Airborne Measurements of Air Mass from O2 A-Band Absorption Spectra

Abstract

Airborne experiments to assess the feasibility of remote sensing surface pressure from a space platform are described. The data are high-resolution spectra in the O2 A band (759?771 nm) of sunlight reflected from the sea surface, measured by a grating spectrograph directed toward sunglint from a research aircraft. It is shown that in the first approximation the reflected radiance is a function of just one variable, the adjusted air mass, defined in terms of geometrical factors, surface pressure, pressure altitude of the aircraft, and an estimate of the mean temperature of the lower atmosphere. This result allows the experiments to be used to determine surface pressure if the pressure altitude is given or vice versa. Since surface pressure varies slowly, whereas the pressure altitude of the aircraft is under experimental control, most of the results apply to retrieval of pressure altitude. The principal difficulty in accurate retrievals of pressure lies in modeling the scattered component of the radiance since this quantity is sensitive to aerosol and thin cloud whose properties often are poorly known. It is shown that low-resolution spectra allow the pressure altitude to be tracked with high precision (0.1%), but the absolute accuracy is low (2%) because from low-resolution spectra it is not possible to determine whether scattered radiance is a significant fraction of the total radiance. However, high-resolution spectra contain additional information that allows the reflected and scattered components of the radiance to be distinguished. Experimental data are presented to demonstrate the sensitivity of high-resolution spectra to scattered radiance. Pressure altitude retrievals based on the use of both low- and high-resolution spectra are shown to achieve an accuracy of 0.1% under a wide range of conditions, including moderate haze below and thin cirrus above the airplane. Finally, data are presented to show that variations in reflectance over the footprint of the spectrograph are unlikely to cause pressure errors exceeding 0.1%.