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GES DISC DAAC Data Guide:UARS ISAMS Level 3A Data Sets Document

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The Improved Stratospheric and Mesospheric Sounder (ISAMS) is one of 10 instruments aboard the Upper Atmosphere Research Satellite (UARS). The ISAMS instrument measured the altitude profiles of temperature, oxides of nitrogen, ozone, water vapor, methane, and carbon monoxide from the tropopause to the mesopause. ISAMS also measured aerosol extinction coefficients. Currently, ISAMS version 10 level 3AT and 3AL data products from 26 September 1991 to 29 July 1992 are available from the Goddard Earth Sciences (GES) Data and Information Services Center (DISC).

Table of Contents:

1. Data Set Overview

Data Set Identification:


Data Set Introduction:

There are two ISAMS level 3A data products archived at the GES DISC:

Level 3AT

ISAMS level 3AT data are daily time-ordered data, arranged at time intervals of 65.536 seconds, or about 495 km intervals along the LOS tangent track. The reference time at which level 3AT data are arranged is common across all UARS level 3AT files. Each data record contains time, latitude, longitude, solar zenith angle, local time, and an array of data, as well as an array of quality (standard deviation) values. Data file structures for these file types are found in the Standard Formatted Data Units (SFDU) documents listed in the References section below.

Level 3AL

ISAMS level 3AL data are daily latitude- and time-ordered data interpolated from the level 3AT data to intervals of 4 degrees latitude at the intersection of the tangent track of the instruments line of sight (LOS). Each record consists of a single array of data of one parameter for a specific time. Level 3AL data records are written to UARS defined standard latitudes, which range from -88 to +88 degrees in 4 degree intervals.


To determine the thermal structure of the middle atmosphere and its fluctuations in space and time (e.g. with season), investigate the photochemistry of nitrogen-containing species, study the water vapor budget, and investigate the role of volcanic and polar stratospheric aerosol in stratospheric chemistry.

Summary of Parameters:

The ISAMS objectives are addressed by measurements of carbon dioxide (CO2) (in four bands, for temperature determination), nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen pentoxide (N2O5, nitric acid (HNO3), ozone (O3), water vapor (H2O), methane (CH4), and carbon monoxide (CO).


The ISAMS Level 3AT and 3AL data files are written in the Standard Data Format Units (SFDU) format. Each file consists of three records called SFDU, LABEL, and DATA. SFDU and LABEL records contain descriptive information about the instrument and the data, such as start/stop time of the data, number of records in the file, etc. The DATA record contains the profile data and their standard deviations. Time, latitude longitude, local solar time, and solar zenith angles are provided with each DATA record. Each data file is accompanied by a short ASCII metadata file, which provides descriptive information such as the start and stop time of the data, file record lengths, and the UARS quality flag.

Related Data Sets:

All UARS level 3AT and 3AL files use the same formats to allow for intercomparisons of atmospheric profiles between the different instruments. Other UARS instruments which measure chemical species include the Cryogenic Limb Array Etalon Spectrometer (CLAES), the Halogen Occultation Experiment (HALOE) and the Microwave Limb Sounder (MLS).

ISAMS is an improved version of the SAMS instrument flown on the Nimbus-7 satellite from 1978 to 1993.

2. Investigators:

Principal Investigator:

Frederic W. Taylor
Oxford University
Department of Atmospheric, Oceanic & Planetary Physics
University of Oxford, Clarendon Laboratory
Parks Rd, Oxford, OX1 3PU, England
Telephone Numbers:
Voice: +44-1865-272902
FAX: +44-1865-272924
Electronic Mail Address:

Title of Investigation:

Improved Stratospheric and Mesospheric Sounder

3. Theory of Measurements:

The ISAMS instrument is an infrared pressure-modulator radiometer that observes thermal emission from the Earth's limb. Measurements are in the 4 to 17 micron range using gas correlation spectroscopy and solid-state detectors cooled to 80 degrees Kelvin by closed cycle refrigerators. ISAMS infers the amounts of gases in the stratosphere from the measurement of the unique infrared emission features associated with each gas.

4. Equipment:

Instrument Description:

ISAMS is a limb sounder, which, at the satellite altitude of 585km, places the tangent point at 23 degrees from the orbital track. When the satellite flies in the +X direction (defined as forwards), viewing to the (usual) anti-sun side (+Y) gives coverage from 80N to 34S. Conversely, when the satellite yaws to fly in the -X direction (backwards), the coverage viewing to +Y is from 34N to 80S. ISAMS also has the ability, when the sun-satellite geometry is favorable, to view to the -Y side for parts of orbits, increasing the potential coverage to 80N-80S for either satellite flight direction.


