NOMAD Observation Types

Every NOMAD HDF5 file is given a letter to denote the type of observation. The following observation types are defined for NOMAD. These can be split into science observations, where each type is assigned a letter, and calibration observations, which are assigned the letter C:

• I = Ingress (during solar occultation). NOMAD performs an observation during a sunset. TGO will point our boresights to the center of the Sun during this observation. The observation will start some minutes before our line of sight enters the atmosphere (full sun reference spectra) and will continue some minutes after our line of sight has hit the Martian disk (dark spectra). These are standard science measurements where a small selection of diffraction orders are cycled through repeatedly. This is the baseline science observation during an ingress.


• E = Egress (during solar occultation). NOMAD performs an observation during a sunrise. TGO will point our boresights to the center of the Sun during this observation. The observation will start some minutes before our line of sight gets free from the Martian disk (dark spectra) and will continue some minutes after our line of sight has left the atmosphere (full sun reference spectra). These are standard science measurements where a small selection of diffraction orders are cycled through repeatedly. This is the baseline science observation during an egress.


• S = Fullscan (during a solar occultation observation). TGO will point the NOMAD boresights to the centre of the Sun during this observation. This has to be done while the FOV passes through the atmosphere, i.e. a normal Ingress or Egress observation has to be sacrificed, or the fullscan has to be combined with the Ingress or Egress observation (only during a long occultation at high beta angle). The SO or LNO channel will perform a sweep over their complete spectral range, one diffraction order at a time. These measurements can be calibrated spectrally and radiometrically, but will not typically pass through the occultation pipeline. If operating simultaneously, UVIS will observe in normal Ingress or Egress mode during this time: a fullscan does not apply to UVIS.


• F = Fullscan (during a nadir observation). TGO will point the NOMAD nadir boresights to nadir during this observation, i.e. a normal dayside or nightside nadir observation has to be sacrificed. The LNO channel will perform a sweep over its complete spectral range, one diffraction order at a time. These measurements can be calibrated spectrally and radiometrically, but will not typically pass through the nadir pipeline (they are typically used for testing purposes). If operating simultaneously, UVIS will observe in normal nadir mode during this time: a fullscan does not apply to UVIS.
• M = Miniscan (during solar occultation). The NOMAD SO and/or LNO channels will perform a sweep over a fraction of their spectral range whilst pointing towards the sun. It is currently unclear why this type of measurement would be performed; however it is listed here in the event that it is used in the future. A miniscan has an AOTF frequency step whereas a fullscan has a diffraction order step.


• D = Day nadir. TGO will be in its nominal pointing conditions, i.e. the –Y direction is aimed approximately towards the centre of Mars, perpendicular to the surface directly underneath it. The sun is positioned such that the surface is illuminated. In principle NOMAD can measure with LNO and UVIS at any time that the TGO axis is pointing nadir. These are standard science measurements where a small selection of diffraction orders are cycled through repeatedly. This is the baseline science observation during a nadir.


• N = Night nadir. TGO will be in its nominal nadir pointing condition, i.e. –Y direction approximately towards the centre of Mars. The region of the surface in nadir is in darkness. These are standard science measurements where a small selection of diffraction orders are cycled through repeatedly. This is the baseline science observation during a nadir.


• L = Limb observation. One or more NOMAD channel FOVs will be pointed towards the limb of Mars. This can be achieved by rotating the spacecraft, so that the nadir (-Y) face of the spacecraft is pointed to the limb, or by using the occultation channels whilst not pointed to the sun.


• U = UVIS full frame (during a solar occultation). TGO will point the NOMAD boresights to the centre of the Sun during this observation. This has to be done while the FOV passes through the atmosphere, i.e. a normal Ingress or Egress observation has to be sacrificed, or the fullscan has to be combined with the Ingress or Egress observation (only during a long occultation at high beta angle). UVIS will operate in full-frame mode, i.e. each pixel will be read out and transmitted to Earth separately; normally the detector is vertically binned to give a single spectrum per readout. These measurements will be calibrated spectrally and radiometrically (with a dedicated pipeline), passing through the science occultation pipeline. SO or LNO can operate if UVIS runs in full frame mode.


• V = UVIS full frame (during a nadir observation). TGO will point the NOMAD nadir boresights to nadir during this observation, i.e. all or part of a normal dayside or nightside nadir observation has to be sacrificed. UVIS will operate in full-frame mode, i.e. each pixel will be read out and transmitted to Earth separately; normally the detector is vertically binned to give a single spectrum per readout. These measurements will be calibrated spectrally and radiometrically (with a dedicated pipeline), passing through the science nadir pipeline. SO or LNO can operate if UVIS runs in full frame mode.


• C = Calibration. This type encompasses all calibration measurements, including:
  • Pointing calibrations, where TGO performs a line or raster scan around a target (typically the Sun). From these measurements the misalignment can be calculated between the S/C pointing axis and the NOMAD boresights. This misalignment value will be used afterwards to correct the S/C pointing vector.
  
  
  • Fullscans, but when the FOV does not pass through the atmosphere. The S/C will point a boresight to the centre of the Sun during this observation, which can be done at any time when the Sun is visible. The NOMAD SO and/or LNO channels will perform a sweep over their complete spectral range. These measurements can be calibrated spectrally but not radiometrically, and hence will not be run through the occultation pipeline.
  
  
  • Miniscans, where the NOMAD SO and/or LNO channels will perform a sweep over a fraction of their spectral range whilst pointing towards the sun (but when the FOV does not pass through the atmosphere). These are used for spectral calibration and will never be run through the pipeline – a set of coefficients will be generated and used to calibrate all the other observations.
  
  
  • Integration time stepping, where the integration time is gradually increased so that the saturation time can be determined. Channels may be pointed towards any target, e.g. the sun, Mars or dark sky. These are used for detector calibration and will never be run through the occultation or nadir pipelines.