The Radiance Standard RASTA of DLR's calibration facility for airborne imaging spectrometers

Abstract

The German Aerospace Center (DLR) operates the Calibation Home Base (CHB) as a facility for the calibration of airborne imaging spectrometers and for field spectrometers. Until recently, absolute radiometric calibration was based on an integrating sphere that is traceable to SI units through calibration at the German Metrology Institute PTB. However, the stability of the radiance output was not monitored regularly and reliably. This was the motivation to develop a new radiance standard (RASTA) which allows monitoring in the wavelength range from 380 to 2500 nm. Radiance source is a diffuse reflector illuminated by a tungsten halogen lamp. Five radiometers mounted in a special geometry are used for monitoring. This setup improves twofold the uncertainty assessment compared to the previously used integrating sphere. Firstly, lamp irradiance and panel reflectance have been calibrated at PTB additionally to the radiance of the complete system. This calibration redundancy allows to detect systematic errors and to reduce calibration uncertainty. Secondly, the five radiometers form a redundant control system to measure changes of the spectral radiance. This enables long-time monitoring of the radiance source including assessment of the uncertainty caused by aging processes. Further advantages concern the reduction of periods of non-availability, applicability to sensors with larger field of view, and the possibility to alter intensity and spectral shape in a well-known way by exchanging the reflector. RASTA has been calibrated at PTB in November 2011 in the wavelength range from 350 to 2500 nm MECHANICAL SETUPThe mechanical configuration of RASTA is shown in Fig. 1. Light source is a 1000-W FEL tungsten-halogen lamp (Gamma Scientific Model 5000-16C), which is widely used as an irradiance standard. It illuminates perpendicularly a reflectance panel with the dimension 25 cm u 25 cm (1). In order to minimize stray light contamination, the lamp is installed inside a lamp housing (2). The lamp housing is mounted moveably on a bar(3) to adjust the distance between lamp and reflectance panel. The distance can be measured accurately by mounting temporarily a lamp alignment jig instead of the lamp. A mounting adapter for measurement devices (4) is installed in front of the reflectance panel. It provides space for seven instruments arranged on a half circle around the optical axis, which is the line perpendicular to the panel center. This setup allows all instruments to look at the panel at the same spot from the same distance and in the same angle of 45° to avoid differences in the radiance caused by spatial inhomogeneities. Five of the adapter spaces are used for radiometers (5,6) which monitor the stability of the system. The two horizontal spaces left and right of the panel (7) can be used for

Deriving aerosol parameters from in–situ spectrometer measurements for validation of remote sensing products

“…The sensor has no temperature stabilization or temperature measurement, thus the dark current is measured frequently during data acquisition and subtracted from the measurements. The spectrometer has been calibrated radiometrically at the RASTA radiance standard [8], which has an absolute uncertainty below 1 % in the spectral range of 350-830 nm. Additional to the radiometric correction a nonlinearity correction is performed.…”

The Calibration Home Base for Imaging Spectrometers

“…For these measurements, our radiometric standard (Schwarzmaier et al, 2012) and two integrating spheres are available. The setup for radiometric measurements is described in Sec.…”

“…The radiance spectrum of both spheres is traceable against German national standard. In order to achieve traceability, the radiance spectrum is constantly monitored against our radiometric standard (RASTA, calibrated at Physikalisch Technische Bundesanstalt) (Schwarzmaier et al, 2012) by the use of a transfer spectrometer (SVC HR1024i). The resulting radiometric spectral (k=1) uncertainty is 2% in the spectral range 400-2500 nm.…”

Characterisation methods for the hyperspectral sensor HySpex at DLR's calibration home base

“…The diameter of the collimated beam is larger than the sensor aperture. The sensor entrance slit is adjusted parallel to the y axis of the laboratory coordinate system (7). Figure 2 shows the measurement principle for the sensor's across (β, rotation around x axis) and along (α, rotation around y axis) track LSFs (8).…”