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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 1 The Delay-Doppler Altimeter R. K. Raney Briefing for the Coastal Altimetry Workshop Johns Hopkins University Applied Physics Laboratory 05-07 February 2008

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 2 What is a Delay-Doppler altimeter? Precision Tracking (Resource material)

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 3 Conventional ALT footprint scan V s/c )))))) RA pulse-limited footprint in effect is dragged along the surface pulse by pulse as the satellite passes overhead ) Footprint: > 2 km; enlarges with SWH

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 4 Delay-Doppler (SAR mode*) Radar Altimetry DDA: near-optimum partially coherent processing Sum range waveforms (incoherent) Range pulse compression N-pulse bursts ~ 2000 looks per second Processing similar to that of Topex: INCOHERENT..... Sum M powers P Dop at each Doppler Along-track FFTs Range pulse compression Sum N P Dop at each x N-pulse bursts.. M bursts per along-track resolution (Doppler) cell Range N Doppler bins 0 Range curve correct... > 6000 looks per second COHERENT INCOHERENT D2P data * CryoSat (Alcatel) terminology for pulse- to-pulse coherent altimetry = “delay- Doppler” Doppler frequency is proportional to along-track angle

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 5 DDA: a fundamentally different method V s/c DDA spotlights each along-track resolved footprint as the satellite passes overhead (One result of Doppler processing) DDA spotlights each along-track resolved footprint as the satellite passes overhead (One result of Doppler processing) ))))))) Pulse-to-pulse coherence => Doppler data decomposition in the along-track direction Footprint ~ 250 m; independent of SWH

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 6 On Precision (wrt Radar Self-Noise) Doppler spectral decomposition => more incoherent averaging All radars are “precision-challenged” N is the number of statistically- independent samples averaged for a given measurement *Delay-Doppler (DDA) = CryoSat “SAR mode” R. K. Raney 30 Jan 2008

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 7 Courtesy: J. R. Jensen * * Delay-Doppler Altimeter (DDA) vs Conventional:

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 8 On Range-gate Tracking The waveform processor is essentially the only difference between a conventional radar altimeter and a delay- Doppler instrument. The “radar” part of the altimeter Doppler spectral decomposition => more selectivity in tracker input

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 9 “Smart” Tracker: Doppler-Beam Limited Waveforms only from preferred Doppler bins

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 10 DDA Beam-Limited Tracker Algorithm V s/c ))))))))))) Tracker “reads” waveforms only from the center (1, 2, or 3) Doppler bins Result? Rejects all reflections from non-nadir sources Tracker “reads” waveforms only from the center (1, 2, or 3) Doppler bins Result? Rejects all reflections from non-nadir sources

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 11 Delay-Doppler Altimeter Summary Precision: The standard deviation of the inherent radar self- noise is better by (approximately) a factor of 2, which reduces by a factor of 4 the extent of in-scene averaging required to get the same results => major advantage in near-shore applications Tracking: Decomposition of the received data into Doppler frequencies corresponds one-to-one to along-track beam-limited selectivity => a major advantage in near-shore applications Other: the DDA altimeter is simple and low-risk, user can choose (along-track footprint) size (in multiples of 250 m); relies on extensive heritage in space and airborne prototypes; and compatible with well-established retrieval algorithms

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 12 Back-up/Resource Material

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 13 On Pseudo-conventional ALT waveforms SAR mode => both DDA and conventional waveforms Transmitter, receiver, range gate tracker and AGC, Stretch demodulation (linear FM deramp) ( Presum n returns, followed by conventional altimeter waveform summations DDA processing 10 kHz28 kHz 4 MHz Averaged waveform output data rates Radar hardware is virtually the same as conventional Processing Full resolution individual waveforms R. K. Raney 30 Jan 2008

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 14 NASA-Funded proof-of-concept Aircraft Altimeter (D2P) Field campaigns 2000, 2001, 2002, 2003, 2005 Johns Hopkins University http://fermi.jhuapl.edu/d2p

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 15 Δx ~250 m Unique Delay-Doppler Characteristics DDA paradigm retains advantages of a pulse-limited altimeter (pulse- limited across-track) SSH accuracy (to first order) does NOT depend on attitude control/knowledge of the s/c Instantaneous pulse-limited footprint along-track length Δx is constant with increasing SWH Sample posting rate @ n /km gives rise to a constant effective footprint along-track length L eff ≈ ( 6.7/n ) km Along-track posting rate 27 Hz for 250-m resolution On-board Doppler selection enables “smart” tracking to ignore near- shore clutter

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 16 Before azimuth FFTs the curvature delays are multi-valued

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 17 After Doppler decomposition range curvature is single-valued Processing consists of an frequency/phase multiply in delay within each Doppler bin, one set of such operations per burst

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 18 An altimeter measures: Sea Surface Height Range measurement Significant Wave Height Slope of return signal Wind Speed Strength of return signal Delay Doppler Altimeter: 1. Low risk, low cost path to altimeter data continuity 2. Smaller footprint: 250m x 2 km 3. Measurement closer to shore: ~0.5 km (vs ~40 km) 4. Less s/c Mass and Power Conventional and Delay Doppler Altimeters (DDA) Measure the same Three Parameters TOPEX Jason-1 ABYSS-Lite ~1 m antenna

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04 Feb 2008_rkraney COASTAL ALTIMETER WORKSHOP 19 Accuracy (cm)Precision (cm) 10 1 0.1 100 10 1 GEOS-3 1975198519952005 Seasat Geosat ERS-1 TOPEX ERS-2 GFO ENVISAT Jason-1 Delay-Doppler Height ACCURACY (orbit-dependent) is the essential attribute of SSH measurements, thence global topographic studies and climatology (e.g annual sea level rise) Height PRECISION (instrument dependent) is the essential measurement attribute for geodesy, bathymetry, and mesoscale oceanography, and enhances near-shore measurements Precision and Accuracy Trends Sun-synchronous orbit lower “limit” Delay-Doppler * break-through Conventional altimeter lower “limit” R. K. Raney 30 Jan 2008 *Delay-Doppler (DDA) = CryoSat “SAR mode”

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