A highly miniaturized satellite payload based on a spatial heterodyne spectrometer for atmospheric temperature measurements in the mesosphere and lower thermosphere

Abstract

Abstract. A highly miniaturized limb sounder for the observation of the O2 A-band to derive temperatures in the mesosphere and lower thermosphere is presented. The instrument consists of a monolithic spatial heterodyne spectrometer (SHS), which is able to resolve the rotational structure of the R-branch of that band. The relative intensities of the emission lines follow a Boltzmann distribution and the ratio of the lines can be used to derive the kinetic temperature. The SHS operates at a Littrow wavelength of 761.8 nm and heterodynes a wavelength regime between 761.9 and 765.3 nm with a resolving power of about 8000 considering apodization effects. The size of the SHS is 38 × 38 × 27 mm3 and its acceptance angle is ±5∘. It has an etendue of 0.01 cm2 sr. Complemented by front optics with an acceptance angle of ±0.65∘ and detector optics, the entire optical system fits into a volume of about 1.5 L. This allows us to fly this instrument on a 3- or 6-unit CubeSat. The vertical field of view of the instrument is about 60 km at the Earth's limb when operated in a typical low Earth orbit. Integration times to obtain an entire altitude profile of nighttime temperatures are on the order of 1 min for a vertical resolution of 1.5 km and a random noise level of about 1.5 K. Daytime integration times are 1 order of magnitude shorter. This work presents the design parameters of the optics and a radiometric assessment of the instrument. Furthermore, it gives an overview of the required characterization and calibration steps. This includes the characterization of image distortions in the different parts of the optics, visibility, and phase determination as well as flat fielding.

Reevaluating the Use of O2 a1Δg Band in Spaceborne Remote Sensing of Greenhouse Gases

“…The volume emission rate (VER, in photons/cm 3 /s) is the integration of r g over the entire band, and the O 2 ( a 1 Δ g ) number density is just the VER divided by the electronic‐vibrational Einstein A coefficient (2.27 × 10 −4 s −1 , calculated from HITRAN2016 individual line intensities using equations 18 and 32 from Gamache & Goldman, ). The O 2 a 1 Δ g state has a long lifetime (73 min), which assures that the molecule is in rotational equilibrium with the ambient atmosphere (Kaufmann et al, ). A simplified assumption is that r g scales with the absorption cross section: where S i j ( T ) is the line intensity for transition i j and f ( ν , ν i j , T , p ) is the normalized line shape function.…”

On the assembly and calibration of a spatial heterodyne interferometer for limb sounding of the middle atmosphere

“…The Fig. 3 Top view of SHI [17]. The blue-shaded components are spacers, which are not in the light path.…”

“…Figure 1 illustrates the broadening of the A-band emission with increasing temperature. As a broad guide, a 1 K temperature change affects spectral intensity ratios by 1% [17]. Typical integration times to obtain an entire altitude profile of temperature are in the order of 1 s for daytime and 20 s for nighttime conditions for an instrument of a volume of 3 l. A concept for such an instrument compatible with a 6-unit CubeSat was recently presented by Kaufmann et al [17], Olschewski et al [21] and references cited therein.…”

AtmoCube A1: airglow measurements in the mesosphere and lower thermosphere by spatial heterodyne interferometry

“…The long radiative lifetime ensures that the molecules are in rotational equilibrium with the ambient atmosphere, such that rotational and ambient temperatures are identical. 15 The vertical distribution of A-band emissions is given by Kaufmann et al 5 Figure 3(a) gives the spectral distribution of the emission lines with selected lines marked in different colors. The line intensities are calculated based on equations given by Song et al 14 Since the rotational structure of the O 2 A-band follows the Boltzmann law, the ratio of different rotational lines allows for temperature retrieval without the need for a precise absolute radiometric calibration of the instrument.…”

“…This reduces power consumption, mass, and costs of such an instrument significantly. 5 For the study of faint radiation from the atmosphere, an interferometer has a significant advantage over conventional grating spectrometers. The throughput is typically more than 2 orders of magnitude larger than for grating spectrometers of the same size.…”