ERS-2 Gridded Level 3 Enhanced Resolution Sigma-0 Measurements from BYU (D. Long)


The C-band scatterometers on ERS-1 and ERS-2 provided normalized radar cross section (sigma0) measurements of the Earth's surface. While originally designed for wind observation, scatterometer sigma0 measurements have proven useful in a variety of land and ice studies. To aid in the application of these measurements, Brigham Young University prepared this dataset consisting of enhanced resolution images generated from ERS sigma0 measurements.

Table of Contents:

1. Data Set Overview:

Data Set Identification:

PO.DAAC Dataset Long Name: ERS-1 Gridded Level 3 Enhanced Resolution Sigma-0 from BYU
PO.DAAC Legacy Product ID: 177

Data Set Introduction:

This data set contains images from 16 regions around the globe. Images are made from fast-delivery products sent to JPL and are processed using the BYU SIR algorithm. Multiple passes of the spacecraft are combined to produce a higher spatial resolution (at a cost of reduced temporal resolution).


Originally developed to measure winds over the ocean from space, microwave radar scatterometers have proven to be applicable over a wide range of vegetation zones and moisture conditions. Due to the penetration of its radar signal, a scatterometer can also observe variability in subsurface/subcanopy climate-related features driven by diurnal, seasonal, and interannual forcing. The C-band scatterometers onboard the ERS-1/2 satellites provide nearly ten years of data for such studies.

Summary of Parameters:

Each data file contains one image and represents a unique combination of:
  • geographical region
  • time span
  • image type
  • reconstruction technique
Landmasked versions of the data files can also be created.


ERS-1 was launched in 1991 and operated until 1995. ERS-2, a copy launched in 1995, operated through 2001. The Advanced Microwave Instrument (AMI) carred by ERS-1/2 operated at C-band in both SAR and Scatterometer modes. The modes were exclusive. In the scatterometer mode (referred to as SCAT or Escat), the AMI made nominally 50 km resolution backscatter measurements on a 25 km grid over a 500 km swath on one side of the spacecraft. Like SASS and NSCAT, Escat was a fan beam but unlike those previous Doppler instruments, Escat used timing to provide along-beam resolution.

For highest possible spatial resolution, multiple orbit passes are combined. There is thus a tradeoff between temporal and spatial resolution. Generally orbit passes of a given point on the Earth occur at nearly the same time of day.

Related Data Sets:

The Scatterometry Climate Record Pathfinder (SCP),, at Brigham Young University is the source of this and other data sets available from PO.DAAC:
  • SeaWinds on QuikSCAT Gridded Level 3 Daily Browse Sigma-0 Measurements from BYU (D. Long)
  • SeaWinds on QuikSCAT Gridded Level 3 Enhanced Resolution Sigma-0 Measurements from BYU (D. Long)
    PO.DAAC Dataset ID: PODAAC-QSBYU-ENR00; formerly, PO.DAAC Product 122
  • SeaWinds on ADEOS-II Gridded Level 3 Daily Browse Sigma-0 Measurements from BYU (D. Long)
  • SeaWinds on ADEOS-II Gridded Level 3 Enhanced Resolution Sigma-0 Measurements from BYU (D. Long)
    PO.DAAC Dataset ID: PODAAC-SEABY-ENR00; formerly, PO.DAAC Product 124
  • SEASAT-A Scatterometer Gridded Level 3 Enhanced Resolution Sigma-0 Measurements from BYU (D. Long)
    PO.DAAC Dataset ID: PODAAC-SASSX-BYSN0; formerly, PO.DAAC Product 175
  • NSCAT Gridded Level 3 Enhanced Resolution Sigma-0 Measurements from BYU (D. Long)
    PO.DAAC Dataset ID: PODAAC-NSBYU-SNEN0; formerly, PO.DAAC Product 177
  • ERS-1 Gridded Level 3 Enhanced Resolution Sigma-0 Measurements from BYU (D. Long)
    PO.DAAC Dataset ID: PODAAC-ERS1B-SNEN0; formerly, PO.DAAC Product 177

