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

49 Airborne base stations | Insight reference voltage generated by a microcontroller running a more complex MPPT algorithm that tracks the output voltage of the solar cells and controls the input voltage of the power manager to match it. The power manager also has to be aware of the different types of batteries, which as mentioned is lithium-ion or lithium-sulphur for the Solara 50 base station. These need to provide constant, clean power to all the systems on the craft, all of which have different load and current requirements, as well as handle any peak currents. To prevent damage to the batteries and extend the flight time of the Solara 50, both the maximum and minimum voltages are monitored against user- programmed undervoltage and overvoltage levels. To further help manage the strict onboard energy budget, the power manager toggles the ‘battery good’ flag to signal an attached microprocessor when the voltage on a battery or capacitor has dropped below a preset critical level. This triggers the shedding of load currents to prevent the system from entering an undervoltage condition. Solara is not the only solar-powered HALE in development, but it is the one with the most focus on base station applications. The 22.5 m wingspan Zephyr-7 for example was developed by UK defence research company Qinetiq and Ascenta Aerospace, and stayed aloft for seven days, and is designed for altitudes of up to 20 km. It is light enough to be hand-launched and uses the same lithium-sulphur batteries as the Solara. The team behind Zephyr worked for Ascenta in Yeovil, England, and was acquired by Facebook in May 2014 to develop the technology in-house to act as the platform for a mobile base station. The Solara project is still a prototype but fits well with another of Google X’s UAV developments, Project Wing. This is looking to use a battery-powered autonomous craft with a wing span of 1.5 m to deliver small packages. Notably it is avoiding two of the key technical challenges with an autonomous craft-based delivery system. Instead of having to navigate in the open air, which requires a high level of accuracy that is not currently supportable by a battery power system, the craft would operate in a pre-programmed corridor, which Google calls an ‘aerial road’. This dramatically simplifies the navigation system’s computing requirements, leaving processing cycles for detecting any other aircraft that might be a collision hazard in the corridor. Avoiding the problem of stabilising the craft to land in an unknown environment, it uses a cable to drop the package to within a few feet of the ground, as measured by a sensor at the end of the cable. This then triggers the release mechanism to deliver the package and signals for the cable to be rewound. Google is keen to point out though that this is purely a prototype to look at different implementation ideas, rather than the technology it would actually use. An example of other potentially useful technology comes from the Department of Aeronautics and Astronautics at MIT in the US, which has been working on health monitoring algorithms that fit well with both Solara and the Wing project, as well as the successor to the Zephyr. Researchers at the department have developed an algorithm called the Partially Observable Markov Decision Process, where each decision node is represented by a probability distribution. This allows the UAV to predict the collision and failure probabilities on different routes, which would be suitable for Project Wing operating in the aerial roads. Using the same algorithm for monitoring the health and likely failure point of components in Solara also allows the system to be recalled and replaced before an actual failure occurs, to ensure consistency of coverage. Facebook and Google are vying to provide internet services to poorly served regions of the world. The combination of autonomous HALE craft with lightweight base station technology and free space optics for the backhaul make this a viable proposition but a major engineering challenge. MIT has been working on health monitoring algorithms that fit well with Solara and the Wing project Google’s Project Wing is exploring different options for unmanned aerial craft (Courtesy of Google) Unmanned Systems Technology | November 2014