Global Ice Sheet Interferometric Radar Concept

1
Global Ice Sheet Interferometric Radar Concept

• Subsurface SAR and InSAR

2
GISMO Instrument

• GISMP Outline
• Science
• Spaceborne Instrument Concept
• Strawman Design
• Simulations
• Aircraft Experiment
• GISIR Team
• K.Jezek, E. Rodriguez, P. Gogineni, J. Curlander,
  X. Wu, J. Sonntag, W. Krabill, P. Kanagaratnam,
  C.Allen, A. Freeman. T. Akins
• D. MacAyeal, R. Forster, S. Tulazek, M.
  Fahnestock, S. Clifford

3
Science
4
Ice Dynamics and Prediction
Force Balance Equations
No Sat. Cover
Satellite Altimetry
Basal Drag, Inferred at best
Terms related to gradients in ice velocity
(InSAR) integrated over thickness
Understanding dynamics coupled with the
continuity equations yields predictions on future
changes in mass balance
5
The Dream Create a new 3-D mosaic stripped of
the icy cover
6
Glaciers and Ice Sheets Mapping Orbiter

• Key Measurements
• Determine total global volume of ice in glaciers
  and ice sheets
• Map the basal topography of Antarctica and
  Greenland
• Determine basal boundary conditions from radar
  reflectivity
• Map internal structures (bottom crevasses, buried
  moraine bands, brine infiltration layers)

7
Glaciers and Ice Sheets Mapping Orbiter

• Key Scientific Drivers
• Ice thickness (with other data) needed to map
  derived stresses and stress gradients force
  balance equations
• Ice thickness to estimate mass balance when
  combined with interferometrically derived ice
  velocity
• Internal layering of the ice sheet to unravel
  climatic history of the polar ice sheet
• Ice thickness to predict response of glaciers and
  ice sheets to changing climate and the impact of
  changing ice volume on global sea level
• Contribute technologies to planetary studies
  understanding the phenomenology of radar sounding

8
Global Ice Sheet Mapping Orbiter Prioritized
Science Requirements

• Measure ice thickness to an accuracy of 20 m or
  better
• Measure ice thickness every 1x1 km (airborne lt
  250 m)
• Measure ice thickness ranging from 100 m to 5 km
• Measure radar reflectivity from basal interfaces
  (rel. 2 dB)
• Obtain swath data for full 3-d mapping
• Measure internal layers to about 20 m elevation
  accuracy
• Pole to pole observations ice divides to ice
  terminus
• One time only measurement of ice thickness
• Repeat every 5-10 years for changes in basal
  properties

9
GISMO Concept
10
A New Technical Approach Required

• Nadir sounding profiler cannot meet science
  requirements
• Beam limited cross-track spatial
• resolution (1km) requires antenna
• size beyond current capabilities
• (420 m at P-band)
• Available bandwidth (and range
• resolution) insufficient for desired
• 10m height accuracy 6Hz gives
• a range resolution of 13.8 m in ice.
• Worse at VHF
• Full spatial coverage requires
• years of mission lifetime (high costs)

11
Other Conventional Approaches Limited

• Conventional Interferometry is Insufficient
• Coverage, spatial resolution, and height accuracy
  suggest a swath SAR interferometer might meet
  concept
• Ambiguous returns from surface clutter and the
  opposite side basal layer make this approach not
  feasible
• An alternate approach multiple baselines to
  resolve subsurface/surface, expensive and hard to
  implement
• Minimum of 3 antennas are required to get
  necessary baselines
• The technique is sensitive to calibration and to
  SNR
• Data rate is high

12
Primary Technical Challenge
Surface Clutter

• Separate basal return from surface clutter

Weak Echos Strong Attenuation
13

• The GISMO Heritage

NASA PARCA (initial radar Developments)
NSF Seed Study (SAR Feasibility and Algorithms)
NSF PRISM Ice Sounding SAR Demonstration
1992
NSF STC/ NASA ESTO Ice Sounding InSAR Radar
Tomography Multiaperture Beam Formation
1996
2001
2006
GISMO
2010
14
GISIR IIP Concept Evaluation Objectives

• Ice sounding performance at P-band and VHF
• SAR imaging of basal ice from aircraft
• Clutter rejection (Interferogram filtering
  tomography multi-aperture beam steering)
• Evaluation of ionospheric effects

15
Interferometric Sounding Concept

• Conventional interferometry uses phase
  information one pixel at a time

• Additional information contained in the spatial
  frequency of the phase
• Because of the difference in incidence angles,
  the near nadir interferometric phase spatial
  frequency from the basal return is much larger
  than the equivalent frequency for surface clutter
  
