Spot3-4 Ib-orbit focus measurement

1. 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)

2. 2
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

3. 3
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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 camera
B0, B2, B3, B4(1km)
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)
SPOT2
SPOT4
SPOT5

4. 4
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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)

5. 5
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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 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

6. 6
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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
1
1.1
1.2
-28 -24 -20 -16 -12 -8 -4 0 4 8 12
Focusing mechanism position
MTF
ratio
Defocus model
Measurement
Vertex
HRG1 refocusing (field center - columns)
-13.7
0.7
0.8
0.9
1
1.1
1.2
-28 -24 -20 -16 -12 -8 -4 0 4 8 12
Focusing mechanism position
MTF
ratio
Defocus Model
Mesurement
Vertex

7. 7
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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
0.8
0.9
1
1.1
1.2
-28 -24 -20 -16 -12 -8 -4 0 4 8 12
Focusing mechanism position
MTF
ratio
Defocus model
Measurement
Vertex
HRG1 refocusing (field center - columns)
-10.0
0.7
0.8
0.9
1
1.1
1.2
-28 -24 -20 -16 -12 -8 -4 0 4 8 12
Focusing mechanism position
MTF
ratio
Defocus Model
Mesurement
Vertex

8. 8
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

9. 9
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

10. 10
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

11. 11
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

12. 12
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

13. 13
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

14. 14
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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)

15. 15
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Point source
Xe lamp: 3kW Xe lamp: 1kW
Spotlights on a grassy uniform area

16. 16
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Point source
1
4
7
1
0
1
3
1
6
1
9
2
2
2
5
S1
S8
S15
0
20
40
60
80
100
120
140
160
180
200
220
240
260
Row-wise MTF (spotlight 17/06/02)
0.34
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Normalized frequency
MTF P2
fs/2

17. 17
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

18. 18
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Natural step edge
Fields near Phoenix (SPOT5 HRS2 10/06/02)
–Example of an almost horizontal edge
 along the track measurement

19. 19
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Natural step edge
HRS2 MTF (Mexicali 25/06/02)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.02 0.04 0.06 0.08 0.1
Frequency (m-1)
Across
track
MTF
MTF
MTF model
Example of result with HRS
• Method improvement: MTF model is fitted on MTF curve

20. 20
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Artificial edge target
Salon-de-Provence target (SPOT5 HRG1 26/07/02)

21. 21
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
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

22. 22
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Periodic target
Opportunity to acquire Stennis Space Center radial target with SPOT5
HM (5m) THR (2.5m)

23. 23
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement methods: Comparison
Comparison of SPOT5 HRG1 MTF measurements
Direction Rows Columns Diagonal
Spotlight 0.35 0.32 0.15
Step edge 0.33 0.30
Radial target 0.38 0.18
Ground 0.31 0.36
Specification 0.25 0.23
– Close results for different methods
– In-flight and ground measurements similar and better than specification

24. 24
D.
LEGER
International
Workshop
on
Radiometric
and
Geometric
Calibration
December
2-5,
2003
MTF measurement : Comments on best methods
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