GISMO Simulation Study Objective Key instrument and geometry parameters Surface and base DEMs Ice...
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Transcript of GISMO Simulation Study Objective Key instrument and geometry parameters Surface and base DEMs Ice...
GISMO Simulation Study
• Objective
• Key instrument and geometry parameters
• Surface and base DEMs
• Ice mass reflection and refraction modeling
• Algorithms used for space-borne phase history data simulation
• Data processing steps
• Results analysis
• Modifications for airborne simulation
Objective
• to perform analysis to validate the interferometric ice sounding technique for measuring the ice mass thickness in polar areas using a P-band space-borne SAR
• Analysis approach– Generate phase history data– Process the data into SLC data and interferograms– Band pass filtering to extract the basal contribution and
to derive the ice thickness from both surface and base interferogram
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
Surface and base DEMs (Greenland)
Surface DEM Base DEM
200 km
200 km
Ice mass reflection and refraction modeling
n1=1
n2 =1.8
n3 =3(for rocks)
1
2
basal DEM(land or water)
surface DEM
ice mass
S
A
B
C
Fig. 1 ice mass reflection and refraction model
H
D
h
d
s
xb
xs
Space-borne SAR phase history data simulation
• Reflectivity map calculation for both reference and slave antennas
• Phase history data generation
Reflectivity map calculation
1
2
basal DEM(n3: land or sea water)
surface DEM (n1)
ice mass (n2)
S(sensor)
A
B
C
ground range grids
Fig. 2 Implementation of reflectivity map calculation
S2
(sensor) B(baseline)
All quantities: slant range, incidence angle, refraction angle and reflection coefficients, are calculated at each ground range grid. A slant range grid will lie between two neighboring ground range grids. The reflectivity coefficient for each slant range grid is calculated through interpolation of these two neighboring ground range bins.
When calculating the reflection from the basal, we still start from the ground range grid on the surface. The refraction vector may or may not hit exactly the ground range grids. Bilinear interpolation is therefore used to calculate the refraction pointing vector from each surface ground grid to the basal. At each surface ground range grid the basal reflection coefficient and the slant range from the sensor to the basal are calculated.
All the calculations for the second orbit are the same as for the reference orbit except the interferometric phase, which is the result of the non-zero baseline and DEMs, is added to the secondary reflectivity map for both surface and basal calculations.
Phase history simulation
• Inverse chirp scaling
Phase history data
HSAR(f) SLC data
Reflectivity map
H-1SAR(f) Phase history data
Data Processing
• SAR processor (Vexcel’s FOCUS)
SLC data• IFSAR processor (Vexcel’s RAMS2)
Interferograms• Interferometric ice sounding processing
– Band-pass filtering to extract the basal interferogram
– Derive surface and base topography
Results Analysis
• Interferogram with both surface and basal contributions
• Interferogram spectrum analysis • Extracted basal interferogram using band-pass
filter• Comparison between the true and derived ice mass
thickness
Results Analysis ……
• 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
Results Analysis ……
• Amplitude images of the phase history data
Results Analysis ……
• Amplitude images of the SLC data
Results Analysis ……
• 80-azimuth-look interferogram 2
Slant range 11.8 km ( ground range 70 km )
Azimuth (130 km)
0
Results Analysis ……
• Interferogram range spectrum
-35
-30
-25
-20
-15
-10
-5
0
-0.01 -0.008 -0.006 -0.004 -0.002 0 0.002 0.004 0.006 0.008 0.01
Frequency (1 / m )
Sp
ec
tra
l A
mp
litu
de
(d
B)
The peak at 0 frequency represents the surface contribution and the peaks at the right side are from base contribution.
