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24 1995 
41 
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 displacement in 
roll direction and a horizontal reference gyro measures the angular displacement 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. 
THE DAIS CHARACTERISTICS 
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). ; 
  
1) 0.4 - 1.0 um : 32 bands, bandwidth = 16 nm (10 nm) detector: Si 
2) 15 .- 1.8 um 8 bands,bandwidth =  36nm detector: InSb 
3) 20 - 25 um : 32 bands, bandwidth = 15 nm detector: InSb 
4) 30 - 9.0 um 1 band, bandwidth = 2.0 um detector: MCT 
5) 87 . - 12.3 um : 6 bands, bandwidth = 0.6 um detector: MCT 
Main geometric parameters 
IFOV: 3.3 mrad (2.5 or 5.0 mrad optional) 
Swath angle: max. t 39? (depending on aircraft) 
Pixels per line: 512 
Main radiometric parameters 
Dynamic range: 15 bit (no gain settings) 
Sensitivity VIS/NIR: NER «0.01 mWcm?sr'um"^ 
SWIR: NER < 0.005 mWoem?sr'um"* 
MIR/TIR: NET <0.1K 
  
Table 1: Spectral band specifications, main radiometric, and geometric instrument parameters of the DAIS-7915 
(Wavelength range: 0.4 - 12 um, 79 bands) 
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. 
One cartridge of the STK recorder can store more than 2300 scan lines. The STK recorder is equipped with an 
autoloader system for 10 cartridges. 
THE DAIS ON-GROUND CALIBRATION 
The on-ground calibration of an VIS / TIR imaging spectrometer consists of a sequence of procedures providing the 
calibration data files and ensuring the radiometric, spectrometric and geometric stability of the instrument: 
© measurement of noise characteristics of the sensor channels, 
M measurement of the dark current of the channels, 
© measurement of the relative spectral response function of the channels, 
. derivation of the effective spectral bandwidth of the channels, 
. definition of the spectral separation of the channels, 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995