An Acousto-Optical Spectrometer (AOS) features the submillimeter-wave limb-emission sounder (SMILES) to be aboard the Japanese Experiment Module (JEM) of International space station (155). The Japanese space agency (NASDA) has contracted ASTRIUM for the development of the AOS. Acousto-Optical Spectrometers are well adapted for analyzing in real time with high resolution a wide band and faint signal embedded in radiometric noise. Their usefulness for instantaneous detection and mapping of multi-species emission was first demonstrated in the field of radio astronomy in 1970s and thanks to their compactness and low power consumption, they are highly well adapted to space application. Several technical concerns related to important instrumental characteristics of AOS are discussed and performances are overviewed.