Radar sounding of ice from orbit has been successful on Mars
[1], is planned for the Galilean satellites [2], and is attractive
for earth [3] as a complement to current airborne instruments
[4], but of major concern is the poorly constrained but
potentially seriously limiting contribution of firn clutter [5].
To inform this issue, we analytically model electromagnetic
scattering in the upper 100 meters of the ice column for
continental ice sheets and evaluate the effects of variable
platform altitude, frequency, and range resolution on clutter
power. Our results show that volume scattering from air
inclusions is insignificant and unlikely to constrain deep ice
sounding. Rather, firn scattering is dominated by quasispecular reflections from layers of varying density which, at
orbital altitudes, may contribute significantly to clutter due to
the small angles of illumination. This layer clutter can be
mitigated by a careful choice of range resolution for center
frequencies below 200 MHz, but is practically unavoidable
above 250 MHz. Firn layer clutter is likely to significantly
constrain UHF orbital ice sounding, making a VHF
instrument the more practical choice.
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Firn Clutter Constraints on the Design and Performance of Orbital Radar Ice Sounders
Radar sounding is a powerful tool for constraining
subglacial conditions, which influence the mass balance of
polar ice sheets and their contributions to global sea-level rise.
A satellite-based radar sounder, such as those successfully demonstrated at Mars, would offer unprecedented spatial and temporal
coverage of the subsurface. However, airborne sounding studies
suggest that poorly constrained radar scattering in polar firn
may produce performance-limiting clutter for terrestrial orbital
sounders. We develop glaciologically constrained electromagnetic
models of radar interactions in firn, test them against in situ
data and multifrequency airborne radar observations, and apply
the only model we find to be consistent with observation to
assess the implications of firn clutter for orbital sounder system
design. Our results show that in the very high-frequency (VHF)
and ultrahigh-frequency (UHF) bands, radar interactions in
the firn are dominated by quasi-specular reflections at the
interfaces between layers of different densities and that off-nadir
backscatter is likely the result of small-scale roughness in the
subsurface density profiles. As a result, high frequency (HF)
or low VHF center frequencies offer a significant advantage in
near-surface clutter suppression compared to the UHF band.
However, the noise power is the dominant constraint in all bands,
so the near-surface clutter primarily constrains the extent to
which the transmit power, pulselength, or antenna gain can be
engineered to improve the signal-to-noise ratio. Our analysis
suggests that the deep interior of terrestrial ice sheets is a difficult
target for orbital sounding, which may require optimizations in
azimuth processing and cross-track clutter suppression which
complement existing requirements for sounding at the margins.
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- Award ID(s):
- 1745137
- NSF-PAR ID:
- 10143105
- Date Published:
- Journal Name:
- IEEE Transactions on Geoscience and Remote Sensing
- ISSN:
- 0196-2892
- Page Range / eLocation ID:
- 1 to 18
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1109/TGRS.2020.2976666
