TRMM Visible and Infrared Scanner (VIRS) Level 1 Calibrated Radiance Produc

The TRMM Visible and Infrared Scanner (VIRS) Level 1B Calibrated Radiance Product (1B01) contains calibrated radiances and auxiliary geolocation information from the five channels of the VIRS instrument, for each pixel of each scan. The data are stored in the Hierarchical Data Format (HDF), which includes both core and product specific metadata applicable to the VIRS measurements. A file contains a single orbit of data with a file size of about 95 MB. The EOSDIS "swath" structure is used to accommodate the actual geophysical data arrays. There are 16 files of VIRS 1B01 data produced per day.

For channels 1 and 2, Level 1B radiances are derived from the Level 1A (1A01) sensor counts by computing calibration parameters (gain and offset) derived from the counts registered during space and solar and/or lunar views. New calibration parameters are produced every one to four weeks. Channels 3, 4, and 5 are calibrated using the internal blackbody and the space view. These calibration parameters, together with a quadratic term determined pre-launch, are used to generate a counts vs. radiance curve for each band, which is then used to convert the earth-view pixel counts to spectral radiances.

Geolocation and channel data are written out for each pixel along the scan, whereas the time stamp, scan status (containing scan quality information), navigation, calibration coefficients, and solar/satellite geometry are specified on a per-scan basis. There are in general 18,026 scans along the orbit pre-boost and 18,223 post-boost, with each scan consisting of 261 pixels. The scan width is about 720 km pre-boost and 833 km post-boost.

TRMM Satellite Operating Altitude Change

The average operating altitude for TRMM was changed from 350 to 403 km during the period of August 7-24, 2001. This orbit boost maneuver extended the mission life significantly. All post-boost data products had been released by the TRMM Science Project, as of early December 2001. All TRMM data products (post- and pre-boost) are available via the TRMM data search-and-order system at http://disc.sci.gsfc.nasa.gov/data/datapool/TRMM_DP/. The time period before August 7, 2001 is referred to as pre-boost, and the time period after August 24, 2001 is referred to as post-boost.

The characteristics of the three rain instruments for pre- and post-boost are shown in following table:

Characteristics of TRMM Instruments
	Swath Width (km)		Ground Resolution (km)
	Pre-boost	Post-boost	Pre-boost	Post-boost
VIRS	720	833	2.2	2.4
TMI	760	878	4.4*	5.1*
PR	215	247	4.3	5.0
* Ground resolutions of TMI are those at 85.5 GHz (highest resolution).

The pre- and post-boost characteristics of TRMM data are different; their details are listed in the Data Characteristics table for each orbital product. Some caveats associated with post-boost TRMM Precipitation Radar (PR) products have been released by the PR algorithm scientists and are available at ftp://disc2.nascom.nasa.gov/data/TRMM/Documentation/TRMM_Boost_PR_Caveats.html.

For TRMM version 5 products, the post-boost filenames all have a "5A" in the product version part of the name, compared with a "5" in pre-boost filenames. For version 6, the filenames have no differences for pre-boost and post-boost.

Sponsor and Acknowledgement

The distribution of this data set is funded by NASA's Earth Science Enterprise (ESE). The data are not copyrighted; however, we request that when you publish data or results using these data, please acknowledge as follows: The data used in this study were acquired as part of the Tropical Rainfall Measuring Mission (TRMM). The algorithms were developed by the TRMM Science Team. The data were processed by the TRMM Science Data and Information System (TSDIS) and the TRMM Office; they are archived and distributed by the Goddard Distributed Active Archive Center. TRMM is an international project jointly sponsored by the Japan National Space Development Agency (NASDA) and the U.S. National Aeronautics and Space Administration (NASA) Office of Earth Sciences. Please send a copy of your publication to Help Desk, Goddard DAAC, Code 610.2, NASA GSFC, Greenbelt, MD 20771 or email the reference of your publication to help-disc@listserv.gsfc.nasa.gov.

Future Updates

It is expected that some of the TRMM algorithms will be refined or improved as new measurements are gathered and analyzed by the TRMM Science Team. The data products are expected to be periodically reprocessed by TSDIS in order to provide the scientific and other user communities with the most current and best available rainfall products. The exact reprocessing schedule will be set by a team designated by the TRMM Project Scientist. This document will be updated in coordination with the TRMM reprocessing schedule and whenever appropriate as determined by the Goddard DAAC Hydrology Data Support Team.

Data Flow Description

VIRS Level 1A (1A01) ==> VIRS Level 1B Calibrated Radiance (1B01)
The data flow of all products are shown in the Satellite Algorithm Flow Diagram.

