Mission

The Atmospheric Chemistry Experiment (ACE) is a Canadian satellite mission for remote sensing of the Earth's atmosphere that was launched into a high-inclination (74°), circular low-earth (650 km from the surface) orbit on August 13, 2003. This orbit gives SCISAT coverage of tropical, mid-latitude, and polar regions, allowing it to study a range of atmospheric processes. The main goal of ACE is to study the atmospheric chemistry and dynamics that affect stratospheric ozone depletion, but ACE measurements are also being used to study ozone depletion in the Antarctic, the relationship between chemistry and climate change, the atmospheric effects of biomass burning, the effects of aerosols and clouds on the global energy balance, and many other areas of atmospheric science.

The primary instrument on SCISAT is the ACE-FTS, a high spectral resolution infrared Fourier Transform Spectrometer (FTS). SCISAT-1 also features a UV-visible-NIR spectrophotometer known as MAESTRO. Working primarily in solar occultation mode, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios (VMRs) of dozens of molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S.

As SCISAT orbits the Earth, the FTS measures sequences of atmospheric absorption spectra during sunrise and sunset. These spectra, measured in the limb viewing geometry with different slant paths and tangent heights are inverted to obtain vertical profiles of temperature, pressure and many atmospheric trace gases with a vertical resolution of about 4 km from the cloud tops up to about 150 km. In addition, a pair of filtered imagers make high signal-to-noise ratio (SNR) measurements of atmospheric extinction for the monitoring of aerosols and clouds and provide an important diagnostic for the variation of the flux over the distorted solar disk in this region. MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is a dual optical spectrophotometer that covers the 285-1030 nm spectral region. It has a vertical resolution of 1-2 km and measures primarily ozone, nitrogen dioxide, and aerosol/cloud extinction.

The ACE-FTS mission concept is based on the ATMOS (Atmospheric Trace MOlecule Spectroscopy) instrument that NASA flew four times (1985, 1992, 1993, and 1994) on the Space Shuttle but the ACE-FTS has been miniaturized by nearly a factor of 10 in mass, power, and volume as compared to ATMOS. With fewer than 400 occultations, ATMOS has made and continues to make valuable contributions to atmospheric science. The ACE-FTS will measure well over 20,000 occultations over its mission lifetime and is augmented by measurements from the two imagers and MAESTRO. In combination, these instruments are now providing a wealth of scientific data that will be used to help improve our understanding of processes in the Earth's atmosphere for years to come.

The raw SCISAT data is downlinked from the satellite to ground stations around the world (see figure below). The data volume, which at times can exceed 3 GB per day, is transferred to the Mission Operations Centre operated by Canadian Space Agency in St. Hubert (near Montreal) then sent to the Science Operations Centre at the University of Waterloo. At Waterloo the data is archived and processed to obtain data products for distribution to the science team members.

Atmospheric temperature profiles are determined as a function of pressure by starting with a priori data from an upper atmospheric model combined with meteorological data from the Canadian Meteorological Centre for improved accuracy in the troposphere and lower stratosphere. An empirically determined CO₂ volume mixing ratio is used for the ACE-FTS and O₂ A-, B- and Gamma-bands for MAESTRO. Both ACE-FTS and MAESTRO solar occultation measurements are converted to height profiles of atmospheric species. The ACE-FTS algorithm for retrieval of atmospheric profiles has some similarities to the ATMOS algorithms but applies a more sophisticated global fit approach, developed by and described in detail by Boone et al. [2005]. The MAESTRO algorithms draw on previous work by McElroy and co-workers with a variety of UV/visible spectrographs that have been deployed

Satellite stations used to downlink data from SCISAT