Turn spectra into profiles

SCISAT-1 carries ACE-FTS, an infrared Fourier transform spectrometer, and MAESTRO, which measures ultraviolet, visible and near-infrared light. Their different spectral regions provide complementary information about gases and particles. Sharing a spacecraft helps coordinate observations, but it does not make the instruments identical.

The ACE-FTS interferometer combines light traveling along different internal optical paths. As the path difference changes, the recorded interference signal forms an interferogram. A Fourier transform converts that pattern into a spectrum showing how the signal varies with wavelength or wavenumber.

Wavelength measures the spacing of a wave; wavenumber measures its reciprocal using a stated length unit. A label in inverse centimetres therefore describes spectral position, not spacecraft altitude. Reading the units is part of understanding what the instrument has measured.

The ACE team distinguishes Level 0 interferograms, Level 1 spectra and Level 2 retrieved profiles of temperature, pressure and gas volume mixing ratios. Moving between these stages involves calibration and physical modeling. A concentration profile is an inference supported by measured spectra, not a number read directly from a photograph of the instrument.

Consider an original two-layer transmission model at one wavelength. Suppose the upper layer transmits 80 percent of the incident light and the lower layer transmits 75 percent of what reaches it. With no added light or scattering in this simplified model, the combined transmission is 0.80 × 0.75 = 0.60. Forty percent has been attenuated overall; adding the two attenuation percentages would incorrectly give 45 percent.

Knowing only the combined value does not uniquely identify both layers. Several combinations can produce 0.60. Observations with different tangent heights, additional spectral features and a model of the atmosphere provide the extra constraints needed for a vertical retrieval.

From a signal to a gas profile

An absorption feature may overlap features from other molecules. Its strength also depends on conditions such as temperature and pressure. A retrieval must account for those effects and assess uncertainty. Otherwise a feature assigned to the target gas could partly belong to an interfering species.

Volume mixing ratio describes a species’ fraction of the gas mixture. In a simple ideal-gas example, five target molecules per million total molecules correspond to five parts per million. This is different from saying five percent of sunlight was absorbed, and it is different from an integrated amount along a path.

The testing photograph documents work on SCISAT before launch, including functional checks described in NASA’s caption. It explains why hardware readiness matters, but it cannot validate a later atmospheric profile. Validation requires comparisons with appropriate independent observations and an understanding of each instrument’s resolution and sampling.