1 / 19

260 likes | 880 Views

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). ). ). ). ). ). ). ).

Download Presentation
## The Delay-Doppler Altimeter

**An Image/Link below is provided (as is) to download presentation**
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.
Content is provided to you AS IS for your information and personal use only.
Download presentation by click this link.
While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

**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)**)**) ) ) ) ) ) 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**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**)**) ) ) ) ) ) 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**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**Delay-Doppler Altimeter (DDA) vs Conventional:*** * Courtesy: J. R. Jensen**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**“Smart” Tracker: Doppler-Beam Limited**Waveforms only from preferred Doppler bins**)**) ) ) ) ) ) ) ) ) ) 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**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**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**NASA-Funded proof-of-concept Aircraft Altimeter(D2P)Field**campaigns 2000, 2001, 2002, 2003, 2005Johns Hopkins Universityhttp://fermi.jhuapl.edu/d2p**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**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**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**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

More Related