40 years of experience with SPOT in-flight Calibration

International Workshop on Radiometric and Geometric Calibration, 2-5 Dec 2003, Gulfport, MSPage 1/30

40 years of experiencewith SPOT in-flight Calibration

C. VALORGE, A. MEYGRET, L. LEBEGUE, P. HENRY (CNES)A. BOUILLON, E. BRETON, R. GACHET (IGN)

D. LEGER, F. VIALLEFONT (ONERA)

+ C. LATRY, V. PASCALF. CABOT, F. DE LUSSY

Ph. KUBIK, A. MEYGRET, E. BRETON, F. MASSONM. PAUSADER, D. LEGER, L. POUTIER

O. HAGOLLE, A. MEYGRETM. DINGUIRARD, D. LEGERF. VIALLEFONT, R. GACHET

AC. DE GAUJAC

P. HENRY, X. BRIOTTETM. DINGUIRARD, D. LEGERAC. DE GAUJAC, P. GIGORD

G. BEGNI, B. BOISSINM. LEROY, D. PRADINES

M. DINGUIRARD, D. LEGERV. RODRIGUEZ, P. GIGORD

JP. DARTEYRE


Overview

SPOT system overview SPOT satellites SPOT system

Geometric calibration and quality assessment Geolocation model and accuracy Internal orientation Image deformation quality assessment

Radiometric calibration and quality assessment Radiometric model Normalization Absolute calibration

Spatial Resolution Refocusing MTF assessment

Summary


SPOT system overview

First generation : 2 identical instruments called HRV: 10m Pan - 20 m multispectral; steering mirror (+/-27°)

SPOT 1: launched 22 February, 1986put on a 560 km orbit in November 2003. Re-entry in 2019

SPOT 2: launched 22 January, 1990no more on-board recording since October, 1993

SPOT 3: launched 26 September, 1993failed on 14 November, 1996

SPOT 4: launched 24 March, 1998 New platform, same resolution New 20m SWIR band (HRVIR) VEGETATION payload

SPOT 5: launched 4 May, 2002 Resolution (HRG): 5 m in panchromatic mode, 10 m in spectral mode

2,5 m in panchromatic mode through processing (THR) Passengers: VEGETATION-2, HRS (High resolution stereo camera), Stellar Sensor

Current operational constellation: SPOT2, SPOT4 and SPOT5 Cumulated life on-orbit: 16 + 14 + 3 + 5.5 + 1.5 = 40 years ;-)


Satellite Operations and Control Center

Network of Direct Receiving Stations

ProgrammingCenter

Processing andArchiving Center

Image QualityExpertise Center


System description: 832km Sun-synchronous orbit; 26 days repeat cycle

900 km wide corridor, daily access Operational architecture



The Image Quality Expertise Center is in charge ofall in-flight activities regarding Image Quality:

Determination of the optimal on-board parameters (focus, radiometric gains, compression parameters…)

Elaboration of the best ground-processing parameters (normalization parameters, interior orientation…), validation and transmission to the processing stations

Periodic assessment of Image Quality Budgets (wrt specifications): « SPOT Image Quality Performances » issued every year, edited by Spotimage, provided to their customers

Analysis and resolution of any image quality problem occuring in-flight This implies specific capacities:

Dedicated programmations of the payloads (even non-nominal) Management of calibration sites and means Dedicated facility, computers, operational interfaces…

This Center is operated by technicians and engineers from CNES, IGN and ONERA (up to 20 people during commissioning phases, 5 for routine operations)


Geometric Calibration

Geolocation model Calibration: GCP database

Established since SPOT1, refurbished for SPOT5 (improved resolution, need of better accuracy)

Planimetric precision: better than 5m for most GCPs Sites covering at least 120 km x 120 km (HRS) + France Special emphasis on the scattering of the location sites around the world

One bundle block adjustment per calibration site involving all calibration acquisitions

