IEEE - Aerospace and Electronic Systems - May 2022 - 38

Hayabusa2 Re-Entry Capsule Tracking by Marine Radars
deviation is 0.46 m2. However, (8) approximates s to a
cubic polynomial by the method of least squares
se cðÞ ¼0:0002c3 þ 0:0136c2 0:2966c þ 3:143
(8)
where se is the approximate RCS (m2) and c is the elevation
angle (deg).
Figure 20 shows the vertical detectable area drawn in
Figure 19.
RCS estimation results at N2 (blue), N3 (red), and S2 (green), and
their approximations (black).
consideration ofthe RCS fluctuation. The black solid line is
the area using se calculated in (8), and the black dashed lines
indicate the standard deviation. This figure also shows tracking
results for each site during the recovery operation. From
the figure, it can be seen that N3 was better able to detect
when the tilt ofthe antenna was set to a slightly higher angle.
In future missions, if the antenna tilt setting and the site layout
are determined based on these results, it should be possible
to track the I/Mwith a higher degree ofcertainty.
CONCLUSION
The I/M was discovered about 2 h after re-entry using the
data of the GOS, the DFS, the MRS, and the DoD radar.
The sample contained in the I/M was transported to Japan
after safety treatment and gas sampling were carried out
locally and arrived in Japan 57 h after re-entry. This rapid
recovery operation led to a substantially reduced recovery
time from the target of 100 h, and the analysis of the sample
is eagerly anticipated.
This article showed that it was possible to track the I/M
Figure 20.
Vertical detectable area considering the RCS fluctuation (black),
and the tracking results (other colors).
landing point was shifted eastward from the predicted
landing point (X¼ Y¼ 0) due to a strong jet stream at an
altitude of 10-15 km. During tracking, the average position
difference compared with the DoD radar was 277 m,
and the difference between the landing point and the final
detection point of the MRS was 240 m.
RCS ESTIMATION OF THE I/M
Here, we discuss the RCS of the I/M during descent (w/t
parachute). In Figure 19, the RCS (s) is calculated using
the measurement results for the received power (Pr), and
detection distance (R) in (5). The horizontal axis is the elevation
angle observed from each site, which was calculated
by the estimation of altitude in the previous section.
Figure 19 shows that the RCS tended to be inversely proportional
to the elevation angle. This tendency is considered
to indicate that the apparent cross-section changed
according to the elevation angle. Although the RCS was
assumed to be 0.2-0.5 m2 in the original plan [7], this value
fluctuated by about 10 times. The dotted line in this figure
is the approximation of the RCS in (8), and its standard
38
(w/t parachute) by using multiple small radars. In the recovery
operation, the MRS tracked the I/M for 18 min. Three
marine radars detected the I/M, and the maximum observed
distance was more than 31.55 km. The distance between the
final detection point of the MRS and the I/M landing point
was 240 m. This was a sufficient result for the systemrequired
accuracy of1 km. The high accuracy of the trajectory
control and estimation developed by JAXA was
indispensable to the success ofthe recovery operation.
In addition, this article proposed an altitude estimation
method by using multiple search radars. Although there
was an error of several percentage points, this can likely
be reduced by improving the measurement accuracy. This
method of combining two or more 2-D search radars to
determine altitude can be applied to other generic civilian
and military surveillance radars. Also, the RCS of the I/M
(w/t parachute) was shown to fluctuate greatly depending
on the elevation angle. Based on this, the detectable area
for future missions was determined.
These results suggest the possibility of a cooperative
search by using multiple small radars. Small radars are
portable and thus take less time to deploy. These features
mean that it is possible to deploy radars just before reentry,
when the estimated trajectory will be more accurate.
Further improvements to the trajectory estimation and the
radar measurement accuracy will lead to area-shared
IEEE A&E SYSTEMS MAGAZINE
MAY 2022
IEEE - Aerospace and Electronic Systems - May 2022

Table of Contents for the Digital Edition of IEEE - Aerospace and Electronic Systems - May 2022
