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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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Radar Scattering in Firn and its Implications for VHF/UHF Orbital Ice Sounding
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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- Award ID(s):
- 1745137
- PAR ID:
- 10143107
- Date Published:
- Journal Name:
- IGARSS
- Page Range / eLocation ID:
- 4137 to 4140
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1109/IGARSS.2019.8898991
