Calibration Methodology for the Airborne DispersivePushbroom Imaging Spectrometer (APEX)

Jens Nieke, Johannes W. Kaiser, Daniel Schläpfer, Jason Brazile, Klaus I. Itten(RSL),

Horst Schwarzer, Peter Strobl (DLR),Michael E. Schaepman (WUR),

Gerd Ulbrich (ESA)

and the APEX team

Recent publications:Recent publications:

UlbrichUlbrich,, Meynart Meynart,, Nieke Nieke, and the APEX team, SPIE, and the APEX team, SPIE Vol Vol. 5570 (2004). 5570 (2004)

NiekeNieke, Kaiser,, Kaiser, Schlaefer Schlaefer,, Brazile Brazile,, Itten Itten, , Strob lStrob l,, Schaepman Schaepman, , UlbrichUlbrich, and the APEX team, SPIE, and the APEX team, SPIE Vol Vol. 5570 (2004). 5570 (2004)

Outline of the Talk

• APEX Project Background• APEX Calibration Specifications (summary)• APEX Calibration Tools

• In-flight Characterization facility IFC• Calibration Homebase CHB• Vicarious and cross calibration• PAF (assimilation scheme)

• Conclusion

Scientific GoalsScientific requirements for APEX have been derived using a performance model forat sensor radiance*.Goal: Development of advanced methods for bio-geophysical parameter retrieval for

• Vegetation,• Soils, rocks & minerals,• Snow and Ice,• Atmosphere,• Coastal and inland waters analysis

and cross calibration with spaceborne EO systems

* Schläpfer & Schaepman,Appl. Optics 41(27), 2002

Hyperspectral Applications

100.0%

0.0%

Precision Farming

Ecology Modeling

Forest Fire Modeling

Water QualityAir Pollution

Natural Hazards

APEX Organization

APEX Selected Calibration Specifications

• Total FOV 28˚ deg, IFOV 0.49 mrad (spatial resolution across slit)

• Total of across-track pixels: 1000 for both detectors

• Total spectral pixels on chip, prior to binning 312 (VNIR) & 199 (SWIR), may beread out for calibration purpose simultaneously

• Total spectral bands in operational mode ~ 300 programmable channels resultingin a spectral sampling interval (and width) of Δλ ≤ 5 nm in VNIR (400 - 1050 nm) andΔλ ≤ 10 nm in SWIR (1050 - 2500 nm)

• Instrument temporal radiometric uncertainty within a flight section: better 0.02• Radiometric performance accuracy: instrument shall allow absolute

calibration accuracy up to 0.03• Interval for instrument re-calibration: after a complete flight season• Center wavelength accuracy: < 0.2 nm

Schematic APEX Instrument Components

APEX

Pro

duct

Tre

eAP

EX P

rodu

ct T

ree

Instrument Set-upInstrument Set-up

Instrument ConfigurationInstrument Configuration

Environmental ThermalControl Box – (ETC)

Stabilizing Platform -STP

ThermalControl Unit

Aircraft I/F

Spectrometer(OMU)

Instrument Set-up in AircraftInstrument Set-up in Aircraft

APEX Instrumentwith Stabilizing Platform integrated inEnvironmental Thermal Control Box

Aircraft I/F

Control and StorageUnit

Power DistributionUnit

Navigation SubSystem

Operator

Monitor

Flight ManagementSystem

APEX Hyperspectral ImagerAPEX Hyperspectral Imager

• Sealed spectrometercompartment

• Thermal stabilized duringoperation: Better 2 ºC, gradientless 0.5 ºC

• Max. deformation of OpticalBase Plate (OBP) less 0.040mm due to air pressurevariations

• Co-registration error VNIR-to-SWIR channels about 0.5 pixel

• Overall mass: 72 kg, withoutcounterweight for COGadjustment

804 mmFits into PAV-30 Stabilizing Platform,Opening diameter 443 mm

Spectrometer - Design ConceptSpectrometer - Design Concept

Type CCD 55-30 from E2V Technologies (GB)