2. Investigator(s):

Investigator:Dr. David G. Long
Title:Director, BYU Center for Remote Sensing
Professor, Department of Electrical & Computer Engineering
Address: 459 Clyde Building
Brigham Young University
Provo, UT 84602

3. Theory of Measurements:

Scatterometers are designed to measure the normalized radar cross-section backscatter (also termed sigma0) of the surface. Over the ocean, the backscatter is due to Bragg scattering of microwaves from centimeter length capillary ocean waves which is related to the wind. The scattering is greater for higher wind speeds and is anisotropic; thus sigma0 measurements from multiple azimuth angles can be used to estimate the near-surface vector wind. Over land, the backscatter is related to the surface roughness and dielectric properties as well as volume scattering from vegetation and snow cover. Over ice, the backscatter is very sensitive to melting and brine inclusions. It is also sensitive to the internal structure of the ice via volume scattering.

4. Equipment:

Sensor/Instrument Description:

Collection Environment:

The ERS-1 and ERS-2 satellites. ERS-1:


The ERS satellites were launched by the European Space Agency (ESA) on 17 July 1991 (ERS-1) and 21 April 1995 (ERS-2) from Guiana Space Center by Ariane 4 rockets.. Both satellites have a sun-synchronous near-polar orbit, with an inclination of 98.5° and mean altitude of 780 km. Descending orbits cross the Equator around 10:30 a.m. local time.

Source/Platform Mission Objectives:

Global measurements of earth's atmospheric and surface properties using microwave (radar) techniques.

Key Variables:

Attema, 1991:
...the AMI ground processors ... take the digital radar echo samples and produce a radar image with the required geometrical and radiometric properties. In such imagery, the different gray-tones represent different levels of radar backscattering, expressed by the dimensionless quantity sigma0... In a later processing step, sigma0 values are related to geophysical quantities such as wind speed over the ocean, ice type, soil moisture, agricultural crop type, stage of growth, forest type, etc.

Principles of Operation:

Attema, 1991:
The AMI is a multimode radar operating at a frequency of 5.3 GHz (C-band), using vertically polarized antennas for both transmission and reception...

In the wind mode, the AMI is configured as a wind scatterometer and provides three radar images of the ocean surface with a spatial resolution of 50 km and a swath width of 500 km. The three images are acquired by three different antennas: one, the mid-beam, looking to the right side of the satellite, perpendicular to the ERS-1 ground track, one looking forward at 45 degrees azimuth projection angle, with respect to the mid-beam, and one looking backwards at 45 degrees azimuth projection angle with respect to mid-beam.

Sensor/Instrument Measurement Geometry:


Manufacturer of Sensor/Instrument:







Frequency of Calibration:


Other Calibration Information:


5. Data Acquisition Methods:

The prime ground station for satellite command and control (S-band) and payload data reception (X-band) is located at Kiruna, Sweden (latitude 67.9 N, longitude 20.9 W). Additional ESA X-band ground stations at Fucino (Italy), Gatineau (Canada), and Maspalomas (Spain), as well as nationally-provided stations, provide thorough coverage of the earth's land surface. Note that ERS-1 has no on-board recorders except for an on-board tape recorder for bitrate data, so AMI data can only be obtained if there is a ground station in view of the orbiting satellite

6. Observations:

Data Notes:

No additional notes.

Field Notes:

No additional notes.

7. Data Description:

Spatial Characteristics:

Spatial Coverage:

Alaska Greenland North America North America Central America Antarctica South America South America Europe Northern Africa Northern Africa Southern Africa Siberia Bering Sea Bering Sea China and Japan Southern Asia Southern Asia Arctic Indonesia Indonesia Australia and New Zealand
The 16 regions of this data set are:
Alaska, Antarctic, Arctic, Australia, Bering Sea, Central America, China and Japan, Europe, Greenland, Indonesia, North America, North Africa, Siberia, South Africa, South America, South Asia

Spatial Coverage Map:

The absolute value of a pixel in a count image indicates the number of sigma-0 measurements that hit the pixel during the imaging interval. Zero indicates no data.

