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A NEW 79-CHANNEL AIRBORNE IMAGING SPECTROMETER:
ITS FUNCTIONING, MAIN CHARACTERISTICS AND CALIBRATION
D. Oertel, F. Lehmann
DLR German Aerospace Research Establishment,
Institute of Optoelectronics, P.O.B. 1116, D-82234 Wessling,
Phone: *49 8153282841, Fax: *49 8153281349 (Oertel)
+49 8153281161, +49 8153231458 (Lehmann)
S. H. Chang
Geophysical & Environmental Research Corp.
One Bennett Common, Millbrook, N.Y.,12545, USA
Phone: +1 914 6776100, Fax: +1 914 6776106
KEY WORDS: Airborne Remote Sensing, Hyper spectral Imaging Spectrometer, Calibration.
ABSTRACT:
The European Union and DLR are funding a new 79-channel Digital Airborne Imaging Spectrometer (DAIS-7915),
which is built by the Geophysical Environmental Research Corp. (GER), USA.
This new sensor covers the spectral range from the visible to the thermal infrared wavelengths at variable spatial
resolution from 3 to 20 m depending on the carrier aircraft flight altitude.
The DAIS-7915 will be used beginning from spring 1995 for remote sensing applications such as environmental
monitoring of land and marine ecosystems, vegetation status and stress investigations, agriculture and forestry
resource mapping, geological mapping, mineral exploration as well as for the supply of data for geographic information
systems (GIS).
Six spectral channels in the 8 - 12 um region could be used for the retrieval of temperature and emissivity of land
surface objects. These and 72 narrow band channels in the atmospheric windows between 450 and 2450 nm allow to
investigate land surface processes with a special emphasis on vegetation/soil interactions.
Based on the requirements for on-ground calibration of the DAIS 7915 a Multipurpose Calibration Facility (MCF) has
been developed at the DLR Institute of Optoelectronics.
The MCF covers the spectral range from 0.4 to 14.5 pm.
The DAIS-7915 has been flown three times on a Do 228 at DLR Oberpfaffenhofen in autumn 1994. First results are
presented.
THE DAIS SYSTEM
The Digital Airborne Imaging Spectrometer DAIS-7915 is a 79 channel high resolution optical spectrometer which
collects information from the Earth's surface in the 0.4 - 12.3 um wavelengths region while scanning from an aircraft,
electronically processes this data into digital format consisting of 16 bit words, and records these digital data on a
cartridge recorder.
The DAIS scan mechanism is a Kennedy type where a cubic polygon mirror scans the terrain below through the
opened window hatch in the bottom of the aircraft. The scan mirror rotates anti clockwise with respect to the aircraft
heading to provide a ground element cross track scanning motion while the forward motion of the aircraft provides a
requested line-by-line scan.
During the scan, the up welling energy collected within the field of view (FOV) of the scanner is reflected by the polygon
by the fixed flat folding mirror and focused by the parabolic mirror on the aperture stop at the entrance of the
spectrometer. (See the Principal Optical Scheme of DAIS-7915 on Figure 1 .)
The beam of divergent radiation is divided by an entrance beam splitter into two separate beams:
» Transmitted IR - radiance for wavelengths greater than 3 um
» Reflected radiance for wavelengths smaller than 3 um
These two diverging beams are collimated to nearly parallel light, directed via optical beamsplitters into the
spectrometer subunits, where they are separated into wavelength bands or channels.
Each spectrometer subunit consists of a grating, a focusing lens and a detector line array, except the 3 - 5 um one-
channel subunit where - instead of a grating a filter is used for the wavelength separation. Each channel contains
a detector with its own preamplifier, an in integrate and hold circuit and an analogue to digital converter (ADC).
To provide a high degree of accuracy in the determination of the average energy level of the recorded data, the scan
head is equipped with two external calibration sources that are also viewed by the rotating polygon mirror.
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995
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