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A practical guide to radio channel measurement techniques Whilst there are numerous books describing modern wireless communication systems that contain overviews of radio propagation and radio channel modelling, few contain detailed information on the design, implementation and calibration of radio channel measurement equipment, the planning of experiments and the in depth analysis of measured data. This work redresses that balance. Beginning with an explanation of the fundamentals of radio wave propagation, the book progresses through a series of topics, including the measurement of radio channel characteristics, radio channel sounders, measurement strategies, data analysis techniques and radio channel modelling. Application of results for the prediction of achievable digital link performance are discussed with examples pertinent to single carrier, multi-carrier and spread spectrum radio links. It addresses specifics of communications in various different frequency bands for both long range and short range fixed and mobile radio links. Key features: Focuses on radio channel measurements and characterization with analysis of MIMO channels Discusses the physical and technical considerations involved in the proper assessment of radio channel characteristics for efficient radio system planning, design, and implementation Provides in-depth information on the planning of experiments and the detailed analysis of measured data from radio propagation and channel modelling Unique practical approach describing how to design and implement channel sounders

Solar radio bursts are strongly affected by radio-wave scattering on density inhomogeneities, changing their observed time characteristics, sizes, and positions. The same turbulence causes angular broadening and scintillation of galactic and extragalactic compact radio sources observed through the solar atmosphere. Using large-scale simulations of radio-wave transport, the characteristics of anisotropic density turbulence from 0.1 R ⊙ to 1 au are explored. For the first time, a profile of heliospheric density fluctuations is deduced that accounts for the properties of extrasolar radio sources, solar radio bursts, and in situ density fluctuation measurements in the solar wind at 1 au. The radial profile of the spectrum-weighted mean wavenumber of density fluctuations (a quantity proportional to the scattering rate of radio waves) is found to have a broad maximum at around (4–7) R ⊙, where the slow solar wind becomes supersonic. The level of density fluctuations at the inner scale (which is consistent with the proton resonance scale) decreases with heliocentric distance as 〈δni2〉(r)≃2×107r/R⊙−1−3.7 cm−6. Due to scattering, the apparent positions of solar burst sources observed at frequencies between 0.1 and 300 MHz are computed to be essentially cospatial and to have comparable sizes, for both fundamental and harmonic emission. Anisotropic scattering is found to account for the shortest solar radio burst decay times observed, and the required wavenumber anisotropy is q ∥/q ⊥ = 0.25–0.4, depending on whether fundamental or harmonic emission is involved. The deduced radio-wave scattering rate paves the way to quantify intrinsic solar radio burst characteristics.

On 6 September 2017, the Sun emitted two significant solar flares (SFs). The first SF, classified X2.2, peaked at 09:10 UT. The second one, X9.3, which is the most intensive SF in the current solar cycle, peaked at 12:02 UT and was accompanied by solar radio emission. In this work, we study ionospheric response to the two X-class SFs and their impact on the Global Navigation Satellite Systems and high-frequency (HF) propagation. In the ionospheric absolute vertical total electron content (TEC), the X2.2 SF caused an overall increase of 2-4 TECU on the dayside. The X9.3 SF produced a sudden increase of 8-10 TECU at midlatitudes and of 15-16 TECU enhancement at low latitudes. These vertical TEC enhancements lasted longer than the duration of the EUV emission. In TEC variations within 2-20 min range, the two SFs provoked sudden increases of 0.2 TECU and 1.3 TECU. Variations in TEC from geostationary and GPS/GLONASS satellites show similar results with TEC derivative of 1.3-1.7 TECU/min for X9.3 and 0.18-0.24 TECU/min for X2.2 in the subsolar region. Further, analysis of the impact of the two SFs on the Global Navigation Satellite Systems-based navigation showed that the SF did not cause losses-of-lock in the GPS, GLONASS, or Galileo systems, while the positioning error increased by 3 times in GPS precise point positioning solution. The two X-class SFs had an impact on HF radio wave propagation causing blackouts at <30 MHz in the subsolar region and <15 MHz in the postmidday sector.

The fine structures of solar radio bursts reveal complex dynamics in the corona, yet the observed characteristics of these subsecond bursts are additionally complicated by radio wave scattering in the turbulent solar corona. We examine the impact of anisotropic turbulence in radio wave propagation simulations with nonradial magnetic field structures in shaping the morphology, time characteristics, and source positions of fine structures. The apparent sources are found to move along the direction of the magnetic field lines and not along the density gradient, whereas the major axis of the scattered source is perpendicular to the local magnetic field (the scattering anisotropy axis). Using a dipolar magnetic field structure of an active region, we reproduce observed radio fine-structure source motion parallel to the solar limb associated with a coronal loop and provide a natural explanation for puzzling observations of solar radio burst position motions with the Low Frequency Array. Furthermore, the anisotropy aligned with a dipolar magnetic field causes the apparent-source images to bifurcate into two distinct components, with characteristic sizes smaller than in unmagnetized media. The temporal broadening induced by scattering reduces the observed frequency drift rate of fine structures, depending on the contribution of scattering to the time profile. The findings underscore the role of magnetic field geometry and anisotropic scattering for the interpretation of solar radio bursts and highlight that anisotropic scattering produces more than a single source.

Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast (∼ms duration) transients. While Galactic sources like pulsars undergo scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening in their lines of sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure and scattering time (τ) contributed by the Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies’ ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts 0.5 ≤ z s ≤ 1, an all-sky, isotropic FRB population has simulated values of τ (1 GHz) ranging from ∼1 μs to ∼2 ms (90% confidence, observer frame) that are dominated by host galaxies, although τ can be ≫2 ms at low Galactic latitudes. A population at z s = 5 has 0.01 ≲ τ ≲ 300 ms at 1 GHz (90% confidence), dominated by intervening galaxies. About 20% of these high-redshift FRBs are predicted to have τ > 5 ms at 1 GHz (observer frame), and ≳40% of FRBs between z s ∼ 0.5–5 have τ ≳ 1 ms for ν ≤ 800 MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale (≪1 au) density fluctuations than observed from nearby halos.

Radio signals propagating via the solar corona and solar wind are significantly affected by compressive waves, impacting the properties of solar bursts as well as sources viewed through the turbulent solar atmosphere. While static fluctuations scatter radio waves elastically, moving, turbulent, or oscillating density irregularities act to broaden the frequency of the scattered waves. Using a new anisotropic density fluctuation model from the kinetic scattering theory for solar radio bursts, we deduce the plasma velocities required to explain observations of spacecraft signal frequency broadening. The inferred velocities are consistent with motions that are dominated by the solar wind at distances ≳10 R ⊙, but the levels of frequency broadening for ≲10 R ⊙ require additional radial speeds ∼(100–300) km s−1 and/or transverse speeds ∼(20–70) km s−1. The inferred radial velocities also appear consistent with the sound or proton thermal speeds, while the speeds perpendicular to the radial direction are consistent with nonthermal motions measured via coronal Doppler-line broadening, interpreted as Alfvénic fluctuations. Landau damping of parallel propagating ion-sound (slow MHD) waves allows an estimate of the proton heating rate. The energy deposition rates due to ion-sound wave damping peak at a heliocentric distance of ∼(1–3) R ⊙ are comparable to the rates available from a turbulent cascade of Alfvénic waves at large scales, suggesting a coherent picture of energy transfer, via the cascade or/and parametric decay of Alfvén waves to the small scales where heating takes place.

Satellite Traces (STs) are the important ionogram signatures for the presence of upwellings in the bottom‐side ionosphere, which provide the necessary seed perturbation for the development of equatorial plasma bubbles (EPBs). In this study, a virtual ionosonde experiment is simulated to investigate the various ST signatures under the presence of shallow, deep, overhead, and off‐centered upwellings in the bottom‐side ionosphere. It is shown that STs occur at higher and lower virtual heights than the main ionogram trace for the off‐centered and overhead upwellings, respectively. The height separation between the main trace and STs increases with the deepening of overhead upwellings. Further, a proof‐of‐concept is demonstrated that multiple STs from ionograms can be used to reconstruct the spatial structure of bottom‐side upwellings, if the precise Angle‐of‐Arrival information can be resolved from the wide beam Ionosonde systems, and can have potential applications in predicting the occurrence of EPBs. Plain Language Summary The ionosphere is the gateway for radio communications between the ground and space. The free electrons and ions in the ionosphere can affect the propagation of radio waves. The structures with turbulent electron density variations in the ionosphere, known as plasma bubbles, can often disrupt radio communication signals beyond the usable levels. Such a situation can lead to severe outages in the satellite‐based (e.g., Global Navigation Satellite System based) positioning, precise navigation, operation of drones, etc. The upwelling structures in the bottom‐side ionosphere are the earliest manifestations of the plasma bubbles, hence, their early detection is vital for forecasting the adverse effects of plasma bubbles. This study illustrates the different characteristic signatures of upwellings on the ionograms and demonstrates a proof‐of‐concept to reconstruct the upwellings from the multiple STs if the precise Angle‐of‐Arrival information is available. Key Points Satellite Traces (STs) occur at lower virtual heights than the main ionogram trace when the upwelling is centered and located overhead of transmitter The precise Angle‐of‐Arrival (AoA) is a crucial parameter to unambiguously interpret ST signatures associated with upwellings of different depths and positions A proof‐of‐concept is demonstrated that multiple STs with AoA values can be used to reconstruct the spatial structure of upwelling

Radio waves undergo scattering by small-scale density fluctuations during propagation through the solar-terrestrial environment, substantially affecting the observed characteristics of solar radio bursts. This scattering process can be effectively modeled as photon diffusion in phase space. In this study, we present a comprehensive comparison between the quasilinear diffusion coefficients and those calculated by ray-tracing the photon trajectories in numerically generated, broadband, isotropic density fluctuation fields in both 2D and 3D configurations. The comparative analysis demonstrates that for weak scattering, the simulated diffusion coefficients agree well with the quasilinear theoretical predictions. However, when the radio frequency approaches the electron plasma frequency and/or the density fluctuation amplitude becomes significant, photons experience strong scattering. Under such conditions, the quasilinear theory tends to underestimate the scattering strength of photons induced by 2D density fluctuations while overestimating the scattering strength in 3D cases. Furthermore, we implement a group velocity correction to the theoretical diffusion coefficients, based on the effective propagation speed averaged over all test photons. The corrected coefficients provide an accurate quantification of the scattering strength for radio waves propagating through 3D density fluctuations. The physical mechanisms underlying these phenomena are elucidated in the discussion.