The MIGHTI instrument measures the horizontal wind speed and direction (the wind vector). Each MIGHTI unit measures the wind along its line of sight. MIGHTI uses two perpendicular fields of view nominally pointing 45 degrees and 135 degrees in azimuth from the spacecraft velocity (MIGHTI A and MIGHTI B). Combining data from both units, this 90 degrees separation between their views allows the wind vector to be determined. For the wind measurements MIGHTI observes the Doppler shift of the atomic oxygen red and green lines at 630.0 nm and 557.7 nm wavelength. The wavelength shift is measured using field-widened, temperature compensated Doppler Asymmetric Spatial Heterodyne (DASH) spectrometers, employing low order échelle gratings operating at two different orders for the different atmospheric lines. The temperature measurement is accomplished by a multichannel photometric measurement of the spectral shape of the molecular oxygen A-band around 762 nm wavelength. For each field of view, the signals of the two oxygen lines and the A-band are detected on different regions of a single, cooled, frame transfer charge coupled device (CCD) detector. On-board calibration sources are used to periodically quantify thermal drifts, simultaneously with observing the atmosphere.