= PDAF-OMI Observation Modules = 

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The implementation of the observations with OMI is done in observation modules (obs-modules). For each observation type a separate module should be created. 

Each obs-module contains four routines:

 - init_dim_obs initializes all variables holding the information about one observation type. The information about the observation type is stored in a data structure (Fortran derived type).
 - obs_op applies the observation operator to a state vector. One can call an observation operator routine provided by PDAF, or one can to implement a new operator.
 - init_dim_obs_l calls a PDAF-OMI routine to initialize the observation information corresponding to a local analysis domain. One can set localization parameters, liek the localization radius, for each observation type.
 - localize_covar calls a PDAF-OMI routine to apply covariance localization. One can set localization parameters, liek the localization radius, for each observation type. 

== obs_f data type == 

To ensure the functionality within each obs-module, we rely on a derived data type called `obs_f` that describes the observation. One instance of this data type is allocated in each obs-module as the variable `thisobs`. a few of the elements of `obs_f` are initialized by the user when the observation information is initialized on `init_dim_obs_f`. Further information is set in a call to the routine `PDAFomi_gather_obs`. This information is then used by all other routines in the obs-module. The template file obs_TYPE_pdafomi_TEMPLATE.F90 shows the different steps needed to initialize thisobs. 

{{{
! ---- Mandatory variables to be set in init_dim_obs_f ----
  INTEGER :: doassim                    ! Whether to assimilate this observation type
  INTEGER :: disttype                   ! Type of distance computation to use for localization
                                        !   (0) Cartesian, (1) Cartesian periodic
                                        !   (2) simplified geographic, (3) geographic haversine function
  INTEGER :: ncoord                     ! Number of coordinates use for distance computation
  INTEGER, ALLOCATABLE :: id_obs_p(:,:) ! Indices of observed field in state vector (process-local)
}}}