Systematic programming of both HRV/IR/G + HRS Between 10 and 20 GCP per image More than 100 images per site in routine phases: robust estimations Possibility to correct for erroneous GCP coordinates Identification of correction parameters for each acquisition Yaw, pitch and roll biases are then analysed in terms of calibration, for

each instrument, with respect to the steering mirror position, latitude, …. Each new acquisition over a given location site is then added to the

corresponding block


Location sites over the world

Kenai

Los Angeles Tulsa Washington

Rothera

Cayenne

Sao Paolo

Spitzberg

Cap Nord

Grenade

Bamako

Izmit

Sanaa

Johannesburg

La Réunion

Perth Sydney

Darwin

Phnom Penh

Syowa

Dumont Durville

Main sites: 12Secundary sites: 4

No more used



Steering mirror viewing model:

Y.4

sin2NNN 2mnnr

Real normal Normal for a perfect mirror

mirror’s axis wedging defaults

levelness default between mirror axis and mirror plane

mirror ’s pointing angle

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// tr

ack

loca

tion

(met

ers)

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mirror angle (degrees)

// tr

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loca

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(m.)



Exterior orientation calibration: After steering mirror calibration: remaining errors translated into biases

between instrument and AOCS reference frames

Interest of the world-wide scattering of our sites: We quickly discovered an orbital variation of these « biases », the same

for each instrument on-board SPOT5 After analysis, due to a wrong reference date in the stellar sensor… Constant biaises after correction of this on-board problem

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Satellite orbital position (degrees)

yaw

(m

icro

rads

)

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-80 -40 0 40 80


pitc

h (m

icro

rads

)

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roll

(mic

rora

ds)

Orbital trends before correction

In yaw In pitch In roll



Geolocation accuracy assessment: Done simultaneously with calibration activities Stringent SPOT5 specifications concerning geolocation accuracy:

50m CE RMS for HRS 15m CE90 after bundle block adjustment without GCPs for Reference3D

=> intensive routine monitoring: at least, each site must be acquired during each repeat cycle (26 days) quasi-real time exploitation of these images

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across track location (meters)

alon

g tr

ack

loca

tion

(met

ers)

alon

g tr

ack

loca

tion

(met

ers)

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across track location (meters)

alon

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loca

tion

(met

ers)



Internal orientation: Absolute method

panchromatic and multispectral reference bands (HMA and B2)

each HRS band comparison image / reference

Relative method THR mode (HMA/HMB bands) Multispectral mode (B1/B2/B3/SWIR bands) Relative panchromatic/multispectral (P/XS)

comparison of pairs of simultaneous images

Quality assessment: Made simultaneously: same methods, different acquisitions, checked on

corrected images



Absolute internal orientation calibration: Reference data: Manosque test-site

Aerial cover of a 60 km 7 km area at 1.50 m resolution with 80% overlap

Triangulation: 0.40 m. accuracy Aerial digital surface model at 1 m

resolution

Method each aerial image is projected into SPOT5

image geometry (taking the MTF into account)

a fine image matching process measures differences between SPOT and the Reference

Filtering and averaging to get each detector orientation

Final modelling of these curves: Drift of along-track orientation = yaw Drift of across-track orientation =

magnification Higher degree tendancies = distortion

SPOT 5

Reference



Absolute internal orientation calibration: Corrections achieved:

Magnification of each instrument

Relative yaw: HRS1/HRS2, HRG1/HRG2 and P/XS

Optical distortion: up to fifth degree polynomials

After calibration: residuals < 15 cm RMS(limitation due to the reference)



Image deformation quality assessment: Dynamic perturbations monitoring

Dynamic perturbations: inertial wheels, magnetic tape recorder (SPOT1-4), steering mirror, attitude restitution errors

Specific programmations: phased pairs (26 days time lag => same viewing conditions), Image Quality mode (simultaneous image of both HRV/IR/G), autotest (SPOT5, mirror in auto-collimation position)

Dense image matching + line-wise averaging => profile vs time First conducted on SPOT1 as technology experiments (87) Nominal activity since SPOT2

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0 20 40 60 80Time (second)

roll shift (pixels)

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0 20 40 60 80

Time (second)

residual roll shift (pixels)

