= U_obs_op_lin = 

The page document the user-supplied call-back routine `U_obs_op_lin`.

The routine `U_obs_op_lin` is a call-back routine that has to be provided by the user. In the simplified interface the predefined name of the routine is `obs_op_lin_pdaf`, but in the full interface, the user can choose the name of the routine.

The routine is used with the all 3D-Var methods. It has to perform the operation of the linearized observation operator acting on a state vector increment that is provided as `state_p`. The observed state has to be returned in `m_state_p`.

The interface is the following:
{{{
SUBROUTINE U_obs_op_lin(step, dim_p, dim_obs_p, state_p, m_state_p)

  INTEGER, INTENT(in) :: step               ! Current time step
  INTEGER, INTENT(in) :: dim_p              ! PE-local dimension of state
  INTEGER, INTENT(in) :: dim_obs_p          ! Dimension of observed state
  REAL, INTENT(in)    :: state_p(dim_p)     ! PE-local model state
  REAL, INTENT(out) :: m_state_p(dim_obs_p) ! PE-local observed state
}}}

The routine is called during the analysis step. It has to perform the operation of the linearized observation operator acting on a state vector increment that is provided as `state_p`. The observed state has to be returned in `m_state_p`.

Hints:
 * For a model using domain decomposition, the operation is on the PE-local sub-domain of the model and has to provide the observed sub-state for the PE-local domain.
 * If the observation operator involves a global operation, e.g. some global integration, while using domain-decomposition one has to gather the information from the other model domains using MPI communication.