THIS PAPER IS REPRODUCED FROM : The Proceedings of the International Meteor Conference, Bollmannsruh, Germany, September 19-21, 2003, Eds.: Triglav-Čekada, M., Trayner, C., International Meteor Organization, p.64-69

SMART PANORAMIC OPTICAL SENSOR HEAD 

A CAMERA FOR OBSERVING METEORS FROM SPACE

Detlef Koschny1, A. Marino2, J. Oberst3

1European Space Agency, SCI-SB, Keplerlaan 1, NL-2200 Noordwijk 
2Osservatorio Astronomico di Torino, Turin, Italy 
3DLR Berlin, Adlershof, Berlin, Germany 

INTRODUCTION

This article presents the results of an industry study performed for the European Space Agency (ESA) to develop a design for a "night-time imager", i.e. a light-sensitive camera with a wide field of view which can be qualified to fly on a space mission. A first presentation about this camera was given at a previous IMC, see Koschny et al. 2003. This camera could address a number of scientific goals better than current space cameras, which are optimised for day-side imaging: 

• Imaging, from orbit, of electric discharges, e.g. lightning flashes, sprites, discharges due to dust storms; 
• Imaging of noctilucent clouds and aurorae, i.e. the light emitted when a dust particle enters an atmosphere; 
• Imaging, from orbit, of space debris entering the Earth's atmosphere;
• Imaging of impacts onto planetary surfaces;
• Imaging from a lander that lands on the dark side of a planet or moon.

The study was financed from the Technology and Research Program (TRP) of ESA. A total of six proposals were received on the Invitation To Tender for this study, two of which were excellent and selected to be financed. One study was performed by Officino Galileo (Italy) with the Observatory of Turin as a subcontractor, the other study was performed by Jena Optronic (Germany) with the DLR Berlin as a subcontractor. 

REQUIREMENTS AND REFERENCE MISSIONS

The science goals given in the previous section are quite diverse. To focus the study and allow industry to develop a proper breadboard, the study was narrowed down to goals (c) and possibly (e), i.e. observing meteors, and impact flashes. 

Derived from this focus, the following top-level requirements were given to industry: 

• Camera sensitivity: Image a visual magnitude Mv = 6 mag object or brighter, moving with an apparent speed of 5 deg/s or slower, with a Signal-to-Noise ratio > 5. - A Signal-to-Noise of larger than 5 should ensure proper detection. The idea was to detect meteors at least to a magnitude a visual observer would detect.
• Field of view: 120 deg x 120 deg or larger. – This requirement stems from the idea that the larger the field of view, the more meteors would be detected. Lenses with 90 deg were already available in space qualification, 120 deg would push the technology a little bit further.
• Duty cycle: The camera shall record the night side of a planet for more than 80 % of the time. – One of the envisaged solutions was to use a standard framing CCD camera. Those need to be read out, which cannot be done infinitely fast, in particular if readout noise should be reduced.
• Time accuracy: The camera shall allow to determine the time of an event to an accuracy of 10 s. – This is not a very stringent requirement. It turned out later that a time accuracy of 1/10 s can easily be achieved.
• Detection efficiency: The applied software algorithm to detect the events shall not have more than 200 % false detections. It shall not miss more than 20 % of the events. – This requirement drives the quality of the software. Based on experience with e.g. MetRec (Molau 1999) it is impossible to achieve 0 % false detections. The idea here is that even if a few images do not contain real meteors that would be acceptable. However, just downlinking everything is not possible in particular for planetary applications – any planetary mission is very limited in downloading capability, typical Mars missions achieve a downlink rate of less than 30 kbit/s.

In addition to these scientific performance requirements, there are technical requirements which must be fulfilled. The radiation environment, maximum and minimum operating temperatures etc. were given. For this paper, the limits for mass and power are of interest. Mass and power are typically severly limited on space missions. The maximum mass and power required are for the complete camera with optics plus the processor running the detection software and the interface to the spacecraft:

• Mass <1.5 kg
• Power < 5 W

The study encompassed to following activities:

• Perform a scientific analysis, in particular estimate the expected number of meteors per time, for the baseline camera in the baseline mission;
• Produce a design for a flight-worthy camera on paper for both hard- and software and estimate the resource requirements (mass, volume, power).
• Build a breadboard, which demonstrates that the design is feasible, using off-the shelf components where possible;
• Test the breadboard on ground, both in the lab and under real-sky conditions.

To calculate the expected number of meteors and to have more boundary conditions for the design of the camera, two reference missions were given. The first reference mission to be studied was to observe the planet Earth at night time from the International Space Station (ISS). Figure 1 shows a graphical visualisation of the field of view of a camera with 120 deg viewing angle as seen from the ISS in an atmospheric height of about 100 km, where meteors occur. Interestingly, the real field of view is very close to that of the all-sky cameras of the European Network using 180 deg fish-eye lenses, namely a circular area of about 1000 km diameter. 

