Design and Calibration of the Bird Payload Platform

The BIRD (Bispectral Infra-Red Detection) is a small satellite mission of the German Aerospace Centre which was succesfully launched in October 2001as piggyback on the Indian PSLV rocket. High technological and scientific performance could be demonstrated under low budget constraints. The satellite is a three-axis stabilised spacecraft with a mass of 92 kg including over 30% of the total mass for the scientific instrumentation. The primary mission objectives are the test of a new generation of infrared array sensors for detection and scientific investigation of High Temperature Events (HTE) such as forest fires and volcanic activities. Together with a VIS/NIR-sensor with stereo capability as a redesign of the MARS-96 mission the diagnostics of vegetation and the discrimination of smoke and water vapor is possible. Due to the highly compact design a strong interaction of interfaces within a small volume leads to special platform technologies mainly driven by optical and thermal requirements.Therefore the calibration of the three image-forming sensors of BIRD in geometrical, radiometrical and spectral way had to be considered from a very early design phase on. The pixelcoregistration of four sensors working in different spectral ranges requires a very complex calibration facility. 1. HOT SPOT RECOGNITION SENSOR (HSRS) – THE MAIN BIRD PAYLOAD The main payload of the BIRD micro-satellite is the newly developed Hot Spot Recognition Sy stem. Its a dual-channel instrument for middle and thermal infra-red imagery based on cooled MCT line detectors. Exact pixel matching and onboard calibration capabilities are required due to the scientific methods of data processing. To avoid saturation and to discriminate high temperature events from environment reliably the sensor electronics has to cover an extended dynamic range which is another challenge of the design. Based on a airborne laboratory model of the two IR cameras for the midand thermal infrared spectral range the requirement for the HSRS for BIRD was to reduce dimensions and power needs of this system significantly. Fig. 1.1 Miniaturised integrated detector/ cooler assembly Thus an integrated detector/ cooler design was derived from military applications technology and has been space-qualified (see figure 1.1). Here the main problem is to provide enough stiffness of the inbound cooling finger beneath the IR-detector chip against dynamical loads during the launch process. A so arising solid structural solution is in contradiction to the demands of thermal de-coupling to minimise power needs. At least the system consumes only 11 Watts which is a factor of 4 less in comparison to a split cooler concept.Together with a special lens design which is optimised by the cold shield arrangement of the detector dewar the inner radiation disturbances could be decreased significantly to improve the instruments radiometric accuracy. Fig. 1.2 – HSRS with IREU in flight configuration (8,7 + 5,8 kg, 0,22 x 0,21 x 0,45 m3