The ISAMS instrument is an infrared spectroradiometer which collects infrared radiation at wavelengths from 4.6 to 16.3 microns. ISAMS is able to scan on either side of the UARS satellite, though the instrument views primarily to the cold (anti-sun) side of the craft. Pressure modulators allow ISAMS to measure selected atmospheric gases.

The primary optics configuration consists of an off-axis (non-obscuring) reflecting telescope which scans the atmosphere vertically at a preprogrammed rate under microprocessor control. An internal calibration target is provided in the primary optics. This, together with views of cold space, provides the radiometric offset and gain.

A rotating, reflecting chopper disc is located at an intermediate focal point in the optical chain and serves both to modulate the beam at several hundred hertz and chop it against the cold space reference. The modulated beam is then modulated again by passage through the pressure modulator cells, one in each of eight channels. The pressure modulators use coupled resonant pistons operating in antiphase; pressure amplitudes of 50 percent of the mean are achieved.

The detector in each channel is a four-element array in a square-shaped configuration. Each of the elements measures 2.6 km high by 18 km wide when projected on to the limb by the instrument optics. The instrument dwells between steps for typically 2 seconds to obtain signal-to-noise ratios of approximately 100:1.

ISAMS collected scientific data from 26 September 1991 to 29 July 1992.

Collection Environment:

Satellite data are collected from a near-circular Earth orbit of about 585 km altitude and 57 degree inclination.


Upper Atmosphere Research Satellite (UARS).

Platform Mission Objectives:

UARS was launched September 12, 1991 with the mission of investigating the chemical and dynamical processes of the Earth's upper atmosphere. See the UARS Project document for more information.

Principles of Operation:

ISAMS has a total of 32 detector elements. The signal processing electronics extracts 2 radiance measurements (Wide-Band and Pressure Modulated) from each detector every Instrument Measurement Period (IMP=2.048 secs), giving a total of 64 measurements per IMP. The detectors are grouped into 4-element arrays at the end of each of 8 optical paths. Each path passes through a different pressure modulator cell. 2 of these paths (#3 and #7), associated with the two CO2 modulators, also contain a filter wheel, enabling selection from one of 4 different spectral regions during any IMP. Thus there are a total of 14 Spectral channels, which, multiplied by the 2 Signal channels (WB and PM), gives a total of 28 radiance channels of which 16 may be sampled each IMP.

Spectral selection is achieved by pressure-modulator cells (PMCs) (Taylor, 1983) whose pressures, therefore transmission characteristics, may be altered in flight. The various observing geometries, plus the ability to program the scan pattern and PMC pressures, lead to a large number of operating modes for ISAMS, which may be characterized by some combination of:

  1. PMC Cell Pressure in each of the 8 Cells
  2. Anti-Sun/Sun side viewing (+/- Y side of spacecraft)
  3. Satellite Direction (+/- X direction of motion)
  4. Day/Night at tangent point
  5. Ascending/Descending node
  6. Limb Scan program

Whether the tangent point is illuminated or not does not affect the operation of the ISAMS instrument, but it is convenient to add it to this list.

5. Data Acquisition Methods:

Data are telemetered from UARS through the Tracking and Data Relay Satellite System (TDRSS) to the Data Capture Facility (DCF) at NASA GSFC. From there the data are given an initial quality check, and are then forwarded to the

UARS Central Data Handling Facility (CDHF)

. The instrument PI teams are connected to the CDHF through remote analysis computers (RACs), where they have developed software to convert the raw data to higher level processed data. The CDHF uses the production software to convert the level 0 (raw) data to level 1, 2, 3A and 3B data. The GES DISC acquired the UARS data from the CDHF.

6. Observations:

Data Notes:

None at this time.

7. Data Description:

Spatial Characteristics:

Spatial Coverage:

ISAMS can view to either side of the UARS spacecraft, allowing for latitudinal spatial coverage from 80N to 80S. Vertical coverage is from approximately 15 to 80 km.

Spatial Coverage Map:

Typical d ata coverage for ISAMS (looking northward on 9/28/91).