Data Set Description

Data Characteristics

VIRS Channels
Channel	Spectral Region	Wavelength (µm)
1	Visible	0.63
2	Near Infrared	1.60
3	Near Infrared	3.75
4	Near Infrared	10.8
5	Infrared	12.0

TRMM 1B01 (VIRS) Data Characteristics
Characteristic	Pre-boost (before 08-07-2001)	Post-boost (after 08-24-2001)
Temporal Coverage	Start Date: 12-20-1997 Stop Date: 08-07-2001	Start Date: 08-24-2001 Stop Date: -
Geographic Coverage	Latitude: 38°S - 38°N Longitude: 180°W - 180°E	Latitude: 38°S - 38°N Longitude: 180°W - 180°E
Temporal Resolution	About 91.5 minutes per orbit About 16 orbits per day More information about revisit frequency	About 92.5 minutes per orbit About 16 orbits per day More information about revisit frequency
Spatial Resolution	2.2 km	2.4 km
Scan Characteristics	Swath Width: 720 km Pixels/Scan: 261 Scans/Second (SS): 298.5/60 Seconds/Orbit (SO): 5490 Average Scans/Orbit: nscan = SSSO = 18026	Swath Width: 833 km Pixels/Scan: 261 Scans/Second (SS): 298.5/60 Seconds/Orbit (SO): 5550 Average Scans/Orbit: nscan = SSSO = 18223
Average File Size	Compressed: ~93 MB Original: ~96 MB	Compressed: ~94 MB Original: ~97 MB
File Type	HDF	HDF

Data Format Structure

The following table summarizes the contents and structure of the VIRS Calibrated Radiance (1B01) product. Further information can be found in Volume 3 of the "File Specifications for TSDIS Products - Level 1" .

Data Format Structure for VIRS Calibrated Radiance (1B01) Product
Name	Type	Record Size (byte)	Dim Size/ # of Records	Scaled by	Range	Unit	Description
ECS Core Metadata	Char Attribute	10,000	-	-	-	-	ECS Core Metadata
PS Metadata	Char Attribute	10,000	-	-	-	-	Product Specific Metadata
Swath Structure	Char Attribute	5,000	-	-	-	-	Specifications for the swath geometry
Scan Time	Vdata Table	8	nscan	-	-	-	Time associated with each scan
Geolocation	Float SDS	4	2261nscan	-	-	degree	Earth location of the center of the IFOV at the altitude of the earth ellipsoid
Scan Status	Vdata Table	19	nscan	-	-	-	Status of each scan
Navigation	Vdata Table	88	nscan	-	-	-	Spacecraft geocentric information
Solar Cal	Vdata Table	32	nscan	-	-	-	Solar unit vector in Geocentric Inertial Coordinates, and the Sun-Earth distance
Calibration Counts	Integer SDS	2	523*nscan	-	-	-	Raw calibration counts data
Temperature Counts	Integer SDS	2	6*nscan	-	0 - 4095	counts	Primary and redundant temperatures for the black body, radiant cooler, and the electronics module
Local Direction	Float SDS	4	2227*nscan	-	-	degree	Angles to the satellite and sun from the IFOV pixel position on the earth
Channels	Integer SDS	2	5261nscan	depends on channels	depends on channels	(mW cm^-2µm^-1 sr^-1)	Scene data for the five channels (mW = milliwatts, sr = steradian). Accuracy depends on channels

Tools for Visualizing Data

The Goddard DAAC provides the following tools to help users visualize data in the Hierarchical Data Format (HDF).

TSDIS Orbit Viewer

The TSDIS Orbit Viewer is a menu-driven graphical interface for dynamically generating images from TRMM HDF files. The viewer can display, at the full instrument resolution, TRMM satellite, Ground Validation, browse, and Coincidence Subsetted Intermediate (CSI) products, as well as other derived products.

The software runs on Microsoft Windows and UNIX.

The source code and installation instructions for the Orbit Viewer are available from the Goddard DAAC's TRMM ftp site (ftp://disc2.nascom.nasa.gov/software/trmm_software/Orbit_Viewer).

Please note: TSDIS can provide technical support for the Orbit Viewer only to certain members of the TRMM Science Team. Other users should contact the DAAC's Hydrology Data Support Team (hydrology-disc@listserv.gsfc.nasa.gov).

EOSView

EOSView is a standalone X-based data visualization tool that displays HDF files. It can be used to view data ordered from the Goddard DAAC. In addition, it provides a secondary mechanism for previewing browse files before ordering data. (The primary mechanism is the preview feature of the TRMM Data Search and Order System.) EOSView serves as a file verification tool. The contents of HDF files are displayed and individual objects can be selected for display. Displayable objects include raster images, data sets in tables, pseudocolor images of data sets, attributes, and annotations. Simple animations can be performed for a file with multiple raster images.

