Frequency Domain Interferometry Technique Research Articles

High‐Resolution Sporadic E Layer Observation Based on Ionosonde Using a Cross‐Spectrum Analysis Imaging Technique

AbstractThe appearance of the sporadic E (Es) layer is an occasional phenomenon in the ionosphere. Limited by range resolution, it is difficult to observe the fine structures and internal evolution processes using existing ionosonde detection techniques. In this study, by analyzing the cross‐spectrum of the echo and the modulation sequence of the transmitted signal, the coherence between frequency components in the signal bandwidth is utilized. Using the Capon spectrum estimation method in the range dimension, the range resolution and use efficiency of the ionosonde are improved. This method does not rely on the joint processing of multifrequency detection data, unlike the frequency domain interferometry technique, maintaining the resulting time resolution. Using this imaging method, observations of the Es layer show that multiple peaks within an intrinsic range bin (3.84 km) can be resolved at a range that is 10‐fold higher (384 m); the multilayer phenomenon inside the Es layer can be clearly imaged in a continuous echo. Semishading between the layers is present. In some cases, the mode of the echo traces can also be directly identified through high‐resolution imaging. Concurrently, observations show that dramatic short‐term evolution processes on the order of seconds inside the Es layer can be captured.

Improved Ionosonde Monitoring of the Sporadic E Layer Using the Frequency Domain Interferometry Technique

The sporadic E (Es) layer is a thin layer of ion plasma enhancement in the E-region ionosphere, typically at altitudes of 90–120 km with vertical and horizontal extent of several or several tens of kilometers. As the transition region between the lower and upper atmosphere, this layer is of critical importance for ionospheric studies. The most economical but effective method to observe this layer is using ionosonde, which, however, is incapable of capturing the finer structure or the internal inhomogeneity of the Es layer as the range resolution is on the order of kilometers. To overcome this limitation, we employ the frequency domain interferometry (FDI) technique, a technique that has been successfully applied to the analysis of some radar and sonar measurements. Here, we use the Es layer measurements near Wuhan, China (114°22′E, 30°30′N) on 8 June 2021 as examples to showcase the capability of this technique. Our results show that the spatial resolution of ionosonde imaging is remarkably increased: the complexity of the internal fine structure in the Es layer can be well observed in the FDI-processed ionograms, whereas the intrinsic range resolution is several kilometers. Moreover, by comparing the ionograms obtained with and without the FDI technique, it is found that the FDI-processed ionogram is particularly suitable for the observation of evolutional processes in the Es layer, as well as the identification of different types of Es layer. With this level of spatial resolution, ionosonde, in combination with the FDI technique, opens the possibility for more refined observations of the Es layer.

Advanced SuperDARN meteor wind observations based on raw time series analysis technique

The meteor observation technique based on SuperDARN raw time series analysis has been upgraded. This technique extracts meteor information as biproducts and does not degrade the quality of normal SuperDARN operations. In the upgrade the radar operating system (RADOPS) has been modified so that it can oversample every 15 km during the normal operations, which have a range resolution of 45 km. As an alternative method for better range determination a frequency domain interferometry (FDI) capability was also coded in RADOPS, where the operating radio frequency can be changed every pulse sequence. Test observations were conducted using the CUTLASS Iceland East and Finland radars, where oversampling and FDI operation (two frequencies separated by 3 kHz) were simultaneously carried out. Meteor ranges obtained in both ranging techniques agreed very well. The ranges were then combined with the interferometer data to estimate meteor echo reflection heights. Although there were still some ambiguities in the arrival angles of echoes because of the rather long antenna spacing of the interferometers, the heights and arrival angles of most of meteor echoes were more accurately determined than previously. Wind velocities were successfully estimated over the height range of 84 to 110 km. The FDI technique developed here can be further applied to the common SuperDARN operation, and study of fine horizontal structures of F region plasma irregularities is expected in the future.

