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The Agile Dual Auroral Radar Network (AgileDARN) radar has been developed as an all-digital high-frequency (HF) coherent scatter radar. Based on phased array antenna and digital signal processing techniques, digital beam forming (DBF) is employed for both transmission and reception, which steers toward the desired direction based on the relative phases within the array elements. However, the amplitude and phase imbalances among channels will lead to substantial degradation of DBF performance, including the beam direction deviation, broadening of the main lobe, deteriorated side lobes, antenna gain degradation, and ultimately affects the radar detection accuracy. Here, we present a complete method for calibrating these imbalances in transceivers and linear array elements in AgileDARN radar: internal calibration and external calibration. During the internal calibration, the calibration signals with the same amplitude and phase as the multi-pulse sequence are sent to all transceivers internally. The variance of the outputs illustrates the transceivers' imbalances and can be processed to implement calibration. Further, the meteor echoes and linear least squares fitting method are employed for external calibration, which calibrates the amplitude and phase imbalances among the cables and phased array elements. The technique has been successfully implemented, and we present the results of measured data observed by the AgileDARN HF radar before and after calibration.

The performance of the high-frequency hybrid sky-surface wave radar is often degraded due to the distortion of radar echoes induced by time-varying ionosphere. Here a modified S-transform method based on the direct wave for ionosphere decontamination of high-frequency hybrid sky-surface wave radar is presented. First, we extract the time-varying-ionosphere-distorted phase of the echoes from the direct wave of high-frequency hybrid sky-surface wave radar by using the modified S-transform. Then this phase function is used to decontaminate the ionosphere distortion of radar echo at each range bins. Simulations and experimental data are conducted to validate the performance of the proposed method on decontaminating the distortion. The experimental data are acquired by Wuhan Universitys ocean state measuring and analyzing radar. A mass of radar data is processed and analyzed, demonstrating that the decontamination effect is statistically significant. The method is not only effective to remove the Doppler shift of direct wave and Bragg peaks caused by ionosphere but also able to narrow the broadened direct wave and Bragg peaks. Meanwhile, the SNR is improved in most range bins; the background noise is reduced detectably, sometimes significantly; and some isolated radar echoes near direct wave or Bragg peaks are recovered or enhanced. These results indicate that the proposed decontamination algorithm works well.

Refractivity-from-clutter (RFC) is a technique for remote sensing of the lowest portion of vertical atmospheric refractivity profile, particularly evaporation and surface-based ducts. It uses the sea-surface backscattered radar signal to estimate the ducting conditions. The Lower Atmospheric Propagation (LATPROP) radar is a software-defined, coherent-on-receive remote sensing system built on a low-cost, commercial off-the-shelf marine radar platform operating at 9.41 GHz. The system is designed to specifically operate as a RFC-capable platform sensitive to low clutter power returns from the sea surface. This paper examines the appropriate radar design, system modifications, and radar signal processing steps, and provides a comparison with existing RFC-capable systems. The LATPROP system was deployed during the Coupled Air-Sea Processes and electromagnetic Ducting Research West Campaign that took place off of the coast of Southern California during October 2017 and was shown to accurately capture the effects of the ducting conditions.

Artificial E-region field-aligned plasma density irregularities (AFAIs) have been generated using the HAARP ionospheric modification facility in Gakona and observed with a 30-MHz coherent scatter radar imager in Homer, Alaska. The AFAIs were generated using a distinctive, twisted-beam antenna pattern that illuminated a particularly broad volume overhead. The broad beam facilitates studies of natural sporadic E layer patches when they are present. The center of the pattern was pointed at different angles between zenith and magnetic zenith to examine the effects on the AFAI morphology. Radar images of AFAIs generally resemble the radiation pattern of the HF source, but the irregularities are strongest within a narrow range of zenith angles bounded approximately by the Spitze angle. A number of factors which might influence AFAI generation and detection are examined. The most important is most likely the requirement for the pump mode to have a standing-wave component for thermal parametric instability to operate.

In this paper we consider the radiolocation equation for the Incoherent Scatter Radar (ISR) method, the equation accounts for refraction effect and Faraday rotation in the VHF frequency domain. Our research shows that we cannot ignore refraction in processing the incoherent scatter data when lag-product is used to obtain the ionospheric characteristics. Effect of refraction can explain some uncertainties that arise during processing and interpretation of the ISR data. This effect shows that together with the "ion line" of incoherent scatter spectrum, a relatively small "electron line" also plays a significant role in forming the ionosphere response. The “electron line” behaves as an additional "radar induced" wideband radiolocation signal affected by refraction and Faraday rotation; it depends on the ionosphere temperature and ion composition. The presence of a wideband “electron line” produces specific distortions of the ISR spectrum during 3D-forming of the ionosphere response inside the ISR antenna diagram pattern. During testing, we found that the signal from the ISR “electron line” can be measured at much higher altitudes comparing to the “ion line”. Hence, to improve the incoherent scatter technique, we can take the “electron line” and refraction into account.

