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Occultation States

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Occultation States:

 

The states observing sun or moon require dynamic control of the scanners. They implement a dedicated sun/moon occultation procedure. At an altitude of about 17 km, refracted and geometrical images overlap significantly and rise with an almost constant rate. The ESM is rotated to this elevation and performs continuous vertical scans of 2 sec each with a vertical range of ±0.33°. The ASM is rotated to an azimuth angle which ensures that the sun or moon is within the field of view of the Sun Follower when their refracted disk appears at the limb. Once the sun/moon has reached an altitude of 17 km above the horizon the ESM tracks the upward motion of the sun/moon in pointing or one of the scanning modes. Nominal scanning moves the ESM in 2 sec ±0.33° around the centre of sun or moon. Because the integration times are shorter than 2 sec, the light can be analysed in horizontal slices of the disk. The fast sweep is a 2.1° wide scan over the solar disk in 0.125 sec. The sweep is centred on the sun. The spectrometer records the integrated intensity at one sweep over the full disk.

 

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Calibration & Monitoring States:

 

Usually, calibration and monitoring states operate either the internal calibration lamps SLS and WLS, measure the dark signal from deep space or observe sun and moon. As long as the line of sight during solar or lunar sunrise traverses the atmosphere, i.e. below an altitude of 100 km, the data serve scientific requirements. Above 100 km, they support calibration & monitoring. Sun measurements above the atmosphere can either observe the solar disk via the scan mirrors or reflect the light via one of the two diffusers. By selecting different light paths – e.g. using the extra mirror – and scanning properties, analysis of solar and lunar states is not only able of providing sun reference spectra for data processing but also information about the status of various optical components.

 

Knowledge of the dark current signal is a prerequisite for successful interpretation of data from all measurement states. Therefore 5 dark current states are specified which cover all relevant integration times. Dark states are executed on the eclipse side during measurement orbits and along the whole orbit during monthly calibration orbits. In a dark current state, the line of sight is directed to and maintained at an altitude of 250 km. It corresponds to pointing into deep space well above the atmosphere. No Earthshine light is expected at this altitude and only the detector dark signal should be recorded.

 

SLS and WLS states are required to derive further pixel-dependent detector properties and to monitor the instrument’s stability. Whenever one of these states is operated, the ESM is rotated to the position where its mirror reflects light from the lamps onto the entrance slit of the spectrometer. Orbital variations may be detected by running SLS or WLS states several times during an orbit. Since each lamp dissipates heat when operated, thermal perturbations have to be kept to a minimum.


2.4.5 Timelines

Concept

SCIAMACHY’s measurement schedule is implemented by executing predefined sequences of measurement states. These sequences are called timelines. A total of 63 timelines can be stored on-board in the TIMELINE table. Each timeline is characterized by the chronological sequence of states and its total duration. Once the timelines are stored in the TIMELINE table, they can be started via a single Macrocommand (MCMD). This MCMD provides the scheduled timeline start time. If the timeline includes a state executing a sun or moon measurement, e.g. sunrise at a given altitude, sub-solar event or moonrise at a given altitude, additional position parameters specifying the solar or lunar celestial positions, are uploaded with the MCMD. They are used by the instrument to correctly position the scan mirrors at the beginning of the particular state to initially acquire the target. Execution of the timeline is a complex interaction between various parameter tables. From triggering the first timeline related instrument internal commands until the sequence of states has finally run to completion, information is extracted from tables and used to control instrument measurement activities. The timeline definition ensures that sun or moon states in the timeline observe their target at the right time and location to meet the scientific requirements. Since the start timeline MCMD can provide only one set of position parameters for one solar or lunar event, there can only exist one sun or moon related state in a timeline. Thus timelines including a sun or moon state are fixed in time and are called sun fixed or moon fixed. All other timelines without a sun or moon state are scheduled relative to sun or moon fixed timelines.

 

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Posted on 4/5/13 2:59 PM.