Calculate the spatial signal probability for each catalog object.

        Parameters:
        -----------
        None

        Returns:
        --------
        u_spatial : array of spatial probabilities per object

def calc_signal_spatial(self):
        """
        Calculate the spatial signal probability for each catalog object.

        Parameters:
        -----------
        None

        Returns:
        --------
        u_spatial : array of spatial probabilities per object
        """
        # Calculate the surface intensity
        self.surface_intensity_sparse = self.calc_surface_intensity()

        # Calculate the probability per object-by-object level
        self.surface_intensity_object = self.kernel.pdf(self.catalog.lon,
                                                        self.catalog.lat)
        
        # Spatial component of signal probability
        u_spatial = self.surface_intensity_object
        return u_spatial