A method has been developed to determine the angle-of-arrival (AoA) of incident radiation using pre-computed lookup tables. The phase difference between two receiving antennas can be used to infer AoA as measured from the pair baseline, but there will be more than one possible solution for antenna spacings greater than or equal to half a wavelength. Larger spacings are preferable to minimize mutual coupling of elements in the receive array and to decrease the relative uncertainty in measured phase difference. We present a solution that uses all unique antenna pairs to determine probabilities for all possible azimuth and zenith values. Prior to analysis, the expected phase differences for all AOAs are calculated for each antenna pair. For a received signal, histograms of possible AoAs for each antenna pair phase difference are extracted and added to produce a two dimensional probability density array that will maximize at the true value of the AoA. A benefit of this method is that all possible antenna pairs are utilized, rather than the restriction to specific pairs along baselines used by some interferometer algorithms. Numerical simulations indicate that performance of the suggested algorithm exceeds that of existing methods, with the benefit of additional flexibility in antenna placement. Meteor radar data has been used to test this method against existing methods, with excellent agreement between the two approaches. This method of AoA determination will allow the construction of low-cost interferometric direction finding arrays with different layouts, including construction of difficult terrain and three-dimensional antenna arrangements.

We present analysis methods for studying the structuring and motion of ionospheric irregularities at the sub-kilometer scale sizes that produce L-band scintillations. Spaced receiver methods are used for Global Navigation Satellite System (GNSS) receivers' phase measurements over approximately sub-kilometer to kilometer-length baselines for the first time. The quantities estimated by these techniques are: plasma drift velocity, diffraction anisotropy magnitude and orientation, and characteristic velocity. Uncertainties are quantified by ensemble simulation of noise on the phase signals carried through to the observations of the spaced-receiver linear system. These covariances are then propagated through to uncertainties on drifts through linearization about the estimated values of the state. Five receivers of the Scintillation Auroral Global Positioning System (GPS) Array (SAGA) provide 100 Hz power and phase data for each channel at L1 frequency. The array is sited in the auroral zone at Poker Flat Research Range, Alaska. A case study of a single scintillating satellite observed by the array is used to demonstrate the spaced receiver and uncertainty estimation process. A second case study estimates drifts as measured by multiple scintillating channels. These scintillations are correlated with auroral activity, based on all-sky camera images. Measurements and uncertainty estimates made over a 30-minute period are made and compared to a collocated incoherent scatter radar, and show good agreement in horizontal drift speed and direction during periods of scintillation for which the characteristic velocity is less than the drift velocity.

The high-power HF (High Frequency) facility (commonly known as Heating) near Tromso, Norway, which is an essential part of the EISCAT (European Incoherent SCATter) Scientific Association, has been upgraded in certain key areas in recent years. It is one of only four similar facilities in the world operating at present. An updated description of the facility is given, together with scientific motivation and some results. The main high-power parts such as transmitters, feed-system and antennas remain essentially the same as built in the late 1970s. The improvements are in the areas of radio frequency waveform generation, computer control and monitoring. In particular, fast stepping in frequency is now possible, an important aspect in examining features close to harmonics of the electron gyrofrequency. One antenna array has been modified to allow reception to implement an HF radar mode for mesospheric and magnetospheric probing. More realistic modelling of the antenna gain gives improved estimates of the total effective radiated power for both wanted and unwanted circular polarizations. Results are presented using these new capabilities, but their full scientific potential has yet to be achieved.

Ultrahigh frequency space-based synthetic aperture radar (SAR) can suffer from the degrading effects of a scintillating ionosphere which modulates both the phase and the amplitude of the radar signal. In this paper, we use Global Navigation Satellite System (GNSS) signals to synthesize an L-band SAR point spread function (PSF). The process of transforming the GNSS signal to the equivalent SAR PSF is described. The synthesized PSF is used to explore the possibility of using a phase correction determined from a point target in a SAR image to correct the ionospheric degradation. GNSS data recorded on equatorial Ascension Island during scintillation events are used to test the feasibility of this approach by applying a phase correction to one GNSS receiver from another located along a magnetic east-west baseline. The peak-to-sidelobe ratios of the synthesized L-band SAR point spread functions before and after the correction are compared, and it is shown that this approach improves the L-band PSF over distances of ~3000 m in the ionosphere, corresponding to 6000 m on the ground.

We propose that by using unmatched decoding for alternating codes, estimates of the autocorrelation function of the backscatter can be estimated at ranges between those resolved by matched filtering. The number of additional correlation estimates obtained in this way is twice that obtained using matched filtering, although each estimate contributes with only a quarter of the power in comparison. This leads to improvements in code performance by a factor of 1.5, and an increase in the speed of incoherent scatter radar experiments by the same factor.

Low-frequency, wide field-of-view (FoV) radio telescopes such as the Murchison Widefield Array (MWA) enable the ionosphere to be sampled at high spatial completeness. We present the results of the first power spectrum analysis of ionospheric fluctuations in MWA data, where we examined the position offsets of radio sources appearing in two datasets. The refractive shifts in the positions of celestial sources are proportional to spatial gradients in the electron column density transverse to the line of sight. These can be used to probe plasma structures and waves in the ionosphere. The regional (10-100 km) scales probed by the MWA, determined by the size of its FoV and the spatial density of radio sources (typically thousands in a single FoV), complement the global (100-1000 km) scales of GPS studies and local (0.01-1 km) scales of radar scattering measurements. Our data exhibit a range of complex structures and waves. Some fluctuations have the characteristics of travelling ionospheric disturbances (TIDs), while others take the form of narrow, slowly-drifting bands aligned along the Earth's magnetic field.