Map showing UARS orbital tracks

Spatial Resolution:

Horizontal resolution is about 495 km along the orbital track for level 3AT files, and 4 degrees for level 3AL files.

Vertical resolution for level 3A files is about 2.5 km between pressure surfaces.


Not Applicable.

Grid Description:

The CLAES, HALOE, HRDI, ISAMS, and MLS instrument data use pressure referenced data arrays. It should be noted that not all species within a data product will contain all 45 pressure levels. The index of the data array defines the pressure level (in millibars) given by:

        P(i) = 1000 x 10**(-i/6) mb, where i=0,1,2,...

Temporal Characteristics:

Temporal Coverage:

Temporal coverage is from 26 September 1991 to 29 July 1992. Listed below are the dates within the above time period for which ISAMS data are missing or unavailable:

Missing Days                  Parameter Subtype
--------------------------    ---------------------------
15-OCT-1991 to 27-OCT-1991    All
04-DEC-1991                   AERO6P23, CH4, CO, N2O, NO2
19-JAN-1992 to 26-MAR-1992    All
05-APR-1992                   O3
03-JUN-1992 to 18-JUL-1992    All

NOTE: Due to a problem with the ISAMS instrument, there are no data after 29 July 1992 (UARS day 322).

Temporal Resolution:

The temporal resolution of ISAMS level 3A data granules is daily.

Data Characteristics:


There are 10 parameters for ISAMS level 3AT/3TP and 3AL/3LP data products. The parameters are classified according to ISAMS subtypes. The measured parameters are listed below with the original ISAMS subtype name, description, and units:

Subtype Description Units
AERO12P1 aerosol extinction profiles at 12.1 µm 1/km
AERO6P21 aerosol extinction profiles at 6.21 µm 1/km
CH4 methane (CH4) volume mixing ratio profiles vmr
CO carbon monoxide (CO) volume mixing ratio profiles vmr
H2O water vapor (H2O) volume mixing ratio profiles vmr
NO2 nitrogen dioxide(NO2) volume mixing ratio profiles vmr
N2O nitrous oxide (N2O) volume mixing ratio profiles vmr
N2O5 dinitrogen pentoxide (N2O5) mixing ratio profiles vmr
O3 ozone (O3) volume mixing ratio profiles vmr
TEMP atmospheric temperature profiles K

NOTE: HNO3 and NO data were not produced in the current version of ISAMS Level 3A data. Also, the H2O subtype is version 9 data.

NOTE: volume mixing ratio (vmr) = 10e-6 parts per million by volume.

8. Data Organization:

Data Granularity:

The granularity of ISAMS level 3A data are defined such that there is one granule for each level and parameter subtype (see the Data Characteristics section above) per day, for a total of 10 granules per day. Each ISAMS level 3A granule is a multi-file granule consisting of four files:

  1. The binary data file (files ending with PROD, or *PROD extension) which contains the data profiles, temperature, pressure, spacecraft and instrument status, number of profile levels, PMC pressure codes, scan program id, view direction and satellite direction, line of sight, latitude and longitude, etc.
  2. An ASCII metadata file (files ending with META, or *META extension) associated with the data file containing items such as the begin date, end date, PI assigned quality flag and record length size of the data file.
  3. A binary parameter file (also *PROD extension) designated 3TP for 3AT granules and 3LP for 2AL granules. ISAMS has a separate 3TP/3LP parameter file for each species in the 3AT/3AL data file. The subtype for these files is PARAM. The ISAMS parameter files contain information about the instrument operating mode line of sight direction and solar angles. This file was added to accommodate the additional ISAMS values after the 3AT/3AL file formats had already been finalized.
  4. An ASCII metadata file associated with the parameter file (also *META extension). The information is identical to the metadata file associated with the data file, except that the record length applies to the parameter file.

The naming convention for UARS granule file names distributed by the GES DISC is as follows:

ISAMS_Llll_Sssss_Ddddd .Vvvvv_Ccc_xxxx,


is the UARS processing level (3AT, 3TP, 3AL, 3LP),
is the subtype or parameter,
is the UARS acquisition day (0001 = 12 September 1991),
is the data version number,
is the data version cycle number, and
is the file extension (PROD for the binary files, or META for the ASCII metadata files)

For a full description of the naming convention see the "meta_desc.doc" file.

