Transcript 23_Raney_Smith_Delay_Doppler

Briefing for the Coastal Altimetry Workshop
The Delay-Doppler Altimeter
R. K. Raney
Johns Hopkins University Applied Physics Laboratory
05-07 February 2008
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• What is a Delay-Doppler altimeter?
• Precision
• Tracking
(Resource material)
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Conventional ALT footprint scan
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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
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Delay-Doppler (SAR mode*) Radar Altimetry
DDA: near-optimum partially coherent processing
N-pulse bursts
* CryoSat (Alcatel)
terminology for pulseto-pulse coherent
altimetry = “delayDoppler”
N-pulse bursts
Range
D2P data
N Doppler bins
INCOHERENT
COHERENT
...
0
Processing
similar to that of
Topex:
Along-track
FFTs
Range pulse
compression
Sum range
waveforms
(incoherent)
~ 2000 looks
per second
Range curve
correct
...
Doppler frequency is
proportional to
along-track angle
Range pulse
compression
M bursts per
along-track
resolution
(Doppler) cell
> 6000 looks
per second
...
. M powers
Sum
PDop at each
Doppler
...
. N
Sum
INCOHERENT
PDop
at each x
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Pulse-to-pulse
coherence => Doppler
data decomposition in
the along-track
direction
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DDA: a fundamentally different method
Vs/c
DDA spotlights each
along-track resolved
footprint as the satellite
passes overhead
(One result of Doppler
processing)
Footprint ~ 250 m;
independent of SWH
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On Precision (wrt Radar Self-Noise)
Doppler spectral decomposition => more incoherent averaging
Gamma Distribution as a function of N
(mean and peak normalized)
All radars are
“precision-challenged”
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Normalized PDF value
0.9
0.8
N = 1 (Single-look SAR)
0.7
N = 4 (Typical SAR image)
0.6
N = 16 (Mini-RF Lunar SAR)
0.5
N = 64 (WS scatterometer)
0.4
N = 200 (Radar ALT @ 10 Hz)
0.3
N = 600 (DDA @ 10 Hz)
0.2
N is the number of statisticallyindependent samples averaged
for a given measurement
0.1
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Sigma-zero distribution (nominal = 1)
R. K. Raney 30 Jan 2008
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*Delay-Doppler (DDA) = CryoSat “SAR mode”
COASTAL ALTIMETER WORKSHOP
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Delay-Doppler Altimeter (DDA) vs Conventional:
Precision at 1 Hz Rate*
Delay/Doppler ~ x2 better than conventional
Height
precision
(cm)
2.5
Wind speed
(Sigma-0)
precision
0.03
(dB)
SWH
precision
(cm)
3.5
3.0
2.0
2.5
0.02
1.5
2.0
1.5
1.0
0.01
1.0
0.5
10 dB SNR
10 dB SNR
No noise
No noise
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0
0
1
2
3
4
Significant wave height (m)
Initatvs\NASA\precisn.wpg
Courtesy:
J. R. Jensen
10 dB SNR
0.5
No noise
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0
1
2
3
4
Significant wave height (m)
*
Based on computer simulations
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1
2
3
4
Significant wave height (m)
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Telemetry
Command
On Range-gate Tracking
Doppler spectral decomposition =>
more selectivity in tracker input
Interface
s/c time
Waveform
Tracker
Processor
Q
I
ADC
ADC
The waveform processor
is essentially the only
difference between a
conventional radar
altimeter and a delayDoppler instrument.
Quadrature
mixers
Fine
height
Synchronizer
CW
90o
rf clock
AGC
Deramp
mixer
The “radar”
part of the
altimeter
Gain
Coarse height
LNA
Chirp
Xmtr
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“Smart”On-Board
Tracker: Doppler-Beam
Processing Limited
...
...
cw / Delay
X
Range deramp
Data store
Line number
(e.g. 64 lines)
Range-gate
tracker
Along-track FFT
Doppler
(e.g. 64 bins)
Range
Delay
phase
coefficients
X
Delay IFFT
Doppler
Along-track position
at zero Doppler
Waveforms
only from
preferred
Doppler bins
Doppler/position mapping
Look integration (e.g. 140 per bin)
...
