First Beowulf Cluster in Space

When President Eisenhower proposed the Open Skies Policy at the 1995 summit meeting to the Soviet delegation in Geneva, it was an unsuccessful move to legitimate the US' plans to launch the U2 spy plane a month later. Five decades later, Open Skies became a reality with the launch of Singapore's X-Sat. What could complement Open Skies better than open source? And it doesn't take a genius to understand that, when reliability is all important, the transparent and open nature of Linux source is an invaluable aid.

At the outset of the X-Sat Project, which focused on developing Singapore's first satellite, arguments were made for Linux but roundly were rejected. At that time, Linux was an esoteric outsider for embedded systems use and hadn't penetrated the consciousness of decision-makers in the area of space developments. Furthermore, Singapore generally is not known for risk taking, and truly there is something to be said in favour of this attitude where satellites are concerned. By contrast, VxWorks has excellent space heritage, although this is no guarantee for success.

Although the satellite's main computer runs VxWorks, Linux powers the data processing computer. This actually is a loosely coupled cluster called the parallel processing unit (PPU), and it is the first distributed example for Linux in space. The concept is to run satellite image-processing applications directly in space after a straightforward re-compilation and uploading procedure from ground-based Linux development platforms. Let's compare the main on-board computer (OBC) and the PPU (Table 1).

The OBC is so expensive because it utilises costly radiation-hardened components, whereas the PPU uses mostly commercial-off-the-shelf (COTS) components. Traditionally, reliability in space is ensured by using the most reliable individual components to survive the hostile environment. But the PPU embodies the relatively new concept—at least in space—of reliability through redundancy. Although each single component of the PPU is less reliable in space than are the OBC components, there are 20 copies of each PPU processor, so even if one after another fails, something still remains. The design almost eliminates single-point failures, where a single component failure could take out the entire system or multiple components. On top of this, the PPU is characterised through graceful degradation from a fully working 20-processor system down to a single processor. So, good design ensures that the probability of a single PPU processor still functioning at the end of design life matches the probability that the OBC still is functioning at the same time. And even with only a single surviving processor, it still thrashes the OBC.

X-Sat is a 100kg micro-satellite, roughly an 80cm cuboid, as shown in the CAD model of Figure 1. The satellite, an educational project at Nanyang Technological University, Singapore, carries three payloads: a 10m resolution multispectral (colour) camera for obtaining images in the Singaporean region, a radio link for an Australian-distributed sensor network and, most notably, the PPU. From the outset of the project, X-Sat has been an open satellite with details publicly available, and what better reason to use open-source software?

Figure 1. X-Sat is about 80cm in size and carries a colour camera, radio link and a Linux cluster.

Communication uses S-band with 4kb/s uplink and 500kb/s downlink as well as a unidirectional 50Mb/s downlink over X-band for image dumps. However, the X-band needs a dedicated 13m dish antenna for reception, and it works only when the satellite is over Singapore. In its intended sun-synchronous 685km orbit, this occurs for only a few minutes every day and leads to a major rationale for the PPU. Assuming a conservative duty cycle of 10% per orbit, the camera can generate 81GB of data per day, but only 12% of this can be downlinked. And with a three-year design lifetime and multimillion-dollar cost, each picture works out to be rather expensive.

If we want maximum value per picture and we have to throw away 88% of the images, which ones do we select? Anyone who has been to Singapore should remember the overcast skies. It turns out that 90% of satellite pictures over Singapore show only clouds and haze. Although this may excite meteorologists, it's a waste of money for us. We'd get value from only 10% of the 12% of pictures we download—a 1.2% success rate! So if there's any way of deciding before downloading whether a picture is obscured by clouds or even a way of cutting out the cloudy bits and downloading the rest, then this is valuable. Well, you guessed it, such applications do exist. They run on Linux Beowulf clusters, require many MIPS and happen to be a perfect fit for our PPU.