An EGNOS Based Navigation System for Highly Reliable Aircraft Automatic Landing

Highly precise navigation is the core technology required for many applications, such as automated aerial refueling (AAR), sea-based joint precision approach and landing systems (JPALS), station-keeping, unmanned aerial vehicles (UAV) swarming and formation flight and unmanned ground vehicles (UGV) convoys. Advances in the above mentioned technology are possible considering the future GNSS framework, given that adequate characterization of new GNSS devices are performed and that new algorithms are developed that fully exploits the functionalities made available by the future GNSS systems. In this paper both aspects are considered, with specific reference to the use of GPS/EGNOS for reliable fixed wing aircraft automatic landing applications. For what concern experimental characterization of the satellite based navigation system GPS/EGNOS, the main aim of the activity was to describe the broadcasted messages to enhance the navigation accuracy and integrity of the core GNSS-1 elements GPS and GLONASS, and to exploit how the data can be used to compute and analyze the performance in terms of Required Navigation Performance (RNP) parameters. The paper describes the algorithm implemented to process the broadcasted EGNOS SIS in order to obtain a position solution and integrity information compliant with RTCA DO229C. Moreover, the paper presents test procedures and experimental results that may be used as a design guideline for monitoring manufacturing compliance and, in certain cases, for obtaining formal DO229C certification of equipment design and manufacture. On the other hand, concerning the development of new algorithms for Guidance, Navigation & Control of fixed wing vehicles, that are already compatible with the future GNSS framework, it was initially considered a suite of navigation sensors with accuracy similar to the one obtainable by EGNOS. In order to overcome the effects due to an insufficient accuracy, the satellite measures can be in fact integrated with different sensor sources allowing a high precision navigation and an improvement of the integrity and reliability of navigation solutions. By means of an appropriate sensor suite, described in the next, and of a sensor fusion algorithm we obtained a high precision level in navigation measurements that, for instance, allows a high autonomous precision approach and landing. A very simple but effective sensor fusion algorithm based on the use of complementary filtering technique has been implemented. This algorithm uses the vehicle position and velocity vectors fed by GPS and the vehicle acceleration vector fed by AHRS. The filter is developed with the aim of trading off the advantages and drawbacks of both sensors: the AHRS has a larger band, a limited signal noise but it is affected by remarkable bias errors, vice versa the GPS. Therefore, it can be thought to integrate the accelerations from AHRS and to process them through a high-pass filter, obtaining the medium-high frequency component of the considered signals. The low frequency components can be obtained by a filtering stage of the GPS measures through a low-pass filter. A simple sum of the above two components gives the final estimation of position and speed. It is important to emphasize that, in both velocity and position measures estimation, the high-pass filter applied to AHRS measures and the low-pass filter applied to GPS measures must be “complementary”, in the sense that the sum of the transfer functions of the two filters must be equal to one. Moreover, some critical autonomous functionality, such as Autolanding, will utilize the GPS integrity signal in its decision-making logic for evaluating the key-decisions regarding the possible execution of an altitude recovery manoeuvre and, in case, also considering a degraded mode by changing the desired performances at touch down, with the aim to be still compatible with the current navigation system precision. In this way the integrity information provided by EGNOS is efficiently used for achieving a higher safety level during autonomous flight operations. The selected on-board software architecture is actually fully compliant to the use of EGNOS based GPS units, without requiring any upgrade and the proposed sensor fusion algorithms have been already developed being basically compatible with integrity information coming from the future GNSS sensors. Anyway, in the presented first phase of flight experiments, we used a coarse DGPS unit, because EGNOS is still in the testing phase. The next steps are to perform autonomous GN&C flight experiment with EGNOS constellation with a runway completely not instrumented. In the first part of the paper, concerning EGNOS system characterization, is presented an overview of EGNOS (chapter 2), are described the processing of the SBAS Signal-In-Space correction and integrity data and the related algorithm to estimate the integrity supplied by the system (chapter 3), the classes of equipment at which the test requirement are referred and the equipment performance and test procedure focusing on processing requirements and the validation performance assessment logic to assess the performance achievable with EGNOS (chapter 4). In its second part, describing the development of GNC algorithms already compatible with the future GNSS framework, the paper deals with the autolanding algorithms (chapter 5), the sensor fusion algorithms to achieve the desired navigation precision and the methodologies developed in order to safely manage the possible presence of sensor failures (chapter 6), the preliminary results of the real time validation with hardware in the loop simulation (chapter 7) and, finally, the algorithm performances achieved during the first experimental flights by using the CIRA experimental flying platform (chapter 8).