At high frequency, diffraction during ionospheric propagation can yield wavefronts whose amplitude and phase fluctuate over the physical dimensions of phased array radars such as those of the Super Dual Auroral Radar Network (SuperDARN). Distortion in the wavefront introduces amplitude and phase scintillation into the geometric beamformed signal while reducing radar performance in terms of angular resolution and achieved array gain. A scintillation correction algorithm based on adaptive optics techniques is presented. An experiment conducted using two SuperDARN radars is presented that quantifies the effect of wavefront distortion and demonstrates a reduction in observed scintillation and improvement in radar performance post scintillation correction.

Electronically steerable array (ESA) technology has recently been applied to incoherent scatter radar (ISR) systems. These arrays allow for pulse-to-pulse steering of the antenna beam to collect data in a three-dimensional region. This is in direct contrast to dish-based antennas, where ISR acquisition is limited at any one time to observations in a two-dimensional slice. This new paradigm allows for more flexibility in the measurement of ionospheric plasma parameters. Multiple ESA-based ISR systems operate currently in the high-latitude region where the ionosphere is highly variable in both space and time. Because of the highly dynamic nature of the ionosphere in this region, it is important to differentiate between measurement-induced artifacts and the true behavior of the plasma. Often, three-dimensional ISR data produced by ESA systems are fitted in a spherical coordinate space and then the parameters are interpolated to a Cartesian grid, potentially introducing error and impacting the reconstructions of the plasma parameters. To take advantage of the new flexibility inherent in ESA systems, we present a new way of analyzing ISR observations through use of the space-time ambiguity function. The use of this new measurement ambiguity function allows us to pose the ISR observational problem in terms of a linear inverse problem whose goal is the estimate of the time domain lags of the intrinsic plasma autocorrelation function used for parameter fitting. The framework allows us to explore the impact of nonuniformity in plasma parameters in both time and space. We discuss examples of possible artifacts in high-latitude situations and discuss possible ways of reducing them and improving the quality of data products from electronically steerable ISRs.

Specular meteor radars (SMRs) have become a widely used tool to observe horizontal winds at the mesosphere and lower thermosphere (MLT). Typically 30 to 120 min mean winds are obtained assuming horizontal homogeneity of the observed area (i.e., few hundreds of kilometer radius). The quality of the measured wind velocity vector depends on the number of detected meteors per altitude and time bins. In order to improve the wind measurements of typical SMRs, here we propose a multistatic and multifrequency approach that consists mainly on adding GPS synchronized receiving stations with interferometric capabilities to existing SMRs. Compared to typical SMRs operating in a monostatic mode, our new approach called MMARIA (Multistatic and Multifrequency Agile Radar for Investigations of the Atmosphere) allows us to (a) increase the number of meteors using the same transmitter (by more than 70%), (b) increase the altitudinal coverage by 5–10 km higher depending on the geometry used, and (c) derive the horizontal wind field in the observed volume by relaxing the assumption of homogeneity. The latter result is facilitated by having common volume observations from at least two different viewing angles. We show the feasibility of these three goals from measurements at two different frequencies using a MMARIA configuration between Juliusruh and Kuhlungsborn in northern Germany.

We develop an accurate and fast forward model for calculating the compact closed-form high-order perturbative solutions of the problem of three-dimensional (3-D) radiation and propagation electromagnetic fields in a layered structure with multilayer rough interfaces. The proposed method for the fast forward model is first demonstrated by strictly theoretical formulas derivation in the framework of classical small perturbation method (SPM) without other else approximation and equivalent process. The kernel functions of high order are proposed for calculating the radar cross sections with more efficiency and clear physical meanings for better use in practice. What is more, we give the clear physical interpretation of the first-order fully polarimetric electromagnetic wave scattering mechanism in the layered structure with multilayer rough interfaces. The proposed forward model is necessary to insure a successful inversion process. Furthermore, the high-order SPM solutions derived by employing the proposed method are validated with existing methods and numerical results. Finally, we study the performance of the high-order fully polarimetric electromagnetic wave scattering according to the numerical results and analyze the scattering enhancement phenomena.

The global precipitation measurement (GPM) mission is an international satellite mission to obtain accurate observations of precipitation on a global scale every 3 h. Its (GPM) core satellite was launched on 27 February 2014 with two science instruments: the microwave imager and the dual-frequency precipitation radar. Ground validation is an integral part of the GPM mission where instruments are deployed to complement and correlate with spacecraft instruments. The dual-frequency, dual-polarization, Doppler radar (D3R) is a critical ground validation instrument that was developed for the GPM program. This paper describes the salient features of the D3R in the context of the GPM ground validation mission. The engineering and architectural overview of the radar is described, and observations from successful GPM ground validation field experiments are presented.

