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:
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 |
