ALISEO: an Imaging Interferometer for Earth Observation

Transcription

1 ALISEO: an Imaging Interferometer for Earth Observation A. Barducci, F. Castagnoli, G. Castellini, D. Guzzi, C. Lastri, P. Marcoionni, I. Pippi CNR IFAC Sesto Fiorentino, ITALY ASSFTS14 Firenze - May 6-8, 2009

2 Contents Imaging interferometry for Earth observation MIOsat: a technological mission with optical payloads on mini-satellite ALISEO payload: optics and electronics configurations Experimental activities: SNR estimatation, spectral range and resolution assessment.

3 Advanced Remote Sensing Imaging spectrometers Imaging interfermoters SAGNAC INTERFEROMETER PUSH-BROOM SPECTROMETER WITH DISPERSIVE GRATING (by Sira) MICHELSON INTERFEROMETER

4 Imaging Interferometry Main advantages: 2. All the Principal critical points: impinging contributes to the radiation 2. Higher dynamic range of the sampled signal: a large information amount is held in the tiny ripple found in the interferogram wings 3. Possible aliasing effects in both image (spatial domain) and interferogram (optical phase difference domain) 4. Complex processing algorithms to retrieve the at-sensor spectral radiance observed interferogram 3. Absence of the entrance slit, a feature characteristic of the leapfrog technique 4. Absence of any moving mechanism 5. Choice of the spectral range and resolution by acting on the instrument sampling step and fieldof-view

5 MIOsat main characteristics Satellite mass: Total: Payloads only: Satellite power: 130 kg 40 kg 150 Watts 1.1 m x 0.8 m x 0.8 m Size: Orbital parameters: Altitude: 500 km Type: circular, polar, Sun synchronous Inclination: 97.4o Descending node: 9:30 AM (local time) Pointing capability: Cross track: Along track: Mach-Zehnder interf. ALISEO from 0o to +15o from +30o to 30o Spot: 5 km Foot print : 10 km x 10 km SSI : 10 m

6 MIOsat payloads ALISEO

7 ALISEO Aerospace Leap-frog Imaging Stationary interferometer for Earth Observation

8 ALISEO on MIOsat main characteristics Spectral range: nm Spectral resolution: 120 cm-1 (5 630 nm) Numbers of samples in each interferogram : 256 Spatial resolution: 10 m Swath: 10 km x 10 km FOV: SNR: > 650 nm Digitalization: 12 bit Data volume: 3.2 Gbit/image set The Phase B (instrument design) has expired on May 30, 2007

9 The Leap-frog operating mode Every frame contains the scene superimposed on the stationary fringe pattern The interferogram of a given pixel is dispersed along a slant direction of the acquired data cube Sensor Comparison with standard spectrometers and interferometers Sensor λ or OPD y x OPD y x Leap-frog technique Standard techniques Energy from each pixel is dispersed spectrally and every plane of a data-cube is a monochromatic image of the scene. In push-broom sensors a single frame is a (x,λ) 2dim domain.

10 The Leap-frog operating mode Leap-frog technique ence u q e s e Fram OPD Different samples of the interferogram are observed under different viewing angles. In aerospace applications this gives rise to differential atmospheric effects along the interferogram that are mitigated by the narrow FOV allowed (roughly 1o).

11 ALISEO development activity Laboratory breadboard and airborne prototype Development and characterization of a laboratory breadboard. Development of algorithms to retrieve spectral at-sensor radiance and reflectance from acquired raw images. Execution of calibration and validation activities. Execution of the first airborne campaign. Execution of in field measurements ALISEO on MIOsat Phase A: Preliminary Requirements review. Phase B: System Requirement Review Preliminary Design Review

12 The Sagnac configuration The two beams travel on opposite paths, and exhibit exactly π phase-delay (for a point source on the optical axis) due to a difference of reflections between the two rays. The Optical Phase Difference (OPD) linearly increases with increasing the slope on the optical axis of the input ray. A relevant critical point is the wide beamsplitter required for holding the two paths allowed by the instrument. To this purpose a light concentrator has been selected as input element