2
Slant range 11.8 km ( ground range 70 km )
Azimuth (130 km)
0
Results Analysis ……
• Band-pass filtered interferogram
Results Analysis ……
Comparison between the true and derived ice mass thickness
+2500 m
+2137 m
2850 m
1714 m
ground range 70 kmground range 70 km
Results Analysis ……
• Comparison between true and extracted basal interferograms
true basal interferogram extracted basal interferogram from band-pass filtering
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
Air Turbulence Simulation
• Along track: x = xa • sin(2fxat + xa)
+ xe • sin(2fxet + xe)
• Horizontal: y = ya • sin(2fyat + ya)
+ ye • sin(2fyet + ye)
• Vertical: z = za • sin(2fzat + za)
+ ze • sin(2fzet + ze)
Varying PRF Simulation
• PRF = PRFn + PRF • sin(2fPRFt + PRF)
Interferometric mode with 2+ receiving antennas
• Current space-borne Scatter: repeat pass mode• Future airborne Scatter:
– Repeat pass mode
– Single pass mode• One transmitting/receiving with others receiving
• Ping-Pong mode ?
Inverse chirp-scaling for varying PRF and varying sensor speed
• The phase history data created from the inverse chirp scaling algorithm apply to a staright line path and uniform along track spacing.
• The data need to be interpolated for a curved path, varying PRF and sensor speed
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
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
Idealized Straight Flight Paths
Ground Reference Point
Multi-Pass SAR Imaging
Synthetic Elevation Aperture
Ground Reference Point
Synthetic Aperture
Simulated Results • Straight & Parallel Paths
40 41 42 43
44 45 46 47
48 49 50
Grazing Angle
Illumination
Illumination
19.2 m
19.2 m
Image Formation
Plane
Simulated Results • Straight & Parallel Paths
40 41 42 43
44 45 46 47
48 49 50
Grazing Angle Illumination
Coherent Sum
19.2 m
19.2 m
Backprojection Plane
Image Formation
Plane
Vexcel’s tomography research
Classified
Tomography results from E-SAR
• L-band• Nominal orbit altitude: 3600 m• Number of flights: 14• Total vertical aperture: 280 m• Vertical resolution: 3.5 m
E-SAR Height/azimuth slice tomogram
E-SAR Height/azimuth slice tomograms
Using MUltiple SIgnal Classification algorithm (MUSIC) with pre-assumed one or five scatterers
GISMO’s Potentials for Tomography Applications
• One flight track– track altitude : 10 km
– 4 ~ 6 receiving antenna elements
– total aperture: 20 m
• Multiple flights– Assume 10 or more flights
– Total 40 ~ 60 measuremes
– total aperture: 400 m
H (flight height)
1
2
Baseline
D (ice thickness)
GISMO’s Potentialsfor airborne case
Ground range (look angle)
Ice thickness0 km
(0o)
1 km(5.7o)
2 km(11.3o)
3 km (16.7o)
4 km(21.8o)
100 m 10.8o 6.4o 4.2o 3.0o 2.3o
500 m 23.4o 18.3o 14.4o 11.5o 9.4o
1000 m 32.0o 26.7o 22.3o 18.6o 15.7o
2000 m 42.6o 37.2o 32.3o 28.1o 24.4o
Angular separations between the surface and base return
Total Baseline / Angular resolution :
20 m / 2o (single pass)
400 m / 0.1o (repeat pass)
GISMO’s Potentialsfor space-borne case
with 600 km orbit
Ground range (look angle)
Ice thickness10 km(0.95o)
30 km(2.86o)
50 km(4.76o)
75 km (7.12o)
100 km(9.46o)
100 m 0.74o 0.33o 0.2o 0.14o 0.1o
500 m 2.3o 1.38o 0.93o 0.66o 0.5o
1000 m 3.5o 2.41o 1.73o 1.26o 0.97o
2000 m 5.4o 4.0o 3.09o 2.34o 1.85o
Angular separations between the surface and base return
Total Baseline / Angular resolution :
45 m / 0.89o (single pass)
1000 m / 0.04o (repeat pass)
SAR Tomographic Ice Sounding
• Would repeat pass SAR tomographic ice sounding WORK ???
• Probably basal returns are still correlated even though the surface returns may corrupt the surface components of the tomogram.