FTMW hyperfine spectrum of 3_1, 2 − 3_0, 3 rotational transition for NCCHCS. Each hyperfine component is split into two Doppler components because the direction of the supersonic jet expansion is parallel to the standing wave in the Fabry-Pérot cavity of the spectrometer. The spectra were achieved by 300 shots of accumulation and a step scan of 1.0 MHz with a repetition rate of 10 Hz.