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Numrering	Referens	Omslagsbild	Hitta
1.	Alayon Glazunov, Andres (författare) A survey of the application of the spherical vector wave mode expansion approach to antenna-channel interaction modeling 2014 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 49:8, s. 663-679 Tidskriftsartikel (refereegranskat)abstract This paper provides an overview of recent advances in the modeling, analysis, and measurements of interactions between antennas and the propagation channel in multiple antenna systems based on the spherical vector wave mode expansion of the electromagnetic field and the antenna scattering matrix. It demonstrates the importance and usefulness of this approach to gain further insights into a variety of topics such as physics-based propagation channel modeling, mean effective gain, channel correlation, propagation channel measurements, antenna measurements and testing, the number of degrees of freedom of the radio propagation channel, channel throughput, and diversity systems. The paper puts particular emphasis on the unified approach to antenna-channel analysis at the same time as the antenna and the channel influence are separated. Finally, the paper provides the first bibliography on the application of the spherical vector wave mode expansion of the electromagnetic field to antenna-channel interactions.
2.	Beghin, Christian, et al. (författare) Modeling of CLUSTER's electric antennas in space : application to plasma diagnostics 2005 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 40:6, s. RS6008- Tidskriftsartikel (refereegranskat)abstract [1] The main characteristics of the long-boom electric antennas installed on board the Cluster satellites are derived from finite element modeling in a kinetic and isotropic space plasma, in the frequency range of about 1–100 kHz. The model is based on the surface charge distribution method in quasi-static conditions. The impedances of both types of antenna, i.e., the double-wire and the double-probe, are computed versus the frequency normalized with respect to the local plasma frequency and for several different Debye lengths. Most of the code outputs are checked using analytic estimations for better understanding of the involved physical mechanisms. As a by-product, the effective length of the double-probe antenna and the mutual impedance between the two antennas are computed by the code. It is shown that if it had been possible to implement such measurements on board, one would have been able not only to determine accurately the electric characteristics of the antennas but also to estimate the local plasma parameters. Nevertheless, an interesting feature predicted by the model has been checked recently in orbit by running a special mode of operation for testing the mutual impedance measurement. The preliminary results are globally consistent with the predictions, except that they suggest that our Maxwellian model for the electron distribution should be revised in order to explain the unexpected low-frequency response. After analysis of the electron flux measurements obtained simultaneously, it appears that a rough adjustment of the electron distribution with a two-component distribution allows us to account for the observations.
3.	Buehler, Stefan, et al. (författare) Expected performance of the superconducting submillimeter-wave limb emission sounder compared with aircraft data 2005 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 40:3 Tidskriftsartikel (refereegranskat)abstract The simulated retrieval performance of a submillimeter wave limb sounder was compared with that of an up-looking instrument with identical observation frequency bands and comparable noise temperature. The frequency bands were 624.32-626.32 and 649.12-650.32 GHz, and the retrieval simulations focused on the key trace gas species O-3, HCl, and ClO. As expected, the limb geometry leads to a better altitude resolution and larger measurement altitude range. The same retrieval setup was applied to measured spectra, taken by the up-looking Airborne Submillimeter Radiometer ( ASUR) instrument on 4 September 2002 at 19.11 degrees E, 71.90 degrees N and on 19 September 2002 at 44.10 degrees E, 4.10 degrees S. The observed structures in the fit residual near the HCl spectral lines at 625.9 GHz lead to the conclusion that the pressure shift parameter of HCl is likely to be higher than the value in the HITRAN spectroscopic database. Depending on the assumed temperature dependence of the shift, the HCl pressure shift value consistent with the ASUR data is 0.090-0.117 MHz/hPa instead of the 0.030 MHz/hPa reported in HITRAN. This result is in good agreement with very recent independent laboratory work which suggests a value of 0.110 MHz/hPa for the shift.
4.	Davis, J.L, et al. (författare) Geodesy by radio interferometry: Effects of atmospheric modeling errors on estimates of baseline length 1985 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 20:6, s. 1593-1607 Tidskriftsartikel (refereegranskat)abstract Analysis of very long baseline interferometry data indicates that systematic errors in prior estimatesof baseline length, of order 5 cm for ~8000-km baselines, were due primarily to mismodeling of theelectrical path length of the troposphere and mesosphere ("atmospheric delay"). Here we discussobservational evidence for the existence of such errors in the previously used models for the atmosphericdelay and develop a new "mapping" function for the elevation angle dependence of this delay. Thedelay predicted by this new mapping function differs from ray trace results by less than ~5 mm, at allelevations down to 5° elevation, and introduces errors into the estimates of baseline length of •
5.	Davis, J.L., et al. (författare) Ground-based measurement of gradients in the “wet” radio refractivity of air 1993 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 28:6, s. 1003-1018 Tidskriftsartikel (refereegranskat)abstract We have used a ground-based microwave radiometer, known as a water vapor radiometer, to investigate the local spatial and temporal variation of the wet propagation delay for a site on the west coast of Sweden. The data were obtained from a wide range of azimuths and from elevation angles greater than 23.6-degrees (air mass 2.5). Visual inspection of the data suggested a simple ‘'cosine azimuth” variation, implying that a first-order gradient model was required. This model was adequate for short time spans up to approximately 15 min, but significant temporal variations in the gradient suggested to us that we include gradient rate terms. The resulting six-parameter model has proven adequate (rms delay residual approximately 1 mm) for up to 30 min of data. Assuming a simple exponential profile for the wet refractivity gradient, the estimated gradient parameters imply average surface wet-refractivity horizontal gradients of order of 0.1-1 N km-1. These gradients are larger, by 1-2 orders of magnitude, than gradients determined by others by averaging over long (approximately 100-km) distances. This result implies that for applications that are sensitive to local gradients, such as wet propagation-delay models for radio-interferometric geodetic studies, the use of meteorological data from widely spread stations may be inadequate. The gradient model presented here is inadequate for times longer than about 30 min. even if no gradients are present, because of the complicated stochastic like temporal behavior of the wet atmosphere. When gradients are present, they can change magnitude by approximately 50% over 10-15 min. Nevertheless, our ability to fit the radiometer data implies that on timescales 23.6-degrees, the local structure of the wet atmosphere can be described with a simple model. (The model is not limited to this range of elevation angles in principle.) The estimated gradient and gradient rate vectors have preferred directions, which indicates a prevailing structure in the three-dimensional temperature and humidity fields, possibly related to systematic behavior in large-scale weather systems and/or the local air-land-sea interaction at this site.