Application of frequency-domain interferometry in the extreme-ultraviolet range by use of high-order harmonics

We apply the frequency-domain interferometry technique in the extreme-ultraviolet (XUV) range using high-order harmonics. The technique is first used to study the influence of ionization of the generating gas on the harmonic emission. We report a thorough study of the fringe contrast as a function of the generating parameters: gas pressure, laser intensity and focus position, harmonic order, delay, and relative intensity of the two laser pulses. We show that two phase-locked time-delayed harmonic pulses can be produced under optimal conditions. However, a strong distortion of the fringe pattern (low contrast and large asymmetry) can be induced by the deleterious effects of the medium ionization, i.e., depletion of the emitters, laser defocusing, and loss of mutual coherence that is due to free-electron dispersion. XUV frequency-domain interferometry is then applied to the measurement of the electron density of a plasma created by optical field-induced ionization of a high-pressure helium jet by use of an intense short-pulse laser. The measured temporal evolution of the electron density shows that the resolution of our experimental setup was of the order of 200 fs, which, to our knowledge, provides the first XUV interferometry measurement on a femtosecond time scale.

System phase calibration of VHF spaced antennas using the echoes of aircraft and incorporating the frequency domain interferometry technique

Using multiple‐receiver arrays, the techniques of spaced antennas and spatial domain interferometry have been widely utilized in mesosphere‐stratosphere‐troposphere (MST) VHF and meteor radars. As these techniques are applied, the phase imbalance between the receiving channels will cause a biased result if it is not considered in the use of phase angles of the radar returns received in different receiving channels. In view of this, many methods have been employed for the phase calibration of receiving channels. In this paper, we propose one more method/procedure for the calibration, in which the commercial airplane flying routinely is used as a radar target. We measure the zenith and azimuth angles of the airplane with a video camera and estimate the range and height of the airplane from observation using the frequency domain interferometry (FDI) technique. Consequently, the track of the airplane in the vicinity of the radar site can be determined. The phase difference between the two radar echoes received by a pair of receiving channels is then predicted in accordance with the track of the airplane. Comparing the predicted phase difference with that observed, the phase imbalance between a pair of receiving channels could be obtained. Tens of cases show consistent results and so verify the reliability of the method. Also shown is the drift of system phase imbalance from season to season, which might be caused by the temperature dependence of antenna system components.

A frequency domain radar interferometric imaging (FII) technique based on high-resolution methods

In the present work, we propose a frequency-domain interferometric imaging (FII) technique for a better knowledge of the vertical distribution of the atmospheric scatterers detected by MST radars. This is an extension of the dual frequency-domain interferometry (FDI) technique to multiple frequencies. Its objective is to reduce the ambiguity (resulting from the use of only two adjacent frequencies), inherent with the FDI technique. Different methods, commonly used in antenna array processing, are first described within the context of application to the FII technique. These methods are the Fourier-based imaging, the Capon's and the singular value decomposition method used with the MUSIC algorithm. Some preliminary simulations and tests performed on data collected with the middle and upper atmosphere (MU) radar (Shigaraki, Japan) are also presented. This work is a first step in the developments of the FII technique which seems to be very promising.

A discussion on the variations of MST/ST radar echo power with an atmospheric layer resolved by frequency domain interferometry technique

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.

Observation of spatial asymmetry of THz oscillating electron plasma wave in a laser wakefield

The asymmetric spatial distribution of electron density perturbation is observed by using a frequency-domain interferometry technique. The wake amplitude of the outside bump is enhanced by the elliptical distribution of the pump laser pulse. This asymmetry can be explained with a two-dimensional analytical model expanded from cylindrically symmetric linear theory.

On the interpretation of the layered structures detected by mesosphere‐stratosphere‐troposphere radars in dual frequency domain interferometry mode

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.

