Radar Altimeter Absolute Calibration Using GPS Water Level Measurements
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Date
2004-01
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Ohio State University. Division of Geodetic Science
Abstract
Recent studies of using long-term island and coastal tide gauges (over 60 years) indicate
that the global sea level rise is at a rate of 1.8 to 1.9 ± 0.1 mm/year (e.g., Douglas 1997, 1991;
Trupin and Wahr, 1990; Warrick and Oerlemans, 1990). Satellite radar altimetry has evolved
into a tool for synoptic observation of the global (±81.5° latitude) oceanic phenomena with
unprecedented accuracy (several cm in sea surface height) and with a temporal resolution of 1-2
weeks and a spatial resolution of 50 km. Its accuracy, global coverage, and temporal
resolution enable its use in studies of global sea level changes. With accurate links among
different satellite radar altimeters, a decadal (~ 15 years) altimeter sea surface height (ssh)
measurements can be obtained. However, the limitations of using altimeters to measure sea
level include inadequate knowledge of the instrumental biases and their potential drifts of each
individual radar altimeter.
The inherent requirements to enable the use of radar altimeters to measure global ssh
include knowledge of the altimeter biases to within 1-cm accuracy and their drifts to less than 1
mm per year. The mechanism is to conduct the absolute radar altimeter calibration with the
eventual goal to obtain the knowledge of its bias and drift with sufficient accuracy for sea level
studies. The goal of the radar altimeter absolute calibration is to determine the altimeter bias
and drift by comparing the altimeter-measured ssh with the accurate ground truth, often referred
to as in situ data sets. However, both systems contain different error sources and it is
necessary to formulate a closure equation and solved for the altimeter bias and drift with least
squares.
In this paper, a GPS buoy campaign in Lake Michigan was conducted by the
Laboratory for Space Geodesy and Remote Sensing Research of the Department of Civil and
Environmental Engineering and Geodetic Science, at the Ohio State University (CEEGS/OSU)
in cooperation with the National Geodetic Survey, National Oceanic and Atmospheric
Administration (NGS/NOAA) from March 20 to 24, 1999. The lake was chosen because of
the relatively calm water conditions such as waves and wind compared with oceans. 1-Hz
kinematic data obtained from a GPS buoy and the lake level record from 1993 to 1999
collected by the Holland West tide gauge were used in this study as the in situ data sets for the
absolute calibration of TOPEX/POSEIDON Side A (TSA) and Side B (TSB).
The GPS buoy and the Holland West tide gauge have not yet met the required accuracy
of absolute radar altimeter calibration. The geoid gradient is the most dominant error source
among others and it should be carefully avoided. The T/P bias and drift estimations in this
study are inaccurate because of the large geoid gradient in using GPS buoy data and the 10-cm
discrepancy in using the Holland West tide gauge data. However, it can be anticipated that the
accuracy will be improved with more data in the future and also with more data collected by
other calibration sites worldwide.
Description
The report was prepared by Kai-chien Cheng, a graduate research associate in the Department of Civil and Environmental Engineering and Geodetic Science, at the Ohio State University, under the supervision of Professor C. K. Shum. This report was supported in part by the National Oceanographic Partnership Program Grant (Dynalysis of Princeton #865618) and National Aeronautics and Space Administration TOPEX/POSEIDON Extended Mission Grant (NAG5-6910/JPL961462), National Aeronautics and Space Administration Earth Science Information Partnership CAN Grant (CIT #12024478), National Aeronautics and Space Administration Interdisplanary Science Project (NAG5-9335), National Science Foundation Digital Government Grant (EIA-0091494, and the Ohio Sea Grant Program (R/CE-5).
This report was also submitted to the Graduate School of the Ohio State University as a thesis in partial fulfillment of the requirements for the Master of Science degree.
This report was also submitted to the Graduate School of the Ohio State University as a thesis in partial fulfillment of the requirements for the Master of Science degree.