H. Luce

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

Abstract 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 Capons 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.

Description and demonstration of the new Middle and Upper atmosphere Radar imaging system: 1-D, 2-D, and 3-D imaging of troposphere and stratosphere

[1]xa0The Middle and Upper atmosphere Radar (MUR) was upgraded in March 2004 for radar imaging capability with 5 frequencies across a 1 MHz bandwidth and 25 digital receivers. Although digitization introduces problems of its own, the uniformity of digitization is a great benefit over the analogue system in place before. This increased reliability will help make the new system an important component of long-term atmospheric science programs. We demonstrate 3-D imaging with Capons method, which can provide information about structure morphology. In addition, we demonstrate an experimental 0.5 μs pulse mode and compare this to Capon method imaging results.

Validation of Winds Measured by MU Radar with GPS Radiosondes during the MUTSI Campaign

Abstract For many years, mesosphere–stratosphere–troposphere (MST) radar techniques have been used for studying the structure and dynamics of the lower and middle atmosphere. In particular, these instruments are unique tools for continuously monitoring vertical and horizontal components of the atmospheric wind at high spatial and temporal resolutions. From the very beginning, many studies have been carried out analyzing the reliability of the MST radar wind measurements and their accuracy. However, until now, very few studies have been presented confirming the high performances of the VHF Middle and upper Atmospheric (MU) radar of Japan (35°N, 136°E) for measuring the wind field. The present paper thus gives original comparisons between horizontal velocities measured by MU radar and by instrumented balloons using global positioning system (GPS) radiosondes. Twelve radiosondes were successfully used during the French–Japanese MU Radar Temperature Sheets and Interferometry (MUTSI) campaign (10–26 May 2000, ...

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.

Extended radar observations with the frequency radar domain interferometric imaging (FII) technique

Abstract In this paper, we present high-resolution observations obtained with the Middle and Upper Atmosphere (MU) radar (Shigaraki, Japan, 34.85°N, 136.10°E) using the frequency radar domain interferometric imaging (FII) technique. This technique has recently been introduced for improving the range resolution capabilities of the mesosphere–stratosphere–troposphere (MST) radars which are limited by their minimum pulse length. The Fourier-based imaging, the Capon method have been performed with 5 equally spaced frequencies between 46.25 and 46.75 MHz and with an initial range resolution of 300 m . These results have been compared firstly to results obtained using the frequency domain interferometry (FDI) technique with Δ f=0.5 MHz and, secondly, to results from a classical Doppler beam swinging (DBS) mode applied with a range resolution of 150 m . Thin echoing structures could be tracked owing to the improved radar range resolution and some complex structures possibly related to Kelvin Helmholtz instabilities have been detected. Indeed, these structures appeared within the core of a wind shear and were associated with intense vertical wind fluctuations. Moreover, a well-defined thin echo layer was found in an altitude range located below the height of the wind shear. The radar observations have not been fully interpreted yet because the radar configuration was not adapted for this kind of study and because of the lack of complementary information provided by other techniques when the interesting echoing phenomena occurred. However, the results confirm the high potentialities of the FII technique for the study of atmospheric dynamics at small scales.

Scattering layer thickness and position estimated by radar frequency domain interferometry: 2. Effects of tilts of the scattering layer or radar beam

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.

Scattering layer thickness and position estimated by radar frequency domain interferometry: 1. Effects of the limited horizontal extent and advection of the scattering layers.

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.

Subcomplementary code pairs: new codes for ST/MST radar observations

A new type of codes, named subcomplementary codes, are introduced. These codes are close to, but not strictly, complementary. Each of the two sequences of the pair has an equal number of opposite elements, which enables the codes to have very high interference-suppression-factor (ISF) performances in and around the radar center frequency. The disadvantage of these codes is the presence of sidelobes of amplitude of -N in their autocorrelation functions for lag 1 (N being the code length). Some properties of these codes are presented along with a technique for generating the code pairs. Subcomplementary code pairs have been found for values of N equal to 4, 8, 16, 20, and 32. A simulation study confirms a major improvement in ISF over complementary code pairs around the zero Doppler frequency. Experimental observations were performed with the middle and upper atmosphere radar in Japan using complementary and subcomplementary code pairs of length 16 and an uncoded pulse for range resolution performance comparisons. The results obtained so far indicate that the effects of the sidelobes in the subcomplementary code pair are minimal for wind observations, although significant for shear velocity observations. The degradation in performance in signal-to-noise ratio observations is found to be noticeable but not severe. The subcomplementary code pairs may, therefore, be used in situations where their advantages for interference suppression are exploited and where the effects of their weaknesses are not so important as in the case of observations for applications in meteorology.