Spatial Resolution:

Each file's reconstruction technique determines its resolution.
SIR, average files: 8.9 km pixel grid
Gridded files: 44.5 km pixel grid
A field in the header also identifies the resolution.


Each file's geographical region determines its projection.
Antarctic, Arctic files: polar stereographic projection
All others: Lambert Equal Area projection
A field in the header also identifies the projection.

Grid Description:

Two auxiliary files for each region at each spatial resolution (i.e. 8.9 km/pix for SIR and average files, 44.5 for gridded) contain the longitude and latitude for the center of each pixel. The files are in the same format as the product but have a latitude or a longitude value instead of a sigma0 value stored in the image. The naming scheme for these auxiliary files is:
Timage type x = longitude, y = latitude
regregion Ala = Alaska, Ant = Antarctica, ...
rcn reconstruction technique sir = SIR, grd = gridded

Two other types of auxiliary files for each region at each spatial resolution contain topography and land mask information. The naming scheme for these files is:
regregion Ala = Alaska, Ant = Antarctica, ...
rcn reconstruction technique sir = SIR, grd = gridded
infotype topo = topography, lmask = land mask
The sirmask program, described in Software, uses the .lmask files to generate land-masked files.

Temporal Characteristics:

Temporal Coverage:

ERS-1: 1 January 1992 - 17 May 1996
ERS-2: 3 June 1996 - 30 January 2001

Temporal Coverage Map:

Not available.

Temporal Resolution:

The imaging time ranges from 6 to 18 days.

Data Characteristics:


Each data file contains one image and represents a unique combination of parameters, which is reflected in the naming scheme.
Timage type
sigma0 average, the normalized sigma0 at a 40° incidence angle
sigma0 standard deviation
sigma0 error
sigma0 slope, sigma0 vs. incidence angle
average incidence angle
incidence angle standard deviation
count, the number of measurements
pixel time
reg geographical
region -
see Spatial

Ala Alaska
Ant Antarctic
Arc Arctic
Aus Australia
Ber Bering Sea
CAm Central America
ChJ China and Japan     Eur Europe
Grn Greenland
Ind Indonesia
NAm North America
NAf North Africa
SAf South Africa
SAm South America
SAs South Asia
Sib Siberia
yy two-digit year, always 78
dd1 three-digit day of year, start of imaging
dd2 three-digit day of year, end of imaging
rcn reconstruction
SIR or SIRF. The Data Manipulations section describes SIR processing generally then specifically for ERS.
average, the first iteration of the SIR algorithm, less enhanced but less noisy (see Processing Steps)
gridded, non-enhanced on a coarser grid
lmsk land mask. Optional. If present, image is land masked.
The FTP site contains .lmsk files, but the orderable DLTs do not. The sirmask program, described in Software, uses auxiliary .lmask files to generate land-masked files.

Variable Description/Definition:

See the previous section

Unit of Measurement:

This depends upon the image type.
sigma0 average: dB
sigma0 standard deviation: dB
sigma0 error: dB
sigma0 slope: dB/deg
average incidence angle: deg
incidence angle standard deviation: deg
count: unitless
pixel time

Data Source:

ERS-1/2 data are obtained from fast-delivery products sent to JPL. No recalibration has been applied.

Data Range:


Sample Data Record:

Each file contains 1 image, which can be visualized using the software described below, such as the modified version of xv.

Each file also has header information. The program xv printed the following sample output as it displayed the file ers1-a-Ala92-001-006.sir.lmsk:

    SIR file header: 'ers1-a-Ala92-001-006.sir.lmsk'
      Title:   'SIR image of alaska'
      Sensor:  'ERS-1/2'
      Type:    'A image  (ers1-a-Ala92-001-006.sir)'
      Tag:     '(c) 2001 BYU MERS Laboratory'
      Creator: 'BYU MERS:ers_meta_sir v4.0 Ai=-20.00 Bi=-0.130 Bacc=  50.0 RefInc= 40.00 It=27'
      Created: '04:26:00 05/18/01'
      Size: 410 x 320    Total:131200  Offset: -33  Scale: 1000
      Year: 1992  JD range: 1-6  Region Number: 2  Type: 1  Form: 2
      Polarization: 2  Frequency: 5.300000 MHz
      Datatype: 2  Headers: 1  Ver:31
      Nodata: -33.000000   Vmin: -32.000000  Vmax: 0.000000
      Lambert form: (local radius)
       Center point:      -155.000000 , 61.500000
       Lon, Lat scale:    8.900000 , 8.900000 (km/pix)
       Lower-Left Corner: -1800.000000 , -1300.000000
      Image Min, Max: -32.000000 , 0.000000

