On-orbit MTF assessment of satellite cameras

1. On-orbit MTF assessment of satellite cameras Dominique Léger (ONERA) Françoise Viallefont (ONERA) Philippe Déliot (ONERA) Christophe Valorge (CNES)

2. 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. 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) SPOT5 SPOT4 SPOT2

4. 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. 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. Refocusing SPOT cameras • Results of HRG1 refocusing operations (First stage) • Vertex outside measurement points • Second stage needed

7. Refocusing SPOT cameras • Results of HRG1 refocusing operations (second stage) • Best focus (field center): p0-13 • Astigmatism: -7 (one focusing step = 1.2 mm)

8. Refocusing SPOT cameras • Best focus and astigmatism vs field area (with respect to p0) • Final focusing • HRG1: p0-12 • HRG2: p0-7

9. 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. 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. 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. 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. 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. 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. MTF measurement methods: Point source Spotlights on a grassy uniform area Xe lamp: 3kW Xe lamp: 1kW

16. MTF measurement methods: Point source

17. 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 • Modulusof 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. 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. MTF measurement methods: Natural step edge Example of result with HRS • Method improvement: MTF model is fitted on MTF curve

20. MTF measurement methods: Artificial edge target • Salon-de-Provence target (SPOT5 HRG1 26/07/02)

21. 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. MTF measurement methods: Periodic target • Opportunity to acquire Stennis Space Center radial target with SPOT5 HM (5m) THR (2.5m)

23. 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. 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