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GES DISC DAAC Data Guide:UARS SUSIM Level 3BS Data Set Document


Link to SUSIM UARS home page


The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) is one of 10 instruments aboard the Upper Atmosphere Research Satellite (UARS). Its primary objective is the daily measurement of the solar UV spectral irradiance. Data collection began 11 October 1991 and has continued, with a few interruptions, to the present. SUSIM data have been processed to levels 1, 2, 3AS and 3BS. Currently, SUSIM level 3BS version 20 data products are available from the Goddard Space Flight Center (GSFC) Distributed Active Archive Center (DAAC).


Table of Contents:

1. Data Set Overview:

Data Set Identification:


Data Set Introduction:

SUSIM Level 3BS data are a subset of the UARS dataset. SUSIM measures solar ultraviolet spectral irradiance at wavelengths between 115 nm and 411 nm to produce approximately 1 nm resolved gridded irradiances. It uses this and other data to also form and report the values of 7 solar indices.


The ultimate goal of SUSIM is to measure the near, middle, and far UV irradiance over a complete 11 year solar cycle.

Summary of Parameters:

Solar UV Spectral Irradiance
Solar UV Indices


The SUSIM Level 3BS data include gridded irradiances representing 1 nm integrations over the whole spectral range and their formal errors derived from the data and the values of 7 solar indices for each mission day. Each file consists of three records called SFDU, LABEL, and DATA. SFDU and LABEL records contain descriptive information about the instrument and the data. The DATA record contains the gridded irradiances and their errors followed by parameter names (seven of which are solar indices) and parameter values.

Related Data Sets:

The UARS SOLSTICE level 3BS gridded irradiance data has the same format and units except the gridded irradiance definitions and the wavelength coverage are different. While there may be some similarity in the parameters and solar indices, the SOLSTICE versions of these, in general, have different definitions and units.

2. Investigator(s):

Investigator(s) Name and Title:

Dr. Guenter Brueckner
Code 7660
Naval Research Laboratory
Washington, DC, 20375
Telephone Numbers:
Voice: (202) 767-3287
FAX: (202) 767-5636
Electronic Mail Address:

Title of Investigation:

Solar Ultraviolet Spectral Irradiance Monitor

Contacts for Scientific Aspects of the Data:

Dianne Prinz
Code 7668
Naval Research Laboratory
Washington, DC, 20375
Telephone Numbers:
Voice: (202) 767-2481
FAX: (202) 767-5636
Electronic Mail Address:


Linton Floyd
Code 7660
Naval Research Laboratory
Washington, DC, 20375
Telephone Numbers:
Voice: (202) 767-2258
FAX: (202) 767-5636
Electronic Mail Address:

3. Theory of Measurements:

The general data gathering technique performed by the SUSIM Instrument is described in the SUSIM Whole Data Set Description metafile.

4. Equipment:

Sensor/Instrument Description:


SUSIM is an absolutely calibrated solar ultraviolet dual dispersion, dual spectrometer instrument. It is mounted on the UARS Solar Stellar Pointing Platform (SSPP) which normally tracks the sun for at least 35 minutes throughout each of the approximately 16 90-minute orbits per day. When illuminated by the sun, SUSIM measures the solar ultraviolet spectral irradiance in the 110 to 411 nm wavelength range.

To calibrate the instrument during its long flight, SUSIM carries four stable deuterium lamps that are periodically scanned. Additional calibration capability is provided by its ability to freely substitute different elements in the optical path. These include entrance filter, entrance slit, primary grating, secondary grating, exit slit, exit filter, and detector. SUSIM measurements are taken using pairs of slits which have 0.15 (narrow), 1.1 (mid) or 5 (wide) nm spectral bandpasses, using one of eight available primary and secondary grating pairs) with one of six entrance filters (2 Quartz, 4 MgF2), one of seven exit filters (1 MgF2, 2 Corning, 2 7% neutral density, 2 15% neutral density) and one of seven detectors (3 RbTe diodes, 2 bialkali diodes, 1 RbTe photoncounter, 1 bialkali photoncounter). The four deuterium lamps can be used matched with each of the eight different grating pairs. Solar irradiance data are acquired either in scanning mode, during which the solar irradiance is measured over a range of wavelengths, or in dwelling mode where the received irradiance is measured continuously at one selected wavelength.

Collection Environment:

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


Upper Atmosphere Research Satellite (UARS).

