Data source:
 
1) Bottom pressure derived from the spherical harmonic transform
   of the Ocean Model for Circulation and Tides (OMCT) model,
   release RL05, truncated at degree/order 100. The coefficients 
   of the OMCT model spherical harmonics transform is provided 
   by the GeoForschingZentrum (GFZ) and downloaded from 
   ftp://podaac.jpl.nasa.gov/allData/grace/L1B/GFZ/AOD1B/RL05/

   The OMCT model contains signals of the atmospheric tide S1
   and its oceanic response to it. The semi-diurnal tide S2 and its 
   oceanic response has been instead removed, since it is partially 
   aliased into a standing wave pattern for a 6 hourly resolved 
   data-set.

   The spherical harmonic transform was underwent the procedure
   of its conditioning in order to mitigate corruption of the bottom
   pressure due leakage due to spherical harmonics truncation.

   The procedure of conditioning has eight steps:

   1) inverse harmonic transform of degree/order 100.

   2) regridding to the equi-angular longitude/latitude grid 10800x5400.

   3) applying fine land/sea mask at 10800x5400 grid, i.e. masking out
      points that are land.

   4) smoothing by convolving the bottom pressure with a kernel
      that exponentially decays with distance. The decay parameter is
      -PI/(2.0*T), where T is the degree/order of truncation of the 
      original harmonic transform (100).     

   5) direct harmonic transform of the grid with degree/order 2699.

   6) replacements of spherical harmonics with degree/order < 101
      with original harmonic transform from the OMCT model

   7) inverse harmonic transform of degree/order 2699.

   8) smoothing by convolving the bottom pressure with a kernel
      that exponentially decays with distance.

   The OMCT model was developed by M. Thomas and his colleagues.

   http://www.gfz-potsdam.de/en/research/organizational-units/departments/
          department-1/earth-system-modelling/topics/omct-model/

   Main publication:

   Dobslaw, H., Flechtner, F., Bergmann-Wolf, I., Dahle, C., Dill, R., 
     Esselborn, S., Sasgen, I., Thomas, M. (2013). 
     Simulating high-frequency atmosphere-ocean mass variability for 
     dealiasing of satellite gravity observations: AOD1B RL05. J.
     Geophys. Res., 118(7), 3704--3711. doi:10.1002/jgrc.20271. 

   Other publications

   Thomas, M., Ozeanisch induzierte Erdrotationsschwankungen
     Ph. D. Thesis, 2002
     http://ediss.sub.uni-hamburg.de/volltexte/2002/608/pdf/dissertation.pdf

   Dobslaw, H., Thomas, M., Simulation and observation of global ocean mass 
     anomalies, JGR, 112, C05040, 2007

   The mean bottom pressure and harmonic variations at each grid cell 
   at the following frequencies were removed from the bottom pressure data 
   used for loading computations:

   PI1     7.232384890619D-05 rad/s
   P1      7.252294578148D-05 rad/s
   S1      7.272206167609D-05 rad/s
   K1      7.292115855138D-05 rad/s
   PSI1    7.312025542667D-05 rad/s
   2T2     1.450459105823D-04 rad/s
   T2      1.452450074576D-04 rad/s
   S2      1.454441043329D-04 rad/s

   The mean, sine, and cosine amplitudes of harmonic variations were
   computed for the time range [1979.0 -- 2016.0], i.e. 37 years.


References:

   Petrov, L., The International Mass Loading Service, 
      Proceedings of the Reference Frames for Applications in 
      Geosciences Symposium, held in Luxembourg in October 2014, 2015. 
      DOI: 10.1007/1345_2015_218, http://arxiv.org/abs/1503.00191

   Petrov L., J.-P. Boy, "Study of the atmospheric
      pressure loading signal in VLBI observations", Journal of
      Geophysical Research, 10.1029/2003JB002500, vol. 109,
      No. B03405, 2004.

   Rienecker, M.M., Suarez M.J., Todling R., Bacmeister J., Takacs L., 
      Liu H.-C., Sienkiewicz W. M., Koster R.D., Gelaro R., Stajner I., 
      and Nielsen E., "The GEOS Data Assimilation System -- Documentation 
      of Versions 5.0.1, 5.1.0, and 5.2.0.", NASA/TM--2008--104606, 2008. 
      http://gmao.gsfc.nasa.gov/pubs/docs/tm27.pdf