A unique interface has been provided for handling HDF-EOS data structures. The Swath/Point/Grid interface uses only HDF-EOS library calls. EOSView users will not see the underlying HDF structures but will be prompted for what parts of the HDF-EOS object they wish to view. The EOSView requires at least 4 megabytes of memory and a larger than 24-bit graphics board.

Download Instructions for the Software:

These tools can be downloaded via anonymous ftp using a command line ftp client, available on all Unix machines.

The source code, installation instructions, and documentation for EOSView and Orbit Viewer are available from the Goddard DAAC's TRMM ftp site (ftp://disc2.nascom.nasa.gov/software/trmm_software).

The following files should be downloaded for EOSView:

EOSView (executable)
eosview.csc (help)
eosview.uid (user interface description file)
eosview.dat (IDL commands file)

How to start EOSView:

Start EOSView by typing 'EOSView' at the command prompt. The current working directory must contain the four EOSView files.

Sample Software

TRMM HDF Data File Read Software

The Goddard DAAC Hydrology Team has developed the TRMM HDF Data File Read Software, first released in February 1999. The software reads TRMM HDF data files and writes out to flat binary files. The software has been tested with most of the TRMM standard products, as well as with some derived subset products. Both C and Fortran versions are available from ftp://disc2.nascom.nasa.gov/software/trmm_software/Read_HDF/.

TSDIS Toolkit

TSDIS developed the TSDIS Science Algorithm Toolkit to assist the TRMM Science Team's algorithm developers. The toolkit provides a library of commonly used routines, constants, and macros. It also allows seamless integration of TRMM algorithms into the TSDIS environment.

The toolkit provides routines for reading and writing data to and from the HDF files; routines are provided for Levels 1-3 products and for both satellite and GV. Each of the routines in the toolkit are callable in either C or Fortran. The toolkit also includes routines for reading land/sea data and topographical data.

Data Access Information

The Goddard DAAC maintains archives of all TRMM data products and many other Hydrology data sets. The archived data can be ordered via FTP network transfer.

Data Volume Limits By Media
CDR		FTP-Pull
Min.	Max.	Min.	Max.	Min.	Max.	Min.	Max.
0 GB	3.17 GB	0 GB	2 GB	1 GB	50 GB	1 GB	50 GB

TRMM 1B01 can be accessed and ordered from
Goddard Earth Sciences Data and Information Services Center (GES DISC) TRMM Data Search and Order System at http://disc.sci.gsfc.nasa.gov/data/datapool/TRMM_DP/01_Data_Products/01_Orbital/01_Virs_Rad_1B_01/.

Points of Contact

For information about or assistance in using any Goddard DAAC data, contact the DAAC Help Desk at:: GES Distributed Active Archive Center (DAAC)
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771

Email: help-disc@listserv.gsfc.nasa.gov.
301-614-5224 (voice)
301-614-5268 (fax)

References

Tropical Rainfall Measuring Mission Science Data and Information System (TSDIS) Interface Control Specification Between the TSDIS and the TSDIS Science User (TSU)
Volume 3: File Specifications for TRMM Products - Level 1.
Volume 4: File Specifications for TRMM Products - Level 2 and Level 3.

Appendix

Solar Unit Vector
Name	Format	Description
Solar Position [3]	3*8-byte float	Sun Unit Vector (X-component) Sun Unit Vector (Y-component) Sun Unit Vector (Z-component) (Geocentric Inertial Coord)
Distance	8-byte float	Sun-Earth Distance (m)

Raw Calibration Counts Data
Dimension	Data Stored
1	Channel number
2	Data word
3	Blackbody, space view, solar diffuser
4	Number of scans

Local Direction Angles
Dimension	Data Stored	Description
1	zenith, azimuth	Zenith angle is measured between the local pixel geodetic zenith and the direction to the satellite. Azimuth angle is measured clockwise from the local North direction around toward the local East direction.
2	object	The object to which the directions point, namely the satellite and the sun
3	pixel number	Angles are given only for every tenth pixel along a scan: pixel 1, 11, 21, ..., and 261.
4	scan number	Scan line number

VIRS Range and Accuracy
Channel	Scale Factor	Minimum (mW cm^-2 µm^-1 sr^-1)	Maximum (mW cm^-2 µm^-1 sr^-1)	Accuracy
1	500	0	65.5	10%
2	1000	0	32.7	10%
3	100000	0	0.111	2%
4	10000	0	1.371	2%
5	10000	0	1.15	2%

