Remote sensing of ocean surface using digital communication signals

Rashmi Shah, Purdue University

Abstract

Accurate measurement of key ocean parameters is essential for atmospheric and weather modeling. Hence, sustained efforts over recent decades have led to the development of several methods, including altimetry, scatterometry, and radiometry for remotely sensing parameters like Significant Wave Height (SWH), Sea Surface Height (SSH), Mean Wave Period (MWP), Sea Surface Salinity (SSS), tidal level, wind speed, and wind direction. These techniques have been demonstrated during a series of satellite missions, including TOPEX/Poseidon, Jason-1, Jason-2, and QuickSCAT, evolving from “pathfinder” or demonstration experiments to operational missions. This dissertation explores a technique for measuring ocean surface topography parameters using an innovative application of reflected navigation and communication signals in a bistatic configuration. This dissertation applies an ocean remote sensing method, first developed for Global Navigation Satellite System Reflectometry (GNSS-R) signals, to other Signals of Opportunity (SoOp), specifically reflected digital communication satellite signals. The fundamental observation is the time series of the Interferometric Complex Field (ICF) of the reflected signal. Relationships are derived between the coherence time of the ICF time series and SWH and MWP of the sea surface. The first part of this dissertation evaluates different forward models based on ICF measurements. For this, direct and reflected signals from S-band satellite transmissions providing the commercial XM Radio service were recorded at Platform Harvest over a 65 day period. In situ measurements from a nearby buoy were used to calibrate this measurement by determining coefficients of a semi-empirical model. SWH retrievals using this model on one minute of reflected signal observations were found to have a standard deviation of 0.38 m over the range of 1 to 4.5 m. An error analysis was done to show that the primary contribution to this error was uncertainty in the relationship between MWP and SWH. The reason for this uncertainty in the relationship between SWH and MWP was the non-fully developed sea during the majority of time period when the experiment was conducted. Also, first and second order SWH retrieval models were evaluated, and it was determined that they both have similar errors. The second part of the dissertation compares the retrieval of the SWH from reflected SoOp in L-, S-, and Ku-bands. The accuracy obtained for S-band and L-band was on the order of 0.4 m, but this model saturated with SWH values of approximately 3 m. A secondary algorithm was developed to retrieve SWH which used root-mean square (RMS) bandwidth. This model showed no saturation effect but the error stayed around 0.4 m. Also, the ICF coherence time and RMS bandwidth of Ku-band signals showed little sensitivity to SWH. Another cause of error that was found in S-band SWH retrieval was the degrading connector due to sea-water induced corrosion. Finally, the last part explores the possibility of performing altimetry measurements using Ku-Band data. Data recorded from 23 days were used for delay estimation. The computed delay was compared with the SSH observed by a tide gauge located at Platform Harvest. A system bias related offset of 33.86 m was found between the expected delay and computed delay. The precision of delay (standard deviation) was found to be 0.18 m with ab averaging time of 55 seconds. A theoretical standard deviation of delay estimation was computed from the chip length and expected signal-to-noise (SNR), and was found to be 0.18 m.

Degree

Ph.D.

Advisors

Garrison, Purdue University.

Subject Area

Physical oceanography|Engineering|Environmental science|Remote sensing

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