Designed to execute mapping and surveillance missions for crisis monitoring on a solar powered High Altitude Long endurance UAV 18 km high up in the stratosphere, the MEDUSA high resolution camera is able to acquire frame images with a ground sampling distance of 30 cm and swath of 3 km. Since mass is a dominant driver for the UAV performance the MEDUSA payload was severely mass optimised to fit within the physical boundaries of 2.6 kg, 12 cm diameter and 1 m length. An inertial navigation system and data transmission equipment is included. Due to the innovative dual sensor on single chip concept the MEDUSA payload hosts two independent frame camera’s of each 10000x1200 pixels (one panchromatic and one colour sensitive). The MEDUSA stratospheric camera has completed its system level test campaign in autumn 2012 and is ready for its maiden flight.
Using the main building blocks of this stratospheric camera a modified version is being developed which is adapted to more conventional UAV’s flying at lower altitude. The current design is targeting a ground resolution of 10 cm and swath of 1 km with each single image. First test flights have been conducted with an engineering model version of the camera generating representative image data. Also the functionality is being expanded by adding hyperspectral sensitivity to high spatial resolution image acquisition within the same compact camera system.
KEYWORDS: Imaging systems, Hyperspectral imaging, Image sensors, Fabry–Perot interferometry, Spectral resolution, RGB color model, Unmanned aerial vehicles, Linear filtering, Data conversion, Control systems
VTT Technical Research Centre of Finland has developed a new miniaturized staring hyperspectral imager with a weight
of 350 g making the system compatible with lightweight UAS platforms. The instrument is able to record 2D spatial
images at the selected wavelength bands simultaneously. The concept of the hyperspectral imager has been published in
the SPIE Proc. 74741. The operational wavelength range of the imager can be tuned in the range 400 - 1100 nm and
spectral resolution is in the range 5 - 10 nm @ FWHM. Presently the spatial resolution is 480 × 750 pixels but it can be
increased simply by changing the image sensor. The field of view of the system is 20 × 30 degrees and ground pixel size
at 100 m flying altitude is around 7.5 cm. The system contains batteries, image acquisition control system and memory
for the image data. It can operate autonomously recording hyperspectral data cubes continuously or controlled by the
autopilot system of the UAS. The new hyperspectral imager prototype was first tried in co-operation with the Flemish
Institute for Technological Research (VITO) on their UAS helicopter. The instrument was configured for the spectral
range 500 - 900 nm selected for the vegetation and natural water monitoring applications. The design of the UAS
hyperspectral imager and its characterization results together with the analysis of the spectral data from first test flights
will be presented.
MEDUSA is the lightweight high resolution camera, designed to be operated from a solar-powered Unmanned Aerial
Vehicle (UAV) flying at stratospheric altitudes. It targets applications such as crisis management and cartography as a
technology demonstrator within the Pegasus program. From an operational altitude of 18 km MEDUSA will deliver
images with a ground sampling distance of 30 cm and cover a swath of 3 km. The innovative UAV poses high demands
on the instrument characteristics such as mass (< 2,5 kg), volume and power consumption. Furthermore the MEDUSA
instrument design needs to guarantee its optical performance within the specific environmental conditions of the
stratosphere.
Well limited chromatic aberrations require the use of anomalous partial dispersion glass. Those glasses unfortunately
suffer from large variations of their refractive index as well as their volume with temperature, causing strong focus
variations of the optical system. Furthermore, the weight constraints impose compact system (minimizing the distance
between lenses for mount weight reduction) and very thin lenses. Manufacturing each of the lenses was challenging. The
very thin aspherical lens has required few manufacture iterations to achieve sufficiently low waviness, after that it
became clear that this lens was the dominant factor for image contrast limitation.
After all subsystem tests have been completed the MEDUSA shall be integrated and finish its ground test program in
autumn 2009. This paper will describe the results of the laboratory characterization of the optical system and an outlook
on the MEDUSA instrument development.
MEDUSA is the lightweight high resolution camera, designed to be operated from a solar-powered Unmanned Aerial
Vehicle (UAV) flying at stratospheric altitudes. The instrument is a technology demonstrator within the Pegasus program
and targets applications such as crisis management and cartography. A special wide swath CMOS imager has been
developed by Cypress Semiconductor Cooperation Belgium to meet the specific sensor requirements of MEDUSA.
The CMOS sensor has a stitched design comprising a panchromatic and color sensor on the same die. Each sensor
consists of 10000*1200 square pixels (5.5μm size, novel 6T architecture) with micro-lenses. The exposure is performed
by means of a high efficiency snapshot shutter. The sensor is able to operate at a rate of 30fps in full frame readout.
Due to a novel pixel design, the sensor has low dark leakage of the memory elements (PSNL) and low parasitic light
sensitivity (PLS). Still it maintains a relative high QE (Quantum efficiency) and a FF (fill factor) of over 65%. It features
an MTF (Modulation Transfer Function) higher than 60% at Nyquist frequency in both X and Y directions The measured
optical/electrical crosstalk (expressed as MTF) of this 5.5um pixel is state-of-the art. These properties makes it possible
to acquire sharp images also in low-light conditions.
MEDUSA is a lightweight high resolution camera, designed to operate at stratospheric altitudes mounted on a solar-powered
unmanned aerial vehicle (UAV). The MEDUSA instrument targets applications such as crisis monitoring and
large scale mapping, requiring high resolution images with regional coverage, flexible flight patterns, high update rates
and long mission lengths (weeks to months). The instrument is subject to severe constraints on mass (< 2,5 kg), volume,
power consumption and survivability in the stratospheric environment. Operating temperatures within the payload vary
over several tens of degrees Celsius over the day-night cycle. Nonetheless, the instrument will be able to provide
panchromatic and color images of 30 cm ground resolution at an altitude of 18000 m and a wide swath of 3000m.
This ESA-PRODEX (PROgramme de Développement d'Expériences scientifiques) funded project successfully passed
the Critical Design Review in September 2007 and the assembly, integration and test (AIT) phase of the subsystems will
be finalized by the end of 2008. Subsystem tests include an optical performance verification performed on optical
compartment with a test-sensor assembly both at ambient and operational environmental conditions. The electronic
subsystems and their interfaces are tested for functionality and performance in the operational temperature and pressure
range. From early 2009 onwards, the MEDUSA system will be fully integrated including a custom designed wide swath
MEDUSA CMOS frame sensor (10000x1200 pixels). The MEDUSA instrument will be ready for its first flight in spring
of 2009. The detailed design of the optical instrument and its performances have been discussed in [1]. In this paper we
will give an overview of the AIT status of the MEDUSA sensor and the optical system and an outlook on the system
integration and test phase.
The ESA-PRODEX funded MEDUSA project aims to develop a light weight high resolution multi-spectral earth
observation instrument, which will be embarked on a solar-powered high altitude long endurance (HALE) UAV,
operated at stratospheric altitudes (15 to 18km). The MEDUSA instrument is designed to fill the gap between traditional
airborne and spaceborne instruments regarding resolution and coverage. It targets applications such as crisis management
and cartography, requiring high resolution images with regional coverage, flexible flight patterns, high update rates and
long mission lengths (weeks to months).
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. The central part of the payload is a focal plane assembly consisting of two frame
sensors (PAN and RGB). The wide swath is realized with a custom designed highly sensitive CMOS sensor of
10000x1200 pixels. A GPS receiver and Inertial Measurement Unit (IMU) provide accurate position and attitude
information. A direct downlink allows near-real time data delivery to the user. The on-board data processing consists
mainly of basic image corrections and data compression (JPEG2000).
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 is operated in a low pressure and low temperature environment. System modeling and
simulation is used to make careful trade-offs between requirements and subsystem performances.
On 27th November 2006 the phase C/D for the design, production and test of the camera has started at VITO with the
support of 9 industrial partners. The MEDUSA camera is expected to transmit its first images the end of 2008.
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