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82 December/January 2017 | Unmanned Systems Technology PS | Guidance, navigation and control systems in space T hanks to the latest advances in sensor technology and navigation hardware such as Lidar and transponders, as well as the ever-improving communications infrastructure around the Earth, advanced autonomous systems can now navigate their way around with reasonable accuracy and repeatability (writes Stewart Mitchell). However, when an autonomous system is launched into deep space, these technologies can no longer determine its position or orientation, as there is little by way of a comms infrastructure and few objects within the range of a Lidar sensor, let alone anything an object recognition system would identify. To that end, novel environment perception and navigation systems are used instead. One example here is the guidance, navigation and control (GNC) technology used on space exploration craft such as the Mars Reconnaissance Orbiter, which is currently searching for evidence of water on the surface of the Red Planet and whether it was there long enough to support life. To function properly, the orbiter must simultaneously keep its solar power arrays aimed at the Sun, and its camera arrays accurately oriented towards the Martian surface to take detailed images of desired targets and remain in a precise orbit around the planet. To do that, the orbiter has long-range camera arrays coupled with sensors called star trackers to take detailed images of celestial bodies in its view at a rate of 10 Hz. These pictures are then cross-referenced with an onboard database of thousands of stars and matched to recognised bodies so that the craft can determine its position in space relative to each one. Despite these cutting-edge camera arrays and star trackers though, the GNC system is not entirely reliable as not all the stars in the orbiter’s view can be identified, so the images must be regularly sent back to Earth for further analysis to ensure the craft is on track. That presents yet another challenge, as Professor Yang Gao of Space Autonomous Systems at the Surrey Space Centre, explains. “Communications between the orbiter and Earth has a certain latency, and the best-case scenario is when the orbiter is in the line of sight of the comms satellites orbiting the Earth. At that point, the comms delay between the orbiter and the Earth is about 30 minutes, which is too long for highly accurate control. “If there is no line of sight, such as when the orbiter is on the far side of Mars, the messages are relayed through a series of satellites orbiting the Earth until the line-of-sight window is open again. In this case, we may never receive the messages, and this is currently the limiting factor for the complexity and navigation accuracy of autonomous systems in space.” The orbiter’s GNC is the forerunner of a far more sophisticated autonomous system being developed for the ExoMars system (detailed in UST 10, October/ November 2016), which is set to go to Mars later this decade. Also, with a series of comms satellites scheduled to launch into orbit around Mars and the Earth between now and 2020, it will not be long before a deep space comms infrastructure is set up, and advanced autonomous systems such as those on Earth will be able to navigate through space and on other planets accurately and reliably. Now, here’s a thing “ ” The comms delay between the orbiter and the Earth is about 30 minutes, which is too long for accurate control