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NOAA/NASA Special Sensor Microwave/ Imager (SSM/I) Pathfinder Daily Precipitation Rate Data

Online Access for SSM/I Pathfinder Daily Precipitation Rate Data Sets

Volume 1
Volume 2


Original Archive
Future Updates
Data Set Description
Data Characteristics
Data Format
Sample Software
Data Access
Anonymous FTP


Points of Contact


This README file contains a description of the Special Sensor Microwave/ Imager (SSM/I) Daily Precipitation data set, including information on the file structure,pertinent scientific references, tools for reading the data, and contactinformation for obtaining the data. This data set is one of the NOAA/NASA Pathfinder Program products.Thedata set currently includes precipitation estimates over both land andocean derived from the SSM/I instrument flown aboard the Defense Meteorological Satellite Program (DMSP) F-8 platform. The precipitation rate is also referred to as the rain rate.


The distribution of this data set is being funded by NASA's Mission To Planet Earth program. The data are not copyrighted; however, we request that when you publish data or results using these data please acknowledge as follows:

The authors wish to thank the Distributed Active Archive Center (Code 610.2) at Goddard Space Flight Center, Greenbelt, MD, 20771, for distributing the data; and the science investigators Drs. Robert Adler and George Huffman, Code 912, NASA Goddard Space Flight Center, Greenbelt, Maryland20771, and Mr. Michael Goodman, NASA Marshall Space Flight Center, Huntsville, Alabama35806 for producing these data products. Goddard's contribution to these activities were sponsored by NASA's Mission to Planet Earth program.

Original Archive

This dataset was originally archived at Marshall Space Flight Center. It was moved to the Goddard Space Flight Center Distributed Active Archive Center (GSFC DAAC) in the fall of 1996.

Future Updates

There are no plans for the Goddard DAAC to receive updated level 2 daily precipitation products from the data producers.

Data Set Description

The NOAA/NASA SSM/I Pathfinder Precipitation Rate (PR) data sets are created using the SSM/I Pathfinder daily HDF Antenna Temperature (TA) files.

Instantaneous global (over land and ocean) precipitation rates are calculated using the Goddard Scattering Algorithm, Version 2 documented in Adler et al. (1991, 1993, 1994). The 19 GHz horizontal and vertical, 22 GHz vertical, and 37 GHz horizontal antenna temperatures are interpolated to the higher resolution of the 85 GHz horizontal channel. These temperatures and the 85 GHz horizontal temperatures are subjected to a discrimination process in which each pixel is tested for the presence of precipitation or no precipitation. At pixels with precipitation, the precipitation rates are calculated using a numerical model-based regression equation.

The algorithm developers strongly suggest some method of pixel weighting be used when interpolating the precipitation rates to a grid. This is because of the overlap of the pixels changes depending on its location within the scan.

Each daily Precipitation Rate file begins with the first A scan after 00:00:00 UTC and contains all data up to 23:59:59 UTC.Each file contains precipitation rate (PRT), confidence/surface type values (CST), latitudes (LAT), longitudes (LON),scan start times (SST), orbital elements (ORB), and a file description.See Data Format for a description of the file structure.

Data Characteristics


Parameters/Units Precipitation Rate (mm/hr * 10 )
Confidence/Surface Type ( N/A )
Latitude ( deg * 100 )
Longitude ( deg * 100 )
Scan Start Time ( seconds of day )
Orbit Parameters ( see Table 8 )
Temporal Coverage August 1987 - December 1988
(some data gaps)
12.5 km

For various reasons (e.g., instrument turned off due to overheating), daily precipitation files were not created for the following periods:


  • December 3, 1987 - January 12, 1988
  • October 6, 1988
  • May 6, 1988 - May 8, 1988
  • September 23, 1988
  • December 25, 1988 - December 27, 1988

The physical file characteristics for the daily Pathfinder precipitation files are as follows:


48 MB (uncompressed)
~ 20 MB (compressed)
Hierarchical Data Format (HDF)


  • YY is the 2 digit year (i.e., 87 or 88)
  • DDD is the day of the year

It should be noted that the data are actually distributed to users as compressed files using the standard UNIX "compress" command; thus, the suffix ".Z" will be appended to the names of all files.

Data Format
The files were created on a Silicon Graphics VGX class computer with version 3.3, release 4 of the HDF library. The file size for an uncompressed file is 48 megabytes.

