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The Delay-Doppler Altimeter

Briefing for the Coastal Altimetry Workshop. The Delay-Doppler Altimeter. R. K. Raney. Johns Hopkins University Applied Physics Laboratory 05-07 February 2008. What is a Delay-Doppler altimeter? Precision Tracking (Resource material). ). ). ). ). ). ). ).

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The Delay-Doppler Altimeter

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

  2. What is a Delay-Doppler altimeter? • Precision • Tracking • (Resource material)

  3. ) ) ) ) ) ) ) Conventional ALT footprint scan Vs/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

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

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

  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

  7. Delay-Doppler Altimeter (DDA) vs Conventional: * * Courtesy: J. R. Jensen

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

  9. “Smart” Tracker: Doppler-Beam Limited Waveforms only from preferred Doppler bins

  10. ) ) ) ) ) ) ) ) ) ) ) Tracker “reads” waveforms only from the center (1, 2, or 3) Doppler bins Result? Rejects all reflections from non-nadir sources DDA Beam-Limited Tracker Algorithm Vs/c

  11. Delay-Doppler Altimeter Summary • Precision: The standard deviation of theinherent 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

  12. Back-up/Resource Material

  13. On Pseudo-conventional ALT waveforms SAR mode => both DDA and conventional waveforms Radar hardware is virtually the same as conventional Full resolution individual waveforms Transmitter, receiver, range gate tracker and AGC, Stretch demodulation (linear FM deramp) 4 MHz ( Presum n returns, followed by conventional altimeter waveform summations DDA processing Processing Averaged waveform output data rates 10 kHz 28 kHz R. K. Raney 30 Jan 2008

  14. NASA-Funded proof-of-concept Aircraft Altimeter(D2P)Field campaigns 2000, 2001, 2002, 2003, 2005Johns Hopkins Universityhttp://fermi.jhuapl.edu/d2p

  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 • Leff ≈ ( 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

  16. Before azimuth FFTs the curvature delays are multi-valued

  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

  18. Conventional and Delay Doppler Altimeters (DDA) Measure the same Three Parameters 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 TOPEX Jason-1 • An altimeter measures: • Sea Surface Height  Range measurement • Significant Wave Height  Slope of return signal • Wind Speed  Strength of return signal ABYSS-Lite ~1 m antenna

  19. Precision and Accuracy Trends Accuracy (cm) Precision (cm) 10 100 Height PRECISION(instrument dependent) is the essential measurement attribute for geodesy, bathymetry, and mesoscale oceanography, and enhances near-shore measurements Conventional altimeter lower “limit” Height ACCURACY (orbit-dependent) is the essential attribute of SSH measurements, thence global topographic studies and climatology (e.g annual sea level rise) 1 10 Sun-synchronous orbit lower “limit” Delay-Doppler* break-through 0.1 1 1975 1985 1995 2005 GEOS-3 GFO ERS-1 Geosat Seasat Delay-Doppler TOPEX ENVISAT ERS-2 Jason-1 *Delay-Doppler (DDA) = CryoSat “SAR mode” R. K. Raney 30 Jan 2008

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