def get_orbit_number(self, utc_time, tbus_style=False):
        """Calculate orbit number at specified time.
        Optionally use TBUS-style orbit numbering (TLE orbit number + 1)
        """
        utc_time = np.datetime64(utc_time)
        try:
            dt = astronomy._days(utc_time - self.orbit_elements.an_time)
            orbit_period = astronomy._days(self.orbit_elements.an_period)
        except AttributeError:
            pos_epoch, vel_epoch = self.get_position(self.tle.epoch,
                                                     normalize=False)
            if np.abs(pos_epoch[2]) > 1 or not vel_epoch[2] > 0:
                # Epoch not at ascending node
                self.orbit_elements.an_time = self.get_last_an_time(
                    self.tle.epoch)
            else:
                # Epoch at ascending node (z < 1 km) and positive v_z
                self.orbit_elements.an_time = self.tle.epoch

            self.orbit_elements.an_period = self.orbit_elements.an_time - \
                self.get_last_an_time(self.orbit_elements.an_time
                                      - np.timedelta64(10, 'm'))

            dt = astronomy._days(utc_time - self.orbit_elements.an_time)
            orbit_period = astronomy._days(self.orbit_elements.an_period)

        orbit = int(self.tle.orbit + dt / orbit_period +
                    self.tle.mean_motion_derivative * dt**2 +
                    self.tle.mean_motion_sec_derivative * dt**3)

        if tbus_style:
            orbit += 1
        return orbit