Observation type and altitude letters

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 points the boresights to the center of the Sun during this observation. The observation starts 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 points our boresights to the center of the Sun during this observation. The observation starts 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 is 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 is 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 = Day limb observation. One or more NOMAD channel FOVs is 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.

 

  • O = Night limb observation. One or more NOMAD channel FOVs is pointed towards the limb of Mars when the tangent point is centred on the nightside of the planet. 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 points 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 performs a sweep over their complete spectral range, one diffraction order at a time. These measurements can be calibrated spectrally and radiometrically, and 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 points 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 performs a sweep over its complete spectral range, one diffraction order at a time. These measurements can be calibrated spectrally and radiometrically, and will pass through the nadir pipeline when ready. If operating simultaneously, UVIS will observe in normal nadir mode during this time; a fullscan does not apply to UVIS.

 

  • P = Phobos. A nadir boresight is pointed towards the centre of Phobos. The surface is illuminated by the sun for the majority of the observation. These are standard LNO science measurements where a small selection of diffraction orders are cycled through repeatedly; UVIS runs a full spectrum.

 

  • Q = Deimos. A nadir boresight is pointed towards the centre of Deimos. The surface is illuminated by the sun for the majority of the observation. These are standard UVIS science measurements, a full spectrum is recorded.

 

  • Calibration. This type encompasses all calibration measurements, including:
    • CL = 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 (Science/Y will remain in counts).
    • CF = Fullscans, but when the FOV does not pass through the atmosphere. The S/C points 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 are used for spectral calibration, and so will not be converted to transmittance (Science/Y will remain in counts).
    • CM = Miniscans, where the NOMAD SO and/or LNO channels performs 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 so will not be converted to transmittance (Science/Y will remain in counts).
    • C = 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 so will not be converted to transmittance (Science/Y will remain in counts).
    • CS = Solar stare (UVIS only). The S/C points a boresight to the centre of the Sun during this observation. These are used for detector calibration and so will not be converted to transmittance (Science/Y will remain in counts).



 

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.

 

  • G = Grazing. During a grazing occultation, the orbital geometry means that the tangent point does not hit the surface at the lowest altitude. These observations pass through the normal pipeline and are converted to transmittance, however the transmittance error may not be as accurate as normal occultations, as there is no zero-signal to provide a second calibration point.

 

The LNO channel also has the ability to measure two different sets of diffraction orders during a nadir or limb 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.

For calibration measurements, both SO and LNO can measure two different sets of diffraction orders. These are also given numbers 1 or 2 in place of the altitude letter.