Source/Platform Mission Objectives:

UARS was launched in September 1991 with the mission of investigating the chemistry, dynamics, and energy inputs of the Earth's upper atmosphere. See the UARS Project and UARS Platform documents for more information.

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. 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 data to level 1, 2, 3AS, and 3BS data. The Goddard DAAC acquires the UARS data from the CDHF.

6. Observations:

Data Notes:

The user is referred to the SUSIM metadata documents for any problems with the data. Further questions may be answered by contacting the SUSIM team at the addresses listed above.

7. Data Description:

Spectral Characteristics:

Spectral Coverage:

115 nm to 411 nm.

Spectral Resolution:

The resolution is that of the SUSIM instrument function convolved with the completely resolved full-disk solar spectrum. The SUSIM instrument function is approximately a triangle having a FWHM of 1.1 nm.

Grid Description:

All SUSIM Level 3BS data are gridded every nm on the half nm.

Temporal Characteristics:

Temporal Coverage:

Temporal coverage is from 11 October 1991 to the present. The GSFC DAAC currently has the first 7 years of SUSIM level 3BS data (to 8 August 1998). Not all days are represented in the dataset, usually due to spacecraft problems. Listed below are the dates within the above period for which SUSIM data are not available:
          03-JUN-1992 to 10-JUL-1992
          14-JUL-1992 to 19-JUL-1992
          17-APR-1993 to 19-APR-1993
          05-AUG-1993 to 10-AUG-1993
          18-SEP-1993 to 22-SEP-1993
          03-OCT-1993 to 04-OCT-1993
          24-MAR-1995 to 25-MAR-1995
          27-MAR-1995 to 28-MAR-1995
          30-MAR-1995 to 06-APR-1995
          18-APR-1995 to 19-APR-1995
          17-MAY-1995 to 24-MAY-1995
          30-JUN-1995 to 02-JUL-1995
          30-JUL-1995 to 01-AUG-1995
          08-SEP-1995 to 09-SEP-1995
          14-OCT-1995 to 19-OCT-1995
          27-DEC-1995 to 28-DEC-1995
          03-FEB-1996 to 13-FEB-1996
          05-MAR-1996 to 06-MAR-1996
          08-MAY-1996 to 10-MAY-1996
          22-JUL-1996 to 30-JUL-1996
          02-SEP-1996 to 04-SEP-1996
          20-OCT-1997 to 26-OCT-1997
          12-MAR-1998 to 16-MAR-1998
          15-APR-1998 to 19-APR-1998
          28-JUN-1998 to 02-JUL-1998
          24-JUL-1998 to 29-JUL-1998

Temporal Resolution:


Data Characteristics:


The principal variable is the gridded solar irradiances of 1.1 nm resolution SUSIM instrument function given every nm on the half nm. There are also parameters related to the data and seven solar indices. These are:
  1. UARS Mean Solar Distance (MSD).
  2. H I Lyman alpha (hydrogen @ 121.6 nm) Index
  3. O I (oxygen @ 130.4 nm) Index
  4. C II (carbon @ 133.5 nm) Index
  5. C IV (carbon @ 154.9 nm) Index
  6. Al Edge (aluminum @ 208 nm) Index
  7. Mg II (magnesium @ 279.9 nm) Index
  8. Ca II (calcium @ 393.3 nm) Index
  9. Average Collection Time of Gridded Irradiances

Variable Description/Definition:

A general description of these indices follows. For more detailed and complete descriptions please refer to the metafile definitions.

The emission line indices, H I Lyman alpha, O I, C II, and C IV, are the flux in the peak minus the level of the flux of the local continuum. The absorption line indices, Mg II and Ca II, are ratios of the depth of the absorption feature to the inferred local continuum. The Al Edge is (roughly) the ratio of the irradiance at the upper level (at longer wavelengths above the Al Edge) to the lower level (at shorter wavelengths). The MSD is the distance from the satellite to the sun at 1200 GMT on the day of the measurement. The Average Collection Time of Gridded Irradiances is the straight average time for the measurements since the start of the day.

Unit of Measurement:

The units of measurement for the parameters and solar indices are as follows:
  1. Solar Irradiances are given in W/m3.
  2. UARS Mean Solar Distance (MSD) is given in km.
  3. The H I, O I, C II, and C IV emission indices are given in mW/m2.
  4. The Al Edge, Mg II, and Ca II indices are dimensionless ratios.
  5. The Average Collection Time is given in seconds.

