Waveform Aliasing in Satellite Radar Altimetry

The full deramp pulse compression scheme employed by satellite radar altimeters digitizes each radar echo at a sampling rate matched to the chirp bandwidth. Echo power is undersampled by a factor of 2 when the power samples are simply obtained by squaring the magnitude of the echo samples, without first resampling, as is done in all altimeters to date. This results in inadequate sampling of the leading edge of the waveform if the significant wave height (SWH) is low. For a typical Ku-band altimeter with a chirp bandwidth of 320 MHz, simple squaring should be inadequate over ocean surfaces with SWH less than 2 m, that is, half of the ocean. Simply zero-padding the digital samples prior to range Fourier transform alleviates the problem introduced by magnitude squaring. Data from the CryoSat altimeter are used to demonstrate this remedy, and it is found that this reduces the variance in estimated range by 10% and in SWH by 20%. These improvements are confined to a range of SWH values between 1 and 4 m. Zero-padding also seems to have some small impact on values estimated over very flat surfaces (SWH ≪ 1 m), although theory suggests that better resolution of such surfaces would require a bandwidth exceeding 320 MHz. The 500-MHz bandwidth of the Ka-band altimeter on the Satellite with ARGOS and AltiKa mission should encounter these difficulties at smaller SWH values. Onboard range tracking and automatic gain control loops in future altimeters might be improved if zero-padding were employed during onboard waveform processing.

[1]  Laurent Rey,et al.  AltiKa: a Ka-band Altimetry Payload and System for Operational Altimetry during the GMES Period , 2006, Sensors (Basel, Switzerland).

[2]  G. Brown The average impulse response of a rough surface and its applications , 1977 .

[3]  Edward J. Walsh,et al.  Pulse‐to‐pulse correlation in satellite radar altimeters , 1982 .

[4]  Richard K. Moore,et al.  Microwave Remote Sensing, Active and Passive , 1982 .

[5]  William J. Caputi,et al.  Stretch: A Time-Transformation Technique , 1971, IEEE Transactions on Aerospace and Electronic Systems.

[6]  Walter H. F. Smith Seafloor Tectonic Fabric from Satellite Altimetry , 1998 .

[7]  L. Phalippou,et al.  CryoSat: A mission to determine the fluctuations in Earth’s land and marine ice fields ☆ , 2006 .

[8]  K. W. Cattermole The Fourier Transform and its Applications , 1965 .

[9]  Remko Scharroo,et al.  RADS: Consistent Multi-Mission Products , 2013 .

[10]  H. Tolman,et al.  Validation of a thirty year wave hindcast using the Climate Forecast System Reanalysis winds , 2013 .

[11]  L. Amarouche,et al.  Improving the Jason-1 Ground Retracking to Better Account for Attitude Effects , 2004 .

[12]  R. Scharroo,et al.  Integrating Jason-2 into a Multiple-Altimeter Climate Data Record , 2010 .

[13]  M. A. Tayfun,et al.  Narrow-band nonlinear sea waves , 1980 .

[14]  Walter H. F. Smith,et al.  Slope correction for ocean radar altimetry , 2014, Journal of Geodesy.

[15]  E. Walsh Analysis of experimental NRL radar altimeter data , 1974 .

[16]  G. S. Hayne,et al.  Corections for the effects of significant wave height and attitude on Geosat radar altimeter measurements , 1990 .

[17]  G. Hayne,et al.  Radar altimeter mean return waveforms from near-normal-incidence ocean surface scattering , 1980 .

[18]  Edward J. Walsh,et al.  Pulse Compression and Sea Level Tracking in Satellite Altimetry , 1989 .

[19]  L. Amarouche,et al.  Comparison of the Ku-Band Range Noise Level and the Relative Sea-State Bias of the Jason-1, TOPEX, and Poseidon-1 Radar Altimeters Special Issue: Jason-1 Calibration/Validation , 2003 .

[20]  A. Fung,et al.  Microwave Remote Sensing Active and Passive-Volume III: From Theory to Applications , 1986 .

[21]  Nasa Cr,et al.  TECHNICAL GUIDANCE AND ANALYTIC SERVICES IN SUPPORT OF SEASAT-A , 1975 .

[22]  John C. Ries,et al.  Chapter 1 Satellite Altimetry , 2001 .

[23]  J. Klauder,et al.  The theory and design of chirp radars , 1960 .

[24]  D. Wingham,et al.  The Rough Surface Impulse Response of a Pulse-Limited Altimeter With an Elliptical Antenna Pattern , 2010, IEEE Antennas and Wireless Propagation Letters.

[25]  IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 34. NO. 4, JULY 1996 Universal Multifractal Scaling of Synthetic , 1996 .

[26]  J. Robert Jensen Radar altimeter gate tracking: theory and extension , 1999, IEEE Trans. Geosci. Remote. Sens..

[27]  P. Beckmann,et al.  The scattering of electromagnetic waves from rough surfaces , 1963 .

[28]  R. Keith Raney,et al.  CryoSat SAR-Mode Looks Revisited , 2010, IEEE Geoscience and Remote Sensing Letters.

[29]  C.E. Shannon,et al.  Communication in the Presence of Noise , 1949, Proceedings of the IRE.

[30]  D. J. Wingham,et al.  A METHOD FOR DETERMINING THE AVERAGE HEIGHT OF A LARGE TOPOGRAPHIC ICE-SHEET FROM OBSERVATIONS OF THE ECHO RECEIVED BY A SATELLITE ALTIMETER , 1995 .

[31]  J. MacArthur,et al.  Design of the SEASAT-A Radar Altimeter , 1976 .

[32]  Ernesto Rodriguez,et al.  Altimetry for non‐Gaussian oceans: Height biases and estimation of parameters , 1988 .

[33]  Duncan J. Wingham,et al.  The mean echo and echo cross product from a beamforming interferometric altimeter and their application to elevation measurement , 2004, IEEE Transactions on Geoscience and Remote Sensing.

[34]  Robert H. Stewart,et al.  Methods of satellite oceanography , 1985 .