Global Ice Sheet Interferometric Radar

NASA ESTO IIPOhio State Univ., JPL, Univ. Kansas,

VEXCEL Corp., E.G&G Corp., Wallops Flight Facility

GISIR Instrument

• GISIR mid-year review– Science– Spaceborne Instrument Concept– Strawman Design– Simulations– Aircraft Experiment– Management

• 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

Science

Glaciers and Ice Sheets ‘Grand Challenges’

• Understand the polar ice sheets sufficiently to predict their response to global climate change and their contribution global sea level rise

•What is the mass balance of the polar ice sheets?

•How will the mass balance change in the future?

Reservoirs of Fresh WaterReservoirs of Fresh WaterIce Thickness Average: 2500m Maximum: 4500mFresh Water Resource

Polar Ice Sheets and Glaciers 77%East Antarctica 80%West Antarctica 11%Greenland 8%Glaciers 1%

National Geographic Magazine

Glacier/Ice Shelf Change• In last 4 years, numerous

observations of glacier/ice shelf reduction– Interior Antarctic Peninsula glaciers

(Scambos, 2004)– Larsen ice shelf (many)– Jacobshaven glacier in Greenland

(Thomas, 2003)– Pine Island and Thwaites (Rignot, 2001)

• None of these observations are predicted by climate models

0.8% decrease in Ice Shelf extentbetween 1963 and 1997

Advance (51,772.2 km2) Retreat (100,099.6 km2) Ice shelves Area without ice shelves

1963-1997

Kim and Jezek

Mass Balance• Ice sheet mass balance is described

by the mass continuity equation

Altimeters

InSAR

Act/Pass. Microwave

No spacebornetechnique available

Evaluations of the left and right hand sides of the equation will yield a far more complete result

Ice Dynamics and Prediction

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

Force Balance Equations

Basal Drag,Inferred at best

Satellite AltimetryNo Sat. Cover

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)

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 < 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

GISMO Concept

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)

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

Primary Technical Challenge

• Separate basal return from surface clutter

Surface Clutter

Weak EchosStrong Attenuation

Interferometric Sounding Concept

• 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)

•Conventional interferometry uses phase information one pixel at a time

•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.

Interferometric Phase and Phase Frequency

b 2kB1

s 2kBs

s 2kB1

xs

hx

H

s 2kB1

xb

dx

n(H D /n)

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

I e ib e i s Complex interferogram

(From E. Rodriguez, 2004)

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 > 50 km, the two are close to being linearly dependent.

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

Clutter Effect on Interferometric Error

Height error as a function of signal to clutter () for a baseline of 45 m and a center frequency of 430~MHz (solid lines) and 130 MHz (dashed lines). The values of () are: 0 dB (black), -10 dB (red), and -20 dB (blue).

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.

Height Error After Interferometric Filtering: 45m Baseline

Assumed clutter to signal ratio (same side): 0dB

Assumed clutter to signal ratio (opposite side): -3dB

Assumed SNR: 0dB

Assumed number of spatial looks: 100

Strawman Design

Desired Swath/Altitude

• A 50 km swath will enable a mission lifetime < 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)

Ground Resolution vs Bandwidth

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?

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

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%

Mission Concept

• 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

• 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

Key Instrument Parameters

Polarimetric 4 channelsCenter frequency 430 MHzBandwidth 6 MHzPulse length 20 sPeak transmit power 5 kWSystem losses -3 dBReceiver noise figure 4 dBPlatform height 600 kmAzimuth resolution 7 mPRF 10 kHzDuty cycle 20 %Antenna length 12.5 mAntenna efficiency -2. dB 1-wayAntenna boresight angle 1.5 degWavelength 0.7 mBaseline 45 mSwath 50 kmMinimum number of looks 500

Estimated Performance

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

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

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 (>15 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

Estimated One-Way Absorption

Absorption losses present severe constraints on system design if an SNR > 0dB is desired.

Together with pulse length considerations, this leads to a desired peak power of ~5kW, which is technologically feasible at P-Band

Sounder Performance 1 km Ice

Sounder Performance 2 km Ice

Sounder Performance 3 km Ice

Sounder Performance 4 km Ice

Spaceborne Simulations

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

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

DEMS in slant range geometry

echo delay caused by the ice thickness at nadir

(a) surface DEM (b) basal DEM

148.8 km (ground range) 148.8 km

137.5

km

• Amplitude images of the phase history data

• Amplitude images of the SLC data

+2500 m

+2137 m

+2850 m

+1714 m

surface and basal interferogram

Band-pass filtered interferogram

original ice thickness

Derived ice thickness (expected performance reduction pass 50 km)

Images are 130 km vertical (azimuth and 70 km ground range (11.9 km slant range)

Simulation Results for GISMO

Error is 0 to 20 m out to 50 Km

Interferogram range spectrum

The peak near 0 frequency represents the surface contribution and the peaks at the right side are from base contribution.