• Opposite side ambiguities have opposite
  interferometric frequencies while the phase in
  one side increases with range, it decreases with
  range in the opposite side (/- spatial
  frequencies of complex interferogram)

• IFSAR sounding concept spatially filter
  interferogram to retain only basal returns from
  one side

Satellite height (H) ice surface height (h)
Depth of the basal layer (D) topographic
variations of the basal layer (d) cross-track
coordinate of the basal layer point under
observation (xb) and, xs is the cross-track
coordinate of the surface point whose two-way
travel time is the same as the two-way travel
time for xb.
16
Interferometric Phase and Phase Frequency
Complex interferogram
Surface interferometric phase difference
Basal interferometric phase difference
Surface interferogram slope as a function of
range. hx is the surface topography cross-track
slope
Basal Interferogram slope as a function of
range. dx is the basal topography cross-track
slope
(From E. Rodriguez, 2004)
17
Surface vs Basal Cross-Track Distance
Surface cross-track distance xs as a function of
basal cross-track distance xb for a platform
height of 600km. Notice that near nadir xs is
nearly independent of xb, while for xb gt 50 km,
the two are close to being linearly dependent.
18
Fringe Spectrum

• Interferogram spectra for
• first 50 km of xb
• - signal to clutter ratio - 1
• - radar freq - 430 MHz
• - bandwidth of 6 MHz
• Basal spectrum is
• colored orange
• Surface spectra for
• - D 1 km (black),
• - D 2 km (red),
• - D 3 km (green),
• - D 4 km (blue).
• Note the basal fringe
• spectrum depends very
• weakly on depth

19
Clutter Effect on Interferometric Error
Height error as a function of signal to clutter
(a/b) for a baseline of 45 m and a center
frequency of 430MHz (solid lines) and 130 MHz
(dashed lines). The values of (a/b) are 0 dB
(black), -10 dB (red), and -20 dB (blue).
20
Height Noise vs SNR
Height error as a function of SNR for a baseline
of 45m and a center frequency of 430 MHz (solid
lines) and 130 MHz (dashed lines). The values of
SNR are 0 dB (blue), -5 dB (red), and -10 dB
(black). The number of looks is assumed to be 100.
21
What is the Effect of Surface Slopes?
Spectral broadening due to surface slopes
Relative spectral broadening is much less than
the spectral peak, as long as the angle of
incidence is small changes in the angle of
incidence are dominated by changes in cross-track
distance rather than changes in slope. This
argument does not hold for higher incidence
angles.
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• Strawman Design

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Desired Swath/Altitude

• A 50 km swath will enable a mission lifetime lt 4
  months
• A 50 km swath is consistent with the
  interferometric sounder approach for a height of
  600 km and a 6 MHz bandwidth
• Heights lower than 600 km will require a decrease
  in swath and a consequent increase in mission
  lifetime (although this is not a strong
  constraint)

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Ground Resolution vs Bandwidth
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Radar Frequency Selection

• Select P-band (430MHz) to minimize baseline
  length, antenna size and range resolution
• Antenna length of 12.5 m consistent with
  demonstrated space technology (TRL 9)
• Baseline range between 30m to 60m consistent with
  SRTM mast (TRL9)
• 6MHz bandwidth possible at P-band
• Higher clutter mitigated by interferometric
  sounder technique
• VHF only feasible using repeat pass
  interferometry, although antenna size (40m) may
  be problematic.
• Can 1 MHz remote sensing band be increased over
  the polar regions?

26
Pulse Length Selection

• In order to minimize contamination from nadir
  surface return, use short 20usec chirp
• surface nadir return sidelobes stops after 1.7 km
  of ice depth
• Much longer pulses will contaminate basal returns
  significantly

27
PRF Selection

• Required PRF for SAR synthetic aperture 1KHz
• Use significantly higher PRF (7KHz - 10KHz)
  together with onboard presum to improve signal to
  noise ratio
• Instrument duty cycle 20

28
Mission Concept

• P-band (430 MHz), 6 MHz bandwidth
• attenuation is essentially same at from 100 MHz
  to 500 MHz
• along-track resolution from SAR processing
• cross-track resolution from pulse bandwidth

• Two antennas, 45 m baseline, off-nadir boresight
  - 1.5 degrees
• mesh dishes, SRTM-like boom, 50 km swath from 10
  to 60 km cross-track
• use conventional nadir sounding for layering
  studies
• Fully polarimetric for ionospheric effects
• 600 km altitude, 1 year minimum mission lifetime

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Estimated Performance
30
Assumed Antenna Pattern

• Assume uniform circular illumination
• Antenna diameter 12.5m
• consistent with available space qualified
  antennas
• Antenna boresight 1.5deg
• Assumed antenna efficiency -2dB 1-way