NOTE: All discussions in this text are for row major applications written in c. If you are using the HDF FORTRAN interface to read the HDF objects, the arrays will be transposed. For example, array A(5,3) in an HDF c interface would become A(3,5) in an HDF FORTRAN interface. This will apply to all HDF data array discussions in this text.

Table 1 shows the contents (HDF objects) of each daily precipitation rate file:





Item HDF Object Type HDF Ref.
Version Descriptor N/A 1
Precipitation Rate Scientific Data Set 2
Confidence/Surface Type Scientific Data Set 3
File Description Annotation 4
Latitude Scientific Data Set 5
Longitude Scientific Data Set 6
Scan Start Times Scientific Data Set 7
Orbit Parameters Scientific Data Set 8
Precipitation image Raster Image Group 9

The DMSP satellite completes just over 14 orbits in a day.An orbit is defined as starting when the satellite crosses the equator going from south to north.Because the Pathfinder PR files are organized by time, it is not uncommon to have a fractional part of an orbit prior to the first full orbit in the daily PR file and a fractional part of an orbit following the last full orbit in the daily PR file.To accommodate this, arrays were set up that store the orbits "side-by-side" from left to right as shown in Table 2 below:

                                 Table 2

                      Orbit Arrangement in Arrays

ORBIT  -->
 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16|
 |   |   |   |   |   |   |   SCAN 1 for each orbit |     |    |    |   |
 |   |   |   |   |   |   |   SCAN 2 for each orbit |     |    |    |   |
S|   |   |   |   |   |   |   SCAN 3 for each orbit |     |    |    |   |
C|   |   |   |   |   |   |   SCAN 4 for each orbit |     |    |    |   |
A|   |   |   |   |   |   |   SCAN 5 for each orbit |     |    |    |   |
N|   |   |   |   |   |   |             o           |     |    |    |   |
 |   |   |   |   |   |   |             o           |     |    |    |   |
 |   |   |   |   |   |   |             o           |     |    |    |   |
 |   |   |   |   |   |   |   SCAN N for each orbit |     |    |    |   |

The first orbit may be a fractional part of an orbit.Its first scan contains the first data for the 24 hour period.That scan may originate at a point other than the beginning of a true DMSP orbit.Following from left to right are up to 15 more orbits.The last orbit to the right may be a fractional portion of a DMSP orbit.If so, it begins before time 23:59:59 but does not complete.The remainder of the orbit will appear as orbit 1 in the daily PR file for the following day.The last DMSP orbit, or fraction of an orbit, may appear as orbit 15 or 16. Each orbit or portion of an orbit is separated from the next by a delimiter column for all objects except the scan start times and the orbit parameters.See Table 4a for the value used as a delimiter in each object.

For each orbit, including the partial orbits, the data has been located in the proper scan position.Any missing scans are identified by missing data flags (see descriptions below for these flag values).This means that if the second orbit of the day is missing there will be a missing data flag in orbit position 2.For the partial orbit at the beginning of the day, any data that falls on the previous day (before 00:00:00 UTCof current day) will not be present in this file, and the scan position will be filled with a missing data flag(see Data Format for a explanation of the exceptions to this). Likewise, for any last partial orbit, the data after 23:59:59 UTC will be flagged as missing for this day but will appear on the next day's file.

The beginning DMSP orbit (or fraction of an orbit)number and ending DMSP orbit (fraction) number can be found by reading the file description stored with each daily PR file.Use the getfiledesc_pr utility described in section 6.0 to get this information from the file.

The number of scans is 3224 for all Pathfinder SSM/I HDF objects except the orbit parameters (see section 5.2).The "width" or number of columns in each product array is 2064.This is calculated from the following:


  • Each scan has 128 samples.
  • There are 14 orbits plus 2 fractions for a total of 16.
  • The orbits are separated by a delimiter column.
  • Thus (128 samples + 1 delimiter) * 16 orbits = 2064 columns.

The valid data and delimiter columns are shown in the Table 3 below:





Orbit Data Columns Delimiter Columns
1 001 - 128 129
2 130 - 257 258
3 299 - 386 387
4 388 - 515 516
5 517 - 644 645
6 646 - 773 774
7 779 - 902 903
8 904 - 1031 1032
9 1033 - 1160 1161
10 1162 - 1289 1280
11 1291 - 1418 1419
12 1420 - 1547 1548
13 1549 - 1676 1677
14 1678 - 1805 1806
15 1807 - 1934 1935
16 1936 - 2063 2064

For example, to get PRT for orbit 2 you would use an HDF utility to read the precipitation rate HDF object into a 3224 x 2064 array, then copy columns 130 through 257 into another array dimensioned 3224x128.MSFC has software available to separate the HDF objects into orbits.The software, getorbpr is described in See Sample Software .

