Physikalisch-Metorologisches Observatorium Davos -- World Radiation Center pmod-wrc


The SOVIM Instrument - Presentation


SOVIM is part of the SOLAR Instruments grouping. During the early utilisation phase of the ISS, the SOLAR Instruments will be accommodated onto one dedicated CEPA (Columbus External Payload Adapter). This CEPA will be uploaded to the ISS. When on-orbit, this CEPA will be de-installed from the Shuttle Cargo Bay and transferred to its final location on the CEPF (Columbus External Payload Facility); the baseline position is the Zenith side.

sovim instrument
Top view of the SOVIM instrument with sunshield mounted on the CPD. Click for a larger view

The SOLAR Instruments need a pointing capability: the CPD (Coarse Pointing Device) is the support equipment, mounted on the CEPA, which accommodates the Instruments and provides this pointing capability. SOVIM will observe and study the irradiance of the Sun with high precision, stability and accuracy. The observation will be performed during a period of about 15 minutes (TBC) per orbit.

The Total Solar Irradiance (TSI) will be observed with active cavity radiometers (PMO6 and DIARAD) which are traceable to the SI scale of irradiance through direct comparison with cryogenic radiometers on the ground and in vacuum for PMO6, and through characterisation in the laboratory for the DIARAD.

Spectral Solar Irradiance (SSI) measurements will be carried out by sun-photometers. A detector based calibration method, directly traceable to the scale established by cryogenic radiometers, will be used to calibrate the SPM with high accuracy.

Scientific Objectives

SOVIM will use the accurate time series of TSI (Total Solar Irradiance) and SSI (Spectral Solar Irradiance) to:
  1. Obtain quasi-continuous high quality measurements of the solar irradiance variation.
  2. Determine with high accuracy the amount of spectral re-distribution of the solar output.
  3. Search for the long periodicity or quasi-periodicity found in other solar parameters.
  4. Study the influence of active regions and other large scale solar structures on the solar irradiance.
  5. Investigate the energy storage in the convection zone in connection with the energy blocking of active regions.
  6. Investigate the mechanisms of solar radiative forcing of climate change on seasonal to decade time scale.
  7. Continue the historical TSI monitoring record by linking the present and future measurements.
SOVIM instrument
Side view of the SOVIM instrument with sunshield mounted on the CPD. Click for a larger view

TSI measurements can define the Sun's total radiative input to the Earth with at least an order of magnitude less uncertainty than the one achievable for narrow spectral bands. Geophysical investigations require the spectral irradiance itself. In particular, the UV radiation below 300nm varies by an order of magnitude more than the TSI and it contributes more than 10% of the TSI irradiance variations Solar UV radiation, absorbed in the Earth's atmosphere, controls the ozone process. This radiation is unavailable for direct radiative forcing of the climate but may contribute indirectly through its influence on the middle atmosphere. More generally, reliable knowledge of solar spectral irradiance and its variability is ultimately required for understanding the mechanisms of solar variability and the wavelength dependent physical processes through which it influences the terrestrial environment.

The incorporation of SOVIM into a joint package with SOVIM will supplement the highly accurate spectral information from the SPM of SOVIM at a few points by the detailed spectrum extending the range to 200-2500 nm.

Functional Description

SOVIM SPM will be calibrated with reference to the Primary Standard (PS) radiometer. The same calibration is foreseen for SOLSPEC: this will guarantee a common calibration base for the instrumentation with an uncertainty of 0.1- 0.3 % depending of the wavelength. An in-flight comparison of solar spectral irradiance can be performed between two different instruments and without any additional uncertainty associated with non-correlated pre-flight calibrations. The technologies of SOLSPEC and SOVIM are very different so the result of the comparison between the solar spectrals will allow an assessment of instrumental changes that may occur during launch and, in longer term, because of solar contamination.

SOVIM instrument consists of one package containing the following instruments:

  • one absolute radiometer (DIARAD)
  • two PMO6V absolute radiometers
  • one PMO6R absolute radiometers
  • two three-channel sunphotometers (SPM)
  • a pointing sensor (TASS)

and also the following electronics:

  • microcomputer based experiment controller
  • data acquisition system
  • power converter
  • data / command interfaces.

More Information

For more information contact Dany Pfiffner ( and see the NASA Solar Project Homepage (external)

Dorfstrasse 33, CH-7260 Davos Dorf, Phone +41 58 467 51 11, Fax +41 58 467 51 00