NewsUsing a laser to measure a distance
of 6 million km

In December 2020, the project celebrated the news of the capsule returning to Earth. But behind the scenes, we were working on another experiment. The Hayabusa2 laser altimeter (LIDAR) that was used to measure the terrain at asteroid Ryugu was about to be used to measure the distance from the Earth.

Recently, products that can measure distance using a laser have become widespread and can be bought by individuals. However, it is not easy to measure the distance to a far away object such as a satellite or spacecraft. For artificial satellites that orbit at distances of hundreds to tens of thousands of kilometers, and also on the Moon’s surface, reflective prisms, known as a corner-cube reflectors are installed. The distance can be obtained by emitting a pulsed laser from a telescope on the ground and measuring the time required for a round trip as the light is reflected from the corner-cube reflectors. However, when an object such as a spacecraft is further than the Moon, the reflected light becomes too weak to use this method. Instead, it is necessary to emit lasers from both the ground and the spacecraft. There are very few successful examples in the world of such a distance measurement that use this method.

The Hayabusa2 LIDAR emitted laser light alone when around asteroid Ryugu and received reflections from objects at a distance of several tens of kilometers. But in this experiment, we set the laser to emit light only when a laser signal is received from the outside. We then emitted laser light from the ground station telescope on Earth directed at Hayabusa2, and accurately measured the time when the laser light left the ground station, and the time when the laser arrived from Hayabusa2.

Since the field of view for the telescope and the beam spread angle of the laser are very narrow for both the Hayabusa2 LIDAR and the ground station, the orbit determination and attitude control of Hayabusa2 and the direction of the ground-based telescope needed to be extremely accurate. It is like matching a needle point to a needle point.

Figure 1: Different types of transmission / reception systems.
Left: near Ryugu. Right: After the Earth swing-by.
(image credit: Hitotsubashi University, Toshimichi Otsubo)

This experiment was also conducted during the 2015 Earth swing-by. At that time, the one-way (outward) transmission from the Stromlo ground station in Australia was successful, but the return signal could not be confirmed. In 2020, ground stations in Grasse, France and Wettzell, Germany, also joined the ones in Koganai, Japan and Stromlo, Australia. As the signal from Hayabusa2 is very weak, a large telescope is required. For Hayabusa2, the stations also needed to be able to observe in the infrared. The experiment continued from December 7 to 23, the day after the spacecraft’s closest approach to the Earth.

Figure 2: The international observation network.
(image credit: Hitotsubashi University, Toshimichi Otsubo)

Because of the previous experiment, the establishment of the outbound route was successful from the first day. But as expected, the return route was not easy. The signal was weak, the weather was changing from moment to moment, the required accuracy of the orbit and attitude of the spacecraft was high, all in addition to the intense noise due to the daytime observation. Even if adjustments are made at the ground station, there can be a delay before the light reciprocates (4 seconds on the first day, 45 seconds on the last day). This all made the experiment difficult.

Furthermore, the experiment was frustrating since the amount of onboard delay (time difference between receiving and transmitting the laser) on the spacecraft side fluctuates so that the success or failure of the return trip is completely unknown at the time of the observation. Only after getting the telemetry data from Hayabusa2 and comparing the signal launch time from the ground station with the arrival time at the spacecraft were we finally able to know the amount of onboard delay. In the initial plan, the team members including the author were scheduled to go to the overseas stations, but as this was not possible, the online conference system to connect with Sagamihara was used and added to the frustration!

On the third day of the experiment, December 9, 2020, while making fine adjustments to the experiment, we succeeded in observing the return trip at the Grasse station. In reality, we had no clues to the success on that day. The next day we saw some maybes and the day after that, we were almost convinced. This graph shows the results of the round-trip time measured at the Grasse station, after applying the onboard delay. The concentration of points at -0.5 microseconds is the return signal from Hayabusa2. The many other points are noise from sunlight. Since Hayabusa2 was headed towards the Sun after the approach to Earth (Sun and Hayabusa2 had an elongation of 30°), the observations were during the daytime and we had to wrestle with a large amount of noise. The ground stations were asked to reduce noise as much as possible without eliminating the signal.

Figure 3: Two-way range observation at the Grasse station on December 9, 2020. Time is in UTC.
(Image credit: JAXA, NAOJ, National Institute of Information and Communications Technology, Hitotsubashi University, Chiba Institute of Technology, Hokkaido University, University of Occupational and Environmental Health, National College of Technology)

When weather permitted, the outbound route from Koganei / Stromlo / Grasse could be established nearly continuously. The return trip was confirmed at Grasse on December 9 (one-way distance 1.4 million km) and December 21 (one-way distance 6 million km). The Wettzell station was unable to make observations due to equipment preparation and weather conditions.

We would like to express our sincere gratitude to the members of the four stations who made preparations during the lockdown and took observations for Hayabusa2 before Christmas. Thank you. Even when there was heavy snow, everyone at the Grasse station commuted to work on the rough roads and found the sunny days to observe.

Figure 4: Everyone who worked with the 1.5m telescope at the Grasse ground station for this experiment.
(image credit: Dr Clément Courde, Observatoire de la Côte d’Azur)

During that period of three weeks, the spacecraft became steadily more distant from the Earth and difficult to observe, the weather changed and some of the ground station systems malfunctioned. On the other hand, for the author who was participating in a deep space mission for the first time, it was an exciting series of experiences at the Sagamihara campus each day. Under the leadership of PI Mizuno, the people of JAXA, NAOJ, the National Institute of Information and Communications Technology, Chiba Institute of Technology, Hokkaido University, the University of Occupational and Environmental Heath and the Oshima National College of Technology did everything from the preparation through to the operation perfectly. I feel this is what gave us our results.

We hope that this experiment can be recognised as one of the achievements of Hayabusa2 and will contribute to high-precision navigation in deep space in the future.

Hayabusa2 Laser Altimeter Science Team
Toshimichi Otsubo (Professor, Graduate School of Social Sciences, Hitotsubashi University)