Table 4a shows the flags used in the geophysical products objects and column delimiter.Valid calculated values of the geophysical product are scaled before being stored.Table 4b shows the scale factor.For example, the precipitation rates are multiplied by 10.0 and then stored as integers, so to retrieve a precipitation rate you must divide the stored number by 10.0.





PRT -10 - 20 -30 -40 -50 -10





  All Surface Types
PRT PRT * 0.0



PRT =Precipitation Rates (0.0 - 100.0 mm/hr.)
FLAG 1=Missing data.
FLAG 2=Erroneous input TA data; either out of bounds or mislocated.
TA values not between 50 and 315 degrees K are considered as out of bounds.
FLAG 3=The GSCAT-2 algorithm flagged the 85H antenna temperature as "bad".
FLAG 4=This pixel could not be interpolated from one of the low resolution arrays.
FLAG 5=The pixel is located within a scan which failed one of the scan tests
(either the scan average check or the number of out of bounds was too high.

The following sections provide further details on each of the HDF precipitation rate objects. Each of these objects can be extracted from the HDF file using special tools whichthe DAAC provides with the data. See Sample Softwarefor instructions on how to get the HDF library and to compile and create these extraction utilities. The following sections provide further details on each of the HDF precipitationrate objects.

GSCAT-2 Precipitation Rate (PRT):

This HDF object contains the precipitation rates.Each value is stored as a 2 byte integer.The values are multiplied by 10 before storing to retain a precision of 0.1 mm/hr.For example, 31.5 mm/hr. is stored as 315.The minimum value for precipitation rate is 0, and the maximum value is 100.Table 5 shows the range of stored values. Flagged values are stored in locations where the precipitation rate could not/should not be calculated. The flagged values are shown in Table 4a.






< 0 > Flagged data
0 - 1000 Precipitation Rate (mm/hr)

The HDF data type used to store the precipitation rate is DFNT_INT16. It will be necessary to use this type in any software written to access the data.The HDF reference number is 2.

Confidence/Surface Type Values (CST):

This HDF object contains labeled values from the discrimination part of the algorithm.Each value is stored as a 1 byte integer and are at full resolution (array size 3224 x 2064).Certain conditions can lead to precipitation rates which are of lower confidence; i.e. although they can not be absolutely defined as raining pixels, there is not a scientific reason to mask them as non raining pixels.These pixels are labeled as ambiguous.

The CST values may also be used to determine the surface type associated with a given pixel. The surface types are a modified version of the Fleet Numerical Oceanographic Center (FNOC) database.The original land/vegetated land classes concatenated into the value 0.The ice/possible ice have been concatenated into the value 4.The database was also modified through the use of a GEMPAK map of the CIA high resolution coastline map, with the coastline thickened to about 75 km in width.An additional surface type named "Coast-Ice" was added; the labeled value is 7. Table 6 gives the labeled values for CST.






Surface Type Precipitation Rate is good
Surface Type + 10 Precipitation Rate is ambiguous
Surface Type + 20 No Precipitation Rate; Cold surface
Surface Type + 100 No Precipitation Rate but good surface type exists.
This occurs when the antenna temperatures are flagged but the geolocation is good
110 No Precipitation Rate or Surface Type available

The HDF data type used to store these values is DFNT_INT16.The HDF reference number is 3.

Latitude Values (LAT):

The latitude values are stored as signed 2 byte integers ranging from -9000 to 9000 (-90.00 to 90.00 degrees).The values are negative south of the equator and positive north of the equator.There is a value for every A/B pixel.Thus the array dimensions are 3224 x 2064; there are 3224 scans, each orbit has 128 samples per scan, and the orbits are separated by a delimiter column.The precision of the latitudes is to 0.01 degrees.Thus a value of 7524 means 75.24 degrees North latitude. Missing scans are denoted by the value -29999.Erroneous or mislocated scan latitudes have 200 subtracted from them before being stored.To retrieve an erroneous or mislocated latitude divide the stored value by 100 and then add 200 to the result, as shown in Table 7 below.






