Abstract
Two nearly identical MODIS instruments are currently operated in space, one on Terra and another on Aqua spacecraft that were launched in December 1999 and May 2002, respectively. The first VIIRS instrument has successfully operated for more than 3 years on-board the S-NPP spacecraft since its launch in October 2011. Both MODIS and VIIRS use an on-board solar diffuser (SD) for their reflective solar bands (RSB) radiometric calibration. The SD BRDF (bidirectional reflectance distribution function) was characterized pre-launch and its on-orbit changes are tracked by an on-board solar diffuser stability monitor (SDSM), which functions as an independent ratioing radiometer through its alternate measurements of direct sunlight through an attenuation screen and the sunlight diffusely reflected off the SD. A separate attenuation screen is also part of the SD calibration system. This paper provides an assessment and comparison of MODIS (Terra/Aqua) and VIIRS (S-NPP) SD on-orbit degradation. Due to different mission operation time, calibration configurations and frequencies, the Terra and Aqua MODIS and S-NPP VIIRS SD have experienced quite different amount of SD degradation. In general, larger SD on-orbit degradation is seen at the shorter wavelengths and with longer exposure time. This paper examines the SD degradation characteristics and its dependency on spectral wavelengths and effective solar exposure time. Lessons learned from this assessment will also help other space borne sensors for their SD calibration system design and on-orbit operation.
Assessment and Comparison of MODIS and VIIRS SD On-orbit Degradation
Two nearly identical MODIS instruments are currently operated in space, one on Terra and another on Aqua spacecraft that were launched in December 1999 and May 2002, respectively. The first VIIRS instrument has successfully operated for more than 3 years on-board the S-NPP spacecraft since its launch in October 2011. Both MODIS and VIIRS use an on-board solar diffuser (SD) for their reflective solar bands (RSB) radiometric calibration. The SD BRDF (bidirectional reflectance distribution function) was characterized pre-launch and its on-orbit changes are tracked by an on-board solar diffuser stability monitor (SDSM), which functions as an independent ratioing radiometer through its alternate measurements of direct sunlight through an attenuation screen and the sunlight diffusely reflected off the SD. A separate attenuation screen is also part of the SD calibration system. This paper provides an assessment and comparison of MODIS (Terra/Aqua) and VIIRS (S-NPP) SD on-orbit degradation. Due to different mission operation time, calibration configurations and frequencies, the Terra and Aqua MODIS and S-NPP VIIRS SD have experienced quite different amount of SD degradation. In general, larger SD on-orbit degradation is seen at the shorter wavelengths and with longer exposure time. This paper examines the SD degradation characteristics and its dependency on spectral wavelengths and effective solar exposure time. Lessons learned from this assessment will also help other space borne sensors for their SD calibration system design and on-orbit operation.