• Frame transfer mode,• 1252 x 1152 p ixel (used 1000 x 393)• Pixel p itch 22.5 x 22.5 µm2, fill factor 100%• Back illuminated• Operated in non-inverted mode• Read out frequency 7 Mpix/s• Operated in dither clocking

mode without cooling• integration time independent

from frame transfer time

VNIR- DetectorVNIR- Detector

0

20

40

60

80

100

380 400 500 650 750 900 1000

Wavelength [nm]Q

ua

ntu

m E

ffic

ien

cy

[%]

SWIR DetectorSWIR Detector

HgCdTe detector array - hybridized on CMOS multiplexer1000 x 256 square pixel, 30 micron, addressable readout, fast operationIntegrated in cryostat cooler assemblyWL range: 0.94 – 2.50 micronQE: > 55 % averageTop.: 130 – 165 K

Dewar

Cooler

Focal Plane Assembly BB

Sapphire Window

Transfer Line

Pre-EM Detector with Cryostat/Dewar Assembly

APEX Electronic: Data Streams Overview

APEX PAF: The Processor FoundationPAF Hardware: Linux,PAF Hardware: Linux,

Archiving SystemArchiving SystemPAF: IDL-PAF: IDL-emacsemacs, XML-tools,, XML-tools,

CVS, TCL/CVS, TCL/webshwebsh

PAF Processor: PAF Processor: IDL, XML, CIDL, XML, C

APEX PAF

SearchAPEX

Archive

Server

Core

Web

Processor

Input

ToolsProcessing

ToolsCollaboration

ToolsBrowsing

Docs

Co-Developers

Users

Operators

CHB Data

APEX higher processing Level 1B, 1C, 2, 3

Atmosph. AuxilaryData

Image Attribute

Data

Quality Description

ParametricOrthorecti-

fication(PARGE)

AtmosphericTopographicCorrection(ATCOR)

Geometry Index

AtmosphericTopographic

Corrected Cube

CubeGeocoding

Geocoded,HDRF Cube

DEM

Calibrated Uniform

Image DataCube

Calibrated Position

and Attitude Custom products

ProductGeneration

Product Description

Level 1B Level 1C Level 2CLevel 2A Level 2B

Bad Pixel Map

Rectification(PARGE)

Rectified Uniform Cube Atmospheric

Normalization

Geometry Index

ApparentReflectance

Cube

Level 3A Level 3B

Standard products

Product Description

Research products

Product Description

Level 3C

APEX Calibration Methodology

APEX Calibration Tools:

• In-flight Characterization facility IFC• Calibration Homebase CHB• Vicarious and cross calibration• PAF (assimilation scheme)

Opto/Mechanical Unit (spectrometer hermetic sealed) with IFC*

Optical Baseplate(actively cooled)

* In-Flight/on-board Characterisation facility

QTH-Lamp, stabilized

Connectors

Baffle

In-Flight/on-board Characterisation Facility - IFC

Baffle

Optical Beampath

Stabilized QTH-lamp

Filter wheel

APEX Spectral Calibration VNIR (312 bands prior binning)

APEX Spectral Calibration SWIR (199 bands)

Calibration Home Base

The CHB will be established at DLR,Oberpfaffenhofen (D) for regular instrument re-calibration during APEX exploitation phase

For laboratory calibration a number ofenvironmental prerequisites are met:

•• Foundation built as seismic blockFoundation built as seismic block•• accessible by large trolleys (no stairs, stepsaccessible by large trolleys (no stairs, steps

or elevators)or elevators)•• close to airfieldclose to airfield•• large enough to host equipment, high ceilinglarge enough to host equipment, high ceiling

and craneand crane•• semi-clean environmentsemi-clean environment•• dark room facilitiesdark room facilities•• heated and temperature controlledheated and temperature controlled•• low humiditylow humiditySeismic block

