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Local Weather
Oberpfaffenhofen

Global Ozone Monitoring by Occultation of Stars

The main scientific objective of GOMOS is to monitor ozone and ozone trends in the stratosphere and mesosphere. This task is achieved by measuring stellar spectra in the UV to near infrared range during limb occultation observations.

The occultation technique has the advantages of being largely self-calibrating, minimizing effects due to long-term drifts in instrument performance and providing good vertical resolution. About 25 stars (magnitudes between -1 and 3) will be monitored each orbit. Contrary to sun occultations this results in a much better global coverage. Depending on the season 150 to 250 stars will be bright enough to be tracked by GOMOS.

The observing strategy is limb viewing of altitudes from 15 to 120 km. The instruments line-of-sight can be oriented towards stars and maintained by tracking the star while the star sets behind the Earths limb as shown in the following figure:

image

During the setting of the star the line-of-sight penetrates deeper and deeper into the atmsophere. The spectra are continuously observed and successively modified and attenuated according to the concentrations of atmospheric constuents. Each trace gas is characterised by a typical, well-defined spectral signature. By deviding these attenuated spectra by the unattenuated stellar spectrum (measured outside the atmosphere) high accuracy spectra are derived. The vertical resolution will be as low as 1.7 km.

Beside two spectrometers (UV-visible, infrared), GOMOS consists of two fast photometers (visible) which have the purpose to monitor input signal scintillation, which enables to derive information on turbulence.

Table: GOMOS spectral properties

Band/device Spectral range Spectral resolution Species, usage
UV-VIS spectrometer 250 - 675 nm 1.2 nm O3, NO2, NO3, Aerosol
Near IR (1) spectrometer 756 - 773 nm 0.2 nm O2 (temperature, air density)
Near IR (2) spectrometer 936 - 952 nm 0.2 nm H2O
´Blue´ photometer 470 - 520 nm 50 nm scintillations, turbulence
´Red´phometer 620 - 700 nm 50 nm scintillations, turbulence

Table: Parameters and performance of GOMOS

Parameter  
Altitude range 15 - 120 km
Vertical resolution 1.7 km
Occultations per orbit ~ 45
Data rate 0.23 Mbps
Data volume ~ 50 Mb/orbit

Table: GOMOS scientific objectives

Long-term objectives Stratospheric and mesospheric ozone monitoring
  Chemistry of the lower and middle stratosphere
  Chemistry of the upper stratosphere
  Large scale dynamics at mid and polar latitudes
  Polar vortex break-up and planetary wave studies
  Dehydration of the equatorial lower stratosphere
  Smaller-scale dynamics: turbulence and gravity waves
  Noctilucent clouds
Campaign objectives Validation of GOMOS data
  Cross validation with MIPAS and SCIAMACHY data
  Special events like solar proton events or vulcanic eruption
  Arctic and Antarctic campaigns
  Stallar spectra
Methodic objectives Synergistic 4D-Var data assimilation with SCIAMACHY and MIPAS