General Information

Top-level requirements

The MEDUSA camera is designed to operate at a ground resolution of 30 cm at 18 km altitude in the visible spectrum (400-650 nm), and a swath of 3000m. To obtain a signal-to-noise ratio (SNR) of 100 at 8.00 AM at equinox a combination of a high resolution panchromatic sensor and a lower resolution color sensor is considered, using pan-sharpening in post-processing. The attitude variations of the platform impose restrictions on the integration time of the sensor to avoid motion blur. The maximum expected data rate for a 60% forward overlap between subsequent images is 170 Mbit/second. On-board JPEG2000 compression is foreseen to fit the 20 Mbit/second data rate of the downlink.

Environmental and Operational conditions

The challenge lies mainly in fulfilling the requirements within the extreme environmental and physical constraints of the HALE UAV. Compared to traditional airborne and spaceborne systems, the MEDUSA camera system is ultra light weight (about 2 kg) and has only a limited amount of power available for the on-board electronics. Moreover, it is operated in a low pressure and low temperature environment which undergoes thermal cycling and needs to compensate for platform attitude variations within the strict weight and power constraints. Optimization of the global payload system design is governed by careful trade-offs between different requirements and subsystem performances.

The camera is operated in a 24h cycle. Power is only available during day time, during which it will perform Remote Sensing operations from an altitude of 18 km. During night time the camera is switched off and the UAV descends to an altitude of 15 km to save energy. It uses the remaining battery power to stay at this altitude. At dawn, the Mercator1 climbs again to 18 km altitude where the camera is switched on to start a new day cycle.

Camera subsystems

SystemDesign

 

The central part of the MEDUSA payload is a combination of two frame sensors, one panchromatic and one with RGB color filtering. The custom design of a highly sensitive wide swath (10000x1200 pixels) CMOS sensor with electronic snapshot shuttering and a small pixel size has been studied. A light-weight optical system consisting of a folding mirror and lenses focuses the image on the sensors. The payload will also contain a GPS receiver and Inertial Measurement Unit (IMU) for position and attitude determination of the camera at the instant of exposure. A direct downlink will allow near-real time data delivery to the user. The on-board data processing consists of time-tagging, basic image corrections, organizing and compressing data. Processing and archiving will be conducted on-ground where data will be received by the ground station and forwarded to a Central Data Processing Centre (CDPC) at VITO, Belgium.

SubsystemPositioning