Atmospheric influences on infrared-laser signals used for occultation measurements between Low Earth Orbit satellites

Abstract

LEO-LEO infrared-laser occultation (LIO) is a new occultation technique between Low Earth Orbit (LEO) satellites, which applies signals in the short wave infrared spectral range (SWIR) within 2 μm to 2.5 μm. It is part of the LEO-LEO microwave and infrared-laser occultation (LMIO) method, recently introduced by Kirchengast and Schweitzer (2011), that enables to retrieve thermodynamic profiles (pressure, temperature, humidity) and accurate altitude levels from microwave signals and profiles of greenhouse gases and further variables such as line-of-sight wind speed from simultaneously measured LIO signals. For enabling trace species retrieval based on differential transmission, the LIO signals are spectrally located as pairs, one in the centre of a suitable absorption line of a target species (absorption signal) and one close by but outside of any absorption lines (reference signal). Due to the novelty of the LMIO method, detailed knowledge of atmospheric influences on LIO signals and of their suitability for accurate trace species retrieval did not yet exist. Here we discuss the atmospheric influences on the transmission and differential transmission of LIO signals. Refraction effects, trace species absorption (by target species, and cross-sensitivity to foreign species), aerosol extinction and Rayleigh scattering are studied in detail. The influences of clouds, turbulence, wind, scattered solar radiation and terrestrial thermal radiation are discussed as well. We show that the influence of defocusing, foreign species absorption, aerosols and turbulence is observable, but can be rendered small to negligible by use of the differential transmission principle and by a design with close frequency spacing of absorption and reference signals within 0.5 %. The influences of Rayleigh scattering and thermal radiation on the received signal intensities are found negligible. Cloud-scattered solar radiation can be observable under bright-day conditions but this influence can be as well made negligible by a design with a close time spacing (within 5 ms) of interleaved laser-pulse and background signals. Cloud extinction loss generally blocks SWIR signals, except very thin or sub-visible cirrus clouds, which can be addressed by a design allowing retrieval of a cloud layering profile from reference signals and its use in trace species retrieval when scanning through intermittent upper tropospheric cloudiness. Wind can have a small influence via Doppler shift resulting in a slightly modified trace species absorption in comparison to calm air, which can be made negligible by using a simultaneously retrieved wind speed profile or a moderately accurate (to about 10 m s<sup>−1</sup>) background wind profile. Considering all these influences, we conclude that the set of SWIR channels proposed for implementing the LMIO method (Kirchengast et al., 2010; Kirchengast and Schweitzer, 2011) provides adequate sensitivity to accurately retrieve eight greenhouse gas/isotope trace species...

Greenhouse gas profiling by infrared-laser and microwave occultation: retrieval algorithm and demonstration results from end-to-end simulations

“…Besides the absorption of the target greenhouse gas, also other influences due to the atmospheric background such as defocusing, foreign species absorption, Rayleigh scattering, aerosol extinction, cloud extinction, signal scintillations from turbulence, Doppler shift of signal frequencies due to line-of-sight winds, and Rayleigh as well as cloud scattering of solar radiation into the receiver are potentially relevant. The effects from these background influences, except for cloud extinction, are practically either negligibly small under most conditions or can be reduced to very small levels of residual error (typically < 0.1 %) as discussed by other studies (Emde and Proschek, 2010;Schweitzer, 2010;Kirchengast et al, 2010a;Schweitzer et al, 2011a). In the LIO forward simulations of the received intensity signals for this study we account for the main effects of attenuation, namely target and foreign species absorption, and defocusing (plus for the small Rayleigh scattering loss since easily comodeled).…”

“…In the LIO forward simulations of the received intensity signals for this study we account for the main effects of attenuation, namely target and foreign species absorption, and defocusing (plus for the small Rayleigh scattering loss since easily comodeled). Cloudy air and a suitable retrieval will be treated in a separate study; a brief discussion of cloud influences, including limitations to tropospheric penetration of part of the events especially in the tropics, is given by Schweitzer et al (2011a). The other effects can be assumed negligible, or are sufficiently corrected to the level of thermal noise that we include.…”

“…To isolate the absorption due to the target GHG from the absorption of foreign species and broadband atmospheric effect, an adjacent pair of signals, one "absorption signal" (at the center of an absorption line of a target species) and one "reference signal" (off-line of any trace species absorption) is employed using a differential absorption principle (Kursinski et al, 2002;Gorbunov and Kirchengast, 2007;Kirchengast et al, 2010a;Kirchengast and Schweitzer, 2011;Schweitzer et al, 2011a), which will be explained in detail in Sect. 3.…”

“…Figure 1 illustrates the LMO and LIO signal propagation paths, with all signals transmitted from one joint platform, LEO Tx , and received at another joint platform, LEO Rx . Both LMO and LIO signals follow closely similar but not identical paths, i.e., the refraction becomes somewhat different for the microwave (MW) and infrared (IR) signals, proportional to the amount of water vapor in the air (Thayer, 1974;Bönsch and Potulski, 1998;Kirchengast and Schweitzer, 2011;Schweitzer et al, 2011a). The corresponding difference in bending of MW and IR ray paths is practically negligible above about 8 km to 12 km, a highly favorable property, and gradually increases downwards into the troposphere (Schweitzer et al, 2011a), leading to a difference of the tangent altitudes of about 0.5 km near 5 km in moist conditions (Kirchengast et al, 2010a).…”

“…Both LMO and LIO signals follow closely similar but not identical paths, i.e., the refraction becomes somewhat different for the microwave (MW) and infrared (IR) signals, proportional to the amount of water vapor in the air (Thayer, 1974;Bönsch and Potulski, 1998;Kirchengast and Schweitzer, 2011;Schweitzer et al, 2011a). The corresponding difference in bending of MW and IR ray paths is practically negligible above about 8 km to 12 km, a highly favorable property, and gradually increases downwards into the troposphere (Schweitzer et al, 2011a), leading to a difference of the tangent altitudes of about 0.5 km near 5 km in moist conditions (Kirchengast et al, 2010a). Figure 1 highlights that this different bending of MW and IR rays, despite generally being a very small effect, formally leads to different bending angles (α MW , α IR ) at any given time during an occultation event and as well also to different impact parameters (a MW , a IR ) and radial distances from the center of curvature to the tangent points (r MW , r IR ), the latter implying as well different tangent point altitudes.…”