Inte

rnat

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al W

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sho

p o

n R

adio

met

ric

and

Geo

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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)

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HRG1 refocusing (field center - columns)

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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)

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HRG1 refocusing (field center - columns)

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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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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)

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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