    Greyscale conversion range:  Min: -32.000000, Max:0.000000

8. Data Organization:

Data Granularity:

The basic granule is one data file. Each file has a unique combination of region, image type, time span, reconstruction technique, and land mask.

The EOSDIS Glossary describes data granularity generally as it applies to the IMS.

Data Format:

The BYU-MERS SIR image format was developed by the Brigham Young University (BYU) Microwave Earth Remote Sensing (MERS) laboratory to store a variety of image types along with the information required to Earth-locate the image pixels.

A SIR format file consists of one or more 512-byte headers followed by the image data and additional zero padding to insure that the file is a multiple of 512 bytes long. The file header record contains all of the information required to read the remainder of the file and the map projection information required to map pixels to lat/lon on the Earth surface. The image pixel values generally represent floating point values and may be stored in one of three ways. The primary way is as 2 byte integers (with the high order byte first), though the pixels may be stored as single bytes or IEEE floating point values. Scale factors are stored in the header to convert the integer or byte pixel values to native floating point units.

The image is stored in row-scanned (left to right) order from the lower left corner (the origin of the image) up through the upper right corner. By default, the location of a pixel is identified with its lower-left corner. The origin pixel (1,1) is the lower left corner of the image. The array index n of the (i,j)th pixel where i is horizontal and j is vertical is given by

n = (j - 1) × Nx + i
where Nx is the horizontal dimension of the image. The last pixel stored in the file is at (Nx, Ny).

The sir file header contains various numerical values and strings which describe the image contents. For example, the value for a no-data flag is set in the header as well as a nominal display range and the minimum and maximum representable value. Optional secondary header records (512 bytes) can be used to store additional, non-standard information.

The standard SIR file format supports a variety of image projections including:

  1. Rectangular array (no projection)
  2. Rectangular lat/lon array
  3. Two different types of Lambert equal-area projections which can be used in either non-polar or polar projections
  4. Polar stereographic projections
  5. EASE grid polar projection with various resolutions
  6. EASE global projection with various resolutions

Any of the programs described in Software below decodes SIR headers.

9. Data Manipulations:


Derivation Techniques and Algorithms:

In general, sir data files are generated using the scatterometer image reconstruction (SIR) resolution enhancement algorithm or one of its variants for radiometer processing. The multivariate SIR algorithm is a non-linear resolution enhancement algorithm based on modified algebraic reconstruction and maximum entropy techniques [Long, Hardin, and Whiting, 1993]. The singlevariate SIR algorithm was developed originally for radiometers [Long and Daum, 1997] but also used for SeaWinds [Early and Long, 2001]. The SIR w/filtering (SIRF) algorithm has been successfully applied to SASS and NSCAT measurements to study tropical vegetation and glacial ice (e.g. Long and Drinkwater, 1999). Variants of SIR have been successfully applied to the ERS-1/2 scatterometer and various radiometers (SSM/I and SMMR). (SIRF is used for SASS, NSCAT, and SeaWinds slice data processing. SIR is used for ERS-1/2 and SeaWinds egg data. The modified median filter [SIRF] is not used with ERS-1/2 data and SeaWinds egg data.)

For scatterometers, the multivariate form of the SIR algorithm models the dependence of sigma0 on incidence angle as sigma0 (in dB) = A + B * (Inc Ang - 40 deg) over the incidence angle range of 15 to 60 deg. The output of the SIR algorithm is images of the A and B coefficients. See the Data Characteristics section.