Data Range:

All values are stored as positive real numbers.

8. Data Organization:

Data Granularity:

SUSIM Level 3BS granules are produced daily. Each granule is a multi-file granule consisting of two files:


  1. The binary data file (files ending with PROD, or *PROD extension) which contains the gridded irradiance data, and quality, as well as observation time, the mean solar distance, and the 7 solar indices and their character string labels.


  2. An ASCII metadata file (files ending with META, or *META extension) associated with the data file containing items such as the PI assigned quality flag and record length size of the data file.

The format for granule file names is SUSIM_L3BS_Ddddd.Vvvvv _Ccc_xxxx, where dddd is the UARS day (0001 = 12 September 1991), vvvv is the data version number, and cc is the data version cycle number. The file extension xxxx is either PROD for the binary data files, or META for the ASCII metadata files.

The average size for a SUSIM Level 3BS granule is about 12 kB. The *META files are small, only about 700 bytes each.

Data Format:

The data are in a native UARS format. The files were originally created on a VAX/VMS system at the UARS CDHF, and now exist as UNIX stream files at the Goddard DAAC. SUSIM 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:

Solar signals are reduced by SUSIM software to produce solar ultraviolet irradiances as a function of wavelength and time. This is accomplished by applying encoder to wavelength assignment, photon counter nonlinearity, darks, gain, temperature, field of view, moves, wavelength, stray light, optical degradation, and 1 AU adjustments. Irradiances from 1.1 nm (MM) scans are then placed on the required 1 nm grid. In addition, several solar indices are derived from the mid resolution and high resolution continuous monitoring scans.

Data Processing Sequence:

Processing Steps:

SUSIM production software comprises five separate job steps or program units, namely, SUS1, SUS2, SUS3, SUS4, and SUS5. These are run sequentially using a full day of data that begins at 00:00:00 GMT. The input data from the spacecraft consists of four level 0 files: the SUSIM science file (SUSIMA or SUSIMB), the ENGINEERING file, the SPACECRAFT file, and the onboard computer (OBC) file. The production software also uses other data - mainly ephemeris information that is not completely derived from UARS telemetry. The production software produces levels 1, 2, and 3 data files. Generally, level 1 data consist of processed instrument engineering reports and science data that remain in engineering units; level 2 data consist of the entire science data output that is not immediately intended for general distribution; and level 3 outputs consist of data that will be available to general science community through the UARS CDHF and GSFC DAAC and also through the SUSIM's FTP site. Please refer to SUSIM Level 2 Data Description and Structure, and SUSIM Level 3 Data Description and Structure documents for a more complete description of the SUSIM level 2 and level 3 processing procedures.

Processing Changes:

An updated stray light algorithm has been inserted for data version 19. Updated calibration files containing responsivity, field-of-view, temperature, and gain corrections were also added for version 19 processing. Responsivity interpolation between calibrations is now based on UV exposure and a function which represents the general degradation behavior.


Special Corrections/Adjustments:


Calculated Variables:

The data record of SUSIM level 3 product contains the 1 nm resolution irradiances and associated qualities for gridded wavelengths between and including 115.5 nm to 410.5 nm, starting wavelength, and nine parameters. Seven of the calculated parameters are various solar indices, one is the mean solar distance at noon GMT, and last is the average time of measurement of the gridded data.

Graphs and Plots:

Example SUSIM level 3BS spectrum and solar indices for a single file:

SUSIM Sample

See also SUSIM Mg II index plots spanning the entire experimental period.

10. Errors:

For each gridded irradiance datum, a formal error is computed accounting for both dark and light signals. Estimation of systematic errors for SUSIM is highly complex and thus is unsuited for this type of computation. See the validation paper by Woods, et al. and the DAAC quality file for a more detailed error analysis. Occasionally, the scans which are used to produce the gridded data did not complete leaving some values for the gridded irradiances unmeasured. These will have a value of exactly zero as will their corresponding error estimate. This latter value distinguishes such a value from an actual zero irradiance. (A zero irradiance can occur when the measured light signal is smaller than the sum of its corresponding dark signal and the estimate of stray light.)

Any apparent anomalies which are found in the SUSIM data would be of interest to the SUSIM team. Reporting of such instances would be greatly appreciated will likely improve the quality the future reprocessing of these data.