6.	Elgered, Gunnar, 1955, et al. (författare) Ground-based microwave radiometry and long-term observations of atmospheric water vapor 1998 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 33:3, s. 707-717 Tidskriftsartikel (refereegranskat)abstract Microwave radiometer data and radiosonde data from the time period 1981-1995 have been used to study long-term trends in the integrated precipitable water vapor (IPWV). The two instruments have operated 37 km apart on the Swedish west coast. Model parameters are estimated for the entire data sets as well as for subsets of the data. The IPWV model parameters are a mean value, a linear drift with time, and the amplitude and phase of an annual component. The radiosonde data, which are uniformly sampled in time, show an increase in the IPWV of 0.03 mm/yr with a statistical standard deviation of 0.01 mm. The microwave radiometer data, which are not at all uniformly sampled in time, show -0.02+/-0.01 mm/yr. We show that the disagreement is caused by the different sampling of the data for the two instruments. When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their statistical uncertainties. This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data. The general quality of the data, in terms of the short time scatter, has been improved over the time period. The root mean square (RMS) difference between the IPWV measured by the radiometer and by the radiosondes was 2.1 mm during the first 5 years and was reduced to 1.6 mm during the last 4 years. These values include the real difference in the IPWV between the two sites. The bias, radiometer-radiosonde, was 0.1 mm for the whole data set and varied between -0.2 and 0.9 mm for smaller data sets of a few years.
7.	Elgered, Gunnar, 1955, et al. (författare) Measurements of atmospheric water vapor with microwave radiometry 1982 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 17:5, s. 1258-1264 Tidskriftsartikel (refereegranskat)abstract A dual-channel, ground-based microwave radiometer, working at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer, and radiosondes have been used for comparative measurements of the integrated amount of precipitable water vapor in the atmosphere over a period with zenith water vapor contents varying between 6 and 26 mm. The microwave radiometer was found to give comparable or better formal accuracy than the radiosondes, the absolute accuracy of which is believed to be about 1 mm. The rms difference of the integrated amount of water vapor in the zenith direction measured with the microwave radiometer and with radiosondes was 1.2 mm for all data, and 0.8 mm for a selected group of good weather data. These are lower formal errors than previously reported. It is shown that the simplified relation between the radiometer antenna termperatures and the integrated amount of water vapor in this case contributes with a formal error of about 0.3 mm. It is suggested that mean ground weather data can be used to adapt this relation to other sites and seasons.
8.	Ellgardt, Anders, 1978-, et al. (författare) A study of edge effects in triangular grid tapered-slot arrays using coupling coefficients 2010 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 45, s. RS2005- Tidskriftsartikel (refereegranskat)abstract Edge effects for tapered-slot elements in triangular grid array antennas are studied by using finite difference time domain codes. The S parameters for finite-by-infinite arrays are computed and evaluated for two different edge models and, to evaluate the edge effects, the results are compared with the S parameters for the infinite array. As expected, the largest difference between the results occurs for the elements closest to the edges, because of the missing coupling from nearby elements and the perturbed element currents due to the edge geometry. By using the proposed method it is possible to distinguish between these two edge effects. A method to combine the finite-by-infinite array results and the infinite array results is presented and used to characterize the perturbation caused by the truncation of the infinite array.
9.	Eriksson, Patrick, et al. (författare) Measurement of tropospheric/stratospheric transmission at 10–35 GHz for H2O retrieval in low Earth orbiting satellite links 2003 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 38:4, s. 8069- Tidskriftsartikel (refereegranskat)abstract Active microwave limb sounding is a possible technique for measuring water vapor in the upper troposphere and lower stratosphere, and here a first assessment of the retrieval capabilities of transmission measurements in the range 10–35 GHz is presented. The proposed observing system consists of a constellation of low Earth orbiters measuring atmospheric transmission at the frequencies 10.3, 17.2, and 22.6 GHz. The use of these relatively long wavelengths guarantees a minimal, for being a remote sensing technique, influence from scattering. The original objective of the measurements was to derive water vapor profiles, but the potential to retrieve the liquid water content of clouds was also identified during the study. Retrieval errors due to thermal noise, gain instability, and spectroscopic uncertainties were considered. With the assumed instrument characteristics a measurement precision for water vapor in the upper troposphere of 5–10% is obtained, with capability to observe through ice clouds and clouds with a low water content.
10.	Fhager, Andreas, 1976, et al. (författare) Comparison of two image reconstruction algorithms for microwave tomography 2005 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 20, s. RS3017- Tidskriftsartikel (refereegranskat)
11.	Fischer, G., et al. (författare) Calibration of the JUICE RWI Antennas by Numerical Simulation 2021 Ingår i: Radio Science. - : American Geophysical Union (AGU). - 0048-6604 .- 1944-799X. ; 56:11 Tidskriftsartikel (refereegranskat)abstract The reception properties of the Radio Wave Instrument (RWI) onboard JUICE (Jupiter Icy Moons Explorer) have been determined using numerical methods applied to a mesh-grid model of the spacecraft. The RWI is part of the RPWI (Radio and Plasma Wave Investigation) and consists of three perpendicular dipoles mounted on a long boom. We determined their effective lengths vectors and capacitive impedances of 8-9 pF. We also investigated the change in effective antenna angles as a function of solar panel rotation and calculated the directivity of the antennas at higher frequencies up to the maximum frequency of 45 MHz of the receiver. We found that the RWI dipoles can be used for direction-finding with an accuracy of 2 degrees up to a frequency of 1.5 MHz. Additionally we calculated the influence of strong pulses from the JUICE active radar on RPWI and found that they should do no harm to its sensors and receivers.