A represents the "incidence angle normalized sigma0" (effectively the sigma0 value at 40 deg incidence angle). The units of A are dB. Typically, +2 < A < -45 dB. However, in the SIR images A is typically clipped to a minimum -32 dB with values of A < -32 used to indicate 'no data'.

B describes the incidence angle dependence of sigma0 and has units of dB/deg. At Ku-band the global average of B is approximately -0.13 dB/deg. Typically, -0.2 < B < -0.1. B is clipped to a minimum value of -3 dB/deg. This value is used to denote 'no data' as well.

Single variable SIR or SIRF algorithms are used for radiometers and produce only an A (in this case, the brightness temperature) image. Typically, this can range from 165 to 320. Single variable SIR and SIRF algorithms are used for SeaWinds egg and slice images, respectively. In both cases the A images are at the nominal measurement incidence angle for the sensor and in the sensor measurement units.

Data Processing Sequence:

Processing Steps:

Enhanced resolution images made from ERS-1/2 data use the Scatterometer Image Reconstruction (SIR) algorithm. This version of the algorithm does not incoporate a median filter and, for ERS-1/2, uses a 2-d Hamming window as the spatial response function for each beam. In the processing, a linear model relating sigma-0 and incidence angle is assumed, i.e. sigma-0(db) = A + B (theta - 40) where A is the "incidence angle normalized sigma-0" at 40 deg incidence in dB, B is the effective incidence slope of sigma-0 versus incidence angle in dB/deg, and theta is the incidence angle of the observation. This simple linear model is used in place of the gamma=sigma-0/cos(theta) as it more accurately represents the sigma-0 versus backscatter response over a wider range of surface and volume scattering conditions. The SIR algorithm makes images of A and B on an 8.9 km pixel grid. The effective resolution is estimated to be 20-30 km resolution, depending on region and sampling conditions. Raw ERS measurements have a quoted nominal resolution of 50 km on a 25 km sampling grid.

Processing Changes:



Special Corrections/Adjustments:

ERS data were obtained from fast-delivery products sent to JPL. No recalibration has been applied. However, several times throughout the mission dataset, the calibration parameters were changed prior to the tapes' being delivered to JPL. These can be observed as "steps" or "jumps" in the A or B time series. In order to make a more uniform data set, calibration corrections for these steps are being developed and will be available soon. In compliance with agreements between JPL and ESA, JPL has not made the original ERS data products publicly available. The original and re-processed ERS data is made available exclusively through the ESA Earthnet Portal:

Calculated Variables:


Graphs and Plots:


10. Errors:

Sources of Error:

Multiple passes of the spacecraft are combined to produce a higher spatial resolution (at a cost of reduced temporal resolution). The overlapped processing enables true resolution enhancement of the images with any "excess sampling" contributing to an improve SNR. Because the AMI SAR and scatterometer modes are mutually exclusive, the resulting coverage gaps, produce reduced quality images when the SAR mode is operated over the study area. In combining the multiple passes, sigma-0 is assumed to be independent of azimuth angle. While true for most areas, some azimuth dependence in sigma-0 has been observed in Antarctic firn, presumably due to sastrugi or snow dunes.

Quality Assessment:

Data Validation by Source:


Confidence Level/Accuracy Judgement:


Measurement Error for Parameters:


Additional Quality Assessments:


Data Verification by Data Center:


11. Notes:

Limitations of the Data:


Known Problems with the Data:


Usage Guidance:


Any Other Relevant Information about the Study:


12. Application of the Data Set:

13. Future Modifications and Plans:

There are no future modifications or plans at this time.