Frequency-based localization methods are widely used to find emitter locations. Several techniques are described in the literature for emitter localization based on Doppler frequency shifts. These techniques can be used efficiently for emitter localization by using narrowband signals. Although these methods are simple and efficient, the application to the radar systems for target localization is very limited. In this paper, a new low-complexity target localization method, Target Localization via Doppler Frequencies (TLDF), for Doppler-only Multi-Input, Multi-Output (MIMO) radar with widely separated stations is described. By using widely separated MIMO radars with unmodulated continuous wave signals, the received frequencies and the Doppler shifts can be estimated efficiently. The position and the velocity of the target can be found from these estimated frequencies by a search in the position space. As the Doppler frequency is estimated efficiently, not only the target velocity but also the direction of the target is estimated accurately with the TLDF method. The Cramer-Rao Bounds (CRB) are calculated for the target velocity and the target position estimations in two-dimensional space. In simulations, the proposed method is compared with the iso-Doppler curves-based traditional method and with the CRB for different geometries. The performance of the proposed method is not affected from the target amplitude fluctuations because of its frequency-based nature. Finally, the comparison between the frequency-only MIMO radar and the pulsed monostatic radar is investigated, and the simplicity and the efficiency of the proposed method are demonstrated.

[1] A new antenna layout for a Super Dual Auroral Radar Network (SuperDARN) HF radar has been developed. The new layout utilizes two auxiliary arrays; one behind and one in front of the main array, rather than the single auxiliary array that existing radars use. The rear auxiliary array consists of three antennas providing beam-steering capability while the front auxiliary array consists of a single antenna. This layout is expected to greatly improve the calculation of elevation angle of arrival. Simulations presented show the advantages and disadvantages of using twin-terminated folded dipole (TTFD) antennas and log-periodic dipole arrays in standard and modified SuperDARN array configurations. TTFD antennas are shown to have superior front-to-back ratio and beam-steering capability but suffer from shadowing effects due to the presence of corner reflectors. Impedance-matching techniques used in SuperDARN radars are discussed, and the results of a new matching method, exhibiting a superior voltage standing-wave ratio over the SuperDARN frequency band, are presented. Shadowing of the main array by the front auxiliary array is investigated, and it is shown that the impact of the front array on the main array gain pattern is significantly less for the case of a single front antenna than for a four-antenna front array. Radar phase calibration techniques are discussed, and it is proposed that the additional single-antenna front array be used for system-wide radar phase calibration. An algorithm for the determination of elevation angle of arrival using the new layout is also given.

[1] We report first results of polyphase-coded incoherent scatter measurements at Millstone Hill. To our knowledge, these are the first incoherent scatter measurements with polyphase alternating codes of Markkanen et al. (2008) and optimal quadriphase sequences of Damtie et al. (2008). The results demonstrate that an arbitrary waveform generator recently installed at the Millstone Hill incoherent scatter radar, part of the National Science Foundation Geospace Facility operated by the Massachusetts Institute of Technology Haystack Observatory, is capable of reproducing the polyphase waveforms with an accuracy sufficient for incoherent scatter measurements. Polyphase codes will allow incoherent scatter radar experiments to be better optimized, because they provide a larger variety of code and code cycle lengths than the traditional binary codes.

[1] Meteor radars have become common and important tools in the study of the climate and dynamics of the mesosphere/lower thermosphere (MLT) region. These systems depend on accurate angle-of-arrival measurements to locate the positions of meteor trails in the atmosphere. Mutual coupling between antennas, although small, produces a measurable error in the antenna pair phase differences used to deduce the angle of arrival of incident radiation. Measurements of the scattering parameter matrix for antennas in an interferometric meteor radar array have been made and applied to the existing angle-of-arrival calculation algorithm. The results indicate that mutual coupling of antennas in the array produces errors in the zenith angle estimate of less than ± 0.5°. This error is primarily in the form of a gradient across the field of view of the radar, which can be removed using existing phase calibration methods. The remaining error is small but will produce small systematic variations in the height estimates for detected meteors.

In the present paper, simultaneous radar observations in the Doppler beam swinging (DBS), spatial interferometry (SI), and spaced antenna (SA) modes with the middle and upper atmosphere (MU) radar are shown. High aspect sensitivities were found after a passage of a cutoff low and an incidental trough owing to observations in the DBS mode. Because it is known that tilted aspect sensitivities introduce a bias in radial velocity measurements, we corrected the horizontal wind directly measured by oblique beams in the DBS mode by taking account of the arrival angle of the oblique beams estimated by the SI technique. On the other hand, the aspect angle is derived from the echo power ratio of the vertical to the oblique beams with the DBS mode, which allows us to estimate an effective pointing angle, and the Doppler wind is corrected by using this effective angle. In addition, the SA technique is performed simultaneously for estimating the horizontal wind. The above four estimating processes are used to determine the horizontal wind and compared for the aspect sensitivity. Finally, a simple computer simulation is performed for the SA technique for constant and variable wind conditions during incoherent integrations.

Atmospheric radar imaging techniques have shown promise in revealing the fine-scale structure of the atmosphere within the resolution volume of the radar. Enhanced resolution can be obtained in both angle and range by using spaced receivers and shifted frequencies, respectively. The distinct techniques have been termed coherent radar imaging (CRI) for angular resolution enhancement and range imaging (RIM) for radial resolution improvement. Because of the mathematical similarities between CRI and RIM it is possible to derive a generalization of both techniques. In this work, the three-dimensional (3-D) imaging technique, which uses multiple receivers and multiple frequencies simultaneously, is developed for the first time. Three-dimensional imaging has the advantage of mitigating the limitations of beam width as well as pulse width of a conventional radar to simultaneously improve both angular and range resolution. It is shown that CRI and RIM are special cases of 3-D imaging. The mathematical problem is formulated as an inverse problem with solutions provided by the Fourier, Capon, and maximum entropy (MaxEnt) methods. These three 3-D imaging methods are verified and statistically tested through numerical simulations.