TRMM Navigation
Name	Format	Description
Spacecraft Geocentric Position [3]	3 X 4-byte float	The position (m) of the spacecraft in Geocentric Inertial Coordinates at the Scan mid-Time (i.e., time at the middle pixel/IFOV of the active scan period). The order of components is: x, y, and z. Geocentric Inertial Coordinates are also commonly known as Earth Centered Inertial coordinates. These coordinates will be True of Date (rather than Epoch 2000 which are also commonly used), as interpolated from the data in the Flight Dynamics Facility ephemeris files generated for TRMM.
Spacecraft Geocentric Velocity [3]	3 X 4-byte float	The velocity (ms ^-1) of the spacecraft in Geocentric Inertial Coordinates at the Scan mid-Time. The order of components is: x, y, and z.
Spacecraft Geodetic Latitude	4-byte float	The geodetic latitude (decimal degrees) of the spacecraft at the Scan mid-Time.
Spacecraft Geodetic Longitude	4-byte float	The geodetic longitude (decimal degrees) of the spacecraft at the Scan mid-Time. Range is -180 to 179.999999.
Spacecraft Geodetic Altitude	4-byte float	The altitude (m) of the spacecraft above the Earth Ellipsoid at the Scan mid-Time.
Spacecraft Attitude [3]	3 X 4-byte float	The satellite attitude Euler angles at the Scan mid-Time. The order of the components in the file is roll, pitch, and yaw. However, the angles are computed using a 3-2-1 Euler rotation sequence representing the rotation order yaw, pitch, and roll for the rotation from Orbital Coordinates to the spacecraft body coordinates. Orbital Coordinates represent an orthogonal triad in Geocentric Inertial Coordinates where the Z-axis is toward the geocentric nadir, the Y-axis is perpendicular to the spacecraft velocity opposite the orbit normal direction, and the X-axis is approximately in the velocity direction for a near circular orbit. Note this is geocentric, not geodetic, referenced, so that pitch and roll will have twice orbital frequency components due to the onboard control system following the oblate geodetic Earth horizon. Note also that the yaw value will show an orbital frequency component relative to the Earth fixed ground track due to the Earth rotation relative to inertial coordinates.
Sensor Orientation Matrix [3 X 3]	3 X 3 X 4-byte float	The rotation matrix from the instrument coordinate frame to Geocentric Inertial Coordinates at the Scan mid-Time.
Greenwich Hour Angle	4-byte float	The rotation angle (degrees) from Geocentric Inertial Coordinates to Earth Fixed Coordinates.

TRMM VIRS Scan Status
Name	Format	Values	Description
Missing	1-byte integer	Value and meaning	Indicates whether information is contained in the scan.
Validity	1-byte integer	Bit and meaning	A summary of status modes.
QAC	1-byte integer	0: No decoding error occurred.	The Quality and Accounting Capsule of the Science packet as it appears in Level-0 data.
Geolocation Quality	1-byte integer	Bit and Meaning	A summary of geolocation quality in the scan.
Data Quality[5]	5 x 1-byte integer		The quality of Channel Data for a given channel on given scan line is the percentage of pixels whose value are within the acceptable range listed in the metadata.
Fractional Orbital Number	4-byte float		The orbit number and fractional part of the orbit at Scan Time. The orbit fraction part is calculated as: (Time-Orbit Start Time)/(Orbit End Time-Orbit Start Time)
Current Spacecraft Orientation	1-byte integer	Value and Meaning	Current spacecraft orientation.
Current ACS Mode	1-byte integer	Value and Meaning	Current ACS mode.
Yaw Update Status	1-byte integer	0: Inaccurate 1: Indeterminate 2: Accurate	Yaw update Status.
VIRS Instrument Status	1-byte integer	Bit and Meaning	VIRS instrument status.
VIRS mode	1-byte integer	Bit and Meaning	VIRS mode.
VIRS Abnormal Conditions	1-byte integer	Bit and Meaning	Bit 0 is the most significant bit.

Value and Meaning of Missing
Value	Meaning
0	Scan data elements contain information.
1	Scan was missing in the telemetry data.
2	Scan data contain no elements with rain.

