## 1 Introduction  
Expectations of growth in the Unmanned Aerial Systems (UAS) industry are quite high, Teal Group predicted that \"Unmanned Aerial Vehicles (UAVs) continue as the most dynamic growth sector of the world aerospace industry this decade\" [1]. This is despite the United States Department of Defence (US DoD), the largest single customer for UAS [2], projecting reduced funding for Research, Development, Testing and Evaluation [2] of UAS. In an environment containing both growth in inventory and reduction in research funds it is important to understand which technologies are absolutely required for progress and which technologies are likely to be advanced by other disciplines outside aerospace. Like any other complex system, UAS are composed of a series of major subsystems. Many of these subsystems are common to other aerospace, communications and intelligence, surveillance and reconnaissance (ISR) systems. Whilst enhancements in these systems will improve UAS performance, they might not enable new capabilities, except where thresholds of specific subsystem parameters must be achieved to undertake a new mission or role. In this paper we will attempt to demonstrate where the core of UAS engineering research lies. We will also outline those areas that require a threshold of functionality or size to affect change in UAS roles. The United States Department of Defense use the following definitions [3]:  
_\"Unmanned aircraft--An aircraft that does not carry a human operator and is capable of flight with or without human remote control. Also called UA.\"_  
_\"Unmanned aircraft system--That system whose components include the necessary equipment, network, and personnel to control an unmanned aircraft. Also called UAS.\"_  
Aboard the UAS are a control system, payloads, communications and a number of other subsystems that conceptually map to standard air vehicle subsystems. It is arguable that, as with a manned system, the ground station or command control element is not really integral to the system or necessary when UAS are more autonomous. The software of the UAS guidance system consists of control systems for stabilisation and navigation, which we will describe as \"control\", and a degree of decision making and sensor processing that we shall describe as \"Autonomy\". The payloads are sometimes UAS specific and are generally used operationally in such a way that payload implementations dominate the role of the entire system. This paper is focused on UAS platform technology. There are many technologies loosely related to high levels of autonomy that could be considered, however, for aerospace vehicle research, the design, performance and control of the air frame to meet requirements is the the emphasis. UAS only became truly effective with the advent of GPS, previous limited successes navigated using high quality inertial measurement units, terrain registration or line of sight video telemetry. The Teledyne-Ryan Firefly and descendant target drones modified by the US Air Force for reconnaissance during the Vietnam War era [4] were conceptually a successor to the high performance Lockheed-Martin D-21 [5]. Navigating on inertial and air-data systems these drones were capable of occasionally capturing imagery from inside heavily defended airspace. GPS has subsequently provided the means for aircraft to be unattended for long periods of time, even the entire mission. Remotely Piloted Vehicles (RPVs) have generally provided a capability only in limited missions, such as being a target. Oddly a very early rotary wing UAV, the Gyrodyne QH-50 DASH achieved a degree of success in the Vietnam war era [6] probably due to its high degree of stabilisation and ability to fly slowly and hover. The aerial domain is generally uncluttered compared to ground environments. Many of the signals used to make decisions are quite simple and artificially derived, for example weather radar, GPS and transponders.