31
Model Backscatter Cross Section

• The backscatter model consists of two
  contributions
• Geometrical optics (surface RMS slope dependent)
• Lambertian scattering
• assumed Lambertian contribution at nadir -25dB

32
Observed Surface Sigma0 Angular Dependence at 120
MHz

• Data obtained with the JPL Europa Testbed Sounder
  in deployment with the Kansas U. sounder over
  Greenland
• Angular decay near nadir (gt15 dB in 5 degrees)
  consistent with very smooth ice surface
• Change in behavior at P-band is still unknown,
  but probably bounded by 1-3 degree slope models

33
Estimated One-Way Absorption
Absorption losses present severe constraints on
system design if an SNR gt 0dB is desired.
Together with pulse length considerations, this
leads to a desired peak power of 5kW, which is
technologically feasible at P-Band
34
Sounder Performance 1 km Ice
35
Sounder Performance 2 km Ice
36
Sounder Performance 3 km Ice
37
Sounder Performance 4 km Ice
38
Spaceborne Simulations
39
Phase History Simulation

• DEM for surface and basal region from Greenland
• Assume homogeneous ice volume
• permittivity 3.4 for ice, 9 for bedrock
• intermediate layers are weakly scattering at
  off-nadir angles
• Attenuation 9 dB/km
• Ice thickness 2.0 to 2.5 km
• Process create reflectivity map of surface and
  subsurface
• Construct phase history data (PHD) using inverse
  chirp scaling
• Process PHD with COTS SAR processor and
  interfeometric processor to 80 looks

40
Key instrument and geometry parameters

• Platform Height 600 km
• Center Frequency 430 MHz
• Chirp Bandwidth 6 MHz
• Pulse Length 20 us
• PRF 2 kHz
• Antenna Length 12.5 m
• Antenna Boresight Angle 1.5o
• Baseline 45 m

41

• DEMS in slant range geometry

148.8 km (ground range)
148.8 km
echo delay caused by the ice thickness at nadir
137.5 km
(b) basal DEM
(a) surface DEM
42

• Amplitude images of the phase history data

43

• Amplitude images of the SLC data

44
Simulation Results for GISMO
surface and basal interferogram
original ice thickness
2500 m
Error is 0 to 20 m out to 50 Km
2137 m
Derived ice thickness (expected performance
reduction pass 50 km)
Band-pass filtered interferogram
2850 m
1714 m
Images are 130 km vertical (azimuth and 70 km
ground range (11.9 km slant range)
45

• Interferogram range spectrum

The peak near 0 frequency represents the surface
contribution and the peaks at the right side are
from base contribution.
46
GISIR Airborne Radar Experiment
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Scaling GISMO to Aircraft Altitudes
GISMO Airborne Experiment

• Preserve the fringe rate separations Basal
  distance for comparable fringe separations scale
  as the square root of the platform heights
  600km elevation satellite imaging a swath from 10
  km to 60 km corresponds to aircraft at 6 km
  imaging 1 to 6 km.
• Preserve number of fringes imposes a
  restriction on the baseline. 45 m spaceborne
  baseline means a 4.5 m baseline on aircraft. 10
  to 20 m better.
• Maintain geometric correlation Limiting phase
  change over a resolution cell imposes a
  restriction on bandwidth. Find 45-90 MHz
  depending on baseline. Larger bandwidth also
  preserves number of samples per swath
• Achievable with P-3 on the edge with Twin Otter

48

Radars for operation on P-3 or Twin Otter
aircraft
Three more antenna elements are being added on
each wing of a Twin Otter to eliminate grating
lobes
49
2006 Airborne Experiment Design

• May 2006 PRISM/CReSIS
• Twin Otter
• 150 MHz
• 3 Km Elevation
• 2 m baseline single pass (7 m?) 25 m baseline
  repeat
• 3 us pulse

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May 2006 Experiment Summary and Objectives

• Flight of opportunity to acquire early GISMO data
• Data acquired using KU 150 MHz radar and WFF
  navigation equipment
• Single pass and repeat pass data acquired
• Objectives in priority order
• investigate whether data of suitable signal
  strength and with suitable knowledge of aircraft
  navigation parameters could be acquired for
  successful InSAR processing for measurements of
  basal topography and reflectivity
• evaluate phase filtering clutter rejection
  concept
• evaluate tomographic imaging concept for clutter
  rejection
• Evaluate clutter rejection using multiple antenna
  elements

51
Single Pass Interferometry
Maximize altitude Maximize antenna array
separation
6 km swath
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Multi-Pass SAR Imaging
Synthetic Aperture
Synthetic Elevation Aperture
Ground Reference Point
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Antenna Array

• Radars
• Installation in Calgary
• Measure antenna pattern
• Antennas
• Twin Otter

