ABOUT_TPJAOS is the jdata module of jLab.

 ABOUT_TPJAOS  Sea surface height anomalies from the Beckley merged dataset.
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    TPJAOS is a matfile containing the sea surface height anomalies
    from the integrated alongtrack altimetry dataset, available online at
 
      http://podaac.jpl.nasa.gov/dataset/MERGED_TP_J1_OSTM_OST_CYCLES_V3
 
    This dataset is produced by Brian Beckley and collaborators on NASA's
    Ocean Surface Topography Science Team (OSTST).  It contains data from
    the TOPEX/Poseidon, Jason-1, and OSTM/Jason-2 satellites.
 
    Currently TPJAOS contains data through October 12, 2016.
 
    TPJAOS.MAT is a slightly reorganized matfile version of the NetCDF 
    data distributed at PO.DAAC, and is redistributed with permission as a
    supplement to the software toolbox JLAB. 
 
    Various processing steps have been carried out to remove bad data 
    points, as described below.
 
    LOAD TPJAOS loads the structure TPJAOS, containing the 
    following fields:
 
      tpjaos.about  Pointer to this document                   
      tpjaos.num    Cycle start in DATENUM format     [887 x 1]
      tpjaos.dnum   Days from cycle start date        [3375 x 254]
      tpjaos.lat    Latitude                          [3375 x 254]
      tpjaos.lon    Longitude                         [3375 x 254]
      tpjaos.atd    Along-track distance (km)         [3375 x 254]
      tpjaos.dfc    Distance from coast (km)          [3375 x 254]
      tpjaos.depth  Bathymetry depth (km)             [3375 x 254]
      tpjaos.mss    Mean sea surface height (cm)      [3375 x 254]
      tpjaos.ssh    Sea surface height anomaly (cm)   [3375 x 254 x 887] 
      tpjaos.flag   True if single-frequency          [3375 x 254 x 887] 
   
    The mean sea surface height is given in the PO.DAAC documentation as 
    being the DTU10 Global Gravity field of Anderson (2010). 
 
    Note that sea surface height is here given in centimeters. 
 
    The along-track distance ATD is computed by SPHEREDIST.  Distance from 
    the coast DFC is provided by the source dataset.  DEPTH is computed by 
    interpolating the one-minute Smith and Sandwell database, see READTOPO.
 
    FLAG is a boolean variable used to indicate when a single-frequency,
    or Poseidon-type, altimeter is being used.  This has an impact on the 
    small-scale noise statistics.  
 
    The first dimension, of length 3375, is the number of measurements
    along each track.  The second, of dimension 254, is the number of 
    tracks split into their descending and ascending portions.  The third
    dimension, of length 887, is the number of cycles. 
 
    After 'load tpjaos', type 'use tpjaos' to map the structure fields into
    named variables in the workspace--num, lat, lon, ssh, etc.
 
    The figure shown at the top is the standard deviation of sea surface 
    height, mapped onto a regular grid with POLYSMOOTH. It can be 
    re-created with 'polysmooth --f2'.  Note that this is computationally 
    expensive and may take a while.
 
    TPJAOS.MAT is distributed as a part of JDATA, a supplement to the 
    software toolbox JLAB, and is available at http://www.jmlilly.net.
 
    Regional datasets can be extracted with the JLAB function TRACKEXTRACT.
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    Time details
  
    NUM is the time at the beginning of each cycle, in Matlab's DATENUM 
    format. DNUM is the mean difference from this time, averaged over all 
    cycles, for each location along each track. 
 
    Specifically, NUM is defined as beginning at 4:05 AM on Sept 23, 1992, 
    and proceeding uniformly with a time interval of 9.915645 days.  This 
    matches the first time for each cycle to within about two minutes. 
 
    The exact measurement time is approximating by add the NUM value for 
    each cycle to the DNUM value appropriate for each alongtrack location. 
    This approximates the exact measurement time to with about one minute.
 
    In order to save space, the exact times for each measurement are not 
    re-distributed here. 
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    Organization
 
    The data has been re-organized somewhat from its original format.  Each
    track has been split into a descending and ascending portion. 
 
    All descending passes are presented first, but flipped north/south to 
    be sorted by the longitude of their southernmost point, beginning with 
    the first track having a southernmost point east of longitude -180.  
 
    Then all ascending passes are presented, also sorted from longitude 
    -180 based on their initial or southernmost point.
 
    This reorganization makes it easier to locate tracks that fall through
    particular latitude / longitude points.  Also, each row of SSH occurs
    at essentially the same latitude, so one can directly average the 
    alongtrack data to get meaningful statistics across latitudes.
 
    The time offset from the cycle start time are therefore scrambled among
    the different tracks.  These time offsets are recorded in DNUM.
 
    The field ATD is the cumulative along-track distance from the initial 
    point of each track, as computed by SPHEREDIST.
 
    A figure illustrating the TPJAOS format is shown below.  This is 
    the standard deviation of sea surface height, as in the figure at the 
    top but without any mapping.  Each column is a track with ascending and
    descending tracks split.  The distorted view reflects the way the 
    satellite tracks actually sample the ocean. 
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    Processing
 
    Several processing steps have been applied to remove bad data points.  
    Firstly, all data over land has been set to NaNs, affecting a small 
    number of data points.  
 
    Secondly, all data at locations missing 3/4 or more of their values are
    set to NaNs.  This affects about 3% of all valid data points, and 
    primarily influences regions with seasonally varying ice cover.
 
    Finally, the data has been despiked, with the spikes set to NaNs.  This
    is done by looking at the statistics of the first central difference.  
 
    A representative standard deviation of this quantity is 4 cm.  All
    points having a first central difference with a magnitude exceeding
    three times this standard deviation, or 12 cm, are set to NaNs.  This 
    also affects about 3% of the valid data points.  
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    Dataset creation
 
    The NetCDF file on which TPJAOS.MAT is based are available for 
    download from PO.DAAC as follows 
 
        ftp  ftp://podaac-ftp.jpl.nasa.gov
        cd   allData/merged_alt/L2/TP_J1_OSTM/all
        get Merged_TOPEX_Jason_OSTM_Jason-3_Version_V4.0.nc.gz
   
    For completeness, the m-file TPJAOS also contains the processing 
    steps used in the creation of the corresponding matfile.  This 
    requires that the current version of JLAB is on your search path.
 
    Then 'tpjaos --create' will recreate TPJAOS.MAT by reading in
    the NetCDF file as downloaded from PO.DAAC. This will take a while.
 
    You'll need to make sure to put the directory containing the file on
    your Matlab search path using ADDPATH, e.g. 'addpath ~/Data/tpjaos'.
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    Further details
 
    From the handbook, cycles 1--355 are TOPEX/Poseidon, 356--582 are 
    Jason-1, and 583 and future cycles are OSTM.
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    See also JDATA, ABOUT_FLOATS, ABOUT_DRIFTERS.
 
    'about_tpjaos --f' generates the two figures shown above.
 
    Usage: about_tpjaos
           about_tpjaos --f 
           about_tpjaos --create     
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    This is part of JLAB --- type 'help jlab' for more information
    (C) 2014--2017 J.M. Lilly --- type 'help jlab_license' for details    

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