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April 11, 2003

National Space Development Agency of Japan

NASDA REPORT NO.129

Part 2 of 3

Research on GPS Receiver

During the rendezvous and docking experiment of Orihime/Hikoboshi,
GPS receivers were used to measure the satellites' positions, the
relative position to the other satellite when it is less than 100m
apart, and their relative velocity. To measure the relative position
and relative velocity, both the chaser satellite Hikoboshi and the
target satellite Orihime were equipped with GPS receivers. The
satellites performed GPS relative navigation with them. Orihime's GPS
data was sent to Hikoboshi through the inter satellite communication
link between them, and the Hikoboshi estimated their relative
position, relative velocity and so on. 

When two satellites are equipped with GPS receivers, relative
position and velocity can be measured as proven in the
Orihime/Hikoboshi experiment, moreover, even when only on satellite
is equipped with the receiver, it is very useful because it enables
to determine the satellites orbital location and velocity can be
determined in real time. 

There are may researches conducted to make performance enhancement of
GPS receivers, such as, the technology to enable the measurement of
location and velocity in shorter time by obtaining signals from more
satellites at a time, the receiver that can be used for
geostationally satellite, and smaller, less power consuming
receivers.


Research on Laser Radar

In Orihime/Hikoboshi rendezvous and docking experiment, laser radar
was used relative distance between the two satellites were from few
meters to few hundred meters. Hikoboshi, the chaser satellite,
generated the laser with laser diode and shone it to the laser
reflector on Orihime, the target satellite. The laser radar measured
the distance between two satellites by measuring the time it takes
for the laser to make a round trip, and it also detected the
direction of the other satellite from the direction of the
reflection. 

The research conducted today for the improvement of laser radar is
pursuing the enlargement of the view angle, size and weight
reduction, the improvement of the accuracy, and so on, to utilize
this technology for the transfer vehicle for International Space
Station. Also, the accuracy enhancement and the measurement range
improvement is being researched to use this sensors for the
navigation of the satellite flying in formation. 


Research on Imagery Sensor

When the distance between Orihime and Hikoboshi were under 10 meters,
Hikoboshi took images of the marker on Orihime with CCD, and
determined the relative location and the relative attitude through
image processing. Current image processing research is aiming to
identify the satellite and measure relateive location and attitude
through the CCD images of the satellite itself, without the marker. 

Photograph 2 shows the image sensor being tested in the rendezvous
and docking test facility in Tsukuba Space Center. Most of the
satellite is attached to the rocket by with a cylinder like
attachment called PAF interface module. The experiment shot a mock up
with this PAF with 5 CCD cameras mounted on the servo table that
simulates the satellite closing in. The stereoscopic images from
these cameras were processed with to determine the relative location
and the attitude of the satellite. The target satellite does not have
the marker or the reflector, but it actually worked very well. 

- Continued -

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