obtained. The formalism is derived for brightness distributions representing total powers. However, the derived expressions have been extended to include “color” images for each of the frequency components of the sampled time series. The frequency filtering is achieved by estimating spectra and cross spectra of the sample time series, in lieu of the power and cross correlations used in the derivation.

Journal ArticleDOI
TL;DR: The mesosphere-stratosphere-troposphere (MST) radar studies have seen significant advances in each of the areas of equipment, techniques, and general radar theory.
Abstract: The last few years have seen several significant advances in mesosphere-stratosphere-troposphere (MST) radar studies. This is true with respect to studies ranging from MF through VHF and up to UHF. These advances have been in each of the areas of equipment, techniques, and general radar theory. In this short paper we will highlight some of these advances and discuss their significance in the longer-term application of MST radar techniques. We will concentrate primarily on discussion about instrumentation and the raw products (powers, radial velocities, and spectral widths) produced by the systems, with some reference to the nature of the radio wave scattering entities. Quantities which are derived from the measurements of these raw quantities like gravity wave fluxes, tidal studies, and so forth will not generally be considered in any detail. Sample advances include new methods of data analysis (both on-line and post-collection), lightning detection, rainfall measurement, and temperature determination. Other interesting applications include simultaneous application of MST techniques with other procedures such as in radio acoustic sounding (RASS) and the artificial periodic inhomogeneity (API) method. Areas of advance in terms of understanding scattering mechanisms include new insights into the controversy about the nature of aspect-sensitive scattering (specular reflection compared with anisotropic turbulence) and improvements and refinements in our measurements of turbulence with these radars. Radars working at frequencies other than those mentioned, especially L and S band (1215–1710 and 1710–2700 MHz respectively), have also been used with good success, and these will be discussed where appropriate. The paper will concentrate on radars which can employ clear-air scattering; we will not consider in much detail systems which rely predominantly on precipitation and other scattering targets embedded in the air.

Journal ArticleDOI
TL;DR: In this paper, the location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere.
Abstract: Troughs in the latitudinal distribution of electron density are a well-known feature of the ionosphere from subauroral to polar latitudes. The location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere. In practice, the precise characteristics of trough-like structures have been difficult to monitor using ground-based methods. Ionospheric tomography represents a new development that is maturing into a technique ideally suited to the study of electron density troughs. Results are presented from a variety of observations made during tomographic campaigns in northern Europe. A long-term investigation has been made of the main trough from a network of stations in the United Kingdom. The position of the trough minimum and the wall gradients have been studied on a diurnal basis using tomographic images reconstructed from measurements for a succession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has also been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography continues to yield images while the limitations of ionosondes are exposed. Studies of narrow troughs with very steep gradients seen at auroral latitudes have been used to investigate some of the successes and limitations of the tomographic method. Measurements made in the polar cap show the depleted densities of the polar hole in the center of the dawn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high-latitude boundary between corotating and counterstreaming flux tubes in the afternoon sector has been revealed in a tomographic image extending over some 30° latitude, made using a chain of six stations in Scandinavia.

Journal ArticleDOI
TL;DR: In this article, the convergence of the higher-order quasi-linear approximations of the Born series is considered and a new approach is proposed to estimate the accuracy of the original QL approximation.
Abstract: We have recently introduced a quasi-linear (QL) approximation for the solution of the three-dimensional (3-D) electromagnetic modeling problem. In this paper we discuss an approach to improving its accuracy by considering the QL approximations of the higher-order. This approach can be considered the natural generalization of the Born series. We use the modified Green's operator with the norm less than 1 to ensure the convergence of the higher orders QL approximations to the true solution. This new approach produces the converged QL series, which makes it possible to estimate the accuracy of the original QL approximation without direct comparison with the rigorous full integral equation solution. It also opens principally new possibilities for fast and accurate 3-D EM modeling and inversion.

Journal ArticleDOI
TL;DR: In this article, a closed form solution is obtained via an asymptotic evaluation of the radiation integral for the fields scattered from the reflector, to within the physical optics approximation that remains valid for the present situation.
Abstract: A closed form solution is obtained to describe, in a physically appealing manner, the reflection and diffraction of a general astigmatic Gaussian beam which is incident on an arbitrary smooth, electrically large, slowly varying curved, perfectly conducting screen (or reflector). This closed form solution is obtained via an asymptotic evaluation of the radiation integral for the fields scattered from the reflector, to within the physical optics approximation that remains valid for the present situation. The analysis developed here is particularly well suited for the fast analysis of electrically large reflector antennas by representing the feed illumination by a relatively small set of Gaussian beams launched from the feed plane. Each of these Gaussian beams after being launched undergoes reflection and diffraction at the reflector; the expressions for the reflected and diffracted fields are developed in this paper and utilized by Chou [1996] to compute the radiation pattern of large reflector antennas in a matter of a few seconds as compared to the conventional numerical physical optics integral method which takes hours on the same computer.