Antenna arrays
Equipment rack
54
Radar - Specifications

• VHF Radar
• To measure thickness, map layers and image
  ice-bed interface
• Center freq 150 MHz
• Bandwidth 20 MHz
• Loop sensitivity gt 210 dB
• Depth resolution 3-4.5 m in ice
• Image
• Resolution
• Cross-track 40-100 m
• Along track 100 m
• Swath width
• Min 1000 m
• Maximum 1500 m

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Nadir Results
56
A-scopes
57

• May 2006 Navigation

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Navigation Techniques

• Two navigation tools available
• Soxmap
• Used for May 2006 GISMO
• Standard Twin Otter tool
• Visual aid for flight crew
• Best for following curved path
• Course Deviation Indicator (CDI)
• Will be used for 2007 GISMO
• Can couple to aircraft steering
• Good repeatability for long straight lines

59
23 May 2006 Mission Plan

• Flight plan was out-and-back
• Thule to Camp Century, then southeast along 18
  May 1999 ATM/KU flight track
• Inbound leg offset 25 m to south of outbound
• Constant 10,000' pressure altitude

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23 May 2006 Steering ErrorOutbound from Thule
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23 May 2006 Steering ErrorInbound to Thule
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Navigation Lessons Learned from May 2006

• 2006 GISMO flight used Soxmap / Twin Otter
  combination
• Configuration more suitable for outlet glacier
  work
• Steering within /-50m, could be better
• 2007 GISMO flights will use CDI with P-3
• Better repeatability for straight flight lines
• Soxmap backup
• More info atm.wff.nasa.gov click Aircraft
  Navigation

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May 06 Data Processing
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May 06 ExperimentFlight Route(MODIS Mosaic)
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Range and Azimuth Compressed Slant Range Images
(log scale)5200 m along track
First Airborne SAR Images of the Base of the
Greenland Ice Sheet
Base
Left Wing
Right Wing
Internal Layers
Surface
66
Ground Range Image(5800 m along track)
1.5 km (ground range)
67
First Airborne SAR Interferogram of the Base of
the Ice Sheet
Base
surface
noise
layers
68
Filtered (left) and Unwrapped (Right)
Interferograms Intensity modulated with
coherenceFirst steps towards computing swath
topography
filtered with an adaptive spectral filter then
unwrapped with a correlation threshold of 0.45.  
Color assignment of 360 deg 1 color cycle
69
Data Processing Steps
70
SAR Image
71
Interferograms from two adjacent channels
72
Interferogram using left wing and right wing
channel combinations
73
Ping Pong ModePing pong mode can be used to
increase the baseline but requires that multiple
transmitters and receiver systems be highly
correlated to maintain usable signal to noise
74
Range Offset SensitivityImage separation is to
small to use traditional image cross correlation
techniques so registration optimized by manually
sliding the images in range
75
Processing steps for bottom topography from
interferogram
Multi-look (10 Az x 1 Rg)
Single-look (intf_3_0_1_1)
76
Subset Phase
Processing steps for bottom topography from
interferogram Multi-look (10 Az x 1 Rg)
Subset Phase/Mag
Adaptive filter
Wrapped simulated baselines
-

Unwrapped simulated baseline
Flattened
Unwrapped
Void filling
No roll /-1 deg roll
77
Tomography

• Combining repeat pass image (Effectively 24
  channels)

surface
ground range (3000 m)
base
depth (1250 m)
78
2007 Airborne Experiment Design
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Technical Objectives for April 07 Experiment

• 1) Acquire data over the May 2006 flight line to
  compare high and low altitude observations and to
  compare interferometry acquired with different
  baselines. Are results consistent with theory?
• 2) Acquire data at 150 MHz and 450 MHz along
  every flight line and compare backscatter and
  interferometric frequency response? Are the
  results consistent with theory?
• 3) Acquire data over areas where we expect to
  find subglacial water. Is water detectable
  either from backscatter maps or from topography?
• 4) Acquire data over regions of increasing
  surface roughness. This may require observations
  over heavily crevassed shear margins such as
  those found around Jacobshavn Glacier. Can we
  successfully implement interferogram phase
  filtering?
• 5) Acquire data for tomographic analysis
• 6) Investigate repeat pass interferometry over
  repeat periods of days.
• 7) Verify volume clutter is weak (all snow zones)
• 8) Collect data over thick and thin ice to test
  for absorption effects

80
Proposed Flight Lines

1. Ice Streams
2. Outlet Glaciers
3. Jacobshavn

81
GISMO Web Connection

• Web site
• www-bprc.mps.ohio-state.edu/rsl/gismo/
• FTP Site
• ftp-bprc.mps.ohio-state.edu
• GISMO transfers under /rsl