Entry: HRVIR
URI: https://terra-vocabulary.org/ncl/FAIR-Incubator/earthsciencesensors/c_2d67f68b
The HRVIR imaging instrument is designed to acquire, instantaneously, one complete line of pixels at a time covering the entire field of view. This is achieved using charged-couple device (CCD) linear array. Special optical devices forming a so-called "spectral separator" separate the in-coming light (from target area on the ground) into four spectral channels. The CCD linear arrays operate in the so-called "push-broom" mode. A wide-angle telescope forms an instantaneous image of adjacent "ground patch areas" on a row of detectors in the instrument's focal plane. Column-wise scanning is a direct result of the satellite's motion along its orbit. The signals generated by the detectors (photodiodes) are read out sequentially at a predetermined clock rate. Thus, although the linear arrays do not "scan" in the line-wise direction to gather light, the detectors are scanned electronically to generate the output signal. The telescope has a field of view of 4°, corresponding to 60 km on the ground covered instantaneously by a line of 6000 detectors. Each HRVIR is thus said to offer a "strip width" of 60 km. As just mentioned, each detector generates one pixel at a time, each pixel corresponding to a ground patch area measuring 10 metres square in the high-resolution mode. When adjacent detectors are electronically scanned in pairs, they yield pixels corresponding to a ground patch area measuring 20 metres square resulting in imagery with 20-m resolution. The satellite's motion along its orbit results in successive scanlines, hence complete images. The HRVIR instrument has two operating modes, depending on whether the detectors are read out singly or in pairs. The image quality is very high, partly because no moving parts are involved in image generation. The light entering the HRVIR is sunlight reflected by the Earth's surface then gathered by a telescope with a focal length of 1.08 m and an aperture of ƒ/3.5. The in-coming beam is split into four spectral channels by a beam-splitter consisting of prisms and filters, then focused onto four rows of detectors. As just mentioned, four rows of detectors simultaneously generate four lines of "registered" pixels; in other words a single line of landscape is simultaneously observed in four perfectly registered spectral bands.