Elevation estimates
(Doppler bin => footprint size; e.g. 250 m)
Unique to
delay/Doppler
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))
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DDA Beam-Limited Tracker Algorithm
Vs/c
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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Delay-Doppler Altimeter Summary
 Precision: The standard deviation of the inherent radar selfnoise 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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Back-up/Resource Material
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On Pseudo-conventional ALT waveforms
SAR mode => both DDA and conventional waveforms
Radar hardware is virtually
the same as conventional
Transmitter, receiver, range
gate tracker and AGC,
Stretch demodulation
(linear FM deramp)
(
Processing
Full resolution individual
waveforms
4 MHz
Presum n
returns,
followed by
conventional
altimeter
waveform
summations
DDA
processing
10 kHz
28 kHz
Averaged waveform
output data rates
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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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Altimeters
Compared
Unique
Delay-Doppler
Characteristics
DDA: More averaging => x2 better precision, x10 better efficiency
Advantage:
along-track
• DDA paradigm retains advantages
incidence and
of a pulse-limited altimeter (pulseDoppler equivalence
(modulo PRF) Delay/Doppler
Conventional
limited across-track)
Pulse length
• SSH accuracy (to first order) does
NOT depend on attitude
control/knowledge of the s/c
Pulse length
• Instantaneous pulse-limited
Pulse
footprint along-track length
limited Δx is
PulseAnnuli of
footprint
Doppler
equalwith
areas increasing SWH
constant
limited
Doppler modulation
Processing: remove
extra delay due to
wavefront curvature,
which converts all
data along-track to
height measurements
On-board Doppler
selection enables “smart”
tracking to ignore nearshore clutter
Along track
footprint
• Sample posting rate @ n /km gives
rise to a constant effective
footprint
0
along-track length 1
2
3
Leff ≈ ( 6.7/n
) km
timefor
delay
• Along-track posting rate Relative
27 Hz
250-m resolution
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0
1
Multi-looks at each location
2
3
Two-dimensional
section of the
angular scattering
function at each
and every
subsatellite point
Δx ~250 m
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Curvature Delay vs Doppler
Before azimuth FFTs the curvature delays are multi-valued
x
xn
Along-track (memory)
x2
x1
~
~
x - xn
R1
R2
Rn
r
Doppler
(slopes)
within
window
{
f1
f2
fn
Observation
window
for one burst
Delay histories
of scatterers
Delay axis
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Compressed pulse length
After Doppler
decomposition
range curvature is
single-valued
Doppler bin number
24
16
8
(a) spacecraft,
Without
delay/Doppler
before
compensation
0
compensation
-8
-16
-24
-32
-2
0
4
6
8
10
relative range (m)
24 24
16 16
88
spacecraft,
(b)
delay/Doppler
After
processed
delay/Doppler
compensation
Doppler bin number
Processing consists of an
frequency/phase multiply in
delay within each Doppler
bin, one set of such
operations per burst
2
00
-8 -8
P ower (per
Dop ple r bin )
-16-16
-24-24
-32-32 -2
-2
Delay
0
0
2
4
6
8
10
2
4
6
8
10
Relative range delay (meters)
relative range (m)
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Conventional and Delay Doppler Altimeters (DDA)
Measure the same Three Parameters
Delay Doppler Altimeter:
TOPEX
Jason-1
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
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
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Precision and Accuracy Trends
Accuracy (cm)
Precision (cm)
10
100
Conventional
altimeter lower “limit”
Height
ACCURACY
(orbit-dependent) is
the essential
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attribute of SSH
measurements,
Sun-synchronous
thence global
orbit lower “limit”
topographic studies
and climatology
1
(e.g annual sea
1975
1985
level rise)
1
Delay-Doppler*
break-through
0.1
1995
2005
Height
PRECISION
(instrument
dependent) is the
essential
measurement
attribute for
geodesy,
bathymetry, and
mesoscale
oceanography,
and enhances
near-shore
measurements
*Delay-Doppler (DDA) = CryoSat “SAR mode”
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