Returns slant range to the object. Call once for each
        new measurement at dt time from last call.

def get_range(self, process_err_pct=0.05):
        """
        Returns slant range to the object. Call once for each
        new measurement at dt time from last call.
        """

        vel = self.vel + 5 * randn()
        alt = self.alt + 10 * randn()

        self.pos += vel*self.dt

        err = (self.pos * process_err_pct) * randn()
        slant_range = (self.pos**2 + alt**2)**.5 + err

        return slant_range