The MOPITT instrument was launched on the Terra platform of NASA's Earth Observing System (EOS) on December 18, 1999. The Terra satellite is in a 705km, sun-synchronous orbit with a 10:30am equator crossing time. MOPITT has been measuring carbon monoxide over the globe since that time.
Despite the fact that we all live in the troposphere, monitoring of the tropospheric composition from space has lagged considerably behind our monitoring of the upper regions of the earth's atmosphere mainly because of the technical difficulty of such measurements. The presence of the earth's surface provides considerable interference to most measurement methodologies and the presence of clouds further impedes the mission. Overcoming these problems requires a very precise instrument with a very high performance.
We monitor carbon monoxide because it helps us understand how the troposphere reacts to various stimuli. These can range from natural phenomena such as the growth of forests, through agricultural sources such as rice paddies, to catastrophic events such as biomass burning. Most of these sources can, and indeed are, being modified by human activity on the planet.
Carbon monoxide is particularly interesting because it shows us how chemicals are transported in the troposphere as well as giving us information about chemical reactions in the troposphere.
MOPITT measurements have already shown us the production of carbon monoxide in biomass burning and its transport by atmospheric circulation systems. This needs to be understood on a global scale and incorporated into models of tropospheric transport.
Measurements are made by intercepting the infra-red radiation coming from the planet and then isolating the required signals. MOPITT is a nadir sounding instrument since this gives the maximal chance of avoiding cloud features, but this implies that it can "see" the surface of the planet and the desired signals must be seen against the background of the surface radiation. The field-of-view of MOPITT is 22 x 22km and it views four fields simultaneously by the use of a 4 x 1 array of detector elements. The field of view is also continuously scanned through a swath about 600km wide as the instrument moves along the orbit increasing both the spatial coverage of the instrument and the chance of finding gaps in the cloud coverage.
The MOPITT instrument makes use of the principle of correlation spectroscopy whereby a cell of the gas to be measured is used as an optical filter in the infra-red to measure the signal from the same gas in the atmosphere. The amount of gas in the instrument cell is modulated by varying either the pressure or the length. In addition to the correlation technique MOPITT makes use of mechanically cooled detectors and filters (at 100K) to enhance the overall performance. The use of this cooling technique, which relies on Stirling Cycle coolers supplied by British Aerospace, is relatively new in satellite instrumentation having been used on only two civilian satellite instruments before. The use of mechanical cooling rather than stored cryogen or radiative cooling permits a relatively large amount of cooling - sufficient for both the detectors and the filter systems - whilst still permitting a five year or greater instrument life.
The MOPITT science team is international, having members from
MOPITT science team
MOPITT is funded by the
Canadian Space Agency
MOPITT Prime Contractor
For further information about MOPITT, contact:
Prof. James R. Drummond
Department of Physics
Last updated May 13, 2007