LAT LAT*100.0 LAT*100.0 -29999 (LAT- 200)*100.0 -10
LON LON*100.0 LON*100.0 -18999 LON*100.0 -10
SST SST*1.0 99999.9 -189.99 SST*1.0 N/A



LON =Longitude in degrees
SST =Scan start time in seconds of day
FLAG 6=Out of bounds
FLAG 7=Missing
FLAG 8=Erroneous/mislocated

Note that the Pathfinder latitudes were updated to reflect the geolocation corrections for yaw, along-track, and pixel 128 identified by Mr. Frank Wentz of Remote Sensing Systems.The HDF data type used with this object is DFNT_INT16.The HDF reference number is 5.

Longitude Values (LON):

Similar to the latitude values described above, the longitudes are stored as signed 2 byte integers ranging from -18000 to 18000 (-180.00 to 180.00) degrees.The values are negative West of the Prime Meridian and positive East.Missing scans are denoted by a value of -18999. Erroneous or mislocated scans are not tagged since that information can be extracted from the latitude object.See the previous table for theLatitude object for the stored longitude values.The HDF object is otherwiseorganized exactly as that for the latitudes described above.The HDF referencenumber is 6.

Note that the Pathfinder longitudes were updated to reflect the geolocation corrections for yaw, along-track, and pixel 128 identified by Mr. Frank Wentz of Remote Sensing Systems.

Scan Start Times (SST):

The scan start times for both the A- and B-scans are stored in this HDF object. The times are stored in seconds of day (0.0 to 86399.9).Each value is stored as a 4 byte real number (floating point).Any missing scans are identified by -189.99.Any out of bounds values are flagged by 99999.9.

Since there are 16 orbits per day and as many as 3224 scans per orbit, the array for SST is 3224 x 16.There are no delimiter columns in this object.The A-scan start times are derived by subtracting 1.9 seconds from the B-scan start times since the antenna temperatures are originally tagged with only the B-scan start times. Note that the B- scan start times were rounded to the nearest second for the period beginning 1 August 1987 and ending on orbit 5 of 16 September 1987. There is the possibility that the first A-scan encountered in the file may actually have occurred on the previous day.If this is the case, the scan start time for the first scan will be between 86398.1 and 86399.9, and it is possible there is a scan near the end of the file with an A-scan start time equal to or greater than the first scan time.

The HDF data type used with this object is DFNT_FLOAT32 and the HDF reference number is 7.

HDF File Annotation:

This object is an ASCII description of the daily PR file. It has the file name, satellite name, Julian date, beginning orbit number, ending orbit number, software version number, file structure version number, HDF version number, MSFC tool set version number, and the email address and phone number for MSFC User Services.The description can be read with the supplied program "getfiledesc_pr", after you compile it with the HDF library.See See Sample Softwarefor instructions on how to get the HDF library. The HDF object referencenumber is 4.

An example of a file description is:


      SSM/I Adler Rain Rates
     Satellite = F8
     Julian Date = 88080
     Beginning Orbit =    3868
     Ending Orbit =       3882
     Time Of First Scan (hhmmss) = 000001
     Time Of Last Scan (hhmmss) =  235959
     SSM/I PATHFINDER Software Version Number 1.0
     MSFC File Structure Version Number 1.0
     HDF Version Number 3.3
     MSFC Tool Set Version Numbers:
          extractpr      1.0
          getorbpr       1.0
          getfiledesc    2.0

     For more information about this file please contact
     Marshall Space Flight Center User Services.
     Phone: 205-922-5932
     email address:

     For more information about this file please contact
     Marshall Space Flight Center User Services.
     Phone: 205-922-5932
     email address:


  • Please note that these files were generated and archived at MSFC. Recently the SSM/I Pathfinder precipitation data set was moved to the Goddard DAAC. The filedescription written into the HDF annotation for each file has not been updated for thenew point of contact and User Services email address,since it is anticipated thatthis data set will be replaced with a new version produced at GSFC by Drs. Adler andHuffman.
  • The file description in the Pathfinder HDF file refers to the SSM/I Adler Rain Rate. After the HDF files were generated, the name was changed to Precipitation Rate, which were created using the GSCAT-2 algorithm.