Journal ArticleDOI
TL;DR: In this article, an experimental approach to derive the energy dissipation rate and eddy diffusion coefficients from ST radar measurements is proposed, using balloon-borne experiment results, in order to test the validity/invalidity of the methods generally used.
Abstract: Different methods have been proposed to derive the energy dissipation rate and eddy diffusion coefficients from ST radar measurements. However, their validity is still questionable because they implicitly assume that the Prandtl number is always equal to one, an assumption which is not verified. An experimental approach to this question, using balloon-borne experiment results, is proposed in this paper in order to test the validity/invalidity of the methods generally used. In situ observations show that the potential temperature gradient is more efficiently (and probably more rapidly) eroded by the turbulent activity than the wind shear. As a consequence of this observational evidence already mentioned by Browning and Watkins [1970], the structure function constant for temperature fluctuations (CT2) is vanishing within fully developed turbulent layers and exibits maxima on their boundaries, while the structure parameter for wind fluctuations (CV2) presents a broad maximum within the same layer and is decreasing at its boundaries. Consequently, the gradient Richardson number Ri strongly varies within fully developed turbulent layers, from Ri close to zero (near their center) up to Ri >1 (at their boundaries). By contrast, the flux Richardson number Rf, which describes the evolution of the ratio between buoyancy flux and turbulent energy production, remains apparently quasi-constant and close to its critical value during the erosion processes, so that the Prandtl number is not a constant close to unity but might also strongly vary during the turbulent life cycle. These results are in good agreement with laboratory experiments in statistically stable fluids reviewed by Thorpe [1973] and with experimental results obtained in the boundary layer [Businger et al., 1971; Gossard and Frisch, 1987]. ST radar are generally not able to observe regions where the potential temperature gradient is eroded by the turbulent activity but may obtain strong responses on the boundaries of fully developed turbulent layers. This behavior does not affect the radar capability of estimating eddy dissipation rate ϵ and eddy diffusivity Kθ (or KM) when complementary information on temperature profiles and humidity are available. It is shown that the “nonlocal” mean potential temperature gradient, the wind shear, and the flux Richardson number are the pertinent parameters allowing a correct estimate of the eddy dissipation rates and eddy diffusion coefficients, from Cn2 (CT2) and rms turbulent vertical wind, in regions where the turbulent activity is observable by ST radars.

Journal ArticleDOI
TL;DR: In this article, the authors used the interferometry technique implemented at the Chung-Li VHF radar to investigate the striated echoes with quasi-periodic characteristics generated from the electron density irregularities associated with sporadic E layer.
Abstract: By using the interferometry technique implemented at the Chung-Li VHF radar, the striated echoes with quasi-periodic characteristics in the range-time-intensity plot generated from the electron density irregularities associated with sporadic E layer are investigated. It is shown that the Es irregularities above 110 km drifting mostly westward along a stationary path of a few kilometer's width are responsible for the striated echoes. Considering the field-aligned property of the Es irregularities and the geometry of the echoing region over the Chung-Li radar site, it indicates that this stationary path is the cross section of a tilted layer which has a sharp electron density gradient in the direction across the layer parallel to the magnetic field line in the E region and orients geographically 72°NW. The observations also demonstrate that the echoing regions of the Es irregularities over the Chung-Li radar station are confined on the right side of a tilted thin plane with the thickness of a few kilometers at the elevation angle of 52° in the radar viewing region. These characteristics can be explained by using the radar backscatter from field-aligned targets in the field-perpendicular direction. The behavior of the sporadic E layer in the equatorial anomalous region is also investigated and discussed, and a descending sporadic E layer modulated by the gravity waves is observed. The descent rate of the layer is about 3.6 m/s, considerably larger than that reported by other investigators. The primary gravity wave modulating the sporadic E layer has a period of 12–15 min and propagates upward in phase with a vertical wavelength of about 50 km. Moreover, a positive correlation between the peak intensity of radar returns from Es irregularities below 110 km and the vertical shear of their horizontal drift velocity is seen. This feature, combined with the positive correlation between radar backscatter and the Doppler spectral width, strongly suggests that the crucial role the neutral wind plays in the excitation of the Es irregularities below 110 km cannot be ignored.

