This study is intended to realize an in-situ gas sensor based upon the principle of millimeter/submillimeter wave spectroscopy. In-situ gas sensor will be attractive because of gas selectivity, multiple parametric measurement such as gas temperature, pressure and density, and of the in-situ measurement capability. One of the major technical problem to be solved is to develop an instrument accurate enough to discern the spectrum change due to the variation of parameters such as temperature. In this paper a proposed gas absorption measurement system is investigated, which schematically consists of Fabry-Perot type gas cell for effective long path length, and vector signal processing to reject leak signal coupled between resonator input and output ports so as to achieve precise absorption measurement. Also included is an parametric study of oxygen absorption characteristics, which is served as the predicted value in the evaluation of the instrument. The experiment at 60 GHz and 120 GHz bands using oxygen demonstrates the effectiveness of the current system configuration.

The reflection signal in the inverse synthetic aperture radar is measured in the polar coordinate defined by the object rotation angle and the frequency. The reconstruction of fixed scene images requires the coordinate transformation of the polar format data into the rectangular spatial frequency domain, which is then processed by the inverse Fourier transform. In this paper a fast and flexible method of coordinate transformation based on the nearest neighbor interpolation utilizing the Delauney triangulation is at first presented. Then, the induced errors in the transformed rectangular spatial frequency data and the resultant fixed scene images are investigated by simulation under the uniform plane wave transmit-receive mode over the swept frequency 120-160 GHz, and the results which demonstrate the validity of the current coordinate transformation are presented.