Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

33 Real-time operating systems | Focus system, and needs to be managed by both the hypervisor and the RTOS. An SMP-enabled RTOS allows an application to be locked onto a particular core and always run on that core at a particular priority. That means you can move directly from a single-core to a multicore environment without reprogramming the code – a key advantage with code that has already been certified. Asynchronous multiprocessing (AMP) uses different types of cores, so you have to map applications to a particular core with different versions of the code for different types of processors. This needs a more complex data link between the cores, so you risk losing timing synchronisation. That is because you end up with a multi-level scheduler, so if you have a task ready and queue eight cores to run the top eight applications, but then want to run a ninth application, you have to flush the cache and load the new application, and that cache exchange is where you lose the determinism. Ways around this include locking the application to a core or flushing the caches each time, but that degrades the performance, so the question then becomes whether you really need determinism or just high performance, and that depends on the requirements of the system. You certainly need it for the flight controls in an aerial vehicle, and even a ground vehicle for the brakes, although if you are travelling at 20 mph it may not make a difference. For example, the Mars Curiosity rover moves at not much more than walking pace; it is sent a given location, then figures out the route and drives itself there. If it then gets into a situation it can’t handle it just waits until it’s told what to do. Curiosity and the earlier Opportunity and Spirit probes use a commercial RTOS for central control. Curiosity also uses another, different, RTOS in some of its instruments, highlighting the need for safety and security across the whole range of RTOS implementations as systems become more complex. Communications The communications element that sits on top of an RTOS is a major challenge for autonomous systems software, as once you add communications into the RTOS stack there is a complex set of partitioning and certification issues. With unmanned systems the challenge becomes split between the ground station and unmanned vehicle with data link – do you include the ground station, link and vehicle as part of the safety case? From a software certification point of view it’s exactly the same issue, but if you move it up to the system level, you have to include how, for example, a UAV interacts with air traffic control and anti-collision systems as well as other aircraft or vehicles. This brings tougher system- level requirements for the software regarding reliability and guarantees of message delivery, as there will be extra requirements to mitigate the risks when components fail – for example, if you lose the data link what does the unmanned vehicle do; what’s the back-up plan? With a data link you are always at risk from adverse weather or multiple transmitter failures, and then it becomes more of an operational issue than one of technology. That’s when you need to decide whether to divert to a crash- landing site or carry on until the link is re-established, but once you have lost the data link you are flying autonomously, leading to the question of what the preferred route would for safe crash areas, such as in the sea. These are more operational requirements that have to be implemented in the software. One promising technique that is emerging for efficient real-time communications is multicast, where data is broadcast from one node to many. This has not yet reached the point of an open standard for high-quality network security, and while the IPSEC and SSL specifications are already The comms element that sits on top of an RTOS is a major challenge for systems software Unmanned Systems Technology | November 2014