An analysis leading to the first and second-order bistatic cross sections of the ocean surface in the context of high-frequency ground wave radar operation is presented. Initially, a pulsed dipole source is introduced into the previously derived electric field expressions for the bistatic reception of vertically polarized radiation scattered from rough surfaces that do not vary with time. To make application to the ocean, a time-varying surface is introduced via a three-dimensional Fourier series with two spatial variables and one temporal variable. The surface randomness is accounted for by allowing the Fourier coefficients to be zero-mean Gaussian random variables. Fourier transformation of the autocorrelations of the resulting fields gives the appropriate power spectral densities. The latter are used in the bistatic radar range equation to produce the cross sections. The features of the bistatic case are seen to reduce to the well-known monostatic results when the appropriate geometry is introduced. Illustrative comparisons of monostatic and bistatic reception are presented.

Regular observation of the polar wind was one of several measurements considered feasible when the European Incoherent Scatter (EISCAT) auroral zone incoherent scatter facility was first conceived. Whereas most of the other measurements proposed with EISCAT have been successful, direct observations of the polar wind using the standard incoherent scatter radar analysis schemes have proven to be elusive. This paper presents the first direct measurement of the H+ velocity by a ground-based instrument situated at high latitudes, where thermal outflow of light ions occurs. We have avoided the difficulties associated with standard analysis by optimizing the detection method to emphasize only those features of the spectrum sensitive to the drift of the O+ and H+ ions. We have tested the analysis procedure on EISCAT VHF observed spectra. The data analyzed show unmistakable evidence of differences in drift velocities of the two major ion populations. The results indicate considerable variation in the drift velocity of the light ion component. The light ion velocity shows an overall increase with altitude consistent with H+ outflows predicted by thermal polar wind models. The observed H+ velocities and the prospects for using the procedure on a routine basis are discussed.

Tropospheric VHF radar data collected using both spaced antenna (SA) and Doppler beam steering (DBS) methods are presented. The SA data are analyzed using full correlation analysis (FCA) on vertical beam data, while DBS analysis is applied to vertical beam data and off-zenith beam data at 3.6°, 7.2°, and 10.8°. A comparison of meridional velocities using SA FCA and DBS analysis is presented. These results indicate good agreement between the DBS and FCA velocities on all off-zenith beams, particularly at 10.8°. The main aim of the study is to compare the aspect sensitivity parameter θs,using the spatial correlation method for the FCA data [e.g., Lesicar and Hocking, 1992] and the power method of Hocking et al. [1986] for the DBS data. This comparison reveals good general agreement between isotropic values of θs with both methods, and aspect sensitive values of θs values with both methods, although there are times when one method produces isotropic values and the other produces aspect sensitive values and visa versa. During aspect sensitive periods the FCA values agree best with the DBS method using the 0.0°–3.6° beam combination.

Complete descriptions of spatial and temporal distributions of the ionospheric plasma are necessary for scientific understanding and for several practical applications involving communication, radar, navigation, etc. Ionospheric diagnostic techniques, on the other hand, provide spatially and temporally incomplete information. A framework for data fusion is needed which would be self-consistent in terms of various input data and would also provide spatial-temporal extrapolations. Incorporation of a physical ionospheric model as a framework for assimilation of ionospheric (ionosonde) data was demonstrated by Ganguly [1990, 1991]. The free parameters of the physical model were adjusted using the observational data, and then the adjusted model was used to predict the state of the ionosphere at a different place and time. Most recently, the approach has been extended to incorporate total electron content (TEC) observation from ground and/or satellite-borne receivers and beacons, as well as data from a variety of other sources such as oblique and vertical sounders, radar, radio propagation, incoherent scatter, optical observations, in situ measurements, etc. This TEC-based reconstruction utilizes novel tomographic techniques and allows integration of any satellite constellation including GPS. Data from other sensors are integrated into the complete system, and the fusion is performed in conjunction with some physical/empirical ionospheric model. This provides a self-consistent four-dimensional ionosphere. The system can accept any reasonable ionospheric model and any amount of diverse input data covering a wide range of space, time, and observational methods.

The prediction of the sea echo Doppler spectrum of VHF and HF radio waves is important for the ocean wave remote sensing by VHF and HF radars. The authors extended the theory of the sea echo Doppler spectrum. In previous theories it is assumed that the scale of the illuminated area is much larger than the dominant wavelength. However, the effect of the finite illuminated area may not be negligible. The authors considered the effect to predict second-order sea echo Doppler spectrum and extended previous theories. Furthermore, the authors developed the numerical technique to compute radar cross sections of the extended theory. As a result, the agreement between measured and predicted Doppler spectra is improved.