First observations of Kelvin‐Helmholtz billows in an upper level jet stream using VHF frequency domain interferometry

In this paper we report the first high‐resolution observations of upper tropospheric Kelvin‐Helmholtz billows using VHF frequency domain interferometry (FDI). The measurements were made using the sounding system VHF radar located in the German Harz Mountains operating at the frequencies of 53.25 and 53.75 MHz. Through an application of the FDI technique it has been possible to track the altitude of thin scattering layers with an accuracy of a few tens of meters and a temporal resolution of 13 s. Taking advantage of the FDI technique, we found several examples of Kelvin‐Helmholtz instability (KHI) in regions coinciding with large values of wind shear. One particularly good example is presented in detail. The KHI occurred just below the axis of the jet at an altitude of 9.1 km. It produced oscillations in the vertical velocity field in the altitude range of 8–10 km. The oscillations had a local period of 90 s and persisted over a time interval of 10 min. On the basis of the magnitude of the horizontal wind at the time and height of the KHI, the billow train had a horizontal extent of about 27 km, while the horizontal length of the individual billow amounted to about 4.0 km. The 90‐s oscillations were also observed in the radar signal power, the FDI‐derived layer height, and the coherence between the received radar signals for the two frequencies. Using the FDI data, we were able to identify the individual billows located within the train and observe their maximum crest‐to‐trough amplitudes to be between 220 and 230 m. This was less than the conventional radar resolution of 300 m used during the experiment.

Effects of anisotropy and power spectrum of refractivity irregularities on the determination of layer thickness and layer position using the frequency domain interferometry technique

In this article we study the effects of the wave number power spectrum of refractivity irregularities with anisotropic structures on the determination of the layer thickness and position by using the frequency domain interferometry (FDI) technique of MST radar returns. Numerical results show that both the power spectral form and the anisotropy of the refractivity irregularity contribute remarkably to the magnitude and phase of a two frequency cross‐correlation function, provided that the radar beamwidth is not too narrow. By approximating the power law spectral form in the scattering integration equation to a modified Gaussian function, we propose a more realistic FDI expression for estimating the layer thickness and position. Furthermore, an FDI experiment at the Chung‐Li coherent scatter 52‐MHz radar was carried out on November 10, 1995. The radar data are used not only to calculate FDI coherence and phase but also to estimate the horizontal correlation length of the refractivity irregularities with conventional space antenna drift (SAD) method. Observational result shows that the horizontal correlation lengths are mostly within 10–40 m, indicating the anisotropic behavior of the irregularities. Substituting the observed values into the theoretical FDI expressions, we can obtain the layer thickness and position accordingly. By comparing the results of the proposed FDI expression with those of the existing FDI equations, we show that a large 50% difference in layer thickness estimation and tens of meters difference in layer position estimation are seen when vertical correlation length is assumed to be 3 m. These results suggest that the effects of the power spectral form and the anisotropy of the refractivity irregularity on the determination of layer thickness and layer position are significant and cannot be ignored.

Implementation of frequency domain interferometry at the SOUSY VHF radar: First results

Frequency domain interferometry (FDI) provides a means of identifying the altitude and width of atmospheric scattering layers with an increased range resolution over conventional pulsed radar measurements. The technique was first formally introduced in 1987 and has since been used in the study of scattering layers from the lower troposphere to the upper mesosphre. Despite its success, FDI has only been implemented on a few atmospheric radars. We present here the first results of FDI measurements conducted at the sounding system (SOUSY) VHF radar in Germany. The flexibility of the SOUSY radar permits simultaneous sampling of regions of the atmosphere using both single and coded pulses, thus facilitating a comparison of the resulting coherences. We present here FDI data obtained during 14 continuous hours of radar operation. The FDI technique is shown to be capable of consistently predicting the location and thickness of atmospheric scattering layers. Such information is important when studying turbulence, scattering mechanisms, and fine‐scale structures in the atmosphere.

