Evidence For Off-Axis Leakage Radiance In High-Altitude Ir Rocketborne Measurements

Abstract

Analysis of IR data obtained from several rocketborne sensors in a limb-viewing geometry clearly show off-axis leakage to be the dominant source of radiance at tangent heights above approximately 100 km in all but the strongest of atmospheric emission bands: CO2 v2 (15 gm) and NO (5.3 gm). Telescope leakage model calculations using BRDF values of -2-4 X 10-3 at 10 are in adequate general agreement with much of these flight data and indicate inflight rejection performance 20-40 times worse than model predictions based on pre-flight laboratory measurements. Laboratory measurements on a number of IR telescopes after many months of vacuum storage indicate an average degradation factor of approximately 35 in excellent general agreement with these rocketborne measurements. However, both laboratory and flight measurements indicate degradation factors which are a function of the off-axis angle with a performance at small angles (< 2°) which can be significantly poorer than these average factors, and can result in a major portion of the leakage energy originating from the lower atmosphere. In at least one case, the off-axis rejection performance at small angles was found to be poor enough for non-rejected radiation from the lower atmosphere to become the dominant source of leakage energy in several IR bands. In this example, the clear presence of anomalous atmospheric spectral features in the data for high tangent heights can only be interpreted in this context. Modelling of the spectra from these experiments throughout the IR region requires proper inclusion of the effects of the atmosphere (both in emission and absorption). Results to date indicate that high-resolution line-by-line calculations may be required to fully explain the detailed structure of the spectra from these measurements.