Orbit Parameters:

The last HDF object is the Orbit Parameters object.This object contains a set of classical orbital elements derived from the two-line elements for the DMSP satellite and time tagged to 00:00:00 UTC.The file format is shown in Table 8 below:






1 Satellite ID (08, 10, or 11)
2 Julian Day of Data (YYDDD)
3 Epoch Time of Elements (DD.DDDD)
4 Inclination Angle Degrees
5 Right Ascension Degrees
6 Eccentricity N/A
7 Argument of Perigee Degrees
8 Mean Anomaly Degrees
9 Mean Motion Orbits/day
10 Semi Major Axis Kilometers
11 Period Seconds/orbit

Sample Software

Three C programs are included with this distribution. They are extractpr.c, getfiledesc_pr.c, and getorbpr.c. Both run on the Silicon Graphics butshould port easily to other platforms. A makefile, named, is included with the distribution. It can be used to compile the three programs on a UNIX platform. You must have the HDF library (HDF3.3r3 or higher) installed on your system before attempting to run this makefile. Information and instructions on how to obtain this library can be found on the Goddard DAAC HDF Information Page.

After making changes to the directory paths and flags in, use thefollowing UNIX command to run the makefile program:


make -f all clean

This will compile both programs, delete the object modules, and produce the followingthree executables:


extractrg , getorbpr , getfiledesc

Extractpr.c will be the most useful.This program extracts an HDF object from the daily PR file.It produces a new HDF file with the name of the HDF object selected.The format for using the utility is as follows:


extractpr < Daily HDF filename > < list of HDF objects >

For example, the command


extractpr rr08mi88.080_Pfndr_daily.hdfPRT

produces a new HDF file called PRT.88080 from the file rr08mi88.080_Pfndr_daily.hdf. Run the program with no arguments, and it will list all of the options as shown below:



Acronym File Object Type Variable Type
Precipitation Rate SDS INT16
CST Values SDS INT16
Latitude SDS INT16
Longitude SDS INT16
Scan Start Time SDS FLOAT32
Orbital Elements SDS FLOAT32

where SDS is Scientific Data Set, INT32 is a 32 bit integer, and FLOAT32 is a 32 bit floating point numbers. Use the 3-letter parameter code when selecting an object to extract.You may select several codes at once, separated by a space.

The getfiledesc_pr program prints the text of the HDF annotation contained in the daily file.Its usage is:


getfiledesc_pr < Daily HDF filename >

The result should look like the example HDF file annotation dump presented earlier.

The getorbpr program extracts an orbit for any object from the daily PR file.It produces a new HDF file with the name and orbit number of the HDF object selected.Usage is as follows:


getorbpr< Daily HDF filename >< HDF object >< orbit #(1-16) >

For example,


getorbpr rr08mi88.080_Pfndr_daily.hdf PRT05

produces a new HDF file called PRT05.88080 from the original file rr08mi88.080_Pfndr_daily.hdf.


Adler, Robert F., H.-Y. M. Yeh, N. Prasad, W.-K. Tao and J. Simpson, 1991, Microwave Simulations of a Tropical Rainfall System with a Three-Dimensional Cloud Model, Journal of Applied Meteorology, vol. 30, pp. 924-953.

Adler, Robert F., A. J. Negri, P. R. Keehn, and I. M. Hakkarinen, 1993, Estimation of Monthly Rainfall over Japan and Surrounding Waters From A Combination of Low-orbit Microwave and Geosynchronous IR Data, Journal of Applied Meteorology, vol. 32, pp. 335-356.

Adler, Robert F., G. J. Huffman, and P. R. Keehn, 1994, Global Tropical Rain Estimates From Microwave-adjusted and Geosynchronous IR Data, Remote Sensing Reviews.

Data Access


Anonymous FTP
The SSM/I Pathfinder precipitation data set also reside on-line at the Goddard DAAC anonymous FTP site and may be accessed either directly from this document,

Link to SSM/I Pathfinder daily data on ftpSSM/I Pathfinder Precipitation Daily Data


or via anonymous FTP at


password:< your internet address >

Points of Contact

Data Investigators:

The investigators of this data set can be contacted as follows:


Dr. Robert Adler
Code 613.1
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
301-286-1762 (Fax)


Dr. George Huffman
Code 613.1
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
301-286-9785 (Voice)
301-286-1762 (Fax)

Data Producers:
The producer of this data set can be contacted as follows:


Mr. Michael Goodman
NASA Marshall Space Flight Center
Huntsville, Alabama35806
301-922-5890 (Voice)
301-922-5723 (Fax)
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