Average granule sizes are about 400 kB for ISAMS_L3AT_DAILY granules (includes 170 kB 3TP parameter file), and 330 kB for ISAMS_L3AL_DAILY granules (includes 140 kB 3LP parameter file). The *META files are small, only about 700 bytes each.

Data Format:

The data are in a native UARS format (SFDU). The files were originally created on a VAX/VMS system at the UARS CDHF, and now exist as UNIX stream files at the GES DISC. ISAMS data file structures are presented in the Standard Formatted Data Units (SFDU) documents listed in the References section.

9. Data Manipulations:


Derivation Techniques and Algorithms:

Data Processing Sequence:

Processing Steps:

ISAMS Level 3A data are generated by interpolating the ISAMS Level 2 profiles to standard UARS surfaces (using the Level 2 pressure files) then interpolating along the tangent track to standard output times (3AT files) or latitude crossings (3AL files). Associated with each 3A data file is a parameter file (3TP with 3AT, 3LP with 3AL), containing additional information on ISAMS operation conditions for each data profile, parameters which are not accommodated in the standard UARS 3A data format.

Level 3ALP data are generated from ISAMS Level 2 Data files at the same time as Level 3AL data files. Where appropriate, the values contained in Level 3ALP (and also 3AL) are derived from the level 2 profiles by linear interpolation in observation time between adjacent profiles. This is necessary because the Level 2 profiles are provided at fixed times where the Level 3AL products are required at fixed latitudes. No attempt is made to interpolate between successive profiles that are in different 'modes'.

Processing Changes:

Reprocessing of the data occur about once a year.


Special Corrections/Adjustments:


Calculated Variables:


10. Errors:

Sources of Error:

Systematic errors in the measurements and the retrievals are expected to change with the operating mode of the instrument.

Quality Assessment:

Data Validation by Source:

All data are checked by the ISAMS science team and assigned quality values. These values appear as the DATA_QUALITY_UARS fields in the ASCII metadata files. The format for DATA_QUALITY_UARS is a 3 character field of the form "p.q" where:

                VALUE       MEANING
         for p      0       Machine inspected
                    1       Qualitative evaluation
                    2       Intensive analysis
         for q      1       less than 50% good data
                    2       50% - 75% good data
                    3       76% - 98% good data
                    4       better than 98% good data

ISAMS uses the DATA_QUALITY_PI field to indicate data coverage on an orbital basis. All parameter/subtypes will have the same value most days. The format for DATA_QUALITY_PI is a 3 character field of the form "abc" where:

a = N     +Y side of UARS pointing north all day
a = S     +Y side of UARS pointing south all day
(the +Y, or cold side of UARS, is the dominant view for ISAMS)
b = 0,1,...F (hex)    Number of +Y tangent tracks [0-15]

c = 0,1,...F (hex)    Number of -Y tangent tracks [0-15]
c = X                 No -Y views attempted

"NAX" Cold side (+Y) pointing north, 10 +Y orbit tracks,
no -Y views attempted

Measurement Error for Parameters:

Systematic errors in the measurements and the retrievals are expected to change with the operating mode of the instrument. While profiles may safely be combined (e.g. gridded) from within any single data mode, or from data modes representing the same instrument operating mode, systematic differences may occur between profiles from data modes representing different instrument modes.

In some cases these differences may be instrumental. One obvious example is if the pressure of one of the PMC cells is changed: this changes the retrieval weighting functions, and therefore the relative influences of the a priori (climatology) and the measurements in the retrieved product. A PMC for one channel may have a knock-on effect on other species; e.g. the CO2 PMCs are used for temperature and pressure determination, so affect all retrieved products. Hence a change of pressure in the CH4 PMC could affect N2O retrievals and vice-versa since the spectra of the two gases overlap and the retrievals are interlinked.

Additional Quality Assessments:


Data Verification by Data Center:

Data files are checked to ensure that they are properly transferred and translated from their original VAX/VMS format at the UARS CDHF to the GES DISC's UNIX format. No additional data checks are performed by the GES DISC.

11. Notes:

Limitations of the Data:

The data files exist as UNIX stream files at the GES DISC. Binary data are IEEE formatted. The binary data files should be read on 32 bit machines running UNIX operating systems. This is especially important for fields which are IEEE floating point values, such as the profile data and quality values. If you are going to use a non 32-bit and/or non-UNIX machine, then you will need to write your own conversion routines to read the data files.

File record length information is only listed in the ASCII metadata files (*META extension) which accompany the data and parameter files.