14. Software:

Software Description:

Sample read and display software for SIR files are available in C, FORTRAN, IDL/PV-WAVE, and MATLAB. These programs can be easily modified to meet the requirements of individual users.
LanguageProgram NameDescription
C csir_dump.cdump SIR file to text output
csirexample.c read SIR file, print values of corner pixels
sir2bmp.cconvert SIR file to BMP
sir2byte.cconvert SIR file to raw, unsigned byte file
sir2gif.cconvert SIR file to GIF
sirmask.cmask one SIR file with another
Fortran fsir_dump.fdump SIR file to text file
fsir_locmap.fread SIR file, create latitude and longitude maps like the auxiliary files
fsirexample.f read SIR file, create an unsigned byte file
sirmask.fmask one SIR file with another load SIR file, save to file, display image, do forward/inverse transforms
PV-WAVE, load SIR file, save to file, display image, do forward/inverse transforms
MATLABloadsir.m, writesir.m, showimage.m, ... load SIR file, save to file, display image, do forward/inverse transforms
IDL is made by Research Systems, Inc.
PV-WAVE is made by Visual Numerics, Inc.
MATLAB is made by The MathWorks, Inc.
All are copyrighted software tools for numerical analysis and visualization.

Software Access:

The latest versions of the sample read and display software can be obtained via anonymous FTP from, where lang = "c", "f", "idl", or "matlab". JPL PO.DAAC also maintains a copy at

The IDL and PV-WAVE programs reside in one directory due to the similarity between the languages.,, and call the same functions, though the file must be modified for PV-WAVE.

15. Data Access:

Contact Information:

For general questions and comments regarding this dataset, please contact
Email is the preferred method of communication.

Dr. David Long of BYU is the source of this dataset. Please contact him with more detailed questions. See Investigator for contact information.

Data Center Identification:

Jet Propulsion Laboratory (JPL)
Physical Oceanography Archive Center (PO.DAAC)

Procedures for Obtaining Data:

This data set is currently available via anonymous FTP at

This data set is publicized courtesy of the PO.DAAC at JPL.

Data Center Status/Plans:


16. Output Products and Availability:

This data set is made available at PO.DAAC on behalf of the BYU SCP:

17. References:

Attema, E.P.W., 1991. "The Active Microwave Instrument On-Board the ERS-1 Satellite," Proc. IEEE, Vol. 79, pp. 791-799

Early, D.S. and D.G. Long, Feb 2001. "Image Reconstruction and Enhanced Resolution Imaging From Irregular Samples," IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No.2, pp. 291-302.

Long, D.G. and D. Daum, 1997. "Spatial Resolution Enhancement of SSM/I Data," IEEE Transactions on Geoscience and Remote Sensing, Vol. 36, pp. 407-417.

Long, D.G. and M.R. Drinkwater, 1999. "Cryosphere Applications of NSCAT Data," IEEE Transactions on Geoscience and Remote Sensing, Vol. 37, No. 3, pp. 1671-1684.

Long, D.G., P. Hardin, and P. Whiting, 1993. "Resolution Enhancement of Spaceborne Scatterometer Data," IEEE Transactions on Geoscience and Remote Sensing, Vol. 31, pp. 700-715.

18. Glossary of Terms:

See the EOSDIS Glossary for a more general listing of terms related to the Earth Observing System project.

19. List of Acronyms:

AMI: Advanced Microwave Instrument
EOS: Earth Observing System
EOSDIS: Earth Observing System Data and Information System
ERS: European Remote Sensing Satellite
ESA: European Space Agency
FTP: File Transfer Protocol
IDL: Interactive Data Language
JPL: Jet Propulsion Laboratory
NASA: National Aeronautics and Space Administration
NRCS: normalized radar cross-section
NSCAT: NASA Scatterometer
PO.DAAC: Physical Oceanography Distributed Active Archive Center
QuikSCAT: the NASA Quick Scatterometer spacecraft, or
QuikSCAT: usually refers to the SeaWinds instrument on the spacecraft
SAR: Synthetic Aperture Radar
SASS: Seasat Satellite Scatterometer
SCP: Scatterometer Climate Record Pathfinder
SMMR: Scanning Multichannel Microwave Radiometer
SSM/I: Special Sensor Microwave/Imager
URL: Uniform Resource Locator

20. Document Information:

Document Creation Date:

21 October 2002

Document Review Date:

21 October 2002

Document Revison Date:

6 December 2011

Document ID:



Document originally written by Richard Chen based heavily on information on the BYU SCP web site,

Document Curator:

PO.DAAC Data Engineering Team

Document URL: