In this thesis, a study of the capacity of a suggested three - dimensional Air-to-Ground cellular system is being made. The Outside Cell Interference Factor (OCIF) is being calculated through simulations for reverse and forward link using seven loops, from the interfering cells around the desired cell for different values of the maximum height of the cell and its radius. Capacity per cell as well as delay and throughput for packet data transmission was calculated for the first time through closed form equations, with the use of the load factor, the activity factor and sectoring gain using the Automatic Repeat Request (ARQ) algorithm for the correction of errors. Moreover, in this thesis, the algorithm which has been created is being analyzed and used for the simulations. Moreover, for the first time, a case study has been made involving the study of capacity of the Air – to - Ground system for the airports of Greece, in three basic scenarios in which the number of the users, the delay and the throughput per cell is being calculated. In the first scenario, we are restricting to the three major airports of the country, while in the second it expands to six airports covering from the radio-coverage side almost all Greece. In the first two scenarios the same cell radius of 175 km is being used, while in the third the radius is reduced to 100 km and the airports are increased to nineteen. In all three scenarios we assume that all the users use the same service. The voice services are also studied of 12.2 kbps and data with transmission rate 64, 128 and 384 kbps. From scenarios 1 and 2 (cell radius 175 km), it was found that we can service at the same time up to 179 voice subscribers per cell at bit rate 12.2 kbps which reduces to 33 users for video call of 64 kbps and in 18 for video call of 128 kbps. In scenario 3 (cell radius 100km),it was found that we can serve at the same time until 126 voice subscribers per cell at bit rate 12.2 kbps which reduces to 23 users for video call of the 64 kbps and in 13 for video call of 128 kbps. In scenario 3 although the capacity per cell is lower than in scenarios 1 and 2, it provides greater total capacity (for all Greece) in relation to these scenarios.

The development of a microprocessor based digital flight control system for a particular R.P.V. is described. The tasks required of this system are defined, and thereafter, the hardware circuits and the software structure necessary to implement a prototype are presented. The autopilot control laws are inferred from z-plane root loci, and then confirmed using digital simulations of the de-coupled roll and pitch attitude loops. The problems of the finite wordlength implementation of the control laws are discussed, and then both hybrid simulation and actual flight results are used to prove the performance of the prototype. To exploit the adaptive capabilities of a software based system, a sliding mode variable structure control law is developed for the roll attitude loop. Digital simulations are used to show that significant improvements in sensitivity reduction can be achieved under some conditions. These improvements are lost if a realistic servo-actuator model is employed. Another objective, namely the reduction of the disturbance error induced by trim imbalance, is maintained provided a reduced order switching function is used.

