GOSS Simulator

The GOMOS instrument has been designed to measure trace gas concentrations and other atmospheric parameters in the altitude range between 20 and 100 km. In addition to the main goal of monitoring ozone long term trends, the GOMOS near-IR spectrometers will measure O2 and H2O, from which density and temperature profiles can be obtained, which are of interest in atmospheric modelling, but will also improve the quality of the ozone retrieval. At the same time two fast photometer channels will produce information about atmospheric fluctuations, again of interest both for basic atmospheric modelling, but above all useful in the reduction of the ozone-related data.

GOMOS Satellit

Figure 1 : GOMOS

The instrument measurement characteristics are listed in tables I. Two features characterise the GOMOS instrument allowing to reach its expected performance :

*The instrument is self-calibrating by measuring the star spectrum outside and through the atmosphere.
*The stellar and background spectra are simultaneously recorded, allowing to correct for background or straylight as well as dark current contributions.
Channel Spectral Range Spectral Resolution Altitude Range Vertical Resolution Analysed Species
UVIS 250 - 675 nm 0.6 nm 20 to 100 km 1.7km O3, NO2, NO3, aerosols
IR 1 756 - 773 nm 0.12 nm 20 to 100 km 1.7 km O2 (temperature information)
IR 2 926 - 952 nm 0.12 nm 20 km to 100 km 1.7 km H2O
PHOT 1 470 - 520 nm broadband 20 to 100 km 1.7 km scintillation
PHOT 2 650 - 700 nm broadband 20 to 100 km 1.7 km scintillation

Table I : GOMOS measurement characteristics

GOMOS System Simulator

The GOMOS system simulator provides support for end-to-end performance evaluation of the GOMOS instrument and on-ground processing. It does so by :
  1. managing a set of targets ;
  2. simulating the star and background spectrum formation process, as well as the scintillation and depointing of the star image, within the instrument ;
  3. performing the ground segment software functions which contribute to product generation.
Performance evaluation is based on qualitative and quantitative evaluation of the difference between the original target parameters set, and the results of processing by instrument and ground segment sub-models. The modular structure of the simulator allows to perform such comparisons at several different stages of the data processing. This is intended to help evaluate the contribution of each stage : satellite, instrument, on-ground processing, to the overall system performance.

Intermediate results, in particular internal parameters of the instrument sub-model, are available as intermediate results to help trouble-shooting.

The selected targets provide a wide choice of stars and atmosphere configuration which may be observed by GOMOS, which is meaningful

*from the point of view of performance evaluation ;
*from the point of view of instrument calibration and testing on ground and in flight.
The complete imaging process is simulated and includes the apparent movement of the star, the limb conditions where the star is viewed, the pointing perturbations resulting from the GOMOS pointing sub-system interaction with the external world, the collection of light by the sensors and the electrical processing on board, and the digital data processing on board and on ground.

Several modes of simulation are provided in order to support :

  1. realistic simulation of the various GOMOS instrument operations : occultation, calibration
  2. investigation of local as well as global effects
  3. investigation of mission strategies by allowing to build and test observation scenarios
The GOMOS System Simulator is a software application capable of running in stand-alone mode and to manage interfaces to the external world. It runs and manages its data on a single host computer. Its user interface allows to prepare, operate simulations of the GOMOS system in various configurations, and to process and analyse the simulation results. Its data management allows to exchange data and results with other components of the GOMOS system environment (e.g. ground processing).

The GOMOS simulator does not operate in real time. Instead, the continuous processes which it models are broken into discrete, elementary operations. The simulation time is modelled by a set of counters, each linked to a pre-set time scale : spacecraft orbital movement, star image formation, CCD sensor cycle, etc.