Instrument Concepts

MOPITT is a nadir sounding instrument, which provides a horizontal resolution of 22 km, but introduces a number of challenges, such as the need for acurately characterizing the surface contribution to the signal.

Fig: Spectral bands

Figure: MOPITT operates by sensing infrared radiation from either the thermal emission/absorption at 4.7 µm for CO profiles, or reflected sunlight at about  2.2-2.4 µm for CO and CH4 column measurements in daylight.    The use of solar channels enhances the instrument sensitivity to the atmospheric boundary layer. 

MOPITT uses correlation radiometry for the detection of CO and CH4.

Fig: correlation radiometry

Figure: Signals pass through a cell containing the target gas, CO or CH4.  The cell pressure or length is varied, which produces a modulation in cell opacity within the lines of the target gas, while the cell opacity at other frequencies remains constant.

MOPITT has both pressure-modulated and length-modulated cells

Pressure Modulated Radiometer (PMR):

Pressure Modulated Radiometer

  • Operates by changing the cell gas pressure using a piston
  • Advantages:
    • Few moving parts
  • Disadvantages:
    • Complex pressure and temperature cycles
    • Upper pressure is limited
  • Used to increase instrument sensitivity to the upper troposphere

Length Modulated Radiometer (LMR):

Length Modulated Radiometer

  • Being used for the first time on MOPITT
  • Operates by changing the gas path thus modulating the gas amount
  • Achieved by rotating a disk of calcium fluoride in the vacuum cell
  • Advantages:
    • Pressure/temperature/time behavior is simple
    • Pressures can be higher than PMRs
  • Disadvantages:
    • Optical components are moved in addition to cell gas leading to potential "imbalances"

MOPITT has 8 channels laid out on 2 optical benches.

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Characteristics of the 8 MOPITT channels
Channel # Primary
Purpose
Modulator 
Type
Cell Pressure
(mb)
Cell Temperature
(K)
Cell Length
(mm)
Spectral Band Center
Wavenumber
(cm-1)
1
CO LMC1
200
300
2-10
CO thermal 2166 (52)
2
CO  LMC1
200
300
2-10
CO solar 4285 (40)
3
CO  PMC1
50-100
300
10
CO thermal 2166 (52)
4
CH4 LMC2
800
300
2-10
CH4 solar 4430 (140)
5 CO LMC3
800
300
2-10
CO thermal 2166 (52)
6 CO LMC3
800
300
2-10
CO solar 4285 (40)
7 CO  PMC2
25-50
300
10
CO thermal 2166 (52)
8 CH LMC4
800
300
2-10
CH4 solar 4430 (140)

 

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The high and low opacity signals from each channel are both averaged and differenced, resulting in 8 D and 8 A signals.

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Average signals are low at the line of interest, providing information about background radiance and interfering signals.

Difference signals have only significant response at the line of interest, providing information about the gas of interest.

 

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The correlation cell pressure determines the portion of the pressure-broadened line wing that is sampled. 

 

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Using PMRs and LMRs with different pressures thus provides information about the vertical profile. 

These are the weighting functions for the different cell pressures of the 6 CO channels.

 

MOPITT diagrams and photos

Schematic
Photo
Photo indicating calibration sources
Terra assembled
Terra being put in rocket
Rocket ready for launch

MOPITT scans across the satellite flight track +/- 26.1 deg in 13 secs.

Figure: scan pattern

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