RADS Utilities

The Radar Altimeter Database System (RADS) was developed by the Delft Institute for Earth-Oriented Space Research, the NOAA Laboratory for Satellite Altimetry, and EUMETSAT. RADS provides a suite of tools or utilities that simplify the reading, editing and handling of data from multiple satellite radar altimeters. These tools abstract the user from having to work directly with the NetCDF datasets. Updates and corrections usually are performed to the data files, allowing the software tools to remain unchanged.

The Radar Altimeter Database System is composed of three elements:

1. Database: ~700GB gigabytes of altimeter data files (routinely synchronized)
2. Software Utilities: A set of software tools (object library and executables);
3. Configuration files: XML file used to describe altimeter missions, variables, and interim variables

• Examples: Command line example of product generation
• Manuals : User and Data Manuals

Once the files, software, and configuration files are local to the user's sytem, setup of the environmental variables is required prior to using the system.

Data Files

RADS data are a collection of NetCDF files, each containing about a half-orbit (ascending/descending) of satellite data. The collection includes data from both past and current satellite missions and are updated twice per day by NOAA and EUMETSAT. The up-to-date set of files is what is referred to as the database.

The satellite pass files are stored as NetCDF with Climate Forecast conventions. Data parameters are dimensioned as a time-based trajectory and typical file-size varies between about 400KB to 1MB. Filenames include abbreviated satellite name, pass number, and cycle number (SSpPPPPcCCC.nc). Passes are sequential, assigning ascending passes with odd numbers and descending with even. These reset each cycle.

Data files are organized into directories for each cycle, which are then grouped for each mission phase for each spacecraft:

Abbreviated satellite names are listed below along with the mission dates. An absent end date indicates an active mission.

ID	Satellite (Mission)	Dates
gs	Geosat	1985-1989
e1	ERS-1	1991-1996
tx	TOPEX	1992-2006
pn	Poseidon	1992-2006
e2	ERS-2	1995-2011
g1	GFO	1998-2008
n1	Envisat	2002-2012
j1	Jason-1	2001-2013
j2	Jason-2	2008-2020
c2	Cryosat-2	2010-
sa	SARAL/Altika	2013-
j3	Jason-3	2016-
3a	Sentinel-3A	2016-
3b	Sentinel-3B	2018-

Mirroring the Database

The RADS database is available from several mirrors. NOAA hosts a mirror at ftp://ftp.star.nesdis.noaa.gov/pub/sod/lsa/rads/data/.

These data are updated twice daily and should regularly be checked for updates. Various tools can be used to mirror the data holdings such as curl, wget, or lftp.

An example using lftp:

$ lftp ftp://ftp.star.nesdis.noaa.gov/pub/sod/lsa/rads/data/
cd ok, cwd=/pub/sod/lsa/rads/data
lftp ftp.star.nesdis.noaa.gov:/pub/sod/lsa/rads/data> mirror --only-newer `3ap0530c001.nc' at 603288 (99%) [Receiving data]

Software Utilities

The requirements to compile and run the code are:

• A unix-type environment (Linux, macOS, etc.).
• The make command.
• A fortran compiler (preferrably gfortran, but others like f90, f95, xlf90, xlf95 and ifort are known to work as well)
• The NetCDF library (version 4.3 or later, compiled) and together with the Fortran 90 interface and module file.
• Optionally, the git program.
• For downloading and synchronizing the data base: the rsync or lftp.

Updated code and documentation are hosted on GitHub: https://github.com/remkos/rads^♦

Examples

The following examples illustrate the potential of the RADS utilities.

1. List the variable names for a mission (Satellite/sensor)
   1. Command: radsvar
   2. Options: -S<satellite/mission code>/<mission phase>
   3. Example: radsvar -Sj3
   4. Output:

      # RADS variable list
      # Created: 2020-06-17 21:40:21 UTC: radsvar -Sj3
      #
      # Possible output records:
      # A  field  alias     var_name             long_name
      # D  field  var_name  default_value        long_name
      # G  field  var_name  grid_file_name       long_name
      # M  field  var_name  math_rpn_statment    long_name
      # N  field  var_name  netcdf_var_name      long_name
      #
      M  0      sla                       alt range SUB dry_tropo SUB wet_tropo SU sea level anomaly
      M         sla_mle3                  alt range_ku_mle3 SUB dry_tropo SUB wet_ sea level anomaly (MLE3)
      N         ssha                      ssha                                     sea surface height anomaly
      N         ssha_mle3                 ssha_mle3                                sea surface height anomaly (MLE3)
      M         adt                       sla mss ADD geoid SUB                    absolute dynamic topography
      N  1  101 time                      time                                     time
      M     102 time_rel_eq               time eqtime SUB                          time relative to equator crossing
      M     103 time_local_solar          time lon 240 MUL ADD 86400 FMOD          local solar time of day
      M     104 time_ymdhms               time YMDHMS                              time
      M     105 time_mjd                  time 86400 DIV 46066 ADD                 Modified Julian Days
      M     106 time_2000                 time 473299200 SUB                       time
      M         time_1950                 time 86400 DIV 12784 ADD                 time
      N  2  201 lat                       lat                                      latitude
      N  3  301 lon                       lon                                      longitude
      N     425 alt_gdre                  alt_gdre                                 CNES GDR-E orbital altitude
      A  4      alt                       alt_gdre                                 CNES GDR-E orbital altitude
      N  5  501 alt_rate                  alt_rate                                 orbital altitude rate
      N     601 range_ku                  range_ku                                 Ku-band range corrected for instr. effects
      A  6      range                     range_ku                                 Ku-band range corrected for instr. effects
      N     602 range_c                   range_c                                  C-band range corrected for instr. effects
      ...

   5. Example: radsvar -Sj3 -Vdry_tropo
   6. Output:

      # RADS variable info
      # Created: 2020-06-17 21:40:28 UTC: radsvar -Sj3 -Vdry_tropo
      #
      userroot="/"
      dataroot="/rads/share"
      sat="j3"
      branch="j3"
      phase="a"
      satellite="JASON-3"
      cycles=(0 999 1)
      passes=(1 254 1)
      alias="dry_tropo"
      var="dry_tropo_ecmwf"
      long_name="ECMWF dry tropospheric correction"
      units="m"
      data="dry_tropo_ecmwf"
      source="ECMWF"
      comment="A dry tropospheric correction must be added to range to correct for dry troposphere delays"
      limits=(-2.4000 -2.1000)
      plot_range=(-2.4000 -2.2000)
      diff_range=(-.1000 .1000)

2. Mean Sea Level Anomalies for 2020 from Jason-3 and SARAL/Altika:
   1. Inputs: 2020 NetCDF data from Jason-3 and SARAL/Altika
   2. Command: rads2grd
   3. Options: --ymd=20200101,20210101 -Sj3 -Ssa -omsla_2020.nc
      where:
      ​ --ymd: year-month-day option for the time start/stop of 20200101 and 20210101.
      -S: satellite identifiers (j3 => Jason-3, sa => SARAL/Altika)
      -o: output NetCDF file named msla_2020.nc
   4. Example: rads2grd --ymd=20200101,20210101 -Sj3 -Ssa -omsla_2020.nc
   5. Output: NetCDF file 'msla_2020.nc' rendered to +/-0.20 m

Manuals

Two manuals have been prepared by Remko Scharroo:

• RADS User Manual: The user manual describes step-by-step installation and configuration details, command-line tool/utilities usage/syntax, and the RADS library (a Fortran90 module providing the main functionalities for the RADS4 software).
• RADS Data Manual: The data manual describes the details of variables used in the RADS database.