12.	Fornieles-Callejon, J., et al. (författare) Extremely low frequency band station for natural electromagnetic noise measurement 2015 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 50:3, s. 191-201 Tidskriftsartikel (refereegranskat)abstract A new permanent ELF measurement station has been deployed in Sierra Nevada, Spain. It is composed of two magnetometers, oriented NS and EW, respectively. At 10 Hz, their sensitivity is 19 V/pT and the signal-to-noise ratio (SNR) is 28 dB for a time-varying signal of 1 pT, the expected field amplitude in Sierra Nevada. The station operates for frequencies below 24 Hz. The magnetometers, together with their corresponding electronics, have been specifically designed to achieve such an SNR for small signals. They are based on high-resolution search coils with ferromagnetic core and 10(6) turns, operating in limited geometry configuration. Different system noise sources are considered, and a study of the SNR is also included. Finally, some initial Schumann resonance measurements are presented in order to validate the performance of the measurement station, including 1 h length spectra, daily variations of resonance amplitudes and frequencies for the different seasons, and a 3 day spectrogram.
13.	Frid, Henrik, et al. (författare) Determining Direction‐of‐Arrival Accuracy for Installed Antennas by Postprocessing of Far‐Field Data 2019 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 54:12, s. 1204-1221 Tidskriftsartikel (refereegranskat)abstract Direction‐of‐arrival (DoA) estimation accuracy can be degraded due to installation effects, such as platform reflections, diffraction from metal edges, and reflections and refraction in the radome. To analyze these effects, this paper starts with a definition of the term installation error related to DoA estimation. Thereafter, we present a postprocessing method, which can be used to determine the DoA estimation accuracy for installed antennas. By computing synthetic signals from the installed far‐field data, it is possible to analyze the installation errors described above, in addition to analyzing array model errors. The method formulation is general, thus allowing generic array configurations, installation configurations, and direction‐finding algorithms to be studied. The use of the presented method is demonstrated by a case study of a wideband four‐quadrant array. In this case study, we investigate the installation errors due to a single‐shell radome. Thereafter, the effects of platform reflections are also analyzed, for an antenna placement in the tail of a fighter aircraft. Simulation results are presented for both the monopulse and the MUltiple SIgnal Classification direction‐finding algorithms.
14.	Glazunov, Andres Alayon, 1969, et al. (författare) Semi-Analytical Model of the Rician K-Factor 2020 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 55:11 Tidskriftsartikel (refereegranskat)abstract The analysis of the performance of 5G wireless communication systems employing Massive MIMO at millimeter-wave frequencies is of great practical relevance. Of special relevance are the signal fluctuations. In the present paper, we introduce a semi-analytical model for a generic scattering environment by using randomly distributed resonant scatterers to investigate the impact of the size of the scattering environment, the scatterer density, and the number of scatterers on the signal variability in terms of the Rician K-factor as a function of frequency. We further present an investigation of the impact of scattering on the frequency dependence of the signal fading statistics in the 500 MHz–100 GHz band. The simplified model is also verified against full-wave simulation using the Method of Moments (MoM).
15.	Gustafsson, Mats, et al. (författare) An optimization approach to two-dimensional time domain electromagnetic inverse problems 2000 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 35:2, s. 525-536 Tidskriftsartikel (refereegranskat)
16.	He, Sailing, et al. (författare) Explicit full identification of a transient dipole source in the atmosphere from measurement of the electromagnetic fields at several points at ground level 2000 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 35:1, s. 107-117 Tidskriftsartikel (refereegranskat)
17.	Hobiger, Thomas, 1978, et al. (författare) GLONASS-R: GNSS reflectometry with an FDMA based satellite navigation system 2014 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 49:4, s. 271-282 Tidskriftsartikel (refereegranskat)abstract The information from reflected Global Navigation Satellite System (GNSS) signals can become a valuable data source, from which geophysical properties can be deduced. This approach, called GNSS Reflectometry (GNSS-R), can be used to develop instruments that act like an altimeter when arrival times of direct and reflected signals are compared. Current GNSS-R systems usually entirely rely on signals from the Global Positioning Service (GPS), and field experiments could demonstrate that information from such systems can measure sea level with an accuracy of a few centimeters. However, the usage of the Russian GLONASS system has the potential to simplify the processing scheme and to allow handling of direct and reflected signals like a bistatic radar. Thus, such a system has been developed and deployed for test purposes at the Onsala Space Observatory, Sweden, that has an operational GPS-based GNSS-R system. Over a period of 2 weeks in October 2013, GPS-based GNSS-R sea level monitoring and measurements with the newly developed GLONASS-R system were carried out in parallel. In addition, data from colocated tide gauge measurements were available for comparison. It can be shown that precision and accuracy of the GLONASS-based GNSS-R system is comparable to, or even better than, conventional GPS-based GNSS-R solutions. Moreover, the simplicity of the newly developed GLONASS-R system allows to make it a cheap and valuable tool for various remote sensing applications.
18.	Isoz, Oscar, et al. (författare) Interference from terrestrial sources and its impact on the GRAS GPS radio occultation receiver 2014 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 49:1, s. 1-6 Tidskriftsartikel (refereegranskat)abstract It is well known that terrestrial GPS/Global Navigation Satellite Systems (GNSS) receivers are vulnerable and have suffered from intentional and unintentional interference sources. Unfortunately, space-based GPS/GNSS receivers are not exempt from radio frequency interference originating from the Earth. This paper explores data recorded by the GNSS Receiver for Atmospheric Sounding (GRAS) instrument onboard MetOp-A in September 2007, which is assumed to be representative of the typical environment for GPS/GNSS instrumentation in LEO orbit. Within these data it is possible to detect both pulsed interference and variations in the background noise. One plausible source of the pulsed interference is identified. We also show that neither the pulsed interference nor the variations in the background noise degrades the performance of the higher level products from GRAS.
