On-orbit MTF assessment of satellite cameras
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Transcript of On-orbit MTF assessment of satellite cameras
Inte
rnat
ion
al W
ork
sho
p o
n R
adio
met
ric
and
Geo
met
ric
Cal
ibra
tio
n -
Dec
emb
er 2
-5,
2003 On-orbit MTF assessment of satellite
cameras
Dominique Léger (ONERA)
Françoise Viallefont (ONERA)
Philippe Déliot (ONERA)
Christophe Valorge (CNES)
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Introduction
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
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SPOT family Overview SPOT1,2,3 • HRV cameras
Pa (10m) B1, B2, B3 (20m)
SPOT4• HRVIR cameras
M (10m) B1, B2, B3, B4 (20m)
• Vegetation cameraB0, B2, B3, B4(1km)
SPOT5• HRG cameras
HM (5m) B1, B2, B3 (10m), B4 (20m)
THR (2,5m)
• HRS cameras (10 m)
• Vegetation cameraB0, B2, B3, B4 (1km)
SPOT2
SPOT4SPOT5
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Refocusing SPOT cameras
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)
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Refocusing SPOT cameras
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 m)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
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Refocusing SPOT cameras
Results of HRG1 refocusing operations (First stage)
– Vertex outside measurement points• Second stage needed
HRG1 refocusing (field center - rows)
-19.8
0.7
0.8
0.9
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1.1
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-28 -24 -20 -16 -12 -8 -4 0 4 8 12Focusing mechanism position
MT
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Defocus modelMeasurementVertex
HRG1 refocusing (field center - columns)
-13.7
0.7
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-28 -24 -20 -16 -12 -8 -4 0 4 8 12Focusing mechanism position
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Vertex
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Refocusing SPOT cameras
Results of HRG1 refocusing operations (second stage)
– Best focus (field center): p0-13• Astigmatism: -7
(one focusing step = 1.2 mm)
HRG1 refocusing (field center - rows)
-16.6
0.7
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-28 -24 -20 -16 -12 -8 -4 0 4 8 12Focusing mechanism position
MT
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HRG1 refocusing (field center - columns)
-10.0
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0.8
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-28 -24 -20 -16 -12 -8 -4 0 4 8 12Focusing mechanism position
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Defocus Model
Mesurement
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Refocusing SPOT cameras
Best focus and astigmatism vs field area(with respect to p0)
Final focusing– HRG1: p0-12
– HRG2: p0-7
HRG1 HRG2
Field area Left Center Right Left Center Right
Mean -9 -13 -11 2 -7 -11
Astigmatism -7 -7 -4 -2 -3 -7
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Relative MTF measurement method
– Both cameras image the same landscape (with and without shift)• Landscapes with a large frequency content (e.g. big cities)
– Three kind of imaging
1 HRG1
HRG2
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
L C R
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Absolute MTF measurement methods
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 SPOT4 HRVIR & SPOT5 HRS• Artificial target SPOT5 HRG
– Bi-resolution• SPOT4 HRVIR (vs airborne) SPOT4 VGT (vs HRVIR)
– Periodic target• SPOT5 HRG
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MTF measurement methods: Visual assessment
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
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MTF measurement methods: Point source
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
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MTF measurement methods: Point source
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
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MTF measurement methods: Point source
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)
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MTF measurement methods: Point source
Xe lamp: 3kW Xe lamp: 1kW
Spotlights on a grassy uniform area
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MTF measurement methods: Point source
1 4 7
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S8
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02 04 06 08 0
1 0 01 2 01 4 01 6 01 8 02 0 02 2 02 4 02 6 0
Row-wise MTF (spotlight 17/06/02)
0.34
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Normalized frequency
MTF P2
fs/2
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MTF measurement methods: step edge
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
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MTF measurement methods: Natural step edge
Fields near Phoenix (SPOT5 HRS2 10/06/02)
–Example of an almost horizontal edge along the track measurement
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MTF measurement methods: Natural step edge
HRS2 MTF (Mexicali 25/06/02)
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Acr
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MTF model
Example of result with HRS• Method improvement: MTF model is fitted on MTF curve
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MTF measurement methods: Artificial edge target
Salon-de-Provence target (SPOT5 HRG1 26/07/02)
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MTF measurement methods: Bi-resolution
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
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MTF measurement methods: Periodic target
Opportunity to acquire Stennis Space Center radial target with SPOT5
HM (5m) THR (2.5m)
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MTF measurement methods: Comparison
Comparison of SPOT5 HRG1 MTF measurements
Direction Rows Columns DiagonalSpotlight 0.35 0.32 0.15Step edge 0.33 0.30Radial target 0.38 0.18Ground 0.31 0.36Specification 0.25 0.23
– Close results for different methods
– In-flight and ground measurements similar and better than specification
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MTF measurement : Comments on best methodsArtificial 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