Chapter 12. Data Reduction and Distribution

The SOHO operations center will be located at the NASA Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The experiment teams will be located in the Experiment Operations Facility (EOF) and the Extended Analysis Facility (EAF). After the initial instrument checkout period, only the coronal instrument teams will be permanently resident at the EOF/EAF. The coronal instruments are LASCO, EIT, CDS, SUMER, and UVCS. The MDI helioseismology experiment will be producing photospheric magneteograms regularly, which will be available at the EOF, but may not have representatives at the EOF/EAF to support daily operations and collaborative programs.

All of the data from SOHO will be sent to the EOF daily. There will be 4 real-time contacts with the spacecraft each day. One of the contacts will be about 8 hours long, and the other three about 1.5 hours each, for a daily total contact of just over 12 hours. During these contacts, real-time experiment data will flow into the EOF as they are being received at the ground station. LASCO experiment data acqired at other times will be recorded by an on board tape recorder and downlinked during the shorter contact periods (the longer 8 hour period will be used for playback by other SOHO experiments, principally MDI). These playback data will be available at the EOF several hours after being downlinked. These two sets of data, the real time and the playback, are called the Quick Look data.

The final experiment data, called the Level 0 data, will be mailed from the GSFC Data Distribution Facility (DDF) to the four consortium institutions about one month after the data are collected. The Level 0 data will be stored on optical disk CD- ROMS, and will be a duplicate of the Quick Look data, but with transmission errors removed. The CD-ROM will also contain summary data from each of the SOHO instruments, spacecraft ephemeris data, command logs, and other ancillary information. The summary data are representative daily images, and catalogs of available data. These summary data will also be available by computer for access by the general scientific community. They will not be used for scientific analysis, since they are processed from the Quick Look data within one day of being received, and may not be properly calibrated, etc.

LASCO workstations at the EOF will be linked to the nearby EAF, and to NRL computers, by a high speed network. The EOF computers will acquire the Quick Look data as they are made available. This direct telemetry data will be reformated and sent to NRL for further processing, including image decompression, calibration, and cataloging. From NRL the data will be sent back to the EOF for use in planning future observations. Data will be transmitted over a high speed link, at 1.544 Mbps (T1 link). Transfer of a complete image (about 1.7 Mbit compressed) should take approximately 1-2 seconds from the EOF to NRL, and about 10-20 seconds for processing at NRL. Since the equivalent of over 200 images will be collected each day, this transfer cycle between the EOF and NRL will be repeated on average every 7 minutes, and utilize about 6% of the time for the Quick Look data. The remainder of the link capacity will be used to query the data base management system, and to transfer images from the data base archive kept at NRL. The LASCO workstations at the EOF will have the capability to decompress the images themselves in case the network link to NRL is interrupted, but will normally leave the image processing to NRL computers.

The LASCO Level 0 data will be processed at NRL similarly to the Quick Look data. Image data and catalogs will be generally available over the network, and also distributed on CD-ROMS to the three European consortium members weekly. Each of the four consortium institutions will participate in the overall data analysis program. They will each become the central repository for the LASCO data in their own respective countries, and will administer the access of the scientific investigators from their own countries, including those who are not affiliated with the consortium institution, according to the policies described in Chapter 13.

12.1 Naval Research Laboratory (NRL)

NRL will be the primary institution for performing the routine processing tasks, and will be responsible for the Experiment Operations Facility (EOF) at the Goddard Space Flight Center. As discussed, the EOF workstations will be linked to the NRL computers via a high speed network. Real-time and tape recorder playback (Quick Look) data will be acquired by the EOF workstations, reorganized into image and housekeeping files, and transmitted to the NRL computers for decompression, calibration, and cataloging into the data base management system at NRL. Images at various stages of calibration will be transmitted back to the EOF to support the mission operations. The final, Level 0 data will be mailed to NRL from the Data Distribution Facility at GSFC for processing. NRL will then perform the routine processing tasks and disseminate the data to the four consortium institutions. NRL will also be responsible for submitting the final data products to the NSSDC.

NRL will provide the calibration data for the C3 optical system, the CCD camera subsystems, the Fabry-Perot interferometer calibration, and the C1 filter and polarizer calibrations.

12.2 European Consortium Institutions

The co-investigator institutions (MPAe, LAS, and UB) will provide the algorithms for the routine processing of data from their respective optical systems at NRL. They will be linked to the EOF and to NRL by computer, and can query the data base management system and down-load any items in the data base such as instrument status, processing status, LASCO images, etc. They will perform the detailed analyses of the performance of their respective subsystems, and provide updates to the calibration tables as necessary. European consortium team members may be in residence at the EOF during special observing campaigns, and at other times of interest.

12.3 Image Calibration and Evaluation

Following restoration of the compressed data (perhaps with some loss of information) at NRL, the images will be characterized statistically in the following ways:

  • Noise standard deviation
  • Histogram
  • Minimum and maximum intensity
  • Average intensity

The following calibration procedures will be applied to the images:

  • Locate stars, determine roll angle and absolute pointing
  • Remove CCD fixed pattern noise
  • Perform photometric calibration
  • Remove instrumental vignetting, stray light, and polarization
  • Calibrate narrow band color images.

The images will be evaluated to determine the effectiveness of the compression algorithm that was used. Stellar and planetary objects (whose positions are well known) that are imaged in the fields of view of each of the telescopes, will provide roll angle, absolute pointing, and geometric distortion determinations. Similarly, the radiometric variations of stars and planets in transit through the fields of view, will yield vignetting and photometric checks. Individual instrument calibration sources will be used to track transmission of filters and optics, and sensitivity of CCDs.

12.4 Standard Data Products

The data will be searched to locate dynamic events. This search will utilize difference images and an image pyramid scheme to perform an initial screening of the data. If a possible dynamic event is seen, then the images will be searched visually to verify the presence of a real event. As events are found, they will be entered into the event data base. Selected images each day will be used to compute the K-brightness values, polarization, electron density, doppler velocity, green and red line intensity, and green line polarization. These images will be used to produce synoptic maps in each of the quantities. Plots will be generated of the radial brightness variations, line profiles, and daily parameters.

Data will be archived on optical disk CD-ROMs to ensure longevity. Sufficient storage capacity on the NRL computers will be available to enable images collected from the most recent 2 weeks to be available over the network. Storage technologies are evolving very rapidly, so that more capacity may be available for the same cost when SOHO actually flies. Juke box storage of optical disk platters could provide 6-12 months of on-line data. Older data will be available on-line upon request. Catalogs of the entire data base will always be available over the network.

For the white light science program the following procedures will be available on demand:

  • Remove F corona
  • Convert to electron density and mass
  • Polarization analysis
  • Electron density profiles of streamers, holes, etc.
  • Identification of CMEs and/or comets

For the emission line science program, the following procedures will be available on demand:

  • Wavelength calibration
  • Compute first, second, and third moments of line profiles for any location in an image
  • Compute doppler shifts and temperature for any location in an image
  • Plot line profiles for selected locations in an image
  • Construct monthly synoptic maps of any of the above physical parameters

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