An extremely sensitive, accurate and simple microwave level meter has been developed. This level meter consists of a continuous wave radar in which carrier is modulated by the M-sequence (Maximal-length-sequence).Employing two M-seq. which are the same in code but generated by the slightly different clock frequencies, the implementation of the crosscorrelation between the received and the reference signals can be simple. This radar successfully gains the sensitivity and suppresses the noise. Moreover, the quantization error of this radar can be designed smaller than the conventional FMCW (Frequency-Modulated-Continuous-Wave) microwave radar. The principle of this microwave level meter is studied and its design criteria are described.This microwave level meter is useful in the case that exclusive performance in terms of sensitivity and accuracy is required for the crucial measurement conditions in the environment such as the iron and steel making process where dust and flare may make it difficult to employ the laser beam or supersonic as sensing waves. The application of the Mseq. modulated microwave level meter is discussed in the experiments measuring the metallurgical slag and the molten metal levels.In the prototype microwave level meter, the carrier frequency is 10GHz, the output power 40mW, the clock frequencies driving the two M-sequence generators about 220MHz, the difference of the two clock frequencies 5.4 kHz or 10kHz, and the lengths of the M-seq. 127bit or 1,023 bit. The water-cooled horn antennas with the gain of 17dB are used.

A method of coded pulse compression employing two maximal length sequences (M-seq.) which are the same in code but generated by slightly different clock frequencies is newly applied to subsurface radars. This method practically gains the sensitivity of the signal without any sacrifice of resolution.The M-seq. pulse-compression underground survey radars are theoretically analyzed and the design criteria are obtained. The waveform made by phase modulation of the M-seq. over the carrier synchronized by the clock frequency generating the M-seq. is found to be more effective than the original M-seq. as transmitting wave.Two prototypes, the surface scanning underground radar and the cross bore-hole radar, are tested and confirmed to be highly sensitive with enough resolution, showing their promising prospect such as inspection of buried pipes in the deeper underground and more precise survey of stratum structure than the conventional subsurface radars with mono-pulse.

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