19.	Jacobsson, Per, 1977, et al. (författare) Shape optimization of the total scattering cross section for cylindrical scatterers 2009 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 44:4 Tidskriftsartikel (refereegranskat)abstract We propose and test a gradient-based shape optimization algorithm for the total scattering cross section of infinitely long cylinders, by means of changing the shape of the cylinder's cross section. On the basis of the optical theorem, we derive sensitivity expressions for both dielectric and metal cylinders given an incident plane wave, where the wave vector is perpendicular to the cylinder axis. Both the transverse electric (TE) case and the transverse magnetic case are considered. The sensitivity expressions are based on the continuum form of Maxwell's equations, and they provide the sensitivity with respect to an arbitrary number of shape parameters in terms of the field solution of the original scattering problem and an adjoint scattering problem. These results are used to construct a gradient-based optimization algorithm that we exploit for the reduction of the total scattering cross section in the TE case for metal cylinders, e.g., struts used in reflector antennas. We present optimized cross sections that are oblong in the direction of the incident wave vector, and some of these designs feature corrugations that are parallel to the cylinder axis. We show designs with asymmetric cross sections that yield a low monostatic scattering cross section for certain directions in combination with a low total scattering cross section, which can be used to reduce the noise temperature contributions from the upper strut in an inverted Y tripod reflector antenna.
20.	Jaron, Frédéric, et al. (författare) Cross-Polarization Gain Calibration of Linearly Polarized VLBI Antennas by Observations of 4C 39.25 2024 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 59:4 Tidskriftsartikel (refereegranskat)abstract Radio telescopes with dual linearly polarized feeds regularly participate in Very Long Baseline Interferometry. One example is the VLBI Global Observing System (VGOS), which is employed for high-precision geodesy and astrometry. In order to achieve the maximum signal-to-noise ratio, the visibilities of all four polarization products are combined to Stokes I before fringe-fitting. Our aim is to improve cross-polarization bandpass calibration, which is an essential processing step in this context. Here we investigate the shapes of these station-specific quantities as a function of frequency and time. We observed the extra-galactic source 4C 39.25 for 6 hours with a VGOS network. We correlated the data with the DiFX software and analyzed the visibilities with PolConvert to determine the complex cross-bandpasses with high accuracy. Their frequency-dependent shape is to first order characterized by a group delay between the two orthogonal polarizations, in the order of several hundred picoseconds. We find that this group delay shows systematic variability in the range of a few picoseconds, but can remain stable within this range for several years, as evident from earlier sessions. On top of the linear phase-frequency relationship there are systematic deviations of several tens of degrees, which in addition are subject to smooth temporal evolution. The antenna cross-bandpasses are variable on time scales of ∼1 hr, which defines the frequency of necessary calibrator scans. The source 4C 39.25 is confirmed as an excellent cross-bandpass calibrator. Dedicated surveys are highly encouraged to search for more calibrators of similar quality.
21.	Johansson, Gustav, et al. (författare) Simulation of post-ADC digital beamforming for large aperture array radars 2010 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 45:RS3001 Tidskriftsartikel (refereegranskat)abstract This paper presents simulations and methods developed to investigate the feasibility of using a Fractional-Sample-Delay (FSD) system in the planned EISCAT_3D incoherent scatter radar. Key requirements include a frequency-independent beam direction over a 30 MHz band centered around 220 MHz, with correct reconstruction of pulse lengths down to 200 ns. The clock jitter from sample to sample must be extremely low for the integer sample delays. The FSD must also be able to delay the 30 MHz wide signal band by 1/1024th of a sample without introducing phase shifts, and it must operate entirely in baseband. An extensive simulation system based on mathematical models has been developed, with inclusion of performance-degrading aspects such as noise, timing error, and bandwidth. Finite Impulse Response (FIR) filters in the baseband of a band-pass-sampled signal have been used to apply true time delay beamforming. It has been confirmed that such use is both possible and well behaved. The target beam-pointing accuracy of 0.06° is achievable using optimized FIR filters with lengths of 36 taps and an 18 bit coefficient resolution. Even though the minimum fractional delay step necessary for beamforming is ∼13.1 ps, the maximum sampling timing error allowed in the array is found to be σ ≤ 120 ps if the errors are close to statistically independent.
22.	Lagovsky, Boris, et al. (författare) Angular superresolution based on a priori information 2021 Ingår i: Radio Science. - Chichester : American Geophysical Union (AGU). - 0048-6604 .- 1944-799X. ; 56:3 Tidskriftsartikel (refereegranskat)abstract The results of theoretical studies and mathematical modeling indicate the possibility of obtaining the angular superresolution and its limits by using algebraic methods of imaging. The necessary algorithms are created on the basis of solution to inverse problems. The limiting levels of the achieved angular superresolution are found depending on the signal‐to‐noise ratio for objects of various types. A number of specific advantages of the method are shown as applied to the solution of two‐dimensional problems. Capabilities of the method are illustrated by examples. High performance of the proposed approach allows one to use it in real time when scanning and tracking targets.
23.	Leyser, Thomas B., et al. (författare) Plasma angular momentum effects and twisted incoherent scatter radar beams 2012 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 47, s. RS5004- Tidskriftsartikel (refereegranskat)abstract Phased arrays provide new possibilities for remote sensing with radars. By imposing an azimuthal phase variation, electromagnetic beams that carry orbital angular momentum can be formed. Such beams have a phase structure that appears twisted and as a result an intensity null in the center of the beam cross section. Here we numerically investigate twisted beams for incoherent scatter radars that are used to study the ionosphere. We discuss the possibility of utilizing such radar beams to probe twisted beams of plasma waves and flows transverse to the beam axis, such as associated with auroral arcs. Transverse plasma flows may give rise to a rotational frequency shift of the scatter from a twisted beam and Doppler broadening due to the beam divergence, the latter also occurring with regular beams. Although the angular momentum effects of the considered large scale flows are generally small, sheared and vortical flows transverse to the beam axis can in principle be discriminated from unidirectional flows with beams carrying orbital angular momentum.
