The instrument is required to meet the science requirements of the MOPITT Science Team, the performance assurance requirements of the Canadian Space Agency, and the instrument interface and system safety requirements of the NASA/EOS program.
The instrument will be launched in a sun-synchronous orbit at an altitude of 704km, with an inclination of 98.4°. The platform and instrument design lifetime is five years.
A block diagram of the instrument is shown in Figure 7.1.
The instrument operates by sensing terrestrial thermal radiation at about 4.7µm and reflected solar radiation at about 2.4µm. Sensitivity to particular gases is provided by the use of "correlation spectroscopy". The technique uses Length-Modulated Cells or Pressure-Modulated Cells, depending upon the region of operation. The 4.7µm channels give information about the carbon monoxide profile in the range 0-15km above the earth's surface using thermal radiation from the earth and atmosphere. The 2.4µm channels give information on the carbon monoxide and methane column amounts using reflected sunlight. The different radiation sources imply that the 4.7µm profile channels are operational during the day and the night, whereas the 2.4µm channels are only operational during the day.
The target accuracy of the measurement is 10% for CO and 1% for CH4 under the signal conditions discussed above with a one pixel integration time. If these performance goals prove impractical then multi-pixel averaging can be invoked to meet the accuracy requirements.
Spatial resolution on the surface is required to be about 22 x 22km. The temporal and spatial coverage are boosted by the use of array detectors and cross-track scanning. A further increase in the received power is obtained by separation of the optics into several parallel channels. Internal calibration sources are used to maintain the instrument calibration whilst it is in flight.
The instrument performance is increased by the use of active cooling systems which provide for a detector temperature of approximately 100K.
The instrument optics utilise viewing ports in the nadir surface to view the earth (earth view) and on the anti-sun surface to provide a radiation zero for the instrument (space view). Doors are used to cover the viewing ports during launch and once opened on-station would only be reclosed in a contingency situation (TBC).
7.1.2 Spacecraft Resource Allocations
The allocations for spacecraft resources given in the MOPITT UIID. For
reference purposes in this document ony, they are currently (UIID July
1993):
Power 206W + contingency from a 120V DC supply
Mass 164kg + contingency
Volume 1102 x 895 x 430mm (LxWxH), with an
extension of 140mm below the baseplate
Data rate 0.025Mbps
Mounting 3-point kinematic mount
7.1.3 Lifetime Requirement
The instrument is required to operate on-orbit for a period of five years,
and is required to meet the probability of success criterion given in
Appendix H, at end of life. Launch and flight environment constraints are as
defined by the GIIS.
7.1.4 Channel Configuration
The discussion in section 3 outlined a set of channel requirements for the
instrument. The minimal required set is given in Table 7.1.
It should be noted that the Level 4 CO thermal channel pressure is above the optimal pressure for the lowest level (see 3). However the choice of a common pressure for the lowest level (highest pressure) CO channel and the CO solar channel permits the combining of the two channels using dichroics. The performance margin on the CO solar channel is much less than that of the CO thermal channel and therefore the optimal pressure is that which gives the solar channel the best performance margin.