Journal ArticleDOI
TL;DR: In this article, the authors used the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 A to determine the nighttime altitude distribution of F region O+ ions and electrons.
Abstract: We present a technique for using the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 A to determine the nighttime altitude distribution of F region O+ ions and electrons. The algorithm uses an iterative scheme based on discrete inverse theory to determine the best fit to the data. We present the results of simulations that demonstrate the convergence properties of the algorithm and the fidelity with which it reproduces the input ionosphere. The algorithm was tested against more realistic simulated “data” generated using the international reference ionosphere (IRI-90) [Bilitza, 1990]. The algorithm accurately retrieved the nighttime F region electron density at midlatitudes (±25°–65°N) over a wide range of solar and geomagnetic activity and local time.

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the stochastic inversion method in ionospheric radiotomography with a special emphasis on regularization used in the inversion process, and concluded that available a priori information, for example, ionosonde or incoherent scatter measurements, should be used in choosing the regularization profile.
Abstract: The stochastic inversion method in ionospheric radiotomography is reviewed with a special emphasis on regularization used in the inversion process. Regularization is used both for preventing vigorous point-to-point oscillations and for controlling the peak altitude and thickness of the inversion result. The latter usually means importing a priori information on the layer height and thickness to the solver. In this paper it is pointed out that due to the curvature of the Earth and of the ionosphere, the measurements contain some information on the ionospheric altitude and the profile shape even in the case of a purely horizontally stratified layer. If this information could be used in choosing an appropriate regularization, no additional information would be needed. Simulation tests are presented which indicate that the altitude of a horizontally stratified layer can be determined with a reasonable accuracy without any a priori information. An attempt is also made to use the data for determining the shape of a proper regularization profile. Although some success is achieved in this effort, it is concluded that available a priori information, for example, ionosonde or incoherent scatter measurements, should be used in choosing the regularization profile. The ideas are tested with true data obtained from difference Doppler measurements carried out in Scandinavia, and the results are compared with simultaneous observations made by the European incoherent scatter radar. The comparison shows a reasonable agreement, although clear discrepancies also occur, for instance, in the shape of the bottomside profile.

Journal ArticleDOI
TL;DR: In this article, a general procedure to quantify the impact of antenna errors on the radiometric accuracy is developed and is then particularized to an L-band Y-shaped interferometer called MIRAS (microwave imaging radiometer by aperture synthesis) currently under study at the European Space Agency.
Abstract: A classification of system errors in aperture synthesis radiometry applied to Earth observation is presented. A general procedure to quantify the impact of antenna errors on the radiometric accuracy is developed and is then particularized to an L-band Y-shaped interferometer called MIRAS (microwave imaging radiometer by aperture synthesis) currently under study at the European Space Agency. This work analyzes in detail the impact of antenna errors on the radiometric accuracy of the instrument. These antenna errors are grouped into amplitude and phase antenna pattern errors, antenna position errors and antenna cross polarization errors. Special attention is paid to antenna coupling effects because of their importance in the selection of a suitable inversion algorithm for large aperture synthesis interferometers: the G-matrix techniques or the Fourier techniques proposed for MIRAS.

Journal ArticleDOI
TL;DR: Ionospheric total electron content (TEC) and slab thickness have been determined for southern Australia from July 1991 to June 1995 using Global Positioning System (GPS) satellite reception and ionograms (at 5-min intervals) recorded at Salisbury, South Australia, and other Australian ionograms of the Ionospheric Prediction Service Radio and Space Services, Australian Department of Administrative Services as mentioned in this paper.
Abstract: Ionospheric total electron content (TEC) and slab thickness (τ) have been determined for southern Australia from July 1991 to June 1995 using Global Positioning System (GPS) satellite reception and ionograms (at 5-min intervals) recorded at Salisbury, South Australia, and other Australian ionograms of the Ionospheric Prediction Service Radio and Space Services, Australian Department of Administrative Services. Seasonal, diurnal, and latitudinal variations in TEC and slab thickness are investigated. The removal of possible error sources in GPS measurements such as satellite and receiver biases is considered. Preliminary procedures are outlined in which protonospheric electron content is separated from GPS TEC measurements (up to 20,000 km height) by subtracting Navy Navigation Satellite System (NNSS) measurements (up to 1000 km height). Although slab thickness is substantially constant, there is a trend for increased values at times of reduced solar influence, such as when approaching sunspot minimum. A preliminary report is given of an extension of the current work at Salisbury by using Australian Surveying and Land Information Group GPS receivers at other Australian locations. A comparison is made between the experimental data and values derived from ionospheric models. The two models considered are the international reference ionosphere model (IRI90) and the parameterized ionospheric model ((PIM) version 1.4, February 1996). The GPS TEC measurements near solar minimum were consistently of the order of 5–10 TEC units greater than the model TEC predictions. This difference is attributed to the inclusion of protonospheric TEC in the GPS measurements but not in the model predictions.