24.	Lomidze, Levan, et al. (författare) Calibration and Validation of Swarm Plasma Densities and Electron Temperatures Using Ground-Based Radars and Satellite Radio Occultation Measurements 2018 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 53:1, s. 15-36 Tidskriftsartikel (refereegranskat)abstract In this study we calibrate and validate in situ ionospheric electron density (N-e) and temperature (T-e) measured with Langmuir probes (LPs) on the three Swarm satellites orbiting the Earth in circular, nearly polar orbits at similar to 500km altitude. We assess the accuracy and reliability of the LP data (December 2013 to June 2016) by using nearly coincident measurements from low- and middle-latitude incoherent scatter radars (ISRs), low-latitude ionosondes, and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites, covering all latitudes. The comparison results for plasma frequency (f alpha root Ne) for each Swarm satellite are consistent across these three, principally different measurement techniques. It shows that the Swarm LPs systematically underestimate plasma frequency by about 10% (0.5-0.6MHz). The correlation coefficients are high (0.97), indicating accurate relative variation in the Swarm LP densities. The comparison of T-e from high-gain LPs and those from ISRs reveals that all three satellites overestimate it by 300-400K but exhibit high correlations (0.92-0.97) against the validation data. The low-gain LP T-e data show larger overestimation (similar to 700K) and lower correlation (0.86-0.90). The adjustment of the Swarm LP data based on Swarm-ISR comparison results removes the systematic biases in the Swarm data and gives plasma frequencies and high- and low-gain electron temperatures that are precise within about 0.4MHz (8%), 150-230K, and 260-360K, respectively. We demonstrate that the applied correction significantly improves the agreement between (1) the plasma densities from Swarm, and from ionosondes and COSMIC, and (2) the T-e from Swarm LPs and International Reference Ionosphere 2016.
25.	Ludwig Barbosa, Vinícius, 1990-, et al. (författare) Evaluation of Ionospheric Scintillation in GNSS Radio Occultation Measurements and Simulations 2020 Ingår i: Radio Science. - : Wiley-Blackwell Publishing Inc.. - 0048-6604 .- 1944-799X. ; 55:8 Tidskriftsartikel (refereegranskat)abstract Like any other system relying on trans-ionospheric propagation, GNSS Radio Occultation (GNSS-RO) is affected by ionospheric conditions during measurements. Regions of plasma irregularities in F-region create abrupt gradients in the distribution of ionized particles. Radio signals propagated through such regions suffer from constructive and destructive contributions in phase and amplitude, known as scintillations. Different approaches have been proposed in order to model and reproduce the wave propagation through ionospheric irregularities. We present simulations considering an one-component inverse power-law model of irregularities integrated with Multiple Phase Screen (MPS) propagation. In this work, the capability of the scintillation model to reproduce features in the signal amplitude of low latitude MetOp measurements in the early hours of DOY 76, 2015 (St. Patrick’s Day geomagnetic storm) is evaluated. Power spectral density (PSD) analysis, scintillation index, decorrelation time and standard deviation of neutral bending angle are considered in the comparison between the simulations and RO measurements. The results validate the capability of the simulator to replicate an equivalent total integrated phase variance in cases of moderate to strong scintillation.
26.	Löfgren, Johan, 1983, et al. (författare) Three months of local sea level derived from reflected GNSS signals 2011 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 46:RS0C05 Tidskriftsartikel (refereegranskat)abstract By receiving Global Navigation Satellite System (GNSS) signals that are reflected off the sea surface, together with directly received GNSS signals (using standard geodetic‐type receivers), it is possible to monitor the sea level using regular single difference geodetic processing. We show results from our analysis of three months of data from the GNSS‐based tide gauge at the Onsala Space Observatory (OSO) on the west coast of Sweden. The GNSS-derived time series of local sea level is compared with independent data from two stilling well gauges at Ringhals and Gothenburg about 18 km south and 33 km north of OSO, respectively. A high degree of agreement is found in the time domain, with correlation coefficients of up to 0.96. The root‐mean‐square differences between the GNSS‐derived sea level and the stilling well gauge observations are 5.9 cm and 5.5 cm, which is lower than for the stilling well gauges together (6.1 cm). A frequency domain comparison reveals high coherence of the data sets up to 6 cycles per day, which corresponds well to the propagation of gravity waves in the shallow waters at the Kattegat coast. Amplitudes and phases of some major tides were determined by a tidal harmonic analysis and compared to model predictions. From the GNSS‐based tide gauge results we find significant ocean tidal signals at fortnightly, diurnal, semi‐diurnal, and quarter‐diurnal periods. As an example, the amplitudes of the semi‐diurnal M2 and the diurnal O1 tide are determined with 1σ uncertainties of 11 mm and 12 mm, respectively. The comparison to model calculations shows that global ocean tide models have limited accuracy in the Kattegat area.
27.	Melsheimer, C., et al. (författare) Intercomparison of general purpose clear sky atmospheric radiative transfer models for the millimeter/submillimeter spectral range 2005 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 40:1, s. RS1007- Tidskriftsartikel (refereegranskat)abstract We compare a number of radiative transfer models for atmospheric sounding in the millimeter and submillimeter wavelength range, check their consistency, and investigate their deviations from each other. This intercomparison deals with three different aspects of radiative transfer models: (1) the inherent physics of gaseous absorption lines and how they are modeled, (2) the calculation of absorption coefficients, and (3) the full calculation of radiative transfer for different geometries, i.e., up-looking, down-looking, and limb-looking. The correctness and consistency of the implementations are tested by comparing calculations with predefined input such as spectroscopic data, line shape, continuum absorption model, and frequency grid. The absorption coefficients and brightness temperatures calculated by the different models are generally within about 1% of each other. Furthermore, the variability or uncertainty of the model results is estimated if (except for the atmospheric scenario) the input such as spectroscopic data, line shape, and continuum absorption model could be chosen freely. Here the models deviate from each other by about 10% around the center of major absorption lines. The main cause of such discrepancies is the variability of reported spectroscopic data for line absorption and of the continuum absorption model. Further possible causes of discrepancies are different frequency and pressure grids and differences in the corresponding interpolation routines, as well as differences in the line shape functions used, namely a prefactor of (nu/nu(0)) or (nu/nu(0))(2) of the Van-Vleck-Weisskopf line shape function. Whether or not the discrepancies affect retrieval results remains to be investigated for each application individually.
