Data source:
 
1) Bottom pressure derived from the spherical harmonic transform
   of the output Max-Planck-Institute for Meteorology Ocean Model 
   (MPIOM06, Jungclaus et al., 2013), release RL06, truncated at 
   degree/order 180. The coefficients of the MPIOM06 model 
   spherical harmonics transform is provided by the 
   GeoForschingZentrum (GFZ) and can be downloaded from 
   ftp://isdcftp.gfz-potsdam.de/grace/Level-1B/GFZ/AOD/RL06/

   The original output of the MPIOM06 model contains signals of 
   the atmospheric tides and its oceanic response to them. 
   The harmonic variations at 18 frequencies, including diurnal, 
   semi-diurnal, ter-diurnal, 4-diurnal frequencies as well as 
   the mean bottom pressure computed over 38 year time series of 
   MPIOM06 bottom pressure over 1980-2018 were estimated by least 
   squares and removed. The bottom pressure dataset used fir loading 
   computation has not harmonic signal at these frequencies.

   Here are the frequencies for which harmonic signal from bottom
   pressure has been removed:

   7.232384890619D-05  rad/s   ( PI1 )
   7.252294578148D-05  rad/s   ( P1  )
   7.272206167609D-05  rad/s   ( S1  )
   7.292115855138D-05  rad/s   ( K1  )
   7.312025542667D-05  rad/s   ( PSI1)
   1.450459105823D-04  rad/s   ( 2T2 )
   1.452450074576D-04  rad/s   ( T2  )
   1.454441043329D-04  rad/s   ( S2  )
   1.456432012082D-04  rad/s   ( R2  )
   1.458423171028D-04  rad/s   ( K2  )
   2.177679627494D-04  rad/s   ( U3  )
   2.179670596247D-04  rad/s   ( T3  )
   2.181661850283D-04  rad/s   ( S3  )
   2.183652533752D-04  rad/s   ( R3  )
   2.185643502505D-04  rad/s   ( K3  )
   2.904900529562D-04  rad/s   ( U4  )
   2.906891498303D-04  rad/s   ( T4  )
   2.908882467044D-04  rad/s   ( S4  )

   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 nine  steps:

   1) inverse harmonic transform of degree/order 180.

   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 (180).     

   5) Subtraction of harmonic signal at 18 frequencies from each
      cell of bottom pressure.

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

   7) replacements of spherical harmonics with degree/order < 181
      with original harmonic transform from the MPIOM06 model

   8) inverse harmonic transform of degree/order 2699.

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

   Credit:

   The MPIOM06 model was developed by M. Thomas and his colleagues.

   The main document that describes the input dataset is available here:

   ftp://isdcftp.gfz-potsdam.de/grace/DOCUMENTS/Level-1/GRACE_AOD1B_Product_Description_Document_for_RL06.pdf

   The main peer-reviewed paper that describes the input dataset
   is Dobslaw et al. (2017).

References:


   Dobslaw, H., Bergmann-Wolf, I., Dill, R., Poropat, L., Thomas, M., Dahle, 
     C., Esselborn, S., Koenig R. & Flechtner, F. (2017). 
     A New High-Resolution Model of Non-Tidal Atmosphere and Ocean
     Mass Variability for De-Aliasing of Satellite Gravity Observations: 
     AOD1B RL06, Geophys. J. Int., 211, 263269, DOI:10.1093/gji/ggx302. 10

   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. 

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

   Jungclaus, J. H., Fischer, N., Haak, H., Lohmann, K., Marotzke, J., 
     Matei, D., Mikolajewicz, U., Notz, D. & von Storch, J. S. (2013). 
       Characteristics of the ocean simulations in the Max Planck
       Institute Ocean Model (MPIOM) the ocean component of the MPI-Earth 
       system model, Journal of Advances in Modeling Earth Systems, 5, 
       422--446, DOI:10.1002/jame.20023. 7, 23

   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.