7 m

6 m12 m

Calibration Home Base

APEX-instrument

Rotary stage withfolding mirror,mounted on linearstage

Mirror-Collimatorsfor spectral andspatial calibration(not shown)Optical bench (granite)

APEX Instrument – Radiometric Calibration

ADS

Integrating

Sphere

Field Spectrometer with Fiber Optics

FOV

QTH Lamps for various radiance levels

APEX

Ø1,60 m

Principle of the radiometriccalibration of APEX instrumentwith DLR’s large integratingsphere

Vicarious and Cross-Calibration

Block diagram for theBlock diagram for theradiance-basedradiance-basedcalval calval method tomethod tocompare TOAcompare TOAradiance predictedradiance predictedfrom APEXfrom APEXmeasurementsmeasurements(TOA radiance 2,3)(TOA radiance 2,3)with those measuredwith those measuredby the spaceborneby the spacebornesensorsensor((TOAradiance TOAradiance 1).1).Hsite Hsite is the altitude ofis the altitude ofthe site, HAPEX isthe site, HAPEX isthe flight altitude ofthe flight altitude ofAPEX and theAPEX and theHTOA the top HTOA the top ofof--atmosphere altitude.atmosphere altitude.

APEX 312 Bands prior binning

APEX Cross-Calibration Capabilities

Calibration Parameter Definition

aiθcentre across-track angleθj,k

aiλcentre wavelengthλj,k

bad/dead pixel mapdj,k

dark current factordcj,k

gain factorGj,k

Parameterised byDescriptionSymbol

for each pixelfor each pixel for all pixels for all pixels

!

b =

Gj ,k

dcj ,k

ai"

ai#

$

%

& &

'

& &

(

)

& &

*

& &

vector of coefficients + their covariance matrix vector of coefficients + their covariance matrix SS

Calibration Coefficient Calculation

Sources of Information: heterogeneous calibration measurements system insight

Goal: combine all information in an optimal way

Solution: data assimilation

• optimal combination according to covariances• accounts for temporal evolution• system insight quantified (correlations) requires covariance matrix

quantitative description with b, Ssystem model for b and Scorrelations in Sparameterizations for b

Example: 1-d Kalman Filter

APEX PAF: The Processor Foundation

PAF Hardware: Linux,PAF Hardware: Linux,Archiving SystemArchiving System

PAF: IDL-PAF: IDL-emacsemacs, XML-tools,, XML-tools,CVS, TCL/CVS, TCL/webshwebsh

PAF Processor: PAF Processor: IDL, XML, CIDL, XML, C

APEX PAF

SearchAPEX

Archive

Server

Core

Web

Processor

Input

ToolsProcessing

ToolsCollaboration

ToolsBrowsing

Docs

Co-Developers

Users

Operators

CHB Data

Calibrated DataCalibration Parameters

Quality Reports

Data Download:- Segregation- Synchronization- Bad Pixel Detection

RawInstrument

Data

Applanix INS Data

Sensor AttributeData

CalibrationProcessor

Position and AttitudeTransformation

Data Calibration:- Electronic smear,- Radiometric response,- Bad p ixels,- Optical effects

VicariousCalibration Data

ConsistencyCheck

Laboratory Calibration

Parameter Files

Archived Instrument Data,

Quicklooks

TrackAir FMS Data

Raw Data Level 1BLevel 0B

LaboratoryCalibration Data

IFC Calibration Parameter Files

Vicarious Calibration

Vicarious CalibrationParameter Files

Independent VicariousQuality Report

Assimilation Processor

Vicarious Validation

Conclusions

APEX timeline is currently before the Critical Design Review (CDR) close-outmeeting

the PAF version 0.3 has been released in Aug. 2004 design and breadboard activities of the In-Flight Calibration facility IFC werefinalized bread boarding phase of the Calibration Home Base CHB is ongoing.

First data for the scientific user community shall be available in 2006!

We are proposing the APEX instrument as a cross-calibration tool to thescience community!

Last but not least…

www.apex-esa.org