331 only slightly different  from the more common additive formulation  for Nye 2001, but was based on Draper  Lab’s long life heritage with PAO oils .  

Table 1.  MOPITT Optical Channels and Mechanism Configuration/Timings   Optical Channel Number  Channel  1 Channel  2 Channel  3 Channel  4 Channel  5 Channel  6 Channel  7 Channel  8  Chopping & Scanning Mechanism  Numbers  Chopper & Scan  Motor 1  Chopper & Scan  Motor 2  Chopper & Scan  Motor 3  Chopper & Scan  Motor 4   Modulating Mechanism  LMC 1  PMC 1  LMC 2  LMC 3  PMC 2  LMC 4   Gas Species in Modulating  Mechanism  CO CO CH 4 CO CO CH 4  Pressure of Modulating  Mechanism [kPa nominal]  20 7.5 80 80 3.8 80  Wavenumber Range [cm-1] 52 40 52 139 52 40 52 139  Mid-Wavelength [µm]  4.617  2.334  4.617  2.258  4.617  2.334  4.617  2.258   Wavelength Range [µm]  0.111  0.022  0.111  0.071  0.111  0.022  0.111  0.071   Modulator Frequency (LMC or  PMC Rate) [Hz nominal]  11.2 48.6 13.5 11.2 43.3 12  Nominal Chopper Frequency [Hz]  401 486 401 433  LMC Angular Encoder Pulses  [pulses per rotation]  72 N/A 72 72 N/A 72  Pressure Control Device Number  Sieve 1  Sieve 2  N/A Sieve 3  Sieve 4  N/A  Stirling C ryocooler System  Compressor & Displacer 1 (Side B)  Compressor & Displacer 2 (Side A)                = indicates Side -B (partially redundant channels)   The input signals are optically chopped/filtered in various ways using gas samples  via two types of  specialized mechanisms:   the Length Modulated Gas Cells (LMC)  (1 per optical chain), and the Pressure  Modulated Cells (PMC) –  in 2 of 4 chains.   

As described previously  [4], each of the four LMC mechanisms contain a pair of optical rotors spinning  inside separate cells at fixed speeds between 672 and 810 RPM. A brushless DC motor directly drives  the rotor in the evacuated cell (‘compensator cell’), which in turn drives a co- linear rotor located in the  second sealed gas cell (‘modulator cell’) charged with CO or CH 4 via a magnetic coupling. The LMC  motor and bearing configuration (including lubrication)  is identical to th at of the choppers , except for the  housing material and bearing preload. All of the LMCs  have achieved ~ 6 billion revolutions  to date.  

The t wo PMC  mechanisms  provided by Oxford University have continued to operate reliably, representing  the longest running in- orbit heritage for PMCs  to date, exceeding 2·1010 cycles of linear motion. The  MOPITT version was specially characterized to run at higher fill pressures ( ≤10 kPa) . These devices  are  lubricant -free, utilizing diaphragm springs and clearance seals , with direct  heritage from the Pressure  Modulated Radiomet er flown on the ISAMS instrument on UARS [5]. As they are  the predecessors that  enabled the current generation of l ong-life space cryocooler  technology , lifetime result s are not surprising.    To maximize signal -to-noise ratio and achieve optimal performance, indium antimonide infrared detectors  were designed to be cooled to < 90K using a pair of 50– 80K Airbus Stirling Cycle Coolers  (formerly  BAe/ Matra Marconi/Astrium)  [3,9]. The coolers are driven by Cooler Drive Electronics manufactured by   Lockheed Martin. The cryogenic cooling system for the detectors represents a duality of  results  in terms  of life , having demonstrated one of the shortest and longest life- times  in-orbit for Stirling devices to date.  The failure of one displacer drive syst em after just over a year in orbit led to adjustments which allowed  the 2 nd cooler to continue running,  with nominal vibration compensation provided by the opposing   compresso r. Partial redundancy of the optical channels  reduced the impact of the failure on science  return. Alongside the TIR ASTER cooler  also on Terra , the prevailing MOPITT cooler system represents