Interview with Dr Guy Meynants, VP R&D at CMOSIS

08 March 2011

Dr Guy Meynants, VP R&D at CMOSIS and speaker at the upcoming ISE 2011 conference, talks about digital Sun-sensor on-a-chip for space navigation

Q: Can you give me an outline of the project.

A sun sensor is used as a part of the attitude and orbit control systems in a satellite or a spacecraft. It searches for the location of the sun in a section of the sky, extracts the centroid of the sun to sub-pixel resolution and then tracks the sun position. This information gives a partial solution to the attitude of the spacecraft. Common sun sensors rely on separate image sensors and an electronic processing unit to extract the sun information. That is rather bulky and complex. We have integrated all of this in a single sensor. This allows a much more compact system. In the presentation, the architecture of that chip is presented and its key performance data is given.

Q: How have you combated the effects of radiation in this harsh environment?

Space is a harsh environment for electronics and image sensors, however. Radiation (gamma rays, protons, ions) deteriorate the performance of image sensors and will create temporary and permanent artifacts in the image. The second part of the talk is about these topics. To illustrate what happens, we show some radiation testing results on several of our imagers, for total dose (gamma), proton and heavy ion irradiation. Then we go back to the sunsensor and talk about measures that we took to reduce the impact of these effects. This is done on several levels. We designed the transistors and pixels for radiation tolerance. We have built in redundancy in the circuits of the chip to allow temporal or permanent failure of one of them. And the sun detection and tracking algorithm is made robust and tolerant against false detections of the sun.

Q: Are there other non-space applications for the technology you have developed here?

Yes, the readout architecture with the column ADCs is also used in sensors that we make for industrial vision cameras, traffic control systems or broadcast cameras. Variants of the large 10 micron pixel are used in some sensors that we make for scientific use. And the radiation tolerant image sensor technology finds its use also in the nuclear industry. And the measures that we take to reduce radiation effects help us also to better understand the device physics in the pixel, and ultimately make better pixels for all of our imagers.

Q: Aside from radiation, what other physical challenges are there for space-bound imaging devices?

The environmental conditions (temperature, vibration, vacuum, etc) are challenging as well. It puts limitations to the design and to how the chip is assembled. This is verified in a qualification campaign, wherein the device undergoes environmental tests in relevant conditions. Our device also has to operate from a 'dirty' power supply with some ripple, without impact on the performance. And low weight and small form factor are obviously also important for space use. This translates into using as little as possible external components to operate the chip.

Q: What are the advantages to the satellite of using your sensor system?

The attitude control system is critical for the mission; if it fails the satellite is lost. The satellite can now use several redundant sensors without large penalty in weight or volume. It makes its attitude control system more reliable. Furthermore, this is also a key enabler for optical navigation on very small micro- or nano-satellites. These are small satellites often no bigger than a washing machine. They are increasingly popular because of the lower cost of the satellite design and the lower cost of bringing it into orbit. They could not afford the large optical navigation systems that existed in the past.

Q: When will this be implemented on its first flight?

We're currently talking about a first flight on which the prototype sensor will be demonstrated. That is a nice step forward of course. To go to a real commercial space product, the space qualification testing must be started. Another next step is integration of star tracker functionality. So we still have some work ahead of us.

You can hear Dr Guy Meynants' presentation on this topic on day 1 at 14.10 at the ISE 2011 conference. Log onto www.image-sensors.com/agenda.aspx for more information.