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  • Author or Editor: D. W. Hancock x
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N. Tran, D. W. Hancock III, G. S. Hayne, D. W. Lockwood, D. Vandemark, M. L. Driscoll, and R. V. Sailor

Abstract

The Geodetic Satellite (Geosat) Follow-On (GFO), Ocean Topography Experiment (TOPEX), and Poseidon altimeter white-noise levels have been evaluated using a technique based on high-pass filtering of 1-Hz sea surface height time series. High-pass filtering removes the geoid and oceanography signals while revealing the random noise. This filtering technique is simpler to use than the repeat-track method, gives essentially the same results, and makes it easier to analyze much larger amounts of data to investigate subtle variations in noise levels. The new noise-level measurements provided here all show stable noise-process characteristics from cycle to cycle, with a linear dependence of the noise level upon significant wave height (SWH). The GFO altimeter noise level is estimated to be 2.5 cm for an SWH of 2 m. The Poseidon noise level is estimated at 2.0 cm for the same value of 2 m SWH. The TOPEX altimeter noise level is 1.8 cm when the dual-frequency ionospheric correction is included; when this noisy correction is not used, the level is reduced to 1.5 cm. Although the dual-frequency ionospheric correction provides an average improvement over the “Doppler orbitography and radiopositioning integrated by satellite” (DORIS) correction, high-frequency noise enters into the dual-frequency correction via noise from the Ku- and C-band ranges. Because the variations in ionospheric refraction are a relatively long wavelength global effect (with strong dependence on latitude), the dual-frequency ionospheric correction should be low-pass filtered before use, and this correction should not be included when estimating the high-frequency noise level of the altimeter.

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R. Kwok, T. Markus, J. Morison, S. P. Palm, T. A. Neumann, K. M. Brunt, W. B. Cook, D. W. Hancock, and G. F. Cunningham

Abstract

The sole instrument on the upcoming Ice, Cloud, and Land Elevation Satellite (ICESat-2) altimetry mission is a micropulse lidar that measures the time of flight of individual photons from laser pulses transmitted at 532 nm. Prior to launch, the Multiple Altimeter Beam Experimental Lidar (MABEL) serves as an airborne implementation for testing and development. This paper provides a first examination of MABEL data acquired on two flights over sea ice in April 2012: one north of the Arctic coast of Greenland and the other in the east Greenland Sea. The phenomenology of photon distributions in the sea ice returns is investigated. An approach to locate the surface and estimate its elevation in the distributions is described, and its achievable precision is assessed. Retrieved surface elevations over relatively flat leads in the ice cover suggest that precisions of several centimeters are attainable. Restricting the width of the elevation window used in the surface analysis can mitigate potential biases in the elevation estimates due to subsurface returns at 532 nm. Comparisons of nearly coincident elevation profiles from MABEL with those acquired by an analog lidar show good agreement. Discrimination of ice and open water, a crucial step in the determination of sea ice freeboard and the estimation of ice thickness, is facilitated by contrasts in the observed signal–background photon statistics. Future flight paths will sample a broader range of seasonal ice conditions for further evaluation of the year-round profiling capabilities and limitations of the MABEL instrument.

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