International Workshop on Radiometric and Geometric Calibration - December 2-5, 2003
On-orbit MTF assessment of satellite
cameras
Dominique Léger (ONERA)
Françoise Viallefont (ONERA)
Philippe Déliot (ONERA)
Christophe Valorge (CNES)
Introduction
2 D. LEGER International Workshop on Radiometric and Geometric Calibration December 2-5, 2003
Objective
– assessment of SPOT camera MTF
• to verify cameras requirements
• to compare in-flight and ground measurements
• to obtain accurate values to adjust deconvolution filters (SPOT5 THR)
Need to focus camera before MTF assessment
– due to possible slight defocus
• vibrations during launch
• transition from air to vacuum
SPOT family Overview
SPOT1,2,3
• HRV cameras
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Pa (10m) B1, B2, B3 (20m)
SPOT4
SPOT4
SPOT5
• HRVIR cameras
M (10m) B1, B2, B3, B4 (20m)
• Vegetation camera
B0, B2, B3, B4(1km)
SPOT2
SPOT5
• HRG cameras
HM (5m) B1, B2, B3 (10m), B4 (20m)
THR (2,5m)
• HRS cameras (10 m)
• Vegetation camera
B0, B2, B3, B4 (1km)
Refocusing SPOT cameras
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Method
–
–
–
–
Both cameras image the same landscape
One is used as a reference
Focusing mechanism of the other is moved
Calculation of the ratio of image spectra
• integration in band 0.25 fs - 0.35 fs
• calculations in row and column directions
• result is a function of position p of mechanism
– The curve looks like a parabola
• a defocus model is fitted on measurements
• the vertex gives the best focus
– Calculations vs field area
• center and edges (SPOT5)
Refocusing SPOT cameras
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Refocusing operation sequence (SPOT5 HRG)
– Before launch, the cameras are set on best vacuum mean focus p0
– First stage: slight defocusing around p0
• p0-8, p0+8, p0
(~±10 mm)
 mechanism validation
 first focus estimation p1
– Second stage: sufficient defocusing to overpass p1
– Final estimation of best focus
• row-wise and columnwise  astigmatism
• field center and field edges
– Setting the focus to best mean position
Refocusing SPOT cameras
1.2
HRG1 refocusing (field center - rows)
1.2
HRG1 refocusing (field center - columns)
-19.8
1.1
1
1
MTF ratio
1.1
MTF ratio
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Results of HRG1 refocusing operations (First stage)
0.9
0.8
0.7
-13.7
0.9
Defocus model
Measurement
Vertex
0.8
0.7
-28 -24 -20 -16 -12 -8 -4
0
4
Focusing mechanism position
8
12
– Vertex outside measurement points
• Second stage needed
Defocus Model
Mesurement
Vertex
-28 -24 -20 -16 -12 -8 -4
0
4
Focusing mechanism position
8
12
Refocusing SPOT cameras
1.2
1.1
HRG1 refocusing (field center - rows)
1.2
-16.6
HRG1 refocusing (field center - columns)
1.1
-10.0
1
MTF ratio
MTF ratio
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Results of HRG1 refocusing operations (second stage)
0.9
0.8
0.7
1
0.9
Defocus model
Measurement
Vertex
0.8
0.7
-28 -24 -20 -16 -12 -8 -4
0
4
Focusing mechanism position
8
12
– Best focus (field center): p0-13
• Astigmatism: -7
(one focusing step = 1.2 mm)
Defocus Model
Mesurement
Vertex
-28 -24 -20 -16 -12 -8 -4
0
4
Focusing mechanism position
8
12
Refocusing SPOT cameras
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Best focus and astigmatism vs field area
(with respect to p0)
HRG1
HRG2
Field area
Left
Mean
-9
-13
-11
2
-7
-11
Astigmatism
-7
-7
-4
-2
-3
-7
Final focusing
– HRG1: p0-12
– HRG2: p0-7
Center Right
Left
Center Right
Relative MTF measurement method
– Both cameras image the same landscape (with and without shift)
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• Landscapes with a large frequency content (e.g. big cities)
– Three kind of imaging
1 HRG1
HRG2
L
C
R
2 HRG1
HRG2
3 HRG1
HRG2
1  Frequency content comparison between homologous areas
• Field centers, field edges
1+ 2 (3)  Frequency content comparison in the field of one instrument
• e.g. 1+2  HRG1 left edge versus HRG1 center
Absolute MTF measurement methods
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Overview of methods from SPOT1 to SPOT5
– Visual assessment
• HRV cameras SPOT1, SPOT2, SPOT3
– Point source method
• SPOT3, SPOT4, SPOT5
– Step edge method
• Natural target
• Artificial target
SPOT4 HRVIR & SPOT5 HRS
SPOT5 HRG
– Bi-resolution
• SPOT4 HRVIR (vs airborne)
– Periodic target
• SPOT5 HRG
SPOT4 VGT (vs HRVIR)
MTF measurement methods: Visual assessment
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SPOT1, SPOT2, SPOT3 HRV cameras
– Only panchromatic band
Aerial imagery of urban sites
– 20 sites chosen in the south of France
Simulation of the corresponding satellite imagery
– For each site, images with decreasing MTF are simulated
– The whole set of images is called MTF catalog
In-flight, visual comparison of actual and simulated images
– MTF of the catalog image nearest to the actual image gives a rough