Airborne SAR processor

• Modify Vexcel’s current fast-back-projection space-borne spotlight SAR processor to be able to process the simulated airborne stripMap SAR data

Modifications for airborne simulation

• Airborne platform with air turbulence• Varying PRF• Interferometric mode with 2+ receiving

antennas• Inverse chirp-scaling modifications for

varying PRF and sensor velocity• Airborne SAR processor

GISIR Airborne Radar Experiment

Scaling GISMO to Aircraft Altitudes

• Preserve the fringe rate separations: Find that basal distance for comparable fringe separations scale as the square root of the platform hieghts: 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

Airborne Radar Depth Sounder: Rely on Design Heritage

Alo

ng-t

rack

(m

)

Cross-track (m)

150 MHz, HH, Greenland (Pass 1)

600 800 1000 1200

500

1000

1500

2000

2500

Cross-track (m)

150 MHz, HH, Greenland (Pass 2)

600 800 1000 1200

Rel

ativ

e P

ower

(dB

)

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

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

GISIR System Block Diagram

Return loss of a single antenna element. Sufficient bandwidth at 150 and 450 MHz

Preliminary Conclusions (1)

• The interferometric sounding concept has the potential to make the desired ice sheet depth measurement for depths > 1km

• For depths < 1km and for ice layering, conventional sounding approach may be sufficient – Results depend on P-band clutter roll-off

Preliminary Conclusions (2)

• At smaller depths (~1km) surface clutter contamination dominates: must be able to show clutter rejection for 10dB clutter to signal ratios for highest attenuation rates

• For other depths, clutter to signal ratios are much better and can be removed using IFSAR filtering

• At ~4 km depths, SNR is poor but sufficient to meet height accuracies

• There are no major technological hurdles in the design, although the power requirements are likely to be high (for deep ice)

• Airborne mission is planned for 2006 with multiple antennas

Preliminary Conclusions (3)

Project Management

GISIR Management Chart

NASA/ESTO

OSUProject Management

ScienceField Experiment

JPLInSAR AlgorithmData Processing

KURadar Development

Field Experiment

VexcelInSAR Simulation

Tomography Simulation

Science TeamScience ReqsTest data setsData validation

EG&G/WFFField ExperimentGPS Navigation

GISIR Schedule – January, 2006

GISMO Project Linkages

Infusion Opportunities• International Polar Year

• IPY Umbrella Proposal: Global Inter-agency IPY Polar Snapshot Year (GIIPSY)

• The goal of this proposal is to develop the most effective mechanism by which to plan and synchronise IPY satellite acquisition data requests • Represents scientists in ESA, US, Canada, China, Korea, Australia, Japan, Norway, Denmark, UK, Germany, Itaty

• IPY Investigator Proposal: A proposed international campaign to map the interior and base of the polar ice sheets• Campaign for Airborne Sounding of Ice Sheets (merged with ASAID)

• Airborne instrumentation development and data acquistion

• Glaciodyn Project (Devon Island overflight in 2007)

• NRC Decadal Survey RFI Response: Glaciers and Ice Sheets Mapping Orbiter (GISMO)

Captain Ashley McKinley holding the first aerial surveying camera used in Antarctica. It was mounted in the aircraft ‘Floyd Bennett’ during Byrd’s historic flight to the pole in 1929. (Photo

from The Ohio State University Archives)

CARISMA: P-band Nadir SoundingPotential for collaboration with GISMO –Flight of opportunity??

Infusion Opportunities: Carbon Cycle and Ice Sheet Mapping Mission ESA earth explorer proposal

Flight Direction

IIP Potential Collaborations

• Keith Raney, Applied Physics Lab Pathfinder Advance Radar Ice Sounder (PARIS) IIP project.

• Delwyn Moller, JPL – Ka-band Radar IIP Project

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

GISIR Publications Presentations

• Jezek, K.C., E. Rodriguez, P. Gogineni, A. Freeman, J. Curlander, X. Wu, J. Paden and C. Allen, in press. Glaciers and Ice Sheets Mapping Orbiter Concept. In press Journal of Geophysical Research - Planets.

• Rodriguez, E., and Wu, X. Interferometric ice sounding: a novel method for clutter rejection, in JPL review, to be submitted to IEEE Trans. Geoscience and Remote Sensing, 2005.

• Jezek, K.C. , The Future of Cryospheric Research. Byrd Polar Research Center Colloquy, The Ohio State University, Columbus, Ohio, October, 2005.

• Jezek, K.C., Spaceborne Observations of Antartica: The RAMP and GISMO Project. USRA Lecture Series, University of Alaska, Fairbanks, October 2005.

• Curlander, J., K. Jezek, E. Rodriguez, A. Freeman, P. Gogineni, J. Paden, C. Allen, X.Wu. Glaciers and Ice Sheets Mapping Orbiter. Advanced SAR Workshop, Montreal, Quebec, 2005.

• Forster, R., Jezek, K., E. Rodriguez, S. Gogineni, A. Freeman, J. Curlander, X. Wu, C. Allen, P. Kanagaratnam, J. Sonntag, W. Krabill, "Global Ice Sheet Mapping Orbiter" , FRINGE 2005 Workshop, Advances in SAR Interferometry from ENVISAT and ERS missions”, European Space Agency, ESRIN Frascati, Italy 28 November - 2 December 2005

• Jezek, K., E Rodriguez, P. Gogineni, A. Freeman, J. Curlander, X. Wu, C. Allen, P. Kanagaratnam, J. Sonntag, and W. Krabill. Glaciers and Ice Sheet Mapping Orbieter, EOS Trans., AGU, 86(52, Fall Meet. Suppl., Abstract IN13B-1092.

Key objectives for next 6 months

• Complete Vexcel airborne simulations

• Complete KU RF/digital design and system testing

• Complete first airborne campaign planning document

• Confirm aircraft availability

• Complete JPL motion compensation programs and back-propagation processor

Backup Slides

SAR Tomography Potentials of GISMO

• SAR tomography background

• SAR tomography simulation

• Results of E-SAR tomography tests

• GISMO potentials for SAR tomography applications

SAR Tomography Background• Conventional SAR Imaging

Ground Reference Point

Idealized Straight Flight Paths

Multi-Pass SAR Imaging

Synthetic Elevation Aperture

Ground Reference Point

Synthetic Aperture