For MERIS, the Instrument Modes are divided into two categories: Support modes and Operation modes. Support modes are used in order to maintain the instrument in full operational condition. Operation modes are those used by the instrument during nominal operations.
Support Modes are: Launch, Off, Reset/Wait, Standby, Standby/Refuse, Heater and Heater/Refuse.
Operation Modes are: Auxiliary, Calibration and Measurement
Of the Operation Modes, the two Measurement Modes (Averaging or Direct & Averaging) outlined above are used for routine acquisition:
AVERAGING (A) mode, intended to generate RR data only, consequently FR data was lost, neither transmitted directly nor recorded on-board on the instrument. Therefore A can be considered as a degraded data and should be used ONLY for the convergence of the OCL. Likewise, such data should not be processed.
DIRECT & AVERAGING (D and A) Mode, was regarded as the nominal mode for MERIS, whereby data was acquired at 300M (FR) and created 1200 m resolution data (RR) by internal averaging, although not all the resulting FR data could be transmitted to ground due to lack of bandpass.
During operations, MERIS underwent Radiometric (RAC) and Wavelength (WV) Calibrations. The radiometric calibration of MERIS was performed at the orbital South Pole, and used one or both Spectralon Diffuser plates that were mounted on a selection disk:
Radiometric Calibration (RAC) measurements occurred every 2 weeks (using Diffuser 1)
Diffuser Ageing Calibration (DAC) monitoring occurred every 3 months (using Diffuser 1 and Diffuser 2 on successive orbits)
For the majority of its operational lifetime time MERIS remained in operational mode. However, there were occasions were MERIS was required to be in the other support roles;
Orbit Control Manoeuvers (OCM) where MERIS should‘ve been commanded to HEATER/OFF mode, however MERIS was left acquiring during most OCMs. As a result such data was then discarded from the catalogue because of non-nominal pointing indicated the data to be of bad quality and should be made unavailable for users.
Wavelength calibrations (WV1, WV2) where MERIS was commanded to STABILISATION mode. Note: Due to the nature of the calibration, products acquired from these orbits were not made available to the User community.
Spectral Campaigns (requiring non-nominal programming of MERIS)