Known Problems with the Data:

None at this time.

Usage Guidance:

Suggested uses of the data include the following:

  1. To determine the thermal structure of the middle atmosphere and its fluctuations in space and time,
  2. To investigate the photochemistry of nitrogen-containing species,
  3. To study the water vapor budget,
  4. To investigate the role of volcanic and polar stratospheric aerosol in stratospheric chemistry.

Any other Relevant Information about the Study:


12. Application of the Data Set:

Anticipated uses of these data are in the fields of understanding unperturbed atmospheric chemistry and dynamics, tracking global change and long term atmospheric trends, predicting atmospheric response to chemical or energetic perturbations, environmental and agricultural planning, weather forecasting, atmospheric energy input and loss studies, and radiation budgets.

13. Future Modifications and Plans:

No future reprocessing of the data are expected.

14. Read Software:

Software Description:

Simple read/dump programs are available for reading the ISAMS level 3A data files. The read programs are available in C and IDL languages.

Software Access:

To get the software use the links below:

15. Data Access:

Contact Information:

Help Desk
NASA Goddard Space Flight Center
Code 610.2
Greenbelt, MD 20771
Telephone Numbers:
Phone: 1-301-614-5224
FAX: 1-301-614-5268
Electronic Mail Address:

Archive Identification:

The UARS ISAMS data are archived at the GES DISC under the UARS Project.

Procedures for Obtaining Data:

The ISAMS level 3A data files can be obtained from the GES DISC by several mechanisms. These include the following:

Data Archive Status/Plans:

The GES DISC currently supports ISAMS level 3AT and 3AL data products.

16. Output Products and Availability:

The ISAMS level 3A data are available. See the section above on Procedures for Obtaining Data for specific information. For more information on ISAMS, please refer to the ISAMS Home Page.

17. References:

Satellite/Instrument/Data Processing Documentation:

Reber, C. A., C. E. Trevathan, R. J. McNeal, and M. R. Luther, The Upper Atmosphere Research Satellite (UARS) Mission, J. Geophys. Res. 98, D6, 10643-10647, 1993.

Taylor, F. W., et al, Remote Sensing of Atmospheric Structure and Composition by Pressure Modulator Radiometry From Space: The ISAMS Experiment on UARS, J. Geophys. Res. 98, D6, 10799-10814, 1993.

Journal Articles and Study Reports:

An extensive list of journal articles are available on the ISAMS Documentation Page.

GES DISC online documentation:



ISAMS Product Quality Summary

Horne, C., UARS Granule Level File (*META) Description, July 1994.

18. Glossary of Terms:

A collection of parameters packaged with associated ancillary and labeling data. Uniformly processed and formatted. Typically uniform temporal and spatial resolution. ISAMS level 3A data products include ISAMS L3AT and ISAMS L3AL. The ISAMS data product class is divided into data product subclasses according to measured parameters.
A logically meaningful grouping or collection of similar or related data. Data having mostly similar characteristics (source or class of source, processing level and algorithms, etc.) ISAMS is a subset of the UARS data set.
A Granule is the smallest aggregation of data which is independently managed.
A measurable or derived variable represented by the data (e.g. air temperature, snow depth, relative humidity). At the GES DISC, parameters are grouped into a Parameter General category, which is broken down into Parameter Specific.

19. List of Acronyms:

CDHF Central Data Handling Facility
CH4 methane
CO carbon monoxide
DCF Data Capture Facility
EOS Earth Observing System
FOV field of view
GSFC Goddard Space Flight Center
H2O water vapor
HNO3 nitric acid
IMS Information Management System
ISAMS Improved Stratospheric and Mesospheric Sounder
K Kelvin
km kilometer
LOS line of sight
m meter
mb millibar
NASA National Aeronautics and Space Administration
NO nitric oxide
NO2 nitrogen dioxide
N2O nitrous oxide
N2O5 dinitrogen pentoxide
O3 ozone
PI Principal Investigator
ppmv parts per million by volume
RAC Remote Analysis Computer
SFDU Standard Formatted Data Units
TDRSS Tracking and Data Relay Satellite System
UARS Upper Atmosphere Research Satellite
USO User Services Office

20. Document Information:

Document Revision Date:Fri May 10 11:53:52 EDT 2002

3 March 1998

Document Review Date:

Document ID:


Document Curator:

Document URL:

UARS Home Page UARS Project


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