# Integrity Analysis for GPS-Based Navigation of UAVs in Urban Environment
## 1 Introduction  
Due to the growing interest in the use of Unmanned Aerial Vehicles (UAV), research in the Guidance, Navigation and Control (GNC) area has become increasingly important. Navigation is one of the key components of GNC, and it carries a number of important requirements to UAV performance that play a key role in ensuring mission safety and success. According to International Civil Aviation Organization (ICAO) Position, Navigation and Timing (PNT) solutions are characterized by accuracy, integrity, continuity and availability within the Performance Based Navigation (PBN) concept [1; 2] as follows:  
* **Accuracy:** The degree of conformance between the estimated or measured position and/or velocity of a platform at a given time and its true position or velocity. * **Integrity:** Integrity is the measure of the trust that can be placed in the correctness of the information supplied by a navigation system. * **Continuity:** The continuity of a system is the ability of the total system (comprising all elements necessary to maintain aircraft position within the defined airspace) to perform its function without interruption during the intended operation. * **Availability:** The availability of a navigation system is the percentage of time that the services of the system are usable by the navigator. These four criteria are linked to each other by definition, as is shown in Figure 1. While accuracy is the initial stage within PBN, the trustworthiness of a navigation system is specified based on the confidence level of accuracy as integrity. Continuity is the simultaneous success rate of accuracy and integrity requirements during an intended operation period. Lastly, the navigation system is available only if accuracy, integrity and continuity requirements are fulfilled. PNT data is the most critical contributor to guidance and control stages, and the Global Navigation Satellite System (GNSS) is one of the primary PNT sources here because of wide coverage, high accuracy, and relatively low cost of the corresponding navigation solutions. GNSS navigation is able to provide accurate positioning and timing data under the open space conditions for a wide range of applications in both military and civilian domains. Vulnerabilities of the GNSS-based PNT are also well known; it can be affected in both intended (jamming, spoofing, etc.) or unintended (multipath, atmospheric effects, etc.) ways. Many current and future applications of UAV are related to dull, dirty and dangerous (3D) tasks, e.g. work at natural disaster scenes, supporting public safety or routine goods delivery [3]. In many such cases, the navigation environment will inevitably become GNSS challenging, or even GNSS denied. The 3D applications of UAVs are mostly in urban environments and at the altitudes below building heights [4]. GNSS performance degrades during low altitude flights in urban areas because of masking and multipath effects from the buildings, structures and vegetation. In order to mitigate the degradation, various alternative PNT solutions are suggested as complementary navigation sources to work with GNSS in cooperation, while others focus on substitutes for GNSS [5]. Due to increased payload capacity of modern UAVs and better availability of high-quality sensors, multi-sensor navigation is one of the most promising complementary solutions for GNSS-based UAV navigation these days [6]. Conventionally, fusion of GNSS with Inertial Measurement Unit (IMU) is used for these purposes [7; 8]. Additional sensors, such as LIDAR, odometer, magnetometer, etc., that can be used for fixing other navigation parameters are utilized for building a more comprehensive and reliable navigation system [9]. Vision-based methods [10], such as Simultaneous Localization and Mapping (SLAM) [11] and ground-based navigation systems [5] are also considered as alternative PNT sources for navigation in GNSS challenging or denied environments.