Use of a phased pair to determine the influence of a steering mirror move



Use of a IQ couple to determine the steering mirror stabilization time

Use of the autotest for the same purpose: only one acquisition during night

Autotest pattern



Image deformation quality assessment: Length distortion assessment:

Conducted along with geolocation activities, using the same GCPs Computed for each pair of GCP by comparing real/modelled distances Analysis as function of orientation and length

Planimetric accuracy assessment: Location accuracy after bundle block adjustment with GCP Can be assessed with high precision GCPs (residual analysis) Can be assessed along with altimetric accuracy (need for reference

DEM) Altimetric accuracy assessment:

Performed by value-added producers: IGN & ISTAR Operational production capacity: optimal conditions, completeness… Crucial point = reference DEM HRS SAP initiative under ISPRS framework

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0 10000 20000 30000 40000 50000

Distance (meters)

dist

ance

err

or (

met

ers)


Radiometric Calibration

Radiometric model: Push-broom sensors => same model for all HRV, HRVIR, HRG, HRS

8-bitsADC

Rad

X(k,n,b,m)

G(m,k)

C(k,n,b,m)A(k)

Optics &filters

Detector (n)

Read-outregister (b)

Amplification

g(k,n,b) (k,b)

R()

X(k,n,b,m)=R[A(k).G(m,k)g(k,n,b).(k,b).Rad(k,n,b)+C(k,n,b,m)]

Absolute calibration Normalization



Normalization Normalized digital count:

Dark currents calibration: Steering mirror in auto-collimation position (HRV, HRVIR, HRG) or night-

acquisitions (HRS) Obtained for each detector of each spectral band with each amplification

gain by averaging its digital counts Short term variation monitoring: 10 minute images Medium term variation monitoring: one acquisition per week, then per

month Difficult case: SWIR band (high increases due to proton collisions)

=> updated every week

),,().,().( bnKRadkmGkA(k,b)g(k,n,b)

C(k,n,b,m)X(k,n,b,m)Y(k,n,b,m)



Normalization Inter-detector coefficients calibration

Problems with on-board lamp (steering mirror positioning accuracy) Use of quasi-uniform landscapes: snowy expanses

Operationally heavy: 10% success (wheather, non-uniformity) Correction of Solar incidence before averaging

180°

80°

70°

Antarctic (winter) Greenland (summer)



Normalization quality assessment:

Made on uniform, normalized images: average line Different criteria:

High Frequency Low Frequency Inter-Array Even-Odd detectors

< 0.3 % for each E/O

HF

IA

LF



Signal-to-Noise Ratio (SNR) assessment: Column-wise Noise:

Use of on-board lamp (SPOT1-4) Use of quasi-uniform sites with 2 simultaneous acquisitions (to separate

instrumental noise from landscape signal) Noise model:

Physical understanding of noise sources (signal noise, digitization…) Simple model: Allows comparison of sensors in a common reference configuration

Line-wise Noise:cf. normalization qualityassessment

Image-Noise: combination of the twoprevious noises

RadKa .

Column-wise noise for SPOT4 HRVIR2 M, may 2003

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Radiance (W/m2/sr/mm)

Co

lum

n-w

ise

no

ise

(W

/m2

/sr/

mm

)

G1

G2

G3

G4

Modele G1

Modele G2

Modele G3

Modele G4Lref(B2)

c(Lref,G2)



Absolute Calibration: Operational use of many different methods Important for users, but also for amplification gain prediction G(m,k)

Gain calibration using IQ mode SPOT Histogram DataBase (started in 87)

Stores every cloud-free scene histogram on a 120km x 120 km grid Gives statistically significant estimation of observed radiances Monthly average used to predict optimal amplification gains



Absolute Calibration: on-board calibration systems Easy to use => frequent estimations On-board lamp: not absolute but temporal variations monitoring

Sensitivity variation measured with the lamp

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05

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24/3/98 6/8/99 18/12/00 2/5/02 14/9/03

Date

Var

iati

on

(%

)

HRVIR1 B1 HRVIR1 B2 HRVIR1 B3

HRVIR1 SWIR HRVIR2 B1 HRVIR2 B2

HRVIR2 B3 HRVIR2 SWIR



Absolute Calibration: on-board calibration systems Sun-sensor

Optical fibers (48 for HRV, 24 for HRVIR) projecting solar radiance onto some detectors of each spectral band

