[1] Multiple echo centers of a mesosphere-summer-echo layer (MSE) observed by the six-receiver OSWIN VHF radar (54.1°N, 11.8°E) were examined with the coherent radar imaging (CRI) technique. The data were collected by different observational modes: vertical and oblique radar beams with the receiving configurations of 3 Â 2, 6 Â 1 (meridional alignment) and 1 Â 6 (zonal alignment) antenna groups. The unique receiving configurations of meridional and zonal aligned antenna groups reveal that the echo centers clustered in three distinct groups above the range height of $86 km. The central group of echo centers was around the direction of radar beam; however, the off-zenith angles of the two side groups, ranging between several and 20 degrees, increased with ascendant range height. Two potential causes of the echoes in the two side groups were examined on the basis of simulation calculation, namely, tilted structures in the layer and additionally, the influence of radar beam pattern. It is indicated that some echoes, originating from the lower part (<$86 km) of the layer, can enter from the first and second sidelobes of the radar beam pattern and then be received at higher range gates (>$86 km) at larger off-zenith angles. The tilted structures, which are considered to be related to wave activities, can also produce the features similar to the observations. This is demonstrated by simulation calculation with wavy reflecting layers, in which the waves are supposed to modulate the multiple reflecting layers, with increasing amplitudes, tilted shapes, asynchronous phases, and horizontal travel.

[1] The frequency domain interferometry (FDI) technique uses two or more frequencies to measure the positions and thicknesses of the atmospheric thin layers embedded in the radar volume, in which the cross-correlation analyses of the radar echoes for the pairs of carrier frequencies are performed and the resultant amplitudes and phases (FDI phase) are both employed. However, in light of the possibility that the characteristics of radar system, mean refractivity gradient, and other factors that would significantly affect the FDI phase, calibration of the FDI phase is required to improve the measurement. In this study we employed three methods in measuring the phase bias in the FDI observation using the Chung-Li VHF radar; namely, (1) histogram of the FDI phases, (2) relationship between echo power and FDI phase, and (3) the FDI phase of aircraft. Both methods 1 and 2 are based on the range weighting effect on the radar echoes returned from the atmospheric scatterers; however, the first produced smaller FDI phase bias than the second. To examine such discrepancy in the results of methods 1 and 2, method 3 was exploited and provided more consistent values of phase biases with those of method 2. Considering that the radar echoes reflected from aircrafts are not related to uncertain conditions of the atmosphere such as mean reflectivity gradients and statistical characteristics, the results of methods 2 and 3 may be more reliable. Besides, the first two methods demonstrated that the FDI phase bias was quasi-linearly dependent on the separation of frequency pair, which not only consolidates the existence of the FDI phase bias but also indicates that a systematic phase compensation for the FDI analysis is possible. For example, considering 0.1-, 0.4-, and 0.8-ms time delays of signals for the returns of 1-, 2-, and 4-ms pulse lengths, respectively, the FDI phase biases can be removed effectively. Same methods and procedures can be applied to other radar systems.

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[1] This paper demonstrates the multiple-frequency range imaging (RIM) which was implemented recently on the OSWIN VHF atmospheric radar (54.1°N, 11.8°E), Germany. A simple but practical phase calibration method is introduced. We validate the RIM technique and the proposed calibration method successfully by examining various radar experiments with different pulse lengths, mono and coded pulses, evenly and unevenly spaced frequencies, and receiver filter bandwidths. The proposed calibration method not only mitigates the phase imbalance between the echoes received at different transmitting frequencies, but also provides a likely value of standard deviation (s z ) of the Gaussian range-weighting function for correcting the range-weighting effect. Moreover, it is found that s z can be adaptive to signal-to-noise ratio when it is employed in practice; this procedure improves the continuity of the imaged powers of RIM around the boundaries of range gates, and an empirical expression has been proposed for this. With the improved power distribution around gate boundaries, we can obtain more available estimates of layer altitudes and exhibit the pass of the layer through gate boundaries clearly. Two observations are shown to demonstrate the maturation of the RIM technique used with the radar: convective cells and double-layer structures. These atmospheric structures cannot be seen clearly in the original presentation of signal-to-noise ratio (or height-time intensity) of the radar echoes having 150-m or 300-m range resolution.Citation: Chen, J.-S., and M. Zecha (2009), Multiple-frequency range imaging using the OSWIN VHF radar: Phase calibration and first results, Radio Sci., 44, RS1010,