In the present paper, simultaneous radar observations in Doppler beam swinging (DBS) and spatial domain interferometry (SDI) modes with the middle and upper atmosphere (MU) radar are shown for the first time. High aspect sensitive echo layers were found at a 6.0 to 8.6 km altitude range owing to observations in DBS mode. From SDI analysis, the angle of arrival of the backscattered signals was also deduced. It was found that the layers were tilted from the horizontal plane along the zonal axis with a tilt angle of approximately 0.7°. Because it is known that tilted aspect sensitive layers introduce a bias in the radial velocity measurements, we corrected the vertical velocities directly measured by the vertical beam in DBS mode, by taking account of the tilt angle of the layers estimated by the SDI technique. On the other hand, from the analysis of horizontal wind profiles, we observed a quasi-monochromatic gravity wave related to the tilted layers. It was found that the tilt angle of the phase of wave front is well associated with the inclination of the echo layers if the corrected vertical velocity is used. Finally, the Richardson number estimated at an altitude range where the aspect sensitive echo layers occur presents large positive peaks at the position of the aspect sensitive echo layers and then seems to be too high for turbulent instabilities. Consequently, the observed echo layers may be stable layers at the most stable phase of the gravity wave.

The relationship between an atmospheric layer and its VHF backscatter resolved by the frequency domain interferometry (FDI) technique is investigated in this article. A theoretical expression connecting the radar echo power with the thickness and position of an atmospheric layer and the range weighting function of a matched filter is derived. It shows that the farther the layer locates from the central height of the radar volume, the weaker the echo power will be. This feature is attributed to the range weighting effect of the radar system. FDI observations also show that the echo power from the atmospheric layer located close to the central height of the radar volume is usually greater than that from the layer located at the edge of the volume, which is in good agreement with the theoretical prediction. Moreover, the correlation between the echo power and layer position is negative (positive) if the layer locates in the upper (lower) part of the radar volume. This behavior is consistent with the theoretical prediction and can also be interpreted by the range weighting effect. It is also suggested that the power-position relation is capable of indicating the system bias causing the systematic error of FDI-estimated layer position. In addition, numerical simulation is performed in this article to examine the difference between the range weighting functions resulting from rectangular and nonrectangular radar pulses for a given receiver impulse response.

In the companion paper (part 1), theoretical studies on the dual frequency domain interferometry (FDI) technique have been presented. Two possible causes of biases in the layer thickness and position estimations by FDI have been considered: the limited extent of the scattering structure in the horizontal plane and the advection of this structure by the wind. In the present work, we study the effects of the tilts of the scattering layer from horizontal. It is shown that in case of large tilt angles, substantial biases on position and thickness can occur. The model, first developed by Liu and Pan [1993] but more extensively described in this paper, can also be used for a prediction of the variations of the FDI coherence with the zenith angle and their relation to the anisotropy of the scatterers. Some preliminary observations of the zenith angle dependence of the FDI coherence and echo power obtained with the middle and upper atmosphere (MU) radar from the vertical up to 28° off zenith with a step of 2° are shown and discussed. In principle, comparisons between the observed power and coherence variations with those given by the model could give more information on the structures that contribute around and far from the zenith.

Radar backscatter is a commonly used tool for studying plasma instabilities in the auroral E region. Analysis of the received signals typically involves moments of the scattered power spectrum such as total power, mean Doppler shift, and spectral width; in some cases the spectral asymmetry may also be of interest. This paper presents the steps required to estimate spectral moments directly from the autocorrelation function, and some advantages and limitations of working in the lag domain are discussed. Recent measurements of auroral spectra at UHF (440 and 933 MHz) are used to motivate the discussion and as test cases. The utility of parametric models is also studied with an emphasis on determining whether spectra are more nearly Gaussian or Lorentzian. A model autocorrelation function is introduced, with spectral characteristics similar to a Voigt distribution but a more convenient functional form.

Lower frequency microwave radar observations can help to measure the properties of snow cover on land by providing information about the soil-snow boundary condition. In this theoretical study, we examine the sensitivities of microwave radar measurements to soil and snow characteristics, and we compare a simple model with previously published data. Depending on the surface roughness, copolarized ratios or single polarization time ratios of radar backscattering may be affected only by the incidence angle and the dielectric contrast at the soil-snow boundary. These measurements can reduce the number of unknowns in any corresponding higher-frequency observations that are also affected by the lower boundary condition. The copolarized ratio is sensitive, first of all, to the snow density, which offers the possibility of measuring this parameter directly if a separate measurement of the soil temperature from a passive microwave system is also available. The thermal insulation provided by snow cover can have a powerful effect on the soil-snow boundary by altering the soil temperature and therefore changing the dielectric contrast. Because the microwave response of snow cover is so sensitive to the conditions of the underlying frozen soil, it is important for future ground truth campaigns that measure snow conditions to also collect data on the temperature, water content, and texture of the soil.

We model the signal received by a FM pulse radar, considering the general case where the transmitter and the receiver are separated and are moving. The electromagnetic wave polarization state change generated by the observed body is also taken into account in the model proposed in this paper. The expression of the received signal is written as the voltage detected on the receiving antenna and is given as a function of time by taking into account the antenna radiation (transmitting antenna and receiving antenna), the polarimetric behavior of the target, and the mobiles velocity effect. Numerical results are presented considering a canonical target or a slightly rough surface using a monostatic or a bistatic configuration to show the accuracy of the theoretical results and to study the velocity effect of the mobiles.

The method of moments (MOM) is applied to the problem of electromagnetic scattering from general three-dimensional dielectric targets in an arbitrary multilayered environment. The dyadic multilayered Green's function is computed via the method of complex images, and the Galerkin MOM solution is effected by employing triangular patch basis functions. Several example frequency and time domain results are presented, with application to radar-based sensing of plastic land mines.

