Interview with Dr. Shinichi Nakasuka, a professor at Tokyo University and a leading figure in the field of development of small satellites in Japan
Operational Status of PRISM and Dream (ambition) of Developing Small Satellites in Asia-Pacific Regions
1. About PRISM
First, we would like to know the operational status of PRISM from its launch to now. It was launched as one of six piggyback satellites (small satellites) with the Greenhouse Gases Observing Satellite (GOSAT) "IBUKI" by H-IIA Launch Vehicle No. 15 at 12:54 p.m. on January 23, 2009 (Japan Standard Time) from the Tanegashima Space Center and is working well.
In fact, we were worried a little at first because we could not confirm that PRISM had a communication link with the armature band, although we could for all the other small satellites in their first path near Japan.
We could, however, confirm that PRISM had separated from the vehicle because at round 17:40 we could receive a signal from the beacon radio wave via a ground station of Lulea University in Kirna, Sweden by remote control from Japan, although the elevation angle was only 1.5 degrees.
And then the satellite had a high elevation angle on the next path over Kirna and so we could receive data on battery voltage, temperature, a part of generation capacity of the solar panel of the satellite and so on. And we could confirm from the receiving data that the satellite performance was normal. Furthermore, at around 22:00 we could clearly receive a signal at the satellite's second path over Japan and at the end of the launch day we could confirm the up and down link of FM. This is a working at the first day.
From the next day, we checked data (NORAD data) supplied by NORAD, the North American Aerospace Defense Command, with our small satellite and that took a few days.
NORAD data is information on unidentified space objects (for example launch vehicles, satellites, and space debris). They are always being watched, their trajectories are calculated, and they are assigned individual identifiers by the NORAD
Satellites that are precisely controlled and tracked (or are measured the distance) from the ground station do not need such NORAD data. However, the trajectory information from NORAD are very important for small satellites that are not tracked like our satellite.
That is, collation work is that we estimate identifiers of eight small satellites on the basis of their launch day and launch time, and finally find out the identifier of each satellite.
We can find out the identifier of each small satellite by comparing the times of Acceptance of signal (AOS) and Loss of signal (LOS) and also by comparing an estimated Doppler shift signal and an actual one.
We could finally determine the identifier of our satellite in the NORAD data by referring also to data being cooperatively sent from overseas' armatures.
Eventually, we determined that an object close to the second stage of the H-IIA vehicle in space was our satellite (PRISM) because it had finally separated from the second stage in comparison with other small satellites.
As the next step, we carried out attitude control to suppress the satellite rotating after we had confirmed the functions of the attitude control system about 10 days after launch, as preparing for the boom deployment which is one of main events.
We successfully decreased the rotation to about zero degree/sec from 2 or 3 degree/sec by using a rate dumping control in which we utilized a magnet torquer reflecting the output of magnet measurements.
Before the boom deployment, we also carried out an experiment to take photographs with a small cameras at the side of the satellite (each camera has a wide visual field and it can see part of the satellite and the Earth), and we could confirm that we had taken fine images with the camera..
After that, we confirmed that the boom faced the Earth about one month after launch and then we had successfully deployed the boom.
We could confirm the completion of the boom deployment by analyzing the reaction movement of the boom and images of the deployment captured by the small camera.
Finally, concerning the confirmation of the quality of the satellite imageries, as we cannot finely adjust the boom's length, we take (adopt) the way of focusing the camera by moving the CMOS device forward and backward using the stepping motor by commands from the ground. We are now focusing the camera and the resolution is gradually becoming 30 m.
We have just finished the initial operation (up to stopping the satellite's rotation) and are entering the mission phase operation.
We initially used a four-person shift, but we are now using a one-person shift about two weeks after the launch.
What are your plans or expectations for the future operations of PRISM?
The most important factor is how effectively the captured images from the satellite can be utilized in real-life cases.
For attaining the purpose, first we are going to confirm the 30m resolution of PRISM and we have already prepared some experiments for applications concerning how we can effectively utilize those images in real-life cases.
I would like to watch from now what would happen if we could get images with 30m resolution with such a small satellite.
For example, we will take many images over the Asia-Pacific region with PRISM and would like to carry out cooperative experimentation with organizations or universities in the countries in that region.
And also we are planning to utilize the PRISM as a test bench for experiments on controlling the oscillation of the deployment boom (a flexible structure).
For this experiment, we have already designed some onboard software which we can rewrite by sending a command from the ground.
So we can verify in the real space a space structure and its modeling and also the way of its control (how to control the oscillation) on the basis of some students' unique ideas.
I believe it is a very valuable experience for students.
2. Suggestions for Developing Small Satellites
Do you have any suggestions for developing small satellites?
As I mentioned before, the value of a satellite depends on how effectively the data it captures can be utilized. It makes no sense to be satisfied only with the successful launch of a satellite. I think we also need to do some research on how to effectively utilize the data from it.
For example, when we use many small satellites as a constellation, even if their precision is rough, we can very frequently observe the daily changes or time changes of CO2 and it is possible to effectively distinguish them from large satellites in terms of the way they are utilized.
We, however, recognize that it is essential to have a certain degree of precision in observations.
Therefore, the most important point in small satellite development in the future will be how to develop small and high performance mission devices.
I think that the resolution of an optical sensor onboard even a small satellite with a weight of 20 kg will be up to about 4 m. There is a possibility of such realization development, although there are still issues to be resolved.
I think it is difficult to develop small radar sensors because they need to be of a certain size to function, but I am keenly aware of the need to develop a small, precise radar sensor because we could not use 80% of the images captured by the optical sensor because of cloud or because it was nighttime, so we have already prepared to carry out some research.
3. Message to APRSAF members
Finally, do you have any message for the interested parties?
First of all, I would like organizations or universities in each country to have an experience of developing even one of any kind of small satellite by themselves and to manufacture a small satellite and launch it with us in the future.
When I say "launch it with us", I mean that we would like to carry out a mission by using satellites which are manufactured by any country, any organization or any university in cooperation with each other (such as for disaster monitoring or greenhouse gas monitoring).
I would like to make a good community in Asia Pacific regions and to carry out a common mission from Asia-Pacific so that we would attain high dense effects which can be attained only by having a constellation of small satellites, each of which has its own mission but has also a common mission added to the own mission and we could contribute those efforts to the world.
We can consider a PARASAT (Parasite Satellite) mission to be an example of a cooperative effort, that is, we put a common (a parasite) sensor on all our small satellites and using them we can conduct very frequent observations.
I would like every organization or every university in the Asia-Pacific region to be able to develop a satellite by themselves.
There are several ways to do this, and a participation in the APRSAF STAR project may be also a one choice.
|University of Tokyo|
|Satellite name:||PRISM (nicknamed Hitomi)|
|Size:||20 cm ?? 20 cm ?? 40 cm (at launch); boom can extend 50 cm|
|Attitude control:||Gravity-gradient stabilized and 3 magnetic torquers (3-axis torquers)|
|Main purpose:||To serve as a first attempt at applying nano-satellites to practical missions|