INT 98-04 NSF/Tokyo Report:   Fine Control of Actively Stabilized Antennas
1/22/98

The National Science Foundation's offices in Tokyo and in Paris periodically
report on developments abroad that are related to the Foundation's mission.
These documents present facts for the use of NSF program managers and policy
makers; they are not statements of NSF policy.

Special Scientific Report #98-03 (January 09, 1998)

Fine Control of Actively Stabilized Antennas

Dr. Hemanshu (Roger) M. Lakhani prepared the following report. In June of
1997, Dr. Lakhani began a 24-month JSPS fellowship at the Tokyo Institute of
Technology (TIT). His host is Professor Toshiaki Ohkami of the Department of
Engineering Sciences at TIT. Dr. Lakhani can be reached via email at
roger@mass.mess.titech.ac.jp.

This research is being conducted in the Mechanical Engineering Sciences
department of the Tokyo Institute of Technology under the supervision of Dr.
Yoshiaki Ohkami. The project is supported by a small company in California
that manufactures actively stabilized antenna platforms. Such antenna
platforms are used onboard moving vehicles, such as ships, so that the antenna
can be made to point toward a given satellite despite undulating motions of
the boat or vehicle. The goal of the project is to modify one of the company's
existing tracking systems so as to achieve more precise control and pointing
accuracy of the antenna platform. The system provided by the US company is
currently being used by customers onboard large shipping vessels to track
INMARSAT geostationary satellites. Such an application requires a pointing
accuracy of approximately 5 [deg], and the current system is capable of
achieving this despite relatively large motions of the ship. In the future,
however, the s! ame system will be required for higher communications
bandwidth applications, such as Ku and even Ka band, and the laboratory at TIT
has been asked to improve the current system so as to enable it to track more
narrow beam widths. The laboratory, which specializes in the dynamics and
control of multibody system, was deemed qualified to handle this research as
the control of the antenna platform can be treated as a classic robotics
problem.

The project was begun in June of 1997 and is being carried out by a team
consisting of one 1st-year doctoral student, one 4th-year undergraduate
student, and me, who was assigned as project team leader. As part of the first
phase of the research, the laboratory was responsible for identifying specific
areas of the system that will require improvement in order to achieve the
higher bandwidth tracking. As INMARSAT receivers were not readily in hand, the
lab chose to conduct evaluation tests outdoors using a satellite dish/tuner
package compatible with the reception from a Japanese BS satellite flying over
Tokyo. Note that tracking of a broadcast satellite (C-band) represents a more
severe problem than tracking of the L-band INMARSAT signal. Additional tests
were carried out onboard a small yacht to test the tracking capabilities of
the system in the presence of swaying motions of the antenna base. One major
finding was that the current system lacks proper structural rigidity in ! the
antenna mount, resulting in a non-collocated control problem due to the
flexing of the antenna post. The lab has also identified several problems
related directly to limitations in the accuracy of the sensors used to detect
the swaying motions of the antenna base (or ship). Here, the laboratory is
attempting to replace the existing sensors with better, more accurate sensors.

The second phase of the research will involve implementation of the
laboratory's own control algorithms, which have been specifically designed to
overcome the current system's shortcomings and thus to demonstrate improved
performance. It is expected that all improvements made through this research
will be directly translated into future generations of this tracking platform,
thus representing a direct contribution to the success of the US company.