28.	Mohammadi, Siavoush M., et al. (författare) Orbital angular momentum in radio : Measurement methods 2010 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 45, s. RS4007- Tidskriftsartikel (refereegranskat)abstract Novel measurement and approximation methodologies for studying orbital angular momentum (OAM) modes in radio beams, i.e., electromagnetic beam modes having helical phase fronts, are presented. We show that OAM modes can be unambiguously determined by measuring two electric field components at one point, or one electric field component at two points.
29.	Olsson, Olle (författare) Scattering of electromagnetic waves by a perfectly conducting half plane below a stratified overburden 1978 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 13:2, s. 391-397 Tidskriftsartikel (refereegranskat)abstract By expanding the electromagnetic field into plane wave eigenfunctions, an integral equation is obtained which describes the coupling between an overburden, consisting of an arbitrary number of plane layers and an arbitrary object beneath a stratified half space. As an example of the technique, a perfectly conducting half plane inclined to the strata in a substratum is discussed. In this case the kernel of the integral equation may be expressed in terms of elementary functions. Numerical results are discussed for a plane wave incident on the ground.
30.	Persson, Patrik, et al. (författare) High-frequency approximation for mutual coupling calculations between apertures on a perfect electric conductor circular cylinder covered with a dielectric layer : Nonparaxial region 2003 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 38:4, s. 1079- Tidskriftsartikel (refereegranskat)abstract An asymptotic Green's function technique is presented for the calculation of the surface fields on a perfect electric conductor circular cylinder covered with a dielectric layer. The sources are apertures located on the perfect electric conductor surface. This method is very efficient and valid for large cylinders in terms of wavelengths. The new representation is obtained via Watson's transformation and integral contour deformations. The efficiency of the method results from the circumferentially propagating representation of the Green's function as well as the efficient numerical evaluation of various integrals. Numerical results are presented showing good agreement when compared to other solutions as well as measurements.
31.	Podlipenko, Yu K., et al. (författare) Guaranteed estimation of solutions to Helmholtz transmission problems with uncertain data from their indirect noisy observations 2017 Ingår i: Radio Science. - : Blackwell Publishing Ltd. - 0048-6604 .- 1944-799X. ; 52:9, s. 1129-1139 Tidskriftsartikel (refereegranskat)abstract We investigate the guaranteed estimation problem of linear functionals from solutions to transmission problems for the Helmholtz equation with inexact data. The right-hand sides of equations entering the statements of transmission problems and the statistical characteristics of observation errors are supposed to be unknown and belonging to certain sets. It is shown that the optimal linear mean square estimates of the above mentioned functionals and estimation errors are expressed via solutions to the systems of transmission problems of the special type. The results and techniques can be applied in the analysis and estimation of solution to forward and inverse electromagnetic and acoustic problems with uncertain data that arise in mathematical models of the wave diffraction on transparent bodies.
32.	Sandström, Sven-Erik, et al. (författare) A study of some FMCW radar algorithms for target location at low frequencies 2016 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 51:10, s. 1676-1685 Tidskriftsartikel (refereegranskat)abstract FMCW (Frequency Modulated Continuous Wave) radar is a simple and inexpensive technique for target location. The resolution is given by the available bandwidth and the directivity of the antenna. Resolution is not a problem at high frequencies, while at low frequencies, and especially for mobile platforms, the required size of the antenna becomes impractical. In order to obtain the bearing of the targets, without relying on directivity, one may use a simple two-dimensional trilateration method that involves several platforms. The problem of ghost targets has been studied for both monostatic and multistatic radar. When there is a confluence of echoes, the relation between accuracy and bandwidth is of particular interest.
33.	Sandström, Sven-Erik, 1956- (författare) On Sparsity Related to the Integral Equations of 2D Electromagnetic Scattering 2023 Ingår i: Radio Science. - New York : John Wiley & Sons. - 0048-6604 .- 1944-799X. ; 58:4 Tidskriftsartikel (refereegranskat)abstract The integral equations of electromagnetic scattering are often solved with the Fast Multipole Method. If the application involves multiple excitations one could instead consider methods that separate factorization and solution. The cost of an elaborate factorization could then be compensated for by an efficient solution. An approach to obtain substantial sparsity in the solution step is considered here. By using the generality provided by the integral representation, the kernel is modified so as to create sparsity in the resulting linear system. In the two modifications that are described here there is an attempt to optimize the shape of the kernel with respect to sparsity and accuracy.
34.	Sandström, Sven-Erik, et al. (författare) Scaling and sparsity in an accurate implementation of the method of moments in 2-D 2014 Ingår i: Radio Science. - : John Wiley & Sons. - 0048-6604 .- 1944-799X. ; 49:8, s. 643-652 Tidskriftsartikel (refereegranskat)abstract The integral equations of electromagnetic scattering are often solved numericallyby means of the method of moments. At high frequencies, this method typically leads to a large linear system with a dense matrix. Higher order basis functions is a means to improve the accuracy. B-splines are used here for a two-dimensional testbed study that avoids the complexity of 3D implementation.For smooth convex scatterers one may use \emph{a priori} knowledge about the oscillatory behaviour of the solution to reformulate the integral equation.This fast scale of variation is included in the kernel of the integral equation. An extension of this idea deals with the variation in the shadow, particularly for circular geometry, and is an improvement that is presented in this study. Generally, the TE-case is less studied at high frequencies and our numerical results therefore relate to this harder problem.A sparse matrix can be obtained by modification of the integration path in the integral equation. The decay of the modified kernel makes this possible for high frequencies but the modified path reduces the accuracy in the deep shadow. This study investigates these modified paths for the case where the shadow region is not omitted from the formulation.
35.	Sheina, Elena, et al. (författare) Advantages of a multi-frequency experiment for determining the dielectric constant of a layer in a rectangular waveguide and free space 2021 Ingår i: Radio Science. - : American Geophysical Union (AGU). - 0048-6604 .- 1944-799X. ; 56:3 Tidskriftsartikel (refereegranskat)abstract An inverse problem of reconstructing real permittivity of a plane‐parallel layer in a perfectly conducting rectangular waveguide or in free space from experimental data using an explicit expression for the scattering matrix is considered. In general, this problem is improperly posed and may be unsolvable due to inaccuracy of the experimental data, and for a perfect noiseless experiment the solution may be not unique because the scattering coefficients curve has self‐intersection points. It is shown that the traditional multi‐frequency method of measurements applied in vector network analyzers can be justified. The following facts are rigorously proved in the paper: nonuniqueness of the solution can be removed if the frequency resolution is sufficiently small; and an algorithm for processing measurement results using least squares provides an approximate solution to the problem that converges to the exact one when the quality of the experiment improves, the convergence rate depends on the number of frequencies used in the experiment.
