Abstract
This study explores performance requirements for the retrieval of the atmospheric aerosol optical depth (AOD) by airborne optical remote sensing instruments. Independent of any retrieval techniques, the calculated AOD retrieval requirements are compared with the expected performance parameters of the upcoming hyperspectral sensor APEX at the reference wavelength of 550nm. The AOD accuracy requirements are defined to be capable of resolving transmittance differences of 0.01 to 0.04 according to the demands of atmospheric corrections for remote sensing applications. For the purposes of this analysis, the signal at the sensor level is simulated by radiation transfer equations. The resulting radiances are translated into the AOD retrieval sensitivity (Δτλaer ) and compared to the available measuring sensitivity of the sensor (NE ΔLλsensor). This is done for multiple signal-to-noise ratios (SNR) and surface reflectance values. It is shown that an SNR of 100 is adequate for AOD retrieval at 550nm under typical remote sensing conditions and a surface reflectance of 10% or less. Such dark surfaces require the lowest SNR values and therefore offer the best sensitivity for measuring AOD. Brighter surfaces with up to 30% reflectance require an SNR of around 300. It is shown that AOD retrieval for targets above 50% surface reflectance is more problematic with the current sensor performance as it may require an SNR larger than 1000. In general, feasibility is proven for the analyzed cases under simulated conditions.
Highlights
It is known that atmospheric aerosols influence the Earth climate system
A spectral radiance simulation at the sensor level has been presented. It is capable of reproducing MODTRAN4 results under the single scattering approximation (SSA) and within typical airborne remote sensing conditions
The results reveal the sensor performance requirements for a sufficiently accurate aerosol optical depth (AOD) retrieval along with a feasibility analysis regarding Airborne Prism EXperiment (APEX)
Summary
It is known that atmospheric aerosols influence the Earth climate system. Various efforts are being made to investigate the global distribution and concentration of these aerosols and to quantify their forcing on the radiation budget. Even long-term studies over the ocean reveal differences between well established satellite instruments of up to 0.1 aerosol optical depth (AOD or τλaer) and 0.45 Ångstrøm exponent [4]. The resulting uncertainty of the surface reflectance is an important contribution to the inaccuracy of the retrieved AOD over land. The increased likelihood of observing a uniform surface within one pixel leads to smaller uncertainties in the assumptions about the surface reflectance. This is a major source of error in most satellite-based AOD retrievals. This study analyses the SNR requirements and limitations of aerosol retrieval with a focus on the influence of the surface reflectance. Flights usually will be carried out at visibilities of more than 10km (τ5a5e0rnm ≈ 0.6)
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