GMI

Introduction

The Global Precipitation Measurement (GPM) satellite has a microwave radiometer onboard called GMI (GPM Microwave Imager).  The GPM satellite was launched on February 27th, 2014 with the GMI instrument started a few days later.  One of the primary differences between GPM and other satellites with microwave radiometers is the orbit, which is inclined 65 degrees, allowing a full sampling of all local Earth times repeated approximately every 2 weeks. The GPM platform undergoes yaw maneuvers approximately every 40 days to compensate for the sun's changing position and prevent the side of the spacecraft facing the sun from overheating.

Instrument Description

GMI is a dual-polarization, multi-channel, conical-scanning, passive microwave radiometer with frequent revisit times.

The GMI instrument was designed with a strict calibration accuracy requirement to a greater precision than any previous microwave satellite sensor, thereby enabling the instrument to serve as a microwave radiometric standard.  Several features of GMI provide the higher calibration accuracy, including protection of the hot load from sun intrusion, noise diodes on the low-frequency channels for a dual calibration system, and a reflective antenna coating.  These features allow us to be very confident of the quality and accuracy of the GMI data we provide here.  More information about GMI is available in the paper by D. Draper et al. paper listed in the reference section below.  Some details of the GMI instrument are listed here:

GMI  
Satellite Platform GPM
Altitude 407 km
Equator Crossing Time
(Local time zone)
varying
Antenna Size 1.22 m
Swath Width 931 km
Orbit Inclination 65 deg
Off-Nadir View Angle 48.5 deg

 The thirteen instrument channels are summarized in the following table.

Band [GHz] Polarization

Spatial Resolution
(3-dB footprint size)
[km x km]

10.65 V,H 32 x 19
18.7 V,H 18 x 11
23.8 V 16 x 10
36.5 V,H 15 x  9
89.0 V,H   7 x  4
165.5 V,H   6 x  4
183.31+/-3 V   6 x  4
183.31+/-7 V   6 x  4

We obtain instrument data from NASA Goddard Earth Sciences Data and Information Services Center, back process the data to raw counts and apply our own on-orbit calibration to convert counts to brightness temperatures. The brightness temperatures are intercalibrated with the RSS Radiative Transfer Model (RTM). We then produce the typical RSS microwave radiometer ocean measurement product suite consisting of: Sea Surface Temperature (SST), Surface Wind Speeds (low and medium frequency), Atmospheric Water Vapor, Cloud Liquid Water, and Rain Rate.  Details are given in the Meissner et al. report. The GMI data are processed with the RSS Version-8.2 algorithm and RTM.  The V8 RTM provides slightly improved brightness temperature calibration over the V7 RTM; however, the ocean products from V7 and V8 algorithms are highly consistent.  Therefore, we consider the V8 products to be comparable to the V7 products from WindSat, AMSRE, AMSR2 and the SSM/I and SSMIS series.  

RSS GMI Data Products

Most microwave radiometer ocean data products at RSS are Version-7, with the decimal values representing the processing stages for that instrument, starting with .1.   The GMI data have been produced using an updated RTM, Version-8.  The V8 brightness temperatures from GMI are slightly different from the V7 brightness temperatures; however, there are essentially no differences between the V7 and V8 ocean products.  The GMI V8.2 data are available from April 3, 2014 to the present.   We continue processing GMI data in near-real time as they arrive, with a  delay of about 3 to 6 hours.

We produce daily binary data files and time-averaged (3-day, weekly and monthly) data files. The daily files consist of over-ocean measurements (geophysical retrievals or parameters) mapped to a regular 0.25-degree grid complete with data gaps between orbits. Two maps exist for each parameter, one of ascending orbit segments and the other of descending orbit segments. Data on each of the segment maps are averaged where successive orbits cross and are overwritten at the "seam" or region where the last orbit of the day overlaps the first orbit of the day. Daily data files contain time maps consisting of the UTC observation time for each set of passes (ascending and descending). Time-averaged data files do not contain any time information.

Gridded data are organized according to observation date. All dates and times are Coordinated Universal Time (UTC), also known as Greenwich Mean Time (GMT), Zulu Time (Z), Universal Time (UT), and World Time. Data products include daily and time averaged geophysical data as follows:

Daily orbital data mapped to 0.25 degree grid divided into 2 maps based on ascending and descending passes, earlier data averaged by successive orbit or overwritten at daily "seam"
3-Day average of 3 days ending on and including file date
Weekly average of 7 days ending on and including the Saturday file date
Monthly average of all data within the calendar month

Each binary data file available from our ftp site consists of fourteen (daily) or six (averaged) 0.25 x 0.25 degree grid (1440,720) byte maps. For daily files, seven ascending maps in the following order, Time (UTC), Sea Surface Temperature (SST), 10 meter Wind Speed using the low frequency channel (WSPD-LF), 10 meter Wind Speed using the medium frequency channel (WSPD-MF), Atmospheric Water Vapor (VAPOR), Cloud Liquid Water (CLOUD), and Rain Rate (RAIN), are followed by seven descending maps in the same order. Time-averaged files contain just the geophysical layers in the same order [SST, WSPD-LF, WSPD-MF,VAPOR, CLOUD, RAIN].

Acronym Product
Name
Product
Description
Scale Offset Valid Data Range Reason for No Data
TIME Time

Minutes since midnight GMT
Fractional hour of day GMT

6.0
0.1

0.
0.