36.	Shestopalov, Yury, 1953- (författare) Resonance scattering by a circular dielectric cylinder 2021 Ingår i: Radio Science. - : American Geophysical Union (AGU). - 0048-6604 .- 1944-799X. ; 56:6 Tidskriftsartikel (refereegranskat)abstract A unified approach is developed for the analysis of singularities of the scattered field and vanishing of scattering harmonics. Rigorous proofs are presented of the existence of complex resonance singularities of the solution to the problem of the plane wave scattering by a circular homogeneous dielectric cylinder. The method employs a mathematically correct approach of the spectral theory of open structures involving generalized conditions at infinity when complex resonance frequencies may be considered. The result is obtained by verifying the existence of complex singularities of the series solution coefficients considered as functions of the problem parameters. The recently developed theory of generalized cylindrical polynomials (GCPs) is used that enables one to reduce determination of singularities (resonances) to finding zeros (real or complex) of a particular subfamily of GCPs.
37.	Shoory, Abdolhamid, et al. (författare) On the accuracy of approximate techniques for the evaluation of lightning electromagnetic fields along a mixed propagation path 2011 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 46, s. RS2001- Tidskriftsartikel (refereegranskat)abstract In this paper we review simplified analytical expressions derived by Wait using the concept of attenuation function for the analysis of the propagation of lightning-radiated electromagnetic fields over a mixed propagation path (vertically stratified ground). Two different formulations proposed by Wait that depend on the relative values of ground surface impedances are discussed. It is shown that both formulations give nearly the same results for the time domain electric field. However, depending on the values of the normalized surface impedance for each ground section, the use of one of the two formulations is computationally more efficient. The accuracy of the Wait formulations was examined taking as reference full-wave simulations obtained using the finite difference time domain technique. It is shown that Wait's simplified formulas are able to reproduce the distant field peak and waveshape with a good accuracy.
38.	Smirnov, Yuri, et al. (författare) Diffraction of a TE-Polarized Wave by a Nonlinear Goubau Line 2019 Ingår i: Radio Science. - : Blackwell Publishing Ltd. - 0048-6604 .- 1944-799X. ; 54:1, s. 151-157 Tidskriftsartikel (refereegranskat)abstract The diffraction of a cylindrical wave by a nonlinear metal-dielectric waveguide filled with nonlinear medium is considered. Two widely used types of nonlinearities (Kerr nonlinearity and nonlinearity with saturation) are considered. The problem is to find amplitudes of the reflected and the transmitted fields when the amplitude of the incident field is known. The analytical and numerical solution techniques are developed. Numerical results are presented. ©2019. American Geophysical Union. All Rights Reserved.
39.	Smolkin, Eugen, et al. (författare) Surface waves in a nonlinear metamaterial rod 2020 Ingår i: Radio Science. - : American Geophysical Union (AGU). - 0048-6604 .- 1944-799X. ; 55:10, s. 1-8 Tidskriftsartikel (refereegranskat)abstract We consider propagation of surface TE waves in a rod filled with nonlinear metamaterial medium. The problem is reduced to the analysis of a nonlinear integral equation with a kernel in the form of the Green function of an auxiliary boundary value problem on an interval. The existence of propagating nonlinear surface TE waves for the chosen nonlinearity (Kerr law) is proved using the method of contraction. For the numerical solution, a method based on solving an auxiliary Cauchy problem (a version of the shooting method) is proposed. New propagation regimes are discovered.
40.	Strandberg, Joakim, 1991, et al. (författare) Improving GNSS-R sea level determination through inverse modeling of SNR data 2016 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 51:8, s. 1286-1296 Tidskriftsartikel (refereegranskat)abstract This paper presents a new method for retrieving sea surface heights from Global Navigation Satellite Systems reflectometry (GNSS-R) data by inverse modeling of SNR observations from a single geodetic receiver. The method relies on a B-spline representation of the temporal sea level variations in order to account for its continuity. The corresponding B-spline coefficients are determined through a nonlinear least squares fit to the SNR data, and a consistent choice of model parameters enables the combination of multiple GNSS in a single inversion process. This leads to a clear increase in precision of the sea level retrievals which can be attributed to a better spatial and temporal sampling of the reflecting surface. Tests with data from two different coastal GNSS sites and comparison with colocated tide gauges show a significant increase in precision when compared to previously used methods, reaching standard deviations of 1.4 cm at Onsala, Sweden, and 3.1 cm at Spring Bay, Tasmania.
41.	Tamburini, F., et al. (författare) Tripling the capacity of a point-to-point radio link by using electromagnetic vortices 2015 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 50:6, s. 501-508 Tidskriftsartikel (refereegranskat)abstract In this paper we report the results from outdoor experiments showing that it is possible to increase the data transmission capacity using at least three coherent, orthogonal beams on the same frequency, 17.128GHz, each in a unique orbital angular momentum state. Each beam was encoded with the digital modulations used in present-day telecommunications. We achieved an error-free throughput of 3x11Mbit/s with four-Quadrature Amplitude Modulation over a 7MHz bandwidth over 100m and 150m long links.