The aviation community has to implement very stringent navigation integrity requirements in a variety of manned and unmanned aircraft applications. This thesis presents the results of the research activities carried out by the Italian Air Force Research and Flight Test Centre (CSV-RSV) in collaboration with the Nottingham Geospatial Institute (NGI) and RMIT University in the area of Avionics Based Integrity Augmentation (ABIA) for mission-essential and safety-critical Global Navigation Satellite Systems (GNSS) applications in the civil/military aviation context. Space and Ground Based Augmentation Systems (SBAS/GBAS) have been developed in recent years to improve GNSS integrity, accuracy and availability for aircraft navigation and particularly for landing applications. SBAS satellites broadcast correction messages back to the earth, where suitably enabled receivers use the information to improve accuracy and integrity. The US, Europe and other nations have developed their own SBAS systems. In the US, the Wide Area Augmentation System (WAAS) exists and is operational. In Europe, SBAS coverage is provided by the European Geostationary Navigation Overlay Service (EGNOS), in Japan by the Multi-functional Satellite Augmentation System (MSAS) and India is developing the GNSS Aided Geo Augmented Navigation (GAGAN) system. An alternative approach to GNSS augmentation is to transmit integrity and correction messages from ground-based systems. An example is the American Local Area Augmentation System (LAAS), which allows a suitably equipped receiver to derive enhanced accuracy and integrity information in a local area. The combination of WAAS and LAAS is targeted to provide the Required Navigation Performance (RNP) in all phases of aircraft navigation, including en-route, terminal, approach/landing and surface operations. Along with SBAS and GBAS, GNSS augmentation may take the form of additional information being provided by other avionics systems. In most cases, the additional avionics systems operate via separate principles than GNSS and, therefore, are not subject to the same sources of error or interference. A system such as this is referred to as an Aircraft Based Augmentation System (ABAS). The additional sensors used in ABAS may include Inertial Navigation Systems (INS), TACAN/VOR-DME, Radar, Vision Based Sensors, etc. Unlike SBAS and GBAS technology, research on ABAS is limited and mainly concentrates on additional information being blended into the position calculation to increase accuracy and/or continuity of the integrated navigation solutions. Additionally, no significant attempts have been made of developing ABAS architectures capable of generating integrity signals suitable for safety-critical GNSS applications (e.g., aircraft precision approach and landing) and no flight certified ABAS products are available at present. During flight test activities with GNSS and Differential GNSS (DGNSS) systems, it was observed that one or more of the following conditions was prone to cause navigation data outages or severe performance degradations: • Antenna obscuration due to aircraft manoeuvring; • Bad satellite geometries and low carrier-to-noise ratios (C/N0); • Doppler shifts caused by aircraft-satellites relative motion; • Interference, at the airborne GNSS antenna, caused by non-GNSS RF signals; • Multipath caused by GNSS signals reflected by the earth surface or the aircraft body. The last two problems can be mitigated by existing technology solutions (i.e., choosing a VHF/UHF Data Link, filtering the radio frequency signals reaching the GNSS antenna, identifying suitable locations for the GNSS antenna and providing adequate shielded of the antenna itself, either by physical devices or via dedicated software masks, etc.). However, there is little one can do in order to prevent critical events during realistic test/training manoeuvres and particular approach procedures (e.g., curved and segmented approaches) performed with high performance military aircraft. Furthermore, although in some cases a careful mission planning may significantly reduce the number of GNSS outages, the adoption of specific aircraft piloting strategies (using the information currently available in the cockpit) cannot effectively avoid the occurrence of these events. ABIA is a new concept that progressively evolved based on research with GNSS-based Time and Space Position Information (TSPI) systems. TSPI research activities included design, integration and ground/flight testing carried out on MB-339CD, TORNADO and TYPHOON military aircraft. As soon as the validity of the TSPI-ABIA (T-ABIA) concept was established, a prototype system was developed for use in flight test applications. This system is capable of alerting the pilot when the critical conditions for GNSS signal loss are likely to occur (within a specified maximum time-to-alert). In this T-ABIA prototype, the aircraft on-board sensors provide information on the aircraft relevant flight parameters (navigation data, engine settings, etc.) to an Integrity Flag Generator (IFG), which is also connected to the on-board GNSS receiver. The IFG can be incorporated into one of the existing airborne computers or can be a dedicated processing unit. Using the available data on GNSS and the aircraft flight parameters, integrity signals are generated which are displayed on one of the cockpit displays and sent to an Aural Warning Generator. At the same time, an alternate flight path is computed taking into account the geometry and the tracking status of the available GNSS satellites, together with the current mission requirements and the information provided by the aircraft Flight Test Instrumentation (FTI) and standard on-board sensors. Based on the results of T-ABIA research a more advanced ABIA system was developed suitable for manned and unmanned aircraft applications. Detailed mathematical algorithms were developed to cope with the main causes of GNSS signal outages and degradation in flight, namely: obscuration, multipath, interference, fading due to adverse geometry and Doppler shift. Adopting these algorithms, the ABIA system is able to provide steering information to the pilot and electronic commands to the aircraft flight control system, allowing real-time avoidance of safety-critical flight conditions and fast recovery of the required navigation performance in case of GNSS data losses. This is achieved by implementing both caution (predictive) and warning (reactive) integrity flags, as well as 4-Dimensional Trajectory (4DT) optimisation models suitable for all phases of flight. The detailed design of the ABIA IFG module was completed and validation activities were performed on TORNADO-IDS, A-320 and AEROSONDE UAV simulated platforms to determine the Time-to-Alert (TTA) performances of the ABIA system in various flight phases from departure to final approach. The results of these activities were encouraging, showing that the system TTA performance is in line with current ICAO, FAA and CAA requirements for the different flight phases, with a potential synergy with SBAS and GBAS systems to support departure, en-route and TMA operations, including CAT-I/III precision approach. Further research concentrated on the 4DT computation module and extended the scope of ABIA applications to Unmanned Aircraft Systems (UAS). In particular, an initial investigation was accomplished to identify the potential synergies of ABIA with UAS Sense-and-Avoid (SAA) architectures for mid-air collision avoidance tasks. In conclusion, although current and likely future SBAS/GBAS augmentation systems can provide significant improvement of GNSS navigation performance, it is shown that the novel ABIA system developed in this research can play a key role in GNSS integrity augmentation for mission-essential and safety-critical applications such as aircraft precision approach/auto-landing and UAS sense-and-avoid. Furthermore, using suitable data link and data processing technologies, a certified ABIA system could play a key role as part of a future GNSS Space-Ground-Aircraft Augmentation Network (SGAAN).

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This thesis addresses the problem of cubesat limitations on transmission power and onboard memory storage. The number of small satellites is continuing to increase. The reduced amount of time and budget required for the development of these satellites has considerable advantages. The short time leads to data becoming available faster than from a larger satellite. Consequently, the communication system is very important to ensure that all data from the cubesat can be transmitted to the receiving station. In this thesis the link budget of a cubesat has been studied to identify the constraints on power and data transmission. As cubesat satellites become more complex, additional constraints and requirements are placed on system components. For more complex missions, greater flexibly of the onboard computer architecture is required to support the mission adaptation or changing specifications of onboard devices. Alternative onboard computer architecture for the next generation of cubesats is presented in this thesis and hybrid onboard computer architecture is proposed. There are many cubesats which have provided remote sensing imagery. An issue is how to store the data onboard and how to transmit these data with limited power. A solution is to reduce the size of the original image by pre-image processing. The potential for using image compression and defining the region of interest to decrease the original satellite image size has been examined in this research. Three approaches are studied and described in the context of the region of interest technique. There is image segmentation based on edge, histogram and texture detection. The presented evaluation is focused on the detection of the land part of the image that contains dynamic information and rejecting the ocean where there is less interest. The technique, however, is equally applicable for any region of interest that can be characterised and this is illustrated by considering some examples. The proposed adaptive image compression system is made up of two parts. The first part consists of the identification of the region of interest and the second part the image compression of this region of interest. The accuracy of the proposed system has been examined by comparing the number of different pixels between the proposed automatic region of interest system and the manual detection of the region of interest. Morphological methods are the main technique that has been used in the system. The morphology structure element has different shapes and size and it is necessary to understand how the shape and size of the structure elements affects the proposed system. A study of structure element has been conducted. In the real implementation of the proposed system on a cubesat, additional power would be required. To quantify this increase, a particular proposed system based on edge segmentation for region of interest automatic detection has been studied. The potential for using the proposed image compression to detect the region of interest and image compression was examined using a standard microcontroller. The result shows that the proposed system could be used on a cubesat satellite with reasonable additional power and mass.

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