Frequency domain interferometry (FDI) is a mesosphere-stratosphere-troposphere radar technique used for probing thin-layered structures of the atmosphere. The position and thickness of a scattering layer embedded within the illuminated volume can be deduced from the complex coherence of received signals at two closely spaced frequencies. This technique has permitted us to identify layered structures (called “FDI layers”) with thicknesses estimated to be around 50–200 m in the troposphere-stratosphere. However, its application needs very restrictive hypotheses. For example, the layers are assumed to have a large extent in the horizontal plane with respect to the horizontal extent defined by the -6-dB angular width of the two-way antenna pattern. This hypothesis seems not always to be verified in light of observations with other radar techniques. In this paper, theoretical calculations are performed in order to estimate the consequences of the limited extent of the scattering structure in the horizontal plane on the FDI parameters, i.e. the vertical extent and position in height of the single layer. The first analysis concerns the effects of the limited extent by itself, and the second one deals with the consequences of the advection of the scattering structure by the wind. It is shown that in some limiting cases, substantial biases in the FDI parameters can occur. Finally, this work stresses the necessity to confront the FDI data with other technique measurement results or to complete it with space domain interferometry data. A future paper will be devoted to the study of the effects of the tilt of a scattering layer on the thickness and position estimations.

This paper describes the estimation of vertical-structure parameters from combined interferometric and polarimetric radar data.

The need exists for measurements with high vertical resolution when observing the variety of atmospheric processes with extremely small vertical extent, such as microscale turbulence and scattering layers associated with inertia gravity waves. For example, recent in situ observations have shown that both humidity and temperature “sheets,” with thicknesses of the order of meters, exist throughout the lower atmosphere. Hampered by bandwidth constraints, however, standard pulsed radar systems have shown only limited usefulness in the detection of such phenomena. Frequency domain interferometry can be used to estimate the position and thickness of a single scattering layer within the resolution volume. Using two closely spaced frequencies, the method is derived under the restrictive assumption of a single, Gaussian-shaped layer. We will now introduce range imaging (RIM), which fully exploits the general advantages of frequency diversity. Using a set of closely spaced transmitter frequencies, a generalized method based on constrained optimization will be used to reconstruct high-resolution images of the average power density as a function of range. The technique will be studied using simulated radar data and will be shown to be capable of resolving complex structures similar to Kelvin-Helmholtz billows, which can be much smaller in vertical extent than the resolution volume.

The frequency domain interferometry (FDI) technique has been developed for probing thin layered structures of the atmosphere. The position and thickness of a single layer embedded within the scattering volume can be deduced from the complex normalized cross correlation (coherence) of received signals at two closely spaced frequencies. Applied in the vertical pointing direction, this technique identified layered structures (“FDI layers”) of 50–200 m in thickness in the lower atmosphere. These structures are 1 order of magnitude thicker than observed temperature sheets (about 10-m thick) which are very likely responsible for the main part of the VHF radar echoes in vertical direction. In this paper, although the ambiguity of the dual FDI technique is well known, we emphasize that the FDI layers do not necessarily correspond to a single atmospheric layer; they can also be interpreted as a more complex structure of very thin atmospheric layers. A simple model, introduced as an example, shows that the FDI layer thickness can also approximately be interpreted as the vertical separation of two very thin atmospheric layers. This result can explain by itself the differences between the estimated thicknesses by balloon and FDI radar techniques. Finally, we stress that comparisons with high-resolution in situ measurements are urgently needed for interpreting the FDI layers.

The “full correlation analysis” (FCA) of spaced antenna (SA) data is sometimes found to be susceptible to the “triangle size effect” (TSE), whereby the magnitude of the FCA “true” velocity increasingly underestimates the actual wind velocity with decreasing antenna spacing. The TSE has been attributed to a number of instrumental effects, including coarsely digitized data, receiver characteristic differences, noise, and antenna coupling. In this paper the sources of the TSE are discussed, and the influence of various previously undocumented instrumental effects upon the FCA are investigated using simulated data. These additional effects are also shown to produce the TSE. The possibility that these instrumental effects may contribute to the lack of agreement exhibited between winds estimated using medium frequency SA radars employing the FCA and alternative in situ, satellite, and other radar techniques is discussed.

This study investigates subtle variations of the zenith and azimuth dependence of VHF-radar echo power in the troposphere and lower stratosphere. Using the middle and upper atmosphere (MU) and Aberystwyth radars, we reanalyze data from two areas of the literature on tilted aspect-sensitive scatterers, linking results from the spatial interferometry (SI) and Doppler beam-swinging (DBS) techniques. Whereas wind profilers commonly use three or five radar beams, we examine a MU radar data set with 64 beam positions, so that maps of echo power distribution can be plotted as far as 5° from zenith. The power distribution pattern is often skewed, with the azimuth of maximum power being closely related to the wind shear caused by, for example, inertia-gravity waves in the lower stratosphere. The results imply that inertia-gravity wave motions are closely coupled to the smaller-scale wind field, causing patches of Kelvin-Helmholtz instability and/or steepening of other shorter-period gravity waves. These effects can alter the distribution of the tilts of aspect-sensitive scatterers and explain the skewed echo power patterns. The deviations of vertical-beam incidence angle measured by SI are found to be inappropriate for off-vertical beams, and it also appears impossible for basic DBS systems to be used for measuring vertical-beam incidence angles. Further tests of mountain wave data are consistent with the tilted layer model and help to confirm that the azimuth of gravity waves may be calculated using radar echo-power imbalances.