Bit and Meaning of Validity
Bit	Meaning if bit=1
0	Spare (always 0)
1	Non routine spacecraft orientation (2 or 3)
2	Non routine RAC mode (other than 4)
3	Non routine yaw update status (0 or 1)
4	Non routine instrument status (other than 1)
5	Non routine QAC (non-zero)
6	VIRS in non-mission mode (non-zero)
7	VIRS condition is abnormal (non-zero)

Bit and Meaning of Geolocation Quality
Bit	Meaning if bit=1
0	Grossly bad geolocation results: * Spacecraft position vector magnitude outside range: pre-boost 6720 to 6740 km, and post-boost 6773 to 6793 km. * Z component of midpoint of scan outside range -4100 to 4100 km. * Distance from S/C to midpoint of scan outside range: pre-boost 340 to 360 km, and post-boost 393 to 413 km.
1	Unexpectedly large scan to scan jumps in geolocated positions in along and cross track directions for first, middle, and last pixels in each scan. Allowed duration from nominal jump in along track motion = 0.06 km (first pixel), 0.04 km (middle pixel), and 0.06 km (last pixel). Allowed duration from nominal jump in cross track motion = 0.05 km (first pixel), 0.04 km (middle pixel), and 0.05 km (last pixel). Bit set in normal mode only.
2	Scan to scan jumps in yaw, pitch, and roll exceed maximum values. Values are : yaw = 0.0001 radians; pitch = 0.0001 radians; roll = 0.0001 radians. Bit set in normal control mode only.
3	In normal mode, yaw outside range (-0.003, 0.003) radians; pitch outside range (-0.007, 0.007) radians; roll outside range (-0.007, 0.007).
4	Satellite undergoing maneuvers during which geolocation will be less accurate.
5	Questionable ephemeris quality (including use of predicted Ephemeris for quicklook) or questionable UTCF quality.
6	Geolocation calculations failed (fill values inserted in the per pixel geolocation products, but not in metadata).
7	Missing attitude data. ACS data gap larger than 20 seconds.

Bit and Meaning of Data Quality
Bit	Meaning if bit=1
0	Missing
5	Geolocation quality is not normal
6	Validity is not normal
Note: Unless this is 0 (normal), the scan data are meaningless to higher processing. Bit 0 is the least significant bit (i.e., if bit i=1 and other bit = 0, the unsigned integer value is 2**i).

Value and Meaning of Current Spacecraft Orientation
Value	Meaning
0	+x forward
1	-x forward
2	-y forward
3	Inertial -- CERES Calibration
4	Unknown Orientation

Value and Meaning of Current ACS Mode
Value	Meaning
0	Standby
1	Sun Acquire
2	Earth Acquire
3	Yaw Acquire
4	Nominal
5	Yaw Maneuver
6	Delta-H (Thruster)
7	Delta-V (Thruster)
8	CERES Calibration

Value and Meaning of VIRS Instrument Status
Value	Meaning
0	Day (no calibration occurring)
1	Night
2	Monitor Scan Stability
3	Day with Calibration

Value and Meaning of VIRS Mode
Value	Meaning
0	Missing Mode
1	Safehold Mode
2	Outgas Mode
3	Activation Mode

Value and Meaning of VIRS Abnormal Conditions
Bit	Value	Meaning
0	0 1	Normal Scan phase error
1	0 1	Normal Selftest error
2	0 1	Normal Thermal data missing
3	0 1	Normal Moon in space view
4	0 1	Normal H/K data drop-out suspected
5	0 1	Normal SV counts for channel 4 or 5 great than L1B01_MIN_DNSV
6	0	Not used
7	0	Not used

TRMM Frequency Analysis Result* (Number of visits for a 30-day period)
Radar Site	Latitude (°)	PR (~215 km)	VIRS (~720 km)	TMI (~760 km)
Kwajalein Atoll	8.72	9	29	31
Darwin, Australia	-12.45	10	31	32
Guam	13.50	9	32	32
Om Koi, Thailand	17.80	9	31	33
Kauai, HI	22.17	13	36	38
Sao Paolo, Brazil	-23.58	12	41	42
Taiwan	23.92	11	40	42
Key West, FL	24.67	13	41	42
Miami, FL	25.75	13	45	45
Brownsville, TX	25.97	13	43	47
Corpus Christi, TX	27.85	15	49	51
Tampa, FL	28.03	13	51	52
Melbourne, FL	28.10	15	49	53
San Antonio, TX	29.53	16	57	59
Jacksonville, FL	30.33	19	63	65
Texas A&M, TX	30.58	18	67	68
Jerusalem, Israel	31.87	20	92	102
PR: Precipitation Radar VIRS: Visible/InfraRed Scanner TMI: TRMM Microwave Imager * This analysis result was derived based on TRMM pre-boost orbital information. The revisit frequency should be slightly higher after TRMM boost (August 24, 2001). If you have questions regarding this table, please send email to: helpdesk@tsdis02.nascom.nasa.gov.

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Last updated: April 08, 2008 18:54:59 GMT