The MLS instrument and data products

Information compiled by the BADC team

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  1. The instrument
  2. MLS data
  3. Data quality overview
  4. Transferring MLS data from the BADC

Go to: [ Contents | Instrument | Data | Quality | Transfer | End ]

1. The instrument

This section contains a brief, simplified outline of the MLS 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 Barath et al. (1993).

1.1 Instrument Description

MLS is a microwave radiometer which measures thermal emission from the Earth's limb. Measurements of the emission spectra are made in three channels at frequencies of 63 GHz, 183 GHz and 205 GHz, from which vertical profiles of mixing ratios, temperature and pressure are derived.

The instrument observes the Earth's limb in a direction normal to the orbital track. The tangent point (at the limb) is a great circle distance of 23 deg. (approx. 2500km) away from the sub-satellite track.

A 1.6m antenna receives radiation from the limb and is mechanically scanned in the vertical every 65 seconds. The optics are diffraction limited, giving a field of view of 3.5km (vertically) at the limb in the 205 GHz channel. A switching mirror in the optical path allows the instrument to accept radiation from the antenna, from an internal calibration source or from cold space. This combination allows an optimal calibration of the detectors.

A dichroic plate separates the signal to the 63 GHz channel and this is followed by a polarization grid which isolates the signals to the 183 GHz radiometer and to the 205 GHz radiometer.

In each channel, the incoming radiation is converted to intermediate frequency bands in the 0-3 GHz range using local oscillators. After amplification the intermediate-frequency signals are divided into six spectral bands, each of which is further divided into 15 contiguous spectral channels by a bank of filters. The spectral resolution of these channels is high enough to resolve emission lines throughout the stratosphere.

1.2 Measurement Techniques

The six intermediate frequency bands are used to derive atmospheric parameters from the atmospheric emission lines detailed below:


The 63 GHz channel
62.998 GHz and 63.569 GHz lines of O2. These lines are relatively temperature-insensitive, and by measuring the intensity and the line-width, the atmospheric pressure is derived. Differentiation with respect to the measured height differential also gives an indication of the temperature via the hydrostatic equation.
The 183 GHz channel
183.310 GHz H2O and 184.378 GHz O3 lines. The intensity of these lines gives a measure of the abundance of the emitting gas
The 205 GHz channel
204.352 GHz ClO, 206.132 GHz O3 and 204.575 GHz H3O2 lines in addition to a number of weak HNO3 lines. The intensity of these lines gives a measure of the abundance of the emitting gas.

1.3 Data Processing

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 MLS are outlined below:


Level 0-1
At the level 0-1 processing step, instrument-specific effects are removed and a set of calibrated data are derived in physical units (eg. voltages and radiances) tagged with their locations.
Level 1-2
The level 1 data are then processed further to produce the level 2 product which contains vertical profiles of temperature, pressure and mixing ratios of chemical constituents at the measurement positions. The retrieval is based on the sequential estimation algorithm with an a priori estimate containing both the NMC daily analysis (when available) and a month-dependent, latitude-dependent climatology developed by the UARS science team.
Level 2-3A
The level 2 data are profiles located at the measurement positions which are determined by the scan pattern and by the track of the tangent point. The level 2-3A processing step takes these data and interpolates them onto a standard set of vertical levels - evenly spaced in log pressure, and onto standard times (level 3AT) and standard latitudes (level 3AL). In the case of MLS, daily parameter files are also produced, containing data on the operation of the instrument which is not allowed by the standard level 3A file definition.

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2. MLS data

2.1 What is available from the BADC?

The public MLS data held at the BADC is at level 3A version 4. The version refers to the processing algorithm used to create the level 3 files. The species, with the equivalent names used in the UARS filenames and headers, are tabulated below:

Measured species                   UARS species

Chlorine monoxide                  CLO
Nitric acid                        HNO3
Ozone (183GHz channel)             O3_183
Ozone (205GHz channel)             O3_205
Temperature                        TEMP
Water vapour                       H3O
Sulphur dioxide                    SO2
Parameter file for 3AL data        PARAM_L3LP
Parameter file for 3AT data        PARAM_L3TP

2.2 Units

The units for the parameters present in the MLS data files are as follows:


Parameter               Units   

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

2.3 Vertical Coverage

The vertical ranges for the parameters contained in the MLS L3A files are tabulated below:

Parameter               Useful pressure range (hPa)
Chlorine Monoxide (ClO)           46 - 1.0                 
Nitric acid (HNO3)               100 - 22
Ozone (O3)                        46 - 0.46                 
Sulphur dioxide (SO2)             46 - 0.46
Water Vapour (H2O)                46 - 0.2                 
Temperature                       46 - 0.46                                                            

2.4 Horizontal Coverage

The latitude coverage of the instrument alternates with the yaw cycle of the satellite, which lasts approximately 36 days. The coverage switches between 80 deg. S to 34 deg. N and 80 deg. N to 34 deg. S at each yaw. The instrument observes the full range of longitudes.

2.5 Temporal Coverage

MLS began operations on the 11th October 1991 (UARS day 30) and, at the time of writing, is still producing data. Since launch there have been a number of periods during which data collection has ceased, as tabulated below:

Dates                   UARS days     

14 Jan 1992             125
14 Feb 1992             156
 9 - 14 Jun 1992        272 - 277
16 - 20 Apr 1993        583 - 587
March-May 1994   (sparse data)       
July 1994 (sparse data)
Oct 1994 - Jan 1995
Feb - July 1995 (sparse data)

Note that the 183 GHz radiometer stopped operating on the 20th of April 1993 (UARS day 587), and consequently there are no water vapour (H3O) or Ozone (O3) data after this date.

