OMEGA, the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (Bibring et al., 2004), is a visible and near-infrared imaging spectrometer onboard ESA’s Mars Express (MEx) spacecraft, which has been operating in Mars orbit since 12/2003.
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== Data description ==
OMEGA covers the wavelengths 0.38-5.1 μm with 352 spectral channels, 7 to 20 nm wide. Spatial resolution varies from ~300 m to 5 km per pixel depending on the spacecraft altitude at the time of acquisition. OMEGA’s Signal-to-Noise ratio (or ‘S/N’) is better than 100 over the whole spectral range (and can reach 1000 for some spectels), allowing detection of absorption bands as shallow as ~1%.
The OMEGA instrument actually consists of two co-aligned grating spectrometers, one working in the visible and near infrared (VNIR) in the range 038-1,05 μm, the other in the short wavelength infrared (SWIR) in the range 0.93-5.1 μm. The VNIR spectrometer builds images in a “pushbroom” mode with a two-dimensional detector, with 96 elements in the spectral dimension. The SWIR spectrograph builds images in a “whiskbroom” mode with two 128-element line detectors over the wavelength ranges 0.93-2.73 μm (SWIR-C) and 2.55-5.1 μm (SWIR-L). A moving mirror generates images either 16, 32, 64 or 128 pixels wide depending on the altitude and speed of the spacecraft relative to Mars: the lower the altitude, the faster the spacecraft and the narrower the images in order to yield contiguous pixels with a sufficient integration time on each of them. MEx orbit is indeed highly elliptical, which yields various observation conditions, with wide coverage at coarse spatial resolution from high altitude or finer spatial resolution but narrow coverage from low altitude, closer to periapsis. The secular precession of MEx orbit since mission start has allowed observation of most of the planet from various altitudes and at various local times. As a consequence, OMEGA observations have various spatial resolutions, various widths and have been obtained under various incidence and emission conditions. This particularity, notably with respect to most other datasets from spacecraft in heliosynchronous low mars orbit, has to be taken into account.
The first hierarchical sorting of the OMEGA data archive is the splitting in various ‘missions’ corresponding to the nominal science phase of MEx (Nom) and to 4 successive mission extensions (Ext1 to Ext4). In each ‘mission’, OMEGA data are sorted by MEx orbit number and observation number in each orbit. Each observation is archived as 2 binary files with plain-text headers at file beginning: a ORBNN/ ORBNNYY_X.QUB file with raw science data at the so-called ‘Level-1B’ (uncompressed, uncalibrated raw digital numbers issued from the detectors), and a GEMNN/ORBNNYY_X.NAV file holding geometrical observation parameters such as incidence angle, latitude and longitude, etc. (NNYY is orbit number, in decimal format up to 9999, then NN in hexadecimal format and YY in decimal format, eg. ‘BA32’; and X is observation number of orbit NNYY). A sequence of observations from one orbit typically starts (resp. ends) with a wide image at low spatial resolution taken from high altitude toward (resp. from) narrower images at higher spatial resolution taken closer to periapsis, ie. from low altitude.
Before OMEGA data can be used for science it must be processed to the so-called ‘Level-2’, which is calibrated radiance (in physical units: W.m-2.sr-1.μm-1) or reflectance (‘I/F’), which is radiance divided by the solar irradiance at Mars at the time of observation. Reflectance SWIR-C data to be used for spectral analysis of the surface can further be processed to remove (most of) the atmospheric CO2 absorptions using the so-called ‘volcano-scan’ correction (eg. Bibring et al., 1989; Mustard et al., 2005).
One final caveat to consider in using OMEGA data is the evolution of the instrument during the course of the nominal and successive extended missions. Several issues incrementally arose during the life of the instrument, mostly in the SWIR-C channel: the sensitivity of some spectels increased or decreased (‘hot’ and ‘cold’ spectels), which was mitigated by modifying their transfer function in the calibration, but some of those spectels eventually failed, resulting in an incremental loss of exploitable channels with time, mostly in the SWIR-C channel. Eventually the whole SWIR-C channel failed in ~2010. These artifacts, along with others, which arose in some observation modes, were mitigated by a continuously evolving pipeline of calibration from Level-1B to Level- 2 data. The calibration pipeline from Level-1B data to Level-2 is not yet available in MarsSI but will eventually be implemented when confidence in the product quality is deemed sufficient.
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Important: please be advised that without the proper calibration routine to Level-2 (available from IAS) OMEGA data as available in ESA’s PSA or in MarsSI cannot readily be used.
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For more information on OMEGA instrument and data, check-out the OMEGA Experiment Archive Interface Control Document available from ESA’s Planetary Science Archive (http://www.sciops.esa.int/index.php?project=PSA&page=mex): ftp://psa.esac.esa.int/pub/mirror/MARS-EXPRESS/OMEGA/MEX-M-OMEGA-2-EDR- FLIGHT-V1.0/DOCUMENT/EAICD_OMEGA.PDF
== References ==
* Bibring, J.-P., et al. (2004), OMEGA: Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité, in Mars Express - The Scientific Payload, European Space Agency Special Publication, SP-1240, edited, pp. 37-49, ESA.
* Bibring, J. P., et al. (1989), Results from the Ism Experiment, Nature, 341(6243), 591-593.
* Mustard, J. F., et al. (2005), Olivine and pyroxene, diversity in the crust of Mars, Science, 307(5715), 1594-1597.
