Selecting Observations
NOMAD operates in solar occultation, nadir and limb modes, observing in both the infrared (with the SO and LNO channels) and ultraviolet-visible (with the UVIS channel).
SO/LNO: Select a diffraction order to analyse
This depends on the molecule and also the altitude range. Typically, we analyse spectra where the absorption lines are not saturated, and so for solar occultations, stronger lines are analysed at higher altitudes and weaker lines at lower altitudes. The choice also depends on the number of observations of that diffraction order, if sufficient coverage throughout the mission is needed.
Coverage of each diffraction order is given here for SO:
https://nomad.aeronomie.be/index.php?option=com_nomad&view=so_order_statistics_chart
And here for LNO:
https://nomad.aeronomie.be/index.php?option=com_nomad&view=lno_order_statistics_chart
For example, for water vapour, the strongest absorption lines are observed in orders 167-169 and weaker lines are found in orders 134 and 136. As can be seen on the coverage charts, orders 134 and 169 are measured very often and so are good choices for measuring water vapour in occultation mode with SO.
For LNO nadir, the weaker lines of orders 134 and 136 are not detectable, and so orders 167-169 are more useful for water vapour mapping.
UVIS: Select spectral binning type
UVIS typically measures an entire spectrum, from 200-650 nm, in every observation. The main difference is therefore the level of spectral binning, which is often necessary in both solar occultation and nadir observations due to data volume limitations, and readout time in occultation mode. In full resolution mode, 1024 spectral points are measured, but the readout time is longer, reducing the number of spectra acquired; this has a negligible effect in nadir mode, but for occultations it is important to measure at as many tangent altitudes as possible. Therefore a mixture of observations are made, with most occultations run at reduced spectral resolution (typically 1/4 or 1/8 resolution) interspersed with high resolution observations.