After 2.3 years in orbit (in late December 1993), the antenna-scanning mechanism began to exhibit signs of wear. March 1994 through May 1994, and July 1994 were periods of testing and significantly reduced data gathering; these months have from one third of the days with bad data to almost all bad days (days with no profiles retrieved) October 1994 through January 1995 again have very few days of useful atmospheric profile data. Very little limb data gathering (typically only a few days per month) occurred from February through July 1995, which was a period during which instrument power sharing began for UARS (in May 1995). Since June 1995, MLS has been in a mode of operation characterized by off periods for power savings and on periods during which typically 2 days of full (reverse) scans are obtained followed by one day of limb tracking at altitudes near 18 km. The August 1995 through September 1996 time period generally contains about one half to one third catalogued good days for atmospheric profiles, and this mode of operation is expected to continue.

JPL's MLS instrument web pages include a complete set of MLS calendars showing the instrument's operational status on daily basis for the entire mission.

Documents detailing the UARS satellite roll, yaw and orbit adjustment manouevres are held in the BADC documentation directory.

2.6 Vertical Resolution

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

    p = 1000x10(-i/6)   [i=0, 1, 2, ...]

The retrieved vertical resolution is two UARS pressure surfaces (delta log10(P) = 0.33, or about 6 km). The retrieved profiles are represented as a piecewise-linear function with breakpoints at alternate (even-numbered) UARS pressure surfaces (e.g. 10, 4.6, 2.2, 1 hPa). The data points on the even-numbered surfaces (level 3AT files only) are the retrieved breakpoint values, while those on the odd-numbered surfaces (e.g 6.8, 3.2, 1.5 hPa) are averages of the temperatures on adjacent even-numbered surfaces. The level 3AL profiles have an additional linear interpolation with respect to latitude to generate an evenly spaced latitude grid.

2.7 Horizontal Resolution

The level 3AT and level 3AL products differ in their horizontal gridding. The 3AT products are interpolated along the tangent-point track at standard output times corresponding to intervals of one UARS Engineering MAjor Frame (1 EMAF = 65.536 s). This period represents an along- track displacement of approximately 400 km between measurements. The profiles are constructed by linear interpolation in observation time from the level 2 profiles. The 3AL products are interpolated onto positions defined by the intersection of the tangent track with a latitude circle at standard 4 deg. latitude intervals.

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3. Data Quality - General Considerations

The information in this section was taken from the product quality summary produced by the MLS team at JPL. The summary document is available online from the MLS documentation directory at the BADC. The `quality' field in the level 3A files is the retrieval's estimated uncertainty, includes random and systematic components, and is obtained by propagating precisions of the radiance measurements, estimates of constrained parameter uncertainties, forward model inaccuracies, and some calibration uncertainties through the retrieval software. The quality should be interpreted as a lower bound on the accuracy. At the conclusion of the retrieval, the estimated uncertainty is compared with the a priori uncertainty. When the ratio is greater than 0.5, or the temperature is more than 25% climatology, the quality is set negative to flag the dependence of the retrieved temperature on the a priori.

Values will appear in the profile vector in the data file whether or not data were received from the instrument. These values are a combination of a priori values (obtained from climatology and contemporary NMC data) and instrument measurements. If no instrument data are received, the retrieval relaxes to the a priori and the estimated uncertainty values will all be negative.

In general, only data having positive quality values should be used for scientific purposes. The level 3 parameter files contain a diagnostic flag ` MMAF_STAT' which gives information on whether each major frame has good data, no data, or bad data. For optimal use of the data, the parameter files should be examined and profiles with `MMAF_STAT' not set to `G', `T', or `t' should be disregarded.

For a full description of data quality for individual species, see the MLS Version 4 data quality document.

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4. Transferring MLS data from the BADC

MLS data is held online in IEEE binary files, exactly as supplied by the Goddard DAAC. Software from Goddard to read UARS binary data is also supplied, as is 3rd party software written by other users of the data.

4.1 Before you start

You will need gzip installed on your local system to uncompress the software packages - the source code is available from software archives. You will also need to know how to use gzip and tar once you have the files on your system and how to transfer files via FTP (File Transfer Protocol) if you don't use the WWW interface for transfers.

4.2 Transferring the files

MLS data can be transferred via FTP (File Transfer Protocol) or directly from these WWW pages. To use anonymous FTP, connect to After logging in, cd to badc to find the dataset directories, or cd /badc/mlsl3 to go directly to the MLS level 3 directory.

4.2.1 Finding data and software files

All MLS data and software is located beneath the directory mlsl3 Here you will find a README file and the following subdirectories

The UARS file name convention will help you to work out which data files you need.

There are 2 level 3A data products for each MLS species per day - one level 3AL and one level 3AT. Each version 4 product file is accompanied by a "meta" file containing information on the record length, which is required by the read software. Make sure you pick up both the PROD and META files when you transfer your data.

For each day there are two parameter files containing data on the operation conditions of MLS which are not allowed by the level 3AT/3AL file definition. There is a one-to-one correspondence between records in the 3AT/3TP and 3AL/3LP files. For more detailed information refer to the level 3A file format documents.

4.3 Using the read software on your local machine

The Goddard read software allows you to read the binary files on a variety of UNIX systems using either C or FORTRAN (depending on the patform - on SGI, HP and Sun machines both languages are supported, on DEC only FORTRAN is supported).

Goddard supply the software in a tar archive compressed with the UNIX compress facility. At the BADC we also hold a gzipped version and the individual source files online. Unzip the software files using the gzip -d command. Un-tar in a suitable directory using the tar -xvf command. A makefile and README file are supplied to help you build the code. The README file explains how to build the code to suit your local operating system.