Progress in Field Spectroscopy

Ted MiltonSchool of GeographyUniversity of Southampton

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1976...

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1976...

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Progress in Instrument Design

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• SNR in SWIR• Temperature effects• Polarisation sensitivity• Field-of-view & PSF• Radiometry

Challenges in Instrument Design

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• Total cost of ownership• Sunlight-unreadable screens• Problems with optical fibres• Steps in spectra

However, users still complain about...

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Roles for Field Spectroscopy in EO

• Scaling-up from individual elements of the scene to areas the size of a pixel

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Mean Reflectance 100W Lamp (%)

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Mean Reflectance Contact Probe (%)

Pima Reflectance

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Roles for Field Spectroscopy in EO

• Scaling-up from individual elements of the scene to areas the size of a pixel;

• Using those pixel-scale data to:• validate numerical models• validate sensor calibration post-launch• correct remotely sensed data for the

effect of the atmosphere

• Field spectroscopy also provides a tool for sensitivity analysis;

• and is an effective aid to teaching the physical principles of remote sensing.

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• Reference panels need calibrating (spectral and angular).

• Reference panels deteriorate over time.• Reflectance factors are not an inherent

property of the target.

... need to pay more attention to the spectral irradiance distribution.

Measurement of reflectance factors

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The problem of sub-visual clouds

• Cause the amount of irradiance to change rapidly.

• Cause the angular distribution of irradiance to vary.

Significant source of error

Possible solutions:• dual-beam methodology• monitor or model the irradiance

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Dealing with sub-visual clouds

Simultaneous measurement Simultaneous estimation

Milton & Rollin, 2006

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The challenge of angular measurements...

NASA JPL PARABOLA III

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Some goniospectrometer designs

The NPLGRASS goniometer

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University of Zurich RSL dual-beam goniometer

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Goniospectrometry using a CCD array

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An Instrument Package for Field Spectroscopy

Instrument Purpose

1. Constant panel set-up Time series of irradiance spectra (plus direct/diffuse).

2. Mobile spectrometer Spatial variation of reflectance

3. Goniospectrometer Angular variation of reflectance

4. Sky camera Permanent visual record

5. Sun photometer AOT and EWT

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Spectral characterisation of large uniform areas

1. White Sands, New Mexico.

2. Railroad Valley, Nevada.

3. Lunar Lake, Nevada.

4. Salar de Uyuni, Bolivia.

5. Salar de Arizaro, Argentina.

6. Tuz Gölü, Turkey.

7. Lake Eyre, Australia.

8. La Crau sèche, France.

9. Dunhuang, China.

10. Dome Concordia, Antarctica.

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Thome et al., 1993

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Seasonal change in playa surface properties

http://en.wikipedia.org/wiki/Image:Jfader_playa.jpg

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Example : How stable are ‘inert’ surfaces?

• Disused airfield• Concrete, Asphalt, Grass• Mobile platform• Dual-beam GER1500• Karen Anderson PhD

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Seasonal change in hemispherical-conical reflectance

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First clue to what we think is affecting the concrete reflectance is found in the met. data...

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Each sequence shows a small variation with solar zenith angle.Not large enough to account for the difference between am and pm.

Neither is the spatial uncertainty of location...

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• Development of an onshore breeze in the afternoon.• Reflectance of concrete affected by what?

… change in air mass? ...surface moisture?

Algae and cyanobacteria on the surface

Anderson and Milton (2006) Int. J. Rem. Sens., 27, 3365-74.

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SpecNet mobile tram system

Gamon et al., 2006Remote Sensing of Environment, 103, 227-235.

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Conclusion

• Progress is slow...• Methodologies still

immature.• Field spectroscopy

remains difficult and expensive.

• Barely begun to address the issues of sampling in time and space.

• Potential contribution of field spectroscopy to a physically-based global remote sensing system is immense.