The instrument's line of sight makes a difference to how atmospheric elements are detected. Advantages to the nadir view are the lower probability of cloud interference, and the good horizontal spatial resolution. The main disadvantage to nadir viewing is limited vertical resolution.
TES uses nadir viewing to measure temperature and the primary species (H2O, HDO, O3, CH4, and CO). Advantages to the limb view are better vertical resolution and enhanced sensitivity to trace constituents. Disadvantages to the limb view are the higher probability of cloud interference and the poorer line-of-sight spatial resolution. As of May 25, 2005, TES uses its limb viewing capability only during special observations to detect trace constituents including HNO3 in addition to the primary species.
One of the features of a Fourier Transform Spectrometer (FTS) is that it readily supports an imaging mode, which, in turn, improves collection efficiency compared with those of the more traditional spatially scanning systems. TES employ 1 X 16 linear arrays, each of whose individual field of view is 0.75 X 7.5 mrad. The footprint of each nadir observation is 5 km by 8 km, averaged over detectors. Limb observations (each detector) have a projection around 2.3 km x 23 km (vertical x horizontal). See the plot below.
Back: Air Pollution
TES observes both straight down (nadir view) and at an angle that just skims the surface of the planet (limb view). Each view has its advantages. Limb viewing provides a much longer path through the atmosphere, and looking through a larger mass of air improves the chances of observing sparsely distributed substances that might be missed in the nadir view. Limb viewing's angle also makes it easier to determine the altitudes of the observed chemicals. But nadir viewing is less obscured by clouds, is able to reach the lowest parts of the troposphere, and enables scientists to study changes across distances as short as tens of kilometers.