Sources of Error:

There are several sources of error in the SUSIM level 3BS gridded irradiances. Most of these are systematic rather than random in nature. They follow, listed in rough order of importance:
  1. Responsivity Degradation Estimation
  2. Stray/Scattered Light Model
  3. Initial (i.e. prelaunch) Responsivity Calibrations
  4. Wavelength Assignment to Hardware Encoder Position
  5. Field-of-View (solar pointing) Errors
  6. Detector Gains and Temperature Correction
  7. Light Signal Measurement Random Error
  8. Dark Signal Measurement and Estimate

Quality Assessment:

Data Validation by Source:

The PI Data Quality value (PI_DQ) is determined for each day's data as follows. A PI_DQ value of 4.0 is currently given to level 3BS products. This indicates that optical calibration for that data was determined using interpolation between reference calibration days. A data quality value of 2.0 is currently given to level 3AS products. This indicates that extrapolation from the two previous calibration files was used to determine the optical calibration coefficients. (At the time of this writing, only 3BS data is archived at the DAAC.) A data quality of 0.0 indicates that there are no usable data in this file.

UARS Data Quality (UARS_DQ) is set to 0.4 if better than 98% of data are valid. It is set to 0.3 if between 76% to 98% of data are valid and is set to 0.2 if between 50% to 76% of data are valid. Finally, UARS Data Quality is set to 0.1 if less than 50% of data are valid. PI_DQ and UARS_DQ flags are located in the catalog entry on the CDHF and in the metafile on the GSFC DAAC.

Measurement Error for Parameters:

Precision information is placed in the form of a measurement standard deviation into the "data quality" field corresponding to each 1nm gridded irradiance. No precision estimates for the solar indices are given.

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 DAAC's UNIX format. No additional data checks are performed by the DAAC.

11. Notes:

Limitations of the Data:

The data files exist as UNIX stream files at the Goddard DAAC. The binary data files are in IEEE big-endian format. 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. For users of non 32-bit and/or non-UNIX machines, conversion routines to read the data files will need to be written. File record length information is only listed in the ASCII metadata files (*META extension) which accompany the data files.

Known Problems with the Data:

The following are known problems with gridded irradiances.
  • Irradiances in the continuum below 140nm and especially below 120nm are often not well determined because the signal levels are very low relative to the other random and systematic effects.
  • Irradiances corresponding to emission line peaks are spread over two or three gridded wavelengths because of the width of the SUSIM instrument function.
  • The irradiance data between the first day of operations (UARS day 30 and UARS day 77) have larger systematic errors because during that period:
    1. The field-of-view dependence of the instrument responsivity was not measured directly and was changing rapidly.
    2. SSPP solar pointing errors were larger during this period than those during the rest of the experiment.
    3. The responsivity degradation rates were changing rapidly and were not measured often enough.
  • Irradiances above 400 nm are based on signal measurements which are known to have additional anomalous stray light which is probably undercorrected.

Usage Guidance:

The variability of irradiance in the various strong emission lines (e.g. H I Ly-a) is better obtained from the solar indices rather than the gridded irradiances. The SUSIM UARS Home page should be consulted for periodic updates about existing SUSIM data and new data products. For example, there is a new algorithm used to form the Mg II index from the V19 irradiances. This improved index is now available directly from SUSIM group at NRL.

Any Other Relevant Information about the Study:


12. Application of the Data Set:

These data are suitable for the study of solar variability as well as providing realistic UV energy input values for atmospheric models.

13. Future Modifications and Plans:

The SUSIM instrument is still (5/8/97) acquiring data. It is hoped that this will continue until the maximum levels of the next solar cycle are reached, perhaps sometime in the year 2000. Improvements in the processing methods are still anticipated in the areas of index redefinition and refinement, degradation, field-of-view, and reconvolution among others. Over time, these improvements will be incorporated into the latest versions of the SUSIM data. Apart algorthmic improvements, new data must be calibrated using data from calibration scans during the same time period. At present, major calibrations are done every six months. Afterwards, new SUSIM irradiances can then be made available. Meanwhile, SUSIM data will be available through the SUSIM UARS Data Page and at times will be more up-to-date than that found on the DAAC.

14. Read Software:

Software Description:

Simple read/dump programs are available for reading the SUSIM level 3A and 3B 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:

Contacts 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 SUSIM data are archived at the GES DISC under the UARS Project.

