Limb Viewing Hyper Spectral Imager (LiVHySI) for airglow measurements onboard YOUTHSAT-1

The Limb Viewing Hyper Spectral Imager (LiVHySI) is one of the Indian payloads onboard YOUTHSAT (inclination 98.73°, apogee 817 km) launched in April, 2011. The Hyper-spectral imager has been operated in Earth’s limb viewing mode to measure airglow emissions in the spectral range 550–900 nm, from terrestrial upper atmosphere (i.e. 80 km altitude and above) with a line-of-sight range of about 3200 km. The altitude coverage is about 500 km with command selectable lowest altitude. This imaging spectrometer employs a Linearly Variable Filter (LVF) to generate the spectrum and an Active Pixel Sensor (APS) area array of 256 × 512 pixels, placed in close proximity of the LVF as detector. The spectral sampling is done at 1.06 nm interval. The optics used is an eight element f/2 telecentric lens system with 80 mm effective focal length. The detector is aligned with respect to the LVF such that its 512 pixel dimension covers the spectral range. The radiometric sensitivity of the imager is about 20 Rayleigh at noise floor through the signal integration for 10 s at wavelength 630 nm. The imager is being operated during the eclipsed portion of satellite orbits. The integration in the time/spatial domain could be chosen depending upon the season, solar and geomagnetic activity and/or specific target area. This paper primarily aims at describing LiVHySI, its in-orbit operations, quality, potential of the data and its first observations. The images reveal the thermospheric airglow at 630 nm to be the most prominent. These first LiVHySI observations carried out on the night of 21st April, 2011 are presented here, while the variability exhibited by the thermospheric nightglow at O(¹D) 630 nm has been described in detail.