13 The airborne prototype Light from the target image produced by objective O is collimated by the achromat L, and, by means of the beam-splitter BS and two folding mirrors (M1 and M2) the interference fringe pattern is originated and focused on a CCD by the lens P. ALISEO - Optical layout M2 M1 BS L O ALISEO - Assembled optical unit P CCD

14 Laboratory activities: Sensor calibration Interferograms have been processed in order to retrieve the at-sensor radiance spectrum 0.07 Noise SNR STEPS: Dark signal subtraction Instrument spatial response compensation Geometrical distortion DC offset subtraction Apodization Inverse Cosine Transform wavelength (nm) 120 progressive counter for local maxima correction SNR DATA PROCESSING Noise m=108 m= nm pixel

15 Calibration activities Horizontal profile nm nm ).a siy(u te In Column Green HeNe laser Wavelength (nm) Horizontal profile nm ).a siy(u te In Column 600 Red HeNe laser nm Wavelength (nm)

16 Field Campaign One of 512 acquired frames A One of 512 re-assambled frames A A Acquired inteferogram, retrieved cypress ilex wood (A) uncalibrated spectrum, and ratio between this spectrum and a building façade (D) uncalibrated spectrum.

17 First airborne test campaign The prototype flew on July 29, 2008 during a test campaign over the Tuscan countryside close to Borgo San Lorenzo ( Florence, Italy).

18 First airborne test campaign The image sequence was acquired flying at an altitude of 500m. The two images show relative sensorobject motion along a 0.5sec time interval.

19 ALISEO on MIOsat : Optical configuration

20 ALISEO on MIOsat : Electronic configuration Cover assembly Block diagram of ALISEO optical and mechanical devices Baffling system Sagnac Imagingoptics interferometer Calibrators Optical bench Shutter Actuator Power Incaming radiation Light source Heaters power Termocouples Settings: IntegrationTime Gain Power test Handshaking commands Detector array Proximity electronics 12 bit ADC DataBus Heaters Termocouples Block diagram of ALISEO electronic

21 ALISEO on MIOsat: Simulating interferogram acquisition and spectrum reconstruction 5 Input angle(rad) Relation between OPD and input nm Interferogram of incoming radiation (O D P µm n 0 4 )@ FusedSilica25mm FusedSilica20mm Computed spectra of light source ranging from 500 nm to 800 nm Blue line: Constant refraction index beam splitter Red and Green lines: Fused silica beam splitter The green spectrum is retrieved using an algorithm compensating for the spectral dispersion of beam splitter refraction index. The achieved spectral resolution is 125 cm-1.

22 Future Activities MIOsat Phase C and D: Development of the spaceborne payload Assessing sensor performance and calibration before the launch. Implementation of the algorithms for autonomous data processing.

23 For further information:

24 Spectral resolution and free spectral range The spectral resolution δλ of a stationary interferometer is: δλ = λ2 2OPDmax The shortest wavelength that can be interpolated (reconstructed) without aliasing is: λ min = 2δ (OPD ) The two parameters δopd and OPDmax are linked by means the number N of acquired samples in the interferogram. Since we need high spectral resolution and short wavelengths we have chosen the most favorable condition, where only one side of the interferogram is digitized and: OPDmax N δ ( OPD )

25 At-sensor 09:00 local time ATSENSORRADIANCE MODTRAN-5 SIMULATION 22 Winter solstice 20 Summer solstice 18-1 ) 12 IC D A R O N E S T c W (m -2 r s 14-1 m n WAVELENGTH (nm)

26 Photon 09:00 local time 5 nm bandwidth PHOTON FLUX 4000 MODTRAN-5SIMULATION Summer solstice h9: Winter solstice h 9: S T H P F O R E B M U N WAVELENGTH(nm)

27 Quantum efficiency for three selected detectors QUANTUM EFFICIENCY DALSA 2020M 0.6 SARNOFF 0.5 DALSA 2021M 0.4 Y IC F E M T N A U Q WAVELENGTH(nm)