The Esrange VHF radar (ESRAD) is a relatively new mesosphere-stratosphere-troposphere (MST) class radar located in northern Sweden (67.88°N, 21.10°E), about 30 km east of Kiruna. The radar operates continuously and has already been included in several campaigns in which multiple profiling instruments have been used, for example, lidars, radiosondes, and scientific balloons. To intercompare the different measurements, it is essential to have precise height estimates. In this paper we begin by presenting a short technical overview of the radar. Then we describe a radar validation conducted in 1997 during a period when numerous scientific balloons were launched from Esrange. All of these balloons were tracked using one or more of the following techniques: Omega navigational system, Global Positioning System (GPS), or the Esrange C-band tracking radar (ESR). A number of these balloons passed close enough to the ESRAD beam that they could be detected by the MST radar. This unique data set has been used to check the range estimation of ESRAD. Furthermore, the data have been used to make a rough check of the geometry of the formed beam.

Data taken during joint observations of the European incoherent scatter (EISCAT) radar and the Karmaselga auroral coherent radar (46.5 MHz) are combined to investigate the dependence of the coherent echo power upon the maximum electron density in the E region. For electron densities in the range (1–3)×1011 m−3, an increase in the echo power with the electron density was found. A few coherent echoes were observed for electron densities larger than 3×1011 m−3, but when such echoes occurred, their power was small even though the ionospheric electric field was strong, more than 30 mV/m. These measurements agree reasonably well with the model of Uspensky and Williams [1988] which takes into account ionospheric refraction and assumes that auroral coherent echo is a superposition of scattering from a thick layer of electrojet irregularities. Results of the comparison indicate that electron density at altitudes around 110 km can be roughly estimated solely from coherent radar data.

It has often been pointed out that the Bragg backscatter of radar waves from elevated turbulent layers is very highly correlated with the height gradient of radio refractive index (RI) through these layers. However, many users need the profiles of RI, or the associated humidity, rather than profiles of their gradients. Simple integration of the gradients is usually not feasible because of ground or sea clutter and because biological scatterers such as insects and birds often severely contaminate the lower range gates. We show that if the total height-integrated RI is independently available (say, from the Global Positioning System (GPS)), and if the surface value of RI is known, the profiles of RI are retrievable with good accuracy. For those profiler systems equipped with a radio acoustic sounding system to measure temperature, the humidity is also retrievable. The method is demonstrated with data collected in southern California, where 7 hours of profiler data were recorded at 449 MHZ along with GPS data. Three radiosonde balloons were launched during the period, and the profiles of RI from the balloon and the profiler are compared. The advantages of the system are its invulnerability to nonprecipitating clouds (at frequencies of 449 MHZ or lower) and that it uses only facilities that will soon be deployed globally. Simulations are used to assess errors from various factors such as loss of sign of the gradient of the potential RI (important especially during some frontal events) and the presence of biological contaminants in some geographical areas (such as coastal zones and some agricultural areas at night).

In August 1990 we participated in the Equatorial Ionospheric Studies sounding rocket campaign near Kwajalein Atoll in the equatorial Pacific region. The campaign included measurements of plasma density using rocket probes and coherent and incoherent scatter radar. During the campaign we fielded high-frequency ionospheric sounders over a bistatic path between Maloelap Atoll and Bikini Atoll in the Marshall Islands. The distance between the transmitters and receivers was 700 km; the ionospheric-reflection region was at 10.18; {degree}N, 168.40;{degree}E, near the magnetic equator. We made three types of measurements: Doppler spread and spatial coherence for a single-frequency CW path; frequency coherence of multiple CW paths; and Doppler spread and time-delay spread for a 60-kHz bandwidth path. We obtained such data over a period of 2 weeks for approximately 2 hours each evening; during this period spread {ital F} was common. Fifty percent of the evenings showed Doppler spread of greater than 6 Hz at the {minus}10 dB level (relative to the peak signal power) and greater than 15 Hz at the {minus}30 dB level. Forty percent of the evenings showed spatial coherence distance of less than 180 m in the direction normal to the bistatic path; 40{percent} of the evenings showed spatialmore » coherence of less than 75 m in the direction parallel to the path. Seventy-five percent of the evenings showed coherence bandwidths of less than 1.5 kHz. {copyright} 1999 American Geophysical Union« less

A comparison of the resonance annihilation filter (RAF) method and the extinction pulse (E-pulse) technique for radar target discrimination is presented in this paper. It is theoretically demonstrated that the RAF synthesized using finite impulse-response digital filters is a particular forced E-pulse waveform expanded using impulse-basis functions. This fact reveals the RAF method to be a very poor technique for discrimination in a noisy environment. Improvement is possible with suitable data windowing equivalent to using E-pulse waveforms synthesized with continuous and smooth basis functions. Numerical simulations are employed to compare the two techniques when different levels of additive white Gaussian noise are present in the target response. This comparison shows that the use of the Gaussian window originally proposed in the RAF method produces results comparable to rectangular E-pulses. However, these results can be improved by the introduction of alternative smoothing windows or subsectional-polynomial E-pulses.