The Digital Airborne Imaging Spectrometer DAIS 7915
	German Aerospace Center

Summary Main Characteristics Design

Summary

The European Union and DLR are funding a new 79-channel Digital Airborne Imaging Spectrometer (DAIS 7915), which was built by the Geophysical Environmental Research corp. (GER). 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 is used since 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 8000 - 12000 nm 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 the Laboratory Calibration Facility (LCF) has been developed at DLR Oberpfaffenhofen. The MCF covers the spectral range from 400 to 14500 nm. The DAIS 7915 has been flown during many campaigns on a Do 228 at DLR Oberpfaffenhofen since autumn 1994.

The DAIS 7915 System Specifications

The main characteristics of the DAIS 7915 are listed in Table 1. They refer to the "Specification of the Airborne Spectrometer" as well as to the "Digital Airborne Imaging Spectrometer DAIS 7915 Requirement Document (IRD)" both agreed by the DLR - Institute of Optoelectronics, Oberpfaffenhofen, Germany and the Geophysical & Environmental Research Corporation (GER)(Chang et al.,1993). The spectral and radiometric properties of the system have been evaluated by DLR since 1995. Details may be found at Strobl et al., 1996.

Spectrometer Characteristics Wavelength range: 400nm - 12.6µm, 4 Spectrometers, 79 bands 1) 400 - 1000 nm : 32 Bands, Bandwidth = 15-30 nm Detector: Si 2) 1500 - 1800 nm : 8 Bands, Bandwidth = 45 nm Detector: InSb 3) 2000 - 2500 nm : 32 Bands, Bandwidth = 20 nm Detector: InSb 3000 - 5000 nm : 1 Band , Bandwidth = 2.0 µm Detector: InSb 4) 8000 -12600 nm : 6 Bands, Bandwidth = 0.9 µm Detector: MCT For details see the Table of center wavelength & bandwidth for all bands Main radiometric parameters Dynamic range: 15 bit (no gain settings) Sensitivity VIS/NIR: NER < 0.025 mW/cm²sr µm SWIR: NER < 0.025 mW/cm²sr µm MIR/TIR: NET < 0.1 K Main geometric parameters FOV: 0.894 rad (+-26 degrees) on DO 228, depending on aircraft max. +- 39 IFOV: 3.3 mrad, (0.189 degrees) GIFOV: depending on aircraft altitude 5 - 20 m Scanfrequency: adjustable according to aircraft altitude between 6 and 24 Hz Image pixels per line: 512 Each channel consists of a detector element, a preamplifier, an integrating electronics and a 16-bit AD-Converter with its output to a common multiplex electronic. The silicon detectors, of the channels 1 to 32, are DC coupled with their preamplifiers, all other detector elements are AC- coupled to their preamplifiers. The multiplexed and encoder clocked 16 bit - word data stream of each scan line with 80 channels (43520 data words) is sent via the S-BUS-S16D-Data interface to the SUN-processor. The SUN-processor transmits the data blocks to the STK 4280 recorder via an 16-bit SCSI - interface. The standard cartridge of the STK recorder can store about 2500 scan lines. A limited number of high capacity cartridges which can hold up to 3300 scanlines is available. The STK recorder is equipped with an autoloader system for 10 cartridges.

The optical system design

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 µm 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. The beam of divergent radiation is divided by an entrance beam splitter into two separate beams: Transmitted IR - radiance for wavelengths > 3000 nm Reflected radiance for wavelengths < 3000 nm 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 camera lens and a detector line array, except the 3000 - 5000 nm 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. In one revolution, the cubic polygon scans four times first the one calibration source (the ambient blackbody) second the terrain and third the other calibration source (the heated blackbody). This polygon scanning principle makes the scan rate four times slower, the channel noise band with four times smaller and the SNR approximately two times higher than in a scanner with a rotating single flat mirror scanner working at the same aircraft velocity and with the same swath over ground. Optical encoding signals from the scanner which are related to the scan motor speed and scan shaft position are used in the electronic circuits so establish clock pulses for system operation control. The scan motor speed is continuously tuneable by the operator. The resulting scan frequency is measured by a counter and is indicated on a display with an accuracy of 4 digits. A gyro subunit is part of the scan head construction. A vertical reference gyro measures the angular displacements in roll direction and a horizontal reference gyro measures the angular displacements in pitch and direction. The gyro data as well as the blackbody temperature sensor data are several times sampled per scan, AD-converted and then included in the housekeeping data of channel 80.

[DLR] [DAIS Home] [People]