1. Nominal operations scenario
1.1 Measurement types
During its nominal operations phase OMIS will continuously perform various types of science and calibration measurements. One can distinguish between the following measurement types:
- Earth measurements
- Tropical measurements
- Mid latitude measurements
- Arctic measurements
- Ozone hole measurements (only during September - November period, replacing the arctic earth measurements in the southern hemisphere)
- Calibration measurements
- Solar calibration/science measurements
- Dark calibration measurements
- White Light Source (WLS) calibration measurements
- Light Emitting Diode (LED) calibration measurements
For all earth measurements types the master clock period is set to 2 seconds in order to meet the requirement on the ground-pixel size. Solar measurements serve to generate both science and calibration data. Measurements using the volume diffuser with BF4 and BF8 are used to generate the Level 1B Irradiance and Level 1B Calibration products. Measurements using the volume diffuser with BF1, the regular diffuser and the backup diffuser are used to generate the Level 1B calibration product. The Global and spatial Zoom-in regular darks always include tropical, midlatitude, arctic and ozone hole settings. The Spectral Zoom-in regular dark always includes tropical, midlatitude and arctic settings but no ozone hole setting.
1.2 Activity definitions
Two options exist fot defining the activities:
- Measurement types, as defined in previous section, can be grouped into so-called measruement-type activities. A measurement-type activity may consist of either just one measurement type or a serie of measurement types in case this series is executed via a SIS. Each measurement-type activity can be scheduled separately on the mission timeline.
- Measurement types can also be grouped into orbital scenarios. These orbital scenarios can be scheduled on the mission timeline. A later section provides more detailed information about the orbital scenarios.
In RD-2 a list of activity definitions is provided, which included both type of activities. In case of the measurement-type activities, related to the individual measurement type, we distinguish between:
- Science (data collection) activities (earth and sun)
- Sun calibration activities
- WLS calibration activities
- LED calibration activities
- Dark calibration activities
In case of orbit-type activities, related to the orbital scenarios, we distinghuish, based on the swath size, ground pixel size and spectral range, between:
- Global orbit-type activities
- Spatial Zoom-in orbit-type activities
- Spectral Zoom-in orbit-type activities
1.3 Nominal orbital scenarios
The measurement types as defined in the previous section can be grouped into eleven orbital scenarios. For the nominal operations baseline nine orbital scenarios are defined:
- Earth Global measurements
- Dark Global measurements with identical settings as earth Global measurements
- Earth Spatial Zoom-in measurements
- Dark Spatial Zoom-in measurements with identical settings as earth measurments
- Earth Global measurements
- Solar Calibration with volume diffuser
- Long Exposure darks
- LED source calibration
- Earth Global measurments
- Solar Calibration with unbinned image, using volume diffuser
- Long exposure darks using different gains
- Earth Global measurments
- Solar Cakibration with aluminium diffuser
- WLS pixel response non-uniformity (PRNU) calibration
- Dark Global measurments with identical settings as earth global measurements
- Earth Global measurments
- LED stability measurements
- WLS radiometric stability measurements
- Dark Global measurments with identical settings as earth global measurements
- Earth spatial zoom-in measurments
- Spatial zoom-in solar calibration with volume diffuser
- Dark spatial zoom-in measurments with identical settings as earth measurments
- LED linearity calibration
- Earth Global measurments
- Solar Calibration using backup diffuser
- LED calibration using different gains
- WLS calibration using different gains
- Long exposure darks with unbinned image evenly distributed over the orbit
1.4 Scheduling of nominal orbit scenarios
The scheduling of the nominal orbit scenarios is based on the OMI orbital repeat cycle
of 466 orbits. After 466 orbits a new cycle starts in which the orbits have an identical
geo-location as the accompanying orbits in the previous orbital repeat cycle. Figure 10
shows the schedule of the nominal orbital scenarios.
Click on the figure for a larger view.
 "OMI baseline operations schedulele")
In this figure the following definitions are used:
- one OMI day consists of 15 orbits
- one OMI week consists of 7 OMI days plus three additional orbits
- one OMI month consists of 4 OMI weeks plus a number of leap orbits to fill up the Aura repeat cycle
- the leap orbits consist of two OMI days plus four additional orbits
- one OMI day consists of 15 orbits
The great advantage of the repeatability of this operations scenario is that it provides a safe way of operating the OMIS system. Once any eventual errors have been removed during testing or during in-flight operations, the next cycle of 466 orbits should run without any operational errors.
Due to the use of the orbital repeat cycle, also the orbits using the spatial zoom-in
scenarios N2 and M1 have a fixed geo-location. As a consequence, always the same parts
of the earth surface will be observed in spatial zoom-in mode once a month. Other parts
of the earth surface will not be observed in spatial zoom-in mode.
As an example of using spatial zoom-in mode, see figure 11. In this example, Washington D.C.
will be observed in spatial zoom-in mode once a month, but e.g Paris will not be observed.
This situation remains the same during the whole nominal operations phase.
Click on the figure for a larger view.
 "spatial zoom-in example of OMI")