IEEE - Aerospace and Electronic Systems - October 2021 - 26

Uncued Detection and Initial Orbit Determination From Short Observations With the Murchison Widefield Array
ODBDIOD
Figure 8.
For the WISE, a comparison of detection performance of the
uncued methods against what is possible with a priori track
information.
Figure 9 details the pass of the WISE, an infrared space telescope
(NORAD 36119), shown in Figure 7. The left subplot
of Figure 9 shows the aggregate delay-Doppler map for the
detections. The orbital-derived higher order Doppler terms
have coherently integrated the targetmotion, as the detections
do not smear. The middle subplot shows each detection's
associated orbit propagated out 30 seconds, the wide fan in
these predicted orbits (when compared to the nominal truth)
because the WISE is only at its point of closest approach
instantaneously, and its Doppler varies by hundreds of hertz
in a matter of seconds. It is no surprise that the output of a
zero-Doppler matched filter will have errors when the target
is not at zero-Doppler. The right subplot of Figure 9 shows
the same orbital predictions, except this time the orbital estimate
has been updated with the detection's Doppler value.
This is done by feeding back the measured Doppler shift into
(12) to adjust the hypothesized orbit. Now, with the correct
Doppler value incorporated, there is a significantly improved
agreement between the detected orbits and the truth.
Figure 10 shows the same detection-level orbital estiFigure
9.
Aggregate detections of the WISE. The top subplot is an aggregate
of the delay-Doppler maps. The next two subplots show the
detection-level IOD from these detections. The left IOD subplot
has no Doppler correction and the right subplot shows orbits after
the detected-Doppler feedback.
The shaping caused by the Doppler mismatch is clear, as
the SNR decreases the further it is from its closest
approach.
Interestingly, Figure 8 shows that the detectability
of the WISE is at its greatest at a point some 30 seconds
after its closest approach. At this point, its
bistatic range is 70 km greater than the minimum, this
is most likely due to the WISE moving into a transmit
beam sidelobe. The baseline distance from the MWA
to Perth is insufficient to ensure that targets will be in
the main beam, even with the large FM elevation
beamwidths [31]. The vast surveillance volume above
the MWA will be punctured by transmit nulls and lowelevation
side lobes. There will also be similar volume
constraints imposed by the receiver beampattern. The
intersection of these limitations along with constraining
the search to only match orbital objects at minimum
range may be overly restrictive for uncued detection.
Limiting the search volume to match the beampatterns
and increasing the parameter search space may significantly
improve detection results.
26
mates as the right subplot of Figure 9, but now they are
propagated forward an hour. This shows good agreement
for such a considerable propagation time; after the 60
minutes, there is less than 1 cross-range error. These predictions
are noteworthy because they are each generated
from a single 3-s detection. Also, the orbits have only
been propagated forward with two-body propagation,
which itself will contribute to errors.
Of course, the goal with this work is not to try to generate
an orbit from a single detection, and integrating
many detections will improve any orbital estimate. However,
having such a rapid IOD step and feature-rich detections
will greatly assist ongoing tracking as well as
measurement association.
OTHERMETHODS
Other IOD methods from the " Initial Orbit Determination "
section have been used with the initial detections of the
Figure 10.
Detection-level IOD for the WISE, propagated 60 minutes
forward.
IEEE A&E SYSTEMS MAGAZINE
OCTOBER 2021

IEEE - Aerospace and Electronic Systems - October 2021

Table of Contents for the Digital Edition of IEEE - Aerospace and Electronic Systems - October 2021