Highly difficult to characterize before launch On-orbit variation of fibers transmission Successful only for SPOT4, abandoned for SPOT5…

SPOT4 HRVIR1 B1

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1 501 1001 1501 2001 2501

Pixel number

Num

eric

al le

vel

0

0 0

).(

).().().()(

)().(

ds

dsTEjjTtu

Lk

kBE

o

FIBk

Eo(): spectral solar illumination u(t): Earth-Sun distance variation o(j): solid angle of fiber j TFIB: transmission of fiber j TBE(): spectral transmission of the

calibration unit Sk(): spectral sensitivity of channel k



Absolute Calibration: use of specific landscapes Rayleigh scattering

For short wavelengths (B1, B2) Specific viewing conditions (clear ocean, off-nadir viewing) Use of B3 for aerosol optical thickness estimation Use of meteorological data (water vapor, wind, pressure) Less convenient that for Vegetation (specific acquisitions, clouds…)

Desert sites Cross-calibration with either Polder, Vegetation or SPOT (SWIR) Sites supposed stable Similar viewing conditions

for the reference sensor Correction for atmospheric

effects and spectralsensitivity differences

Replaces the lamp forSPOT5: high frequencyacquisitions are possible



Absolute Calibration: use of specific landscapes Vicarious calibration over test-sites

Simultaneous image acquisition and ground characterization of reflectance and atmosphere

Cooperation with University of Arizona (86-98)

White Sands test-site (NM) French laboratories (LOA & LISE)

La Crau (France) Recent achievement of an automatic

radiometer CIMEL station: Continuous ground and atmosphere

characterization Phone link transmission of the data Enables calibration of any sensor,

each time it overpasses the site Operational in La Crau Others are planned



Absolute Calibration: Cross-calibration: simultaneous acquisitions

IQ mode HRVIR or HRG / Vegetation

Synthesis Use of all these methods

to match a sensitivitycurve

Discrepancy betweenmethods:6% visible8% SWIR

Ak SPOT4 HRVIR1 B1

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0 500 1000 1500

Days since launch

Lamp

Sun sensor

La Crau

Rayleigh

Sphere

White Sands


Spatial resolution

Refocusing: Bi-image method:

Simultaneous viewing of the same landscape with both instruments Fixed focus for the reference camera Focusing mechanism of the

other is moved Determination of the position

giving the highest ratio of theFourier transform ofcorresponding images

First try on SPOT1 (1994) Operationnally used for

SPOT4 and SPOT5

Use of the autotest device Only for SPOT5 Periodic square target No absolute measurement Monitoring of focus over time

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1

1.1

1.2

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Focusing mechanism position

MT

F r

atio

(H

RG

1/H

RG

2)

Defocus modelMeasurementVertex


Spatial resolution

Modulation Transfer Function assessment: Bi-image method for relative comparisons Point Source target (since SPOT3): needs on-ground team Edge target: natural (pb of edge quality) or artificial (SPOT5 THR) Final synthesis => MTF @ Nyquist in both directions for each band

One of our Xenon lamps SPOT5 THR imageof our Salon-de-Provence target


Summary

Lessons learned: Continuity needed between pre-flight and post-flight activities Need for an operational Center in charge of all in-flight image quality

activities Strong involvement (means, people) Continuous improvement of our methods & means:

More accurate More versatile Easier to perform

For the future: Pléiades = SPOT High Resolution Follow-on On-board simplification (no calibration device, no on-board

registration, non-continuous detection lines, non-linear radiometric response, …) + improvement of performances (resolution, geolocation accuracy…)=> complexification of ground image processing & calibration

Geometry: new test sites with high resolution/accuracy GCPs Radiometry: non-linear normalization => specific steered acquisitions,

new methods (histograms) Resolution: Artificial Neural Networks (focus & MTF), bi-resolution

Interest of sharing reference data over test-sites (cf HRS-SAP)

Documents

40 years of experience with SPOT in-flight Calibration