42.	Zhang, Jiliang, et al. (författare) Two-Ray Reflection Resolution Algorithm for Planar Material Electromagnetic Property Measurement at the Millimeter-Wave Bands 2020 Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 55:3 Tidskriftsartikel (refereegranskat)abstract Electromagnetic (EM) reflection properties of building materials and structures have been investigated through practical measurements. However, the finite thickness of measured materials makes it challenging to resolve the rays reflected from the front and the back surfaces. In this paper, we therefore present a three-step minimum least squares-based algorithm to resolve two closely adjacent rays reflected from the front and the back surfaces of a board-shaped material. Our analytical and numerical results show that the proposed algorithm achieves the Cramér-Rao lower bound. The proposed algorithm is validated using measurement data for various materials and incident angles in the 40–50 GHz frequency band. The validation results show that the proposed algorithm is capable of resolving two closely adjacent rays with a root-mean-square deviation that is smaller than 0.08. Main applications of the proposed algorithm can be found in the frequency domain measurements of the EM wave reflections by typical building structures, e.g., window glass, doors, ceiling, and floors.
43.	Enstedt, M., et al. (författare) A spectral expansion-based Fourier split-step method for uncertainty quantification of the propagation factor in a stochastic environment 2016 Ingår i: Radio Science. - 0048-6604. ; 51:11, s. 1783-1791 Tidskriftsartikel (refereegranskat)abstract A chaos expanded Fourier split-step method is derived and applied to a narrow-angle parabolic equation. The parabolic equation has earlier been used to study deterministic settings. In this paper we develop a spectral-based Fourier split-step method that will take a limited degree of information about the environment into account. Our main focus is on proposing an efficient method for computational electromagnetics in stochastic settings. In this paper we study electromagnetic wave propagation in the troposphere in the case when the refraction index belongs to a uniform distribution.
44.	Ericsson, Andreas, et al. (författare) A resonant circular polarization selective structure of closely spaced wire helices 2015 Ingår i: Radio Science. - 0048-6604. ; 50:8, s. 804-812 Tidskriftsartikel (refereegranskat)
45.	Forslund, Ola, et al. (författare) Scattering from dielectric frequency selective structures 2003 Ingår i: Radio Science. - 0048-6604. ; 38:3, s. 1037-1037 Tidskriftsartikel (refereegranskat)abstract A plane wave impinges on a laterally double-periodic inhomogeneous lossy dielectric structure. Using a wave splitting approach, and an expansion of the fields and permittivity in entire domain basis functions, the scattered fields are determined via a propagator. The formulation is originally intended for constitutive parameters which vary continuously but is here applied to structures with an abruptly varying permittivity. The method resembles the so called coupled-wave analysis method. Calculated results are compared with experimental data for a slab with domains of a piecewise homogeneous permittivity. The agreement is good considering the error sources in the experiment. The results are compared with calculated results obtained by an entirely different method showing very good agreement for the examples given. The method is also used on a pyramidal absorber-like structure.
46.	Gustafsson, Mats, et al. (författare) State-space models and stored electromagnetic energy for antennas in dispersive and heterogeneous media 2017 Ingår i: Radio Science. - 0048-6604. ; 52:11, s. 1325-1343 Tidskriftsartikel (refereegranskat)abstract Accurate and efficient evaluation of the stored energy is essential for Q factors, physical bounds, and antenna current optimization. Here it is shown that the stored energy can be estimated from quadratic forms based on a state-space representation derived from the electric and magnetic field integral equations. The derived expressions are valid for small antennas embedded in temporally dispersive and inhomogeneous media. The quadratic forms also provide simple single frequency formulas for the corresponding Q factors. Numerical examples comparing the different Q factors are presented for dipole and meander line antennas in conductive, Debye, and Lorentz media for homogeneous and inhomogeneous media. The computed Q factors are also verified with the Q factor obtained from the stored energy in Brune synthesized circuit models.
47.	HOLMBERG, B, et al. (författare) MAGNETIC-FIELDS AND CANCER DEVELOPMENT IN ANIMAL-MODELS 1995 Ingår i: RADIO SCIENCE. - 0048-6604. ; 30:1, s. 223-231 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
48.	Karlsson, Anders (författare) Some results extracted from the time domain version of the optical theorem 2002 Ingår i: Radio Science. - 0048-6604. ; 38:2, s. 8008-8008 Tidskriftsartikel (refereegranskat)abstract The time domain version of the optical theorem is discussed. The theorem is derived from the optical theorem in the frequency domain by the Parseval's relation. It expresses the sum of the scattered and absorbed energies in terms of the scattered farfield in the forward direction. From the theorem, causality, and reciprocity, a number of results concerning the scattered and absorbed energies from a plane pulse that is scattered from bounded objects are obtained. Some of these results are verified by numerical calculations.
49.	Kristensson, Gerhard (författare) Homogenization of the Maxwell equations in an anisotropic material 2003 Ingår i: Radio Science. - 0048-6604. ; 38:2, s. 1-19 Tidskriftsartikel (refereegranskat)abstract This paper contains an overview of the homogenization of anisotropic materials at fixed frequency using the concept of two-scaled convergence. The homogenized electric and magnetic parameters, the relative permittivity, and the relative permeability are found by suitable averages of the solution of a local problem in the unit cell. A comparison between the exact homogenization method presented in this paper and the traditional mixture formulae, which are based on physical arguments, is made.
50.	Pettersson, Rickard (författare) Frequency dependence in detection of the cold-temperate transition surface in a polythermal glacier 2005 Ingår i: Radio Sience. - Washington D.C. : American Geophysical Union. - 0048-6604. ; 40:RS3007 Tidskriftsartikel (refereegranskat)abstract Radar surveys of the cold-temperate transition in polythermal glaciers have been used to investigate the polythermal structure, as well as the stability of the temperature regime, in glaciers. The mapping is based on the detection of small water-filled pockets present in the temperate ice beneath the cold surface layer. I compare radar profiles recorded at different frequencies on a polythermal glacier to show that the apparent depth to the cold-temperate transition reflection at a center frequency of 155 MHz is significantly greater than that detected at frequencies above 345 MHz. This increase appears to be the result of a reduction of scattering efficiency from the uppermost part of the temperate ice, as the theory of electromagnetic scattering from small objects indicates a strong decrease in scattering efficiency from objects much smaller than the wavelength of the incident wave. Thus I interpret this apparent depth increase to indicate that the average radii of the water pockets of the cold-temperate transition is on a subdecimeter scale. Spatial variation in the difference between cold-temperate transition depths at 155 MHz and higher frequencies may arise from spatial variations in size or the number density of scatterers.

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