0 to 1440
0.0 to 24.0

no data
SST Sea surface temperature Temperature of top layer (skin) of
water ~1 mm thick
0.15 -3.0 -3 to 34.5 deg high winds (<20 m/s), sun glint, rain, RFI, near sea ice or land 
WSPD_LF 10-m wind speed Wind speed using 10.7, 18.7, 23.8, and 36.5 GHz channels 0.2 0. 0. to 50.0 m/s sun glint, rain, RFI, near sea ice or land 
WSPD_MF 10-m wind speed Wind speed using 18.7, 23.8, and 36.5 GHz channels 0.2 0. 0. to 50.0 m/s sun glint, rain RFI, near sea ice or land
VAPOR Columnar atmospheric water vapor Total gaseous water contained in a
vertical column of atmosphere
0.3 0.

0. to 75.0 mm
1 gm/cm2= 10mm

heavy rain or near land 
CLOUD Columnar cloud liquid water Total cloud liquid water contained in a
vertical column of atmosphere
0.01 -0.05 -0.05 to 2.45 mm near land 
RAIN Rain rate Rate of liquid water precipitation 0.1 0. 0. to 25.0 mm/hr near land

The data values between 0 and 250 need to be scaled to obtain meaningful geophysical data. To scale the data, multiply by the scale factors listed in the table above.  Read routines provided perform the scaling.

 

The daily, 3-day and monthly maps are stored in appropriate year and month subdirectories. The weekly data files are stored in the /weeks directory.

The file names have the following naming conventions:

Time Directory Path File Name
Daily [year]/[month]/ fs_yyyymmddvv.gz
3-Day [year]/[month]/ fs_yyyymmddvv_d3d.gz
Weekly weeks/ fs_yyyymmddvv.gz
Monthly [year]/[month]/ fs_yyyymmvv.gz

Where "fs",  "yyyy", "mm", "dd", and "vv" stand for:

fs file specifier f35

yyyy 

 year 2014, 2015 etc.
mm   month 01 (Jan), 02 (Feb), etc.
dd  day 01, 02, etc.
vv version V.v

The file specifier is an internal numbering code used by RSS to reference the large array of satellite sensors.  The file specifier for GMI is f35.  The GMI filenames all begin with f35.  File specifiers for the thirteen other RSS microwave radiometers is provided in our data access table.  The filenames for all radiometer data files begin with f## to identify that sensor.

The center of the first cell of the 1440 column and 720 row map is at 0.125 E longitude and -89.875 latitude. The center of the second cell is 0.375 E longitude, -89.875 latitude. The data values fall between 0 and 255. Specific values have been reserved:

0 to 250 =  valid geophysical data
251 = missing SST or wind speed due to rain, or missing water vapor due to heavy rain
252 = sea ice
253 = observations exist, but are bad (not used in composite maps)
254 = no observations
255 = land mass

 

Missing Data

There are gaps within these data. Missing data generally affect Daily and 3-Day products, but can also reduce the number of observations in Weekly and Monthly averages.

When browsing imagery, the navigation may skip dates with no data, or you may see a blank map stating that no data are available for that time.

Binary data files for dates with completely missing data are not produced; they will be absent from our FTP server.

Data gaps are generally due to missing data upstream from our processing facility, such as the instrument being turned off. Occasionally, there are delays in obtaining and/or processing recently recorded data; beyond several weeks, it is unlikely that missing data will become available.  For information on a current outage, be sure to look at the web page announcements.

Dates for which GMI data are completely missing include:

date range # days
2014.04.27 1
2014.10.23 1

Browse Images

Each daily, 3-day, weekly and monthly browse image map displays one geophysical parameter: Sea Surface Temperature (SST), 10 meter Wind Speed (WSPD-LF), 10 meter Wind Speed (WSPD-MF), Columnar Water Vapor (VAPOR), Cloud Liquid Water (CLOUD), or Rain Rate (RAIN). The daily maps display the daytime or nighttime satellite passes separately. The date of the data displayed is the UTC date when the data were collected. The scale for each browse image is located next to the map for reference. Though the valid data range (minimum to maximum) is given in the geophysical variable table above, the scale bars in the browse images are set to visually enhance the data and may vary.

Read Routines

Binary file read routines and verification files are available on our ftp server in the gmi/support directory.  Routines are written in IDL, Matlab, Fortran, C++ and Python.

Related Data

There is no alternative source of RSS GMI ocean data products. 

 

References

Draper, D. W., D. Newell, F. J. Wentz, S. Krimchansky, and G. M. Skofronick-Jackson, 2015: The Global Precipitation Measurement (GPM) Microwave Imager (GMI): Instrument overview and early on-orbit performance. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. doi:10.1109/JSTARS.2015.2403303.

Meissner, T., F. J. Wentz, and D. Draper, 2012: GMI Calibration Algorithm and Analysis Theoretical Basis Document, Remote Sensing Systems, Santa Rosa, CA, report number 041912, 124 pp.

Acknowledgements

GMI data are produced by Remote Sensing Systems and sponsored by the NASA Physical Oceanography Project.

How to Cite These Data

Continued production of this data set requires support from NASA.  We need you to be sure to cite these data when used in your publications so that we can demonstrate the value of this data set to the scientific community.  Please include the following statement in the acknowledgement section of your paper:

"GMI data are produced by Remote Sensing Systems and sponsored by NASA Earth Science funding. Data are available at www.remss.com. "

An official data citation for use in publications is given below.  Insert the appropriate information in brackets.

Wentz, F.J., T. Meissner, J. Scott, K.A. Hilburn, 2015:  Remote Sensing Systems GPM GMI [indicate whether you used Daily, 3-Day, Weekly, or Monthly]  Environmental Suite on 0.25 deg grid, Version 8.2, [indicate subset if used]. Remote Sensing Systems, Santa Rosa, CA. Available online at www.remss.com/missions/gmi. [Accessed dd mmm yyyy].

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