Theoretical study of two‐frequency coherence of MST radar returns

The theoretical expressions of two‐frequency coherence of the MST radar returns from an atmospheric thin layer with sharp boundaries, which embeds within the radar volume and cannot be resolved by conventional radar techniques, are derived in this article. It shows that the derived frequency domain interferometry (FDI) coherence is not only the function of layer thickness and the components of radar wave vector, but also related to the wavenumber power spectrum of refractivity irregularities. A quantitative examination of the derived FDI coherence indicates that the effect of the irregularity power spectrum on FDI coherence is negligible for power law spectral model, while it cannot be ignored for other spectral forms, for example, a Gaussian model. The zenith angle dependence of FDI coherence is also investigated in this article. According to the observations made with Chung‐Li VHF radar, it shows that the ratio of the observed FDI coherence at the vertical to that at 17° off‐zenith angle is between 1.3 and 2.4. This feature can be illustrated satisfactorily by using the theory developed in this article. The effect of echoing mechanism on FDI coherence is also studied, showing that the expression of FDI coherence derived from the turbulent scattering theory can be treated to be identical to that from the Fresnel reflection theory as long as the condition Δk/k ≪ 1 is met. This result implies that it seems to be impossible to identify the echoing mechanism of MST radar by using FDI technique. The problem of estimating the thickness of a thin layer having sharp boundaries by using a Gaussian function with the expression of exp (−Δk2σ2r) is also discussed. It suggests that the pertinent formula used for layer thickness estimate is L = 2.364σr. A comparison of this work with other results is also made in the text.

Frequency domain interferometry of polar mesosphere summer echoes with the EISCAT VHF radar: A case study

During the polar mesosphere summer echo (PMSE) campaign in 1988 the first multiple‐frequency mesospheric measurements were carried out using the European incoherent scatter (EISCAT) 224‐MHz radar. A case study of nearly simultaneous measurements of coherent backscatter, collected on two closely spaced frequencies on July 3, 1988, is presented in this paper. The data are used to investigate the frequency coherence of the radar echoes and to perform frequency domain interferometry (FDI) analysis. The FDI technique provides precise information about the thickness and relative position of isolated scattering layers. The results indicate that scattering layers with thicknesses in the range 85–120 m are sometimes present in the polar summer mesosphere. Such a layer is shown to exist for a period of approximately 10 min, and its position is tracked as it descends over more than 1 km in altitude and transits from one range gate to the next. In addition, the FDI technique is used to study a case where a sudden frequency jump is observed in the Doppler spectrum. Spectral jumps appear to be a characteristic of the 224‐MHz PMSE echoes observed at EISCAT. The FDI analysis and Doppler sorting are employed to study the coherence spectrum of the two‐frequency data before, during, and after the jump. The frequency jump as well as the signatures that are observed in the coherence spectrum can be explained if the radar is observing a thin scattering layer which has been rippled by a steepened gravity wave.

Frequency domain interferometry studies of mesospheric layers at Jicamarca

Equatorial mesospheric scattering layer widths and motions were measured using the high‐resolution frequency domain interferometry technique at Jicamarca. Layer widths as small as ∼200 m were identified, as well as oscillatory vertical layer motions with 1–1.5 km peak‐to‐peak amplitudes. Layers are shown to oscillate with the vertical component of simultaneously observed gravity wave perturbation velocities. There is some suggestion in the data that lower mesospheric layers are more horizontally structured and patchy than higher mesospheric layers.

Pulse compression and frequency domain interferometry with a frequency‐hopped MST radar

The use of a coherent frequency‐hopped radar for pulse compression and frequency domain interferometry is considered. An analytical model for the two‐frequency correlation at the output from a matched filter receiver is developed that includes the effects of finite beam width. The analysis is restricted to the case of vertically directed radar beam. This result is used to derive the effective height weighting function that can be obtained by appropriate superposition of the receiver output from multiple frequencies. The result is also used to study the normalized correlation, or coherence, which is used in the frequency domain interferometry technique. The influence of finite beam width on the altitude resolution and coherence function is illustrated, and practical applications and limitations of the technique are discussed.