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 all 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 for SO/LNO. UVIS runs either a full spectrum or reduced spectrum measurement. 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.

 

  • 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 surface is illuminated by the sun for the majority of the observation, however the start and end may cross the day/night terminator. These are standard science measurements where a small selection of diffraction orders are cycled through repeatedly for SO/LNO. UVIS runs either a full spectrum or reduced spectrum measurement. This is the baseline science observation during a dayside nadir.

 

  • N = Nightside 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 for SO/LNO. 

 

  • 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.

 

  • 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.

 

  • 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.




 

Altitude Letters

The SO channel has the ability to measure a different set of diffraction orders at the top and bottom of the atmosphere during an occultation observation. Therefore an additional letter is required in the filename to signify this as follows:

 

  • A = All. The diffraction order is measured for the whole occultation.

 

  • H = High. The diffraction order is measured just at high altitudes, typically 50km and above.

 

  • L = Low. The diffraction order is measured just at low altitudes, typically below 50km only.

 

The LNO channel also has the ability to measure two different sets of diffraction orders during an observation. These are given numbers 1 or 2 in place of the altitude letter. At present, LNO uses only one set of diffraction orders for a science observation, and so this number will typically be 1.