## 2 Classes of UAS from An Aerospace Perspective
### Micro UAS  
We will assume that micro UAS are represented at the large end by the Aerovironment Wasp AE [14] and at the lowest end, as of this writing, by the Proxdynamics Black Hornet [15]. A legal definition exists in Australian Civil Aviation Regulations [16], which imposes a 100 g weight limit, below which systems are defined as a \"micro UAV\" and not regulated. There are numerous programs to reduce the size of UAS well below the Black Hornet's 18 g. Currently there is nothing acknowledged that is smaller and claimed to have achieved operational utility.  
In the currently dominant military and law enforcement contexts, very small UAS are expected to be useful for a range of ISR tasks, including: personal ISR by individual infantry, counter terrorism and law enforcement personnel; base and airfield surveillance; detection of hazardous materials.  
Other roles are speculative and there is increasing interest from a range of potential users including the media, but at this scale payload capability is necessarily limited. Other roles will develop as the systems become better understood and emerge from the current state. Generally, the small size of the aircraft satisfies requirements for convenience and limited carriage/stowage burden and offers the promise of mission agility (quick to deploy) and physical agility (the ability to manoeuvre in small spaces such as narrow streets and even inside rooms). Critically, these systems offer the promise of safe operation, for example, operation in a metropolitan area over civilians when threat levels are too low to justify the hazard presented by a large high capability UAS.  
These systems are limited as a direct result of their size. It is clear that multidisciplinary technical hurdles are non-trivial and functional limitations are unavoidable.  
The low Reynolds number domain requires specialized airfoils and wing shapes [17]. This regime may favour flapping wing flight with interactions between aerodynamic surfaces and vortices [18], once efficiency of the aero-electro-mechanical system is understood and improved [19]. Currently there is little concrete progress towards a practical flapping wing UAS.  
Performance improvements will be driven by traditional aeronautical research. The empirical approach of hobbyists and the market driven engineering of small companies providing hobby components may also play a role. Performance improvements are multiplicative for flight systems, such as wing aerodynamics and propeller efficiency, so a series of modest optimizations of different systems might increase performance substantially.  
Steering, pointing and zooming of electro-optical (EO) payloads is limited by the size of robust, accurate mechanics. This limit is constrained by the precision achievable in mass produced items. Other issues include optical limits set by lens size and focal plane size. There is one main market driving small EO payloads: mobile telephones. Features such as face tracking and internal zoom mechanisms can be directly applied to UAS applications. High resolution thermal cameras, laser radar and other sensors found in larger UAS are currently a challenge for micro UAS. This could change if these sensors become part of mass market products such as mobile telephones.  
The physical size of guidance hardware (the autopilot) is a problem in this class. Advances in micro-electro-mechanical systems (MEMS) and their integration into multisensor integrated circuits is resulting in dramatically decreased size.  
A notable deficiency in all UAS to date is the Human Machine Interface (HMI), this is particularly the case in the micro UAS domain. A significant conceptual advance is required, possibly including display technologies which will improve continuously thanks to mobile devices. It seems reasonable that the human interface should be proportionate to the size of the air vehicle. Ultimately HMI should use natural human communication and largely eliminate the ground control element.