Description

The reflected solar bands show a reduced response over time. Early analyses show the existence of both SD and "scan mirror" degradation.

Impact to L1B data set

The SD degradation is about 2.5% at 410nm (Band 8), 1-1.5% at 466nm (Band 3), and 0.5% at 554nm (Band 4) after the first year of operation. The scan mirror degradation (including aft optics) is about 7-8% for Band 8, 5% for Band 9, 4% for Band 3, and 3% for Band 10. There is small degradation for other RSB bands.

Example of degradation:

Estimated Impact to L2 science products

The SD degradation is about 2.5% at 410nm (Band 8), 1-1.5% at 466nm (Band 3), and 0.5% at 554nm (Band 4) after the first year of operation. The scan mirror degradation (including aft optics) is about 7-8% for Band 8, 5% for Band 9, 4% for Band 3, and 3% for Band 10. There is small degradation for other RSB bands.

Cause of Effect

On orbit degradation has been observed frequently in the past. The "common wisdom" is that this degradation is caused by thin film depositions on optics followed by chemical reaction of that film due to ultraviolet exposure. The spectral degradation pattern for Terra is consistent with this scenario. These thin film effects will depend on the angle of incidence of light onto the optics. Consequently, the presence of scan mirror degradation is expected to have an impact on the RVS behavior for MODIS bands below about 600nm. The films are sufficiently thin that effects on the TEB are likely to be insignificant.

Mitigation Approach/ Added Testing to Characterize Effect

System degradation, including the SD and scan mirror, can be removed using SD-based calibration. Independent observations of the SD reflectance can be used to distinguish between system degradation and SD degradation. SDSM analysis is being used to distinguish between SD and scan mirror degradation. Effects associated with transmission/attenuation screens that are present in the Solar Diffuser Stability Monitor (SDSM), and effects associated with the deployable screen that is sometimes in front of the SD are being quantified, and may have a minor impact on the inference of the amount of SD degradation. Data taken during a series of spacecraft yaw maneuvers are being used to model SDSM screen characteristics.

Estimated Schedule to Complete

The reduced response of the reflected solar bands over time was not accounted for in the algorithm for product versions V2.5.5 or lower. Fixed in version 3.0.0.2 (5/31/01).

History of Characteristic