assessment of the in-flight MTF
MTF measurement methods: Point source
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SPOT3 HRV, SPOT4 HRVIR, SPOT5 HRG
– Pa and XS bands
Image of a spotlight aimed at the satellite
– In SPOT5 THR mode, the PSF is sufficiently sampled
• MTF is obtained by Fourier transform of the PSF
In other modes, two ways to overcome PSF undersampling
– To use a MTF model
– To combine several images to rebuild sufficiently sampled image
• or to use several spotlights
MTF measurement methods: Point source
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Unique point source method
– Integrating point image (row-wise or columnwise)
• 1D problem
– Reference LSF = FT(parametric 1D MTF model)
• Two parameters: MTF and phase (versus sampling grid)
– Matching LSF samples with reference
 Value of the MTF parameter
• Corresponding MTF = 1D in-flight MTF
 Value of the phase parameter
Stability of MTF
– Possibility to mix the various sets of LSF samples
• If different phase parameters
MTF measurement methods: Point source
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Two point source method
– Simplified version of point source array
– Integrating point image (row-wise or columnwise)
• 1D problem
– Hypothesis MTF is negligible beyond frequency sampling
 Two points are sufficient
– Experiment with two spotlights (SPOT5)
MTF measurement methods: Point source
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Spotlights on a grassy uniform area
Xe lamp: 3kW
Xe lamp: 1kW
S15
25
22
19
16
13
10
7
S8
4
260
240
220
200
180
160
140
120
100
80
60
40
20
0
1
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MTF measurement methods: Point source
S1
Row-wise MTF (spotlight 17/06/02)
1
MTF P2
fs/2
0.9
0.8
0.7
0.6
0.5
0.4
0.34
0.3
0.2
0.1
0
0
0.1
0.2
0.3 0.4 0.5 0.6 0.7
Normalized frequency
0.8
0.9
1
MTF measurement methods: step edge
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Step edge method
– Image of a target (artificial or natural) with a sharp transition between dark
and bright area
– With a slight edge inclination, we can interleave successive rows (or columns)
to rebuild a sufficiently sampled response to Heaviside function
• Again, this is not necessary with THR mode
– Modulus of ratio of FT (edge response) to FT (edge) = in-flight MTF
Two kinds of edge
– Natural edge: agricultural fields
• Difficulty to find a good one and to validate it
– Artificial edge
• A checkerboard target has been laid out (Salon-de-Provence in south of France)
• 60 x 60 m
MTF measurement methods: Natural step edge
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Fields near Phoenix (SPOT5 HRS2 10/06/02)
–Example of an almost horizontal edge

along the track measurement
MTF measurement methods: Natural step edge
Example of result with HRS
HRS2 MTF (Mexicali 25/06/02)
1
0.9
0.8
Across track MTF
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• Method improvement: MTF model is fitted on MTF curve
MTF
MTF model
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.02
0.04
0.06
Frequency (m-1)
0.08
0.1
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MTF measurement methods: Artificial edge target
Salon-de-Provence target (SPOT5 HRG1 26/07/02)
MTF measurement methods: Bi-resolution
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Principle
– Same landscape acquired with two spatial resolutions (same spectral band)
• High resolution image = reference
• Low resolution image = sensor under assessment
– In-flight MTF = Modulus of ratio of FT (LR image) to FT (HR image)
Two situations
– Satellite image versus aerial image
• Attempt with SPOT4 HRVIR
– Both sensors on the same satellite
• Attempt with SPOT4: VGT1 versus HRVIR
MTF measurement methods: Periodic target
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Opportunity to acquire Stennis Space Center radial target with SPOT5
HM (5m)
THR (2.5m)
MTF measurement methods: Comparison
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Comparison of SPOT5 HRG1 MTF measurements
Direction
Spotlight
Step edge
Radial target
Ground
Specification
Rows
0.35
0.33
0.38
0.31
0.25
Columns
0.32
0.30
Diagonal
0.15
0.18
0.36
0.23
– Close results for different methods
– In-flight and ground measurements similar and better than specification
MTF measurement : Comments on best methods
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Artificial step edge
– Well suited to high-resolution satellites (GSD < 5 m Salon-de-Provence target)
 Target building and maintenance expensive
 Only two measurement directions
Spotlight
– Suitable to GSD up to 30m
– No orientation constraint
 Needs a team on ground
Bi-resolution
– Attractive with different GSD cameras aboard the same satellite
Radial target
– Interest of visual assessment in addition to MTF measurements
– No orientation constraint
 Target building and maintenance expensive