Documentation of the BYU-MERS "SIR" image format
This document briefly describes the standard file format of a BYU .SIR file
The BYU-MERS "sir" image format was developed by the Brigham Young University
(BYU) Microwave Earth Remote Sensing (MERS) research group to store images of
the earth along with the information required to easily earth-locate the image
A SIR file consists of one or more 512 byte headers containing all the information
required read the remainder of the file and the map projection information
required to map pixels to lat/lon on the Earth surface. Pixel values
are generally stored as 2 byte (high order byte first) integers
though can be stored as bytes or IEEE floating point. Unfortunately,
the latter is not portable to all machines and so is not recommended.
Scale factors to convert the integer or byte pixel values to native
floating point units are stored in the file header. Utilities to
read SIR format files are available in C, Fortran, Matlab, and IDL/PV-WAVE
from the BYU MERS web site
or the NASA Scatterometer Climate
Record Pathfinder web site or their corresponding ftp sites:
BYU MERS ftp site and the
NASA Scatterometer Climate
Record Pathfinder ftp site Details of the header byte format
are documented in the the various file readers.
The origin of the images are in the displayed lower left corner. The earth
locaion of a pixel is identified with its lower-left corner.
The standard sir format supports a variety of image projections including:
(though typically images are produced in only one projection)
- Rectangular array (no projection)
- A rectangular lat/lon array
- Two different types of Lambert equal-area projections which can be use in both non-polar
and polar projections
- Polar stereographic projections
- EASE grid polar projection with various resolutions
- EASE global projection with various resolutions
In general, *.sir image data files have been 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
has been successfully applied to 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.)
- D.G. Long, P. Hardin, and P. Whiting, "Resolution Enhancement of Spaceborne
Scatterometer Data," IEEE Trans. Geosci. Remote Sens., Vol. 31, pp.
- D.G. Long and D. Daum, "Spatial Resolution Enhancement of SSM/I Data,"
IEEE Trans. Geosci. Rem. Sens., Vol. 36, pp. 407-417, 1997.
- D.G. Long and M.R. Drinkwater, "Cryosphere Applications of NSCAT Data,"
IEEE Trans. Geosci. Remote Sens., Vol. 37, No. 3, pp. 1671-1684,
- D.S. Early and D.G. Long,"Image Reconstruction and Enhanced Resolution
Imaging From Irregular Samples," IEEE Trans. Geosci. Remote Sens.,
Vol. 39, No.2, pp. 291-302, Feb. 2001.
For scatterometers, the multivariate form of the SIR algorithm models
the dependence of sigma-0 on incidence angle as sigma-0 (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.
A represents the "incidence angle normalized sigma-0" (effectively the
sigma-0 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'.
The B coefficient describes the incidence angle dependence of sigma-0
an has the units of dB/deg. At Ku-band 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.
SIR images arestored 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
of 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.
Be sure to use binary ftp to transfer *.sir files!
Code to read BYU SIR files can be found on the BYU NASA Scatterometer
and ftp sites. Code to compute
pixel location information is included.
Note: All BYU-produced data products and associated
documentation and software are copyright BYU. The code may be freely
copied and used for non-commercial purposes. BYU-produced data products
may not be used for commercial purposes without written authorization
by Dr. David G. Long (further authorization may be required from
NASA). Appropriate acknowledgement for BYU MERS and the JPL
should be given when using data products in published
works, with a copy of the publication sent to Dr. David G. Long
and to the JPL PO.DAAC.