The second reference mission is a low Mars orbiter in an orbit of 300 km altitude.

Figure 1: Apparent field of view of a camera with a viewing angle of 120 deg (circular) observing the Earth from the International Space Station.

SCIENTIFIC ANALYSIS

Both studies gave a detailed analysis of the detection probability of meteors with the proposed cameras. It can be expected that between 5 to 50 meteors per hour of recording can be seen from an orbit as the ISS. The expected numbers around Mars are very similar – on Mars, there is also a chance to see impact events, which are more abundant compared to the Earth due to the thinner atmosphere.

During meteor showers, the rate would be accordingly higher.

These numbers fully justify the operations of such a camera and would allow statistical analysis of meteor activity.

THE ITALIAN SOLUTION

Galileo Avionica (GAL) based their design on a so-called Electron-Multiplied CCD (EMCCD). A CCD (Charged Coupled Device) is a sensor to convert light into electrons, which can be read out by a computer. The EMCCD incorporates a stage which multiplies the charge generated by the incoming photons, it acts like an on-chip photomultiplier. However, it does not have the disadvantages of a normal photomultiplier, which is mechanically sensitive and would need high voltage in the order of kilovolts. Thus it is particularly suited for space applications.

The EMCCD is a rather new technology development by the British company E2V. A commercial camera is available from the company Andor, the so-called iXon camera. GAL built their breadboard around this commercial camera, using an off-the-shelf fast C-mount lens. They developed special software, which for the breadboard runs on a standard Windows PC.

THE GERMAN SOLUTION

Jena Optronic designed a special, very fast lens with 6.6 mm focal length and f/0.95. This fast lens allows to fulfil the requirements without a special sensor and their camera uses a standard back-illuminated (thus more light-sensitive) CCD. For their breadboard, they built one of these lenses, however using standard glass materials rather than special radiation-hard glasses which would be required for a space application. They also built a special optical head with a back-illuminated E2V CCD 42-20 (1024 x 1024 pixel). Their meteor detection software runs on a Linux laptop.

PROPOSED FLIGHT UNITS

Figure 2 shows a photograph of the possible layout of the GAL camera – the image actually shows a camera designed to act as a star tracker for the European Space Agency's Mercury mission BepiColombo. The flight camera would look very similar. Table 1 summarizes the main characteristics of the flight models. It can be seen that it is feasible to build such a camera, even if the mass and power requirements are not quite met.

Figure 2: This is how the camera could look like.

TESTING ON GROUND

Both companies performed ground testing of their breadboards: in the laboratory, in a planetarium with simulated meteors, and under real-sky conditions. It was demonstrated that the cameras work according to specification.

The Jena/DLR camera was used in several ground-based campaigns from August to December 2004. In December, the Jena/DLR team attempted coordinated observations with one of the intensified video cameras used by the meteor group of the Research and Scientific Support Department (RSSD) of the European Space Agency. The campaign was hampered by bad weather, but in the night from 14/15 Dec 2005, two hours could be used for observations through holes in the clouds. Two examples for simultaneously observed meteors are shown in Figure 3, showing that the breadboard camera reaches comparable results as a proven concept with an image-intensifier. The software correctly identified the meteors and the data was stored on hard disk.

Figure 3: Comparison between meteors as observed by the Jena/DLR camera and an image-intensified camera (on the left side, ICC) in use by the European Space Agency's meteor group. The field of view of the ICC is 22 deg, of the Jena/DLR camera almost 180 deg.

CONCLUSIONS

Two parallel studies were performed to study a design for a night-time imager which can be built to space flight standards. It was shown that such a camera could be built for about 2.5 kg mass or less, needing about 5 W of power. Software to detect meteors on board and only downlink potential meteor events was developed. This is extremely important as it is currently impossible to downlink video data streams from planetary space missions. Breadboards were built and demonstrated that the designs are feasible.

One of the breadboard cameras was operated in parallel to a standard image-intensified meteor video camera and demonstrated that it can record comparable data. One drawback is that the frame rate is less than the video rate and typically around 5 to 10 frames per second.

ACKNOWLEDGEMENTS

Most of this work was funded by the Technology and Research Programme of the European Space Agency, ESTEC Contract No.: AO/1-4229/02/NL/CP.

We thank all the people involved from Jena Optronic, DLR Berlin, Galileo Avionica, and the Observatory Turin for their enthousiastic support.

REFERENCES

Koschny, D., di Marino, M., Oberst, J. (2003), Meteor observation from space – The Smart Panoramic Optical Sensor (SPOSH), IMC Proceedings 2003, pp. 64-69.

Molau, S., Nitschke., M. (1996), Computer-based meteor search, WGN 24, 1996, pp. 119-123.