Procedures for Obtaining Data:

The SUSIM level 3A and 3B 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 SUSIM level 3A and 3B data products.

16. Output Products and Availability:

The SUSIM level 3BS data are available. See the section above on Procedures for Obtaining Data for specific information.

Additional output products are available via ftp at the SUSIM RAC. For more information, please refer to the corresponding SUSIM Data Page.

17. References:

Satellite/Instrument/Data Processing Documentation:
  • Floyd, L. E., D. K. Prinz, and G. E. Brueckner, "SUSIM/UARS Optical Responsivity Evolution: Character and Implications", Proceedings of the International Workshop on VUV and X-Ray Radiometry for Space-Based Instruments, Berlin, pp. 51-54, 1994.
  • Floyd, Linton E., Lynn C. Herring, Dianne K. Prinz, and Guenter E. Brueckner, "Maintaining calibration during the long term space flight of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)", Proceedings, SPIE, 2831, pp. 36-46, 1996.
  • Prinz, D.K., L.E. Floyd, L.C. Herring, and G.E. Brueckner, "On-orbit performance of deuterium calibration lamps during the four years of SUSIM operations on UARS", Proceedings, SPIE, 2831, pp. 25-35, 1996.
  • Reiser, P., J. Lumpe, C. Wang, and D. K. Prinz, "Post Launch Characterization of Stray Light in the UARS/SUSIM", Proceedings of the International Workshop on VUV and X-Ray Radiometry for Space-Based Instruments, Berlin, pp. 47-50, 1994.
Journal Articles and Study Reports:
  • Brueckner, G. E., K. L. Edlow, L. E. Floyd IV, J. L. Lean, and M. E. VanHoosier, "The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on Board the Upper Atmosphere Research Satellite (UARS)", J. Geophys. Res. 98, D6, pp. 10695-10711, 1993.
  • Brueckner, G.E., L.E. Floyd, P.A. Lund, D.K. Prinz, and M.E. VanHoosier, "Solar ultraviolet spectral-irradiance observations from the SUSIM-UARS experiment", Metrologia, 32, pp. 661-665, 1995/96.
  • Floyd, L.E., P.A. Reiser, P.C. Crane, L.C. Herring, D.K. Prinz, and G.E. Brueckner, "Solar Cycle 22 UV Spectral Irradiance Variability: Current Measurements by SUSIM UARS", accepted for publication, Solar Physics, 1997.
  • Lean, Judith, Michael VanHoosier, Guenter Brueckner, Dianne Prinz, Linton Floyd, and Kenneth Edlow, "SUSIM/UARS Observations of the 120 to 300 nm Flux Variations During the Maximum of the Solar Cycle: Inferences for the 11-year Cycle", Geophysical Research Letters, 19, 22, pp. 2203-2206, 1992.
  • 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, pp. 10643-10647, 1993.
  • Woods, T. N., et al., "Validation of the UARS Solar Ultraviolet Irradiances: Comparison with the ATLAS-1, -2 Measurements", J. Geophys. Res., 101, D6, pp. 9541-9569, 1996.
Goddard DAAC online documentation:

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.
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.).
A granule is the smallest aggregation of data which is independently managed.

19. List of Acronyms:

CDHF Central Data Handling Facility
cm centimeter
DAAC Distributed Active Archive Center
DCF Data Capture Facility
EOS Earth Observing System
FOV Field of View
FWHM Full Width, Half Maximum
GSFC Goddard Space Flight Center
IMS Information Management System
km kilometer
m meter
MSD mean solar distance
mW milliwatts
NASA National Aeronautics and Space Administration
nm nanometer
PI Principal Investigator
RAC Remote Analysis Computer
SFDU Standard Formatted Data Units
SOLSTICE Solar-Stellar Irradiance Comparison Experiment
SSPP Solar Stellar Pointing Platform
SUSIM Solar Ultraviolet Spectral Irradiance Monitor
TDRSS Tracking and Data Relay Satellite System
UARS Upper Atmosphere Research Satellite
W Watts

20. Document Information:

Document Revision Date:Fri May 10 11:54:23 EDT 2002

01 May 1997

Document Review Date:

04 Jul 1995

Document ID:


Document Curator:


Document URL:



UARS button UARS Project Authors: Linton Floyd --
Ed Esfandiari --
James Johnson --

Change History

Version 2.0
Version baselined on addition to the GES Controlled Documents List, July 4, 1995.
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