BADC Help File: CLAES data

This file contains a summary of the instrument documentation to help you use the CLAES data held at the BADC


  1. The Instrument
  2. The Data

1. The Instrument

This section contains a brief, simplified outline of the CLAES instrument and the measurement techniques which are employed. Users requiring a more detailed description of the design and operation of the instrument should consult the paper by Roche et al. (1993).

1.1 Instrument Description

CLAES is a cryogenically cooled infrared spectrometer which measures thermal emission from the Earth's limb. Characteristic vibration-rotation spectral line radiances are obtained between 3.5 and 13 microns and these are then fed into retrieval algorithms to obtain pressures, temperatures and species mixing ratios.

The instrument observes the Earth's limb in a direction normal to the orbital track on the anti-sun side of the spacecraft.The tangent point is a great circle distance of approximately 23° (2500km) away from the sub-satellite track, and is swept horizontally along the limb by the motion of the spacecraft. The field of view of the instrument is centred on a point at approximately 35km altitude at the limb tangent point. The optical design allows observations to be performed over an altitude range of 10 to 60 km with a spacing of 2.5km.

The instrument observes radiation through a 6-inch aperture Mersene telescope. The altitude range of the observations is adjusted by means of a motorised limb acquisition and adjustment mirror (LAAM). The CLAES spectrometer consists of one of four tilt-scanned Fabry-Perot etalons which are used in conjunction with one or more of nine interference filters which define the spectral channels. Radiation emerging from the spectrometer is focussed onto the focal plane assembly (FPA) consisting of two linear arrays of SiGa detectors. The main array has 20 elements and provides 2.5 km spacing at the limb. A separate three-element array is used with the 3.5 mm HCL channel, which sacrifices vertical resolution to increase signal-to-noise for the weak atmospheric thermal emissions at this short wavelength.

CLAES carries a full aperture black body calibration source on the inside of the telescope aperture door, which is passively heated by radiation from the Earth when the door is open during data acquisition. When the door is closed the black body slowly cools allowing an end-to-end absolute radiometric calibration over a large part of the instrument's dynamic range.

The Cryogens which slowly evaporated as they cooled the instrument, were designed to last about 18 months in orbit, allowing CLAES to make scientific measurements from 1st October 1991 through 5th May 1993, when the cryogens finally evaporated and the instrument warmed up.

1.2 Measurement Techniques

Spectroscopy of the incoming radiation is performed by tilt-scanning one of the four Fabry-Perot etalons between 0 and 23° in conjunction with blocking filters to define the spectral channel. The spectrometer can operate in one of two modes :

1.3 Data Processing Techniques

The UARS data processing is carried out at the Central Data Handling Facility at the Goddard Space Flight Center using software supplied by the instrument's Principal Investigator group. The data processing for UARS instruments consists of a progression through a sequence of `levels' from the raw telemetry at level 0 to geophysical quantities interpolated onto standard grids at level 3. The processing steps for CLAES are outlined below :

2. The Data

The public CLAES data held at the BADC is at level 3A version 8. Some files from earlier processing version (v0007) are also held where there are gaps in the version 8 data. The version number refers to the processing algorithm which is used to create the level 3A files.

The data is stored in UARS binary format. Software is available to convert the files into ASCII data.

2.1 Spatial Coverage

Vertical Coverage:
The table below gives the vertical ranges over which geophysical parameters are retrieved. The retrieved quantities are not necessarily valid over this range, and users should consult the CLAES Data Set Change History Document before using any CLAES data.

      Parameter        |     Retrieved Range (mb) 
      Temperature      |     150-0.1
      O3               |     150-0.1
      CH4              |     100-0.1 
      N2O              |     100-0.15
      NO2              |     100-0.1 
      HNO3             |     100-0.1 
      NO               |     100-0.1 
      H2O              |     100-0.1 
      ClONO2           |     100-1.0  
      CF2Cl2           |     100-2.0
      AEROSOL          |     68-20 
      N2O5             |     46-1.0 
      CFCl3            |     100-10

The vertical coverage of the CLAES instrument is shown schematically in UARS Figure 3 which compares the vertical coverage of the UARS instruments.

Horizontal Coverage:
Views of the limb from the UARS orbit extend to 10° of the pole. To achieve maximum latitude coverage, the instrument views at right angles to the orbit track. The spacecraft performs a 180° yaw approximately every 36 days so that the spacecraft orientation relative to the velocity vector is reversed. (eg. the spacecraft flies "backwards" rather than "forwards").

Each spacecraft yaw manoeuvre changes the coverage from high latitudes in one hemisphere to high latitudes in the other. Thus, when the spacecraft files "forwards", the coverage is 80°S to 34°N, and when it flies "backwards" the coverage is 80°N to 34°S.

The latitudinal coverage of the CLAES instrument is compared with the other UARS instruments in UARS Figure 2.

2.2 Temporal Coverage

CLAES operated from shortly after the UARS launch in October 1991 until the 5th May 1993, when the instrument's cryogen was exhausted. The following list gives days for which data are missing or unavailable at version 7.

  Date                     UARS day         Comments
  13th-16th Jan 1992       124-127          Aperture door closed
  31st Jan 1992            142              Aperture door closed
  13th-17th Feb 1992       155-159          Aperture door closed
  4th Mar 1992             175              Aperture door closed
  8th Mar 1992             179              Processing failed
  22nd-25th Mar 1992       193-196          Aperture door closed
  13th Apr 1992            215              Aperture door closed
  17th May 1992            249              Aperture door closed
  30th May-10th Jun 1992   262-273          Solar array problem
  19th Jun 1992            282              Processing failed
  11th-16th Jul 1992       304-309          Aperture door closed
  18th Jul 1992            311              Processing failed
  29th Jul 1992            322              Aperture door closed
  11th-15th Aug 1992       335-339          Aperture door closed
  18th-23th Sep 1992       373-378          Aperture door closed
  10th Oct 1992            395              Aperture door closed
  27th-31st Oct 1992       412-416          Aperture door closed
  13th Nov 1992            429              Aperture door closed
  27th Nov-1st Dec 1992    443-447          Solar array problem
  8th-11th Jan 1992        485-488          Aperture door closed
  28th Jan 1992            505              Aperture door closed
  7th-11th Feb 1992        515-519          Aperture door closed
  17th-21st Mar 1992       553-557          Aperture door closed
  27th Mar 1992            563              Processing failed
  30th Mar 1992            566              Processing failed
  4th Apr 1992             571              Aperture door closed
  16th-20th Apr 1992       583-587          Aperture door closed
  25th-29th Apr 1992       592-596          Aperture door closed
  6th May 1992             603              Cryogen exhausted.

2.3 Resolution

Vertical Resolution:
The UARS level 3A data are interpolated onto a standard set of logarithmically spaced pressure levels. There are 6 levels per decade in pressure given by the following relation :

         p = 1000 x 10(-i/6),        i=0, 1, 2, ...

Horizontal Resolution:
The level 3AT and level 3AL products differ in their horizontal gridding:

2.4 Data Quality

The quality of the CLAES data is comprehensively reviewed in the CLAES Data Set Change History Document, which is available online in the BADC's CLAES documentation directory. Details of the data quality change from one data version to the next, so the contents of this document are not reproduced here.

2.5 Units

The units for the parameters present in the CLAES data files are tabulated below :

         Parameter               |    Units 
         Temperature             |    K
         Pressure                |    mb 
         Gas Mixing Ratios       |    by volume
         Aerosol Extinction      |    km-1