X-ray pulsar-based GNC system for formation flying in high Earth orbits

We describe the design and implementation of a Guidance, Navigation and Control (GNC) System for fixed wing Unmanned aerial vehicles (UAVs). This system allows an autonomous navigation by following a desired trajectory defined by a profile of airspeed, altitude and a list of waypoints. The GNC system has a hierarchical architecture and integrates the Controller Area Network data bus, which facilitate the process of implementation and testing. The results of Hardware In the Loop simulations and real-time performances demonstrate the soundness of the implemented system. This article gives the necessary details so the proposed implementation will be reproducible by other searchers.

In this paper, a new method of the hardware-in-the-loop (HWIL) simulation test of aircraft guidance, navigation and control (GNC) system is proposed based on cable-driven parallel robot and triaxial turntable. The traditional HWIL simulation test methods of the aircraft GNC system, which depends on the five axes motion platform or the six-degree-of-freedom motion system, have a series of problems such as expensive and complex test equipment, single test mode, and monotonous simulation trajectory. Aiming at the needs of teaching and scientific research, this paper proposes a low-cost and efficient HWIL simulation test scheme for GNC system. Through simulation analysis and verification, the scheme has some advantages over traditional methods in workspace and control accuracy.KeywordsHardware-in-the-loopCable-driven parallel robotGNC system simulation

PRISMA—A formation flying project in implementation phase

The properties of coulomb force (CF) that nearly propellantless and exhaust plume contaminatless compared to the conventional chemical liquid propulsion thrusters (e.g., micro PPT, FEEP…etc.), make it attractive use for the close formation flying of the spacecraft at high Earth orbit (HEO). This paper presents the charged feedback controller using the optimal control based on the Linear Quadratic Regulator (LQR) method. The interaction between the CF that generated from charged sphere devices on the Target and Chase crafts are exploited to deploy the Chaser from the Target. Finally, the simulation results showed the effectiveness coulomb feedback-based LQR controller can be efforted the initial position of the Chaser entry to the design orbit as expected. The command force expenditure is on the order of -9 to 48 milli-Newton, in which the magnitude of coulomb charged has bounded.

In the past decades, short multipath effect is one of the most important error sources in global navigation satellite systems (GNSS) navigation, and seriously affects the positioning precision. Therefore, some algorithms based on traditional Kalman filtering (KF) have been proposed to overcome the short multipath effect, for example extended KF (EKF), adaptive KF (AKF) and constrained KF (CKF). However, height positioning accuracy of EKF and AKF is lower than that of horizontal. On the other hand, CKF requires additional height information from a barometer or location service system. To overcome the limitations of current algorithms, this paper first proposes an empirical multipath residual model after GNSS baseband processing, which shows that pseudo range error exhibits “rectangular” distribution or “shock” distribution. This paper also demonstrates that GNSS positioning error has the same distribution as the pseudo range error, and the height positioning error is higher than the horizontal positioning error in influence of short multipath effect. To improve the height positioning accuracy and suppress the short multipath effect, we propose a practical climbing motion model to constrain the height estimation in the AKF, which can effectively suppress short multipath effect and achieve an improved positioning result. Field tests illustrate that the proposed height constrained AKF (HCAKF) improves the height positioning accuracy by 85.1% over traditional EKF, 85.4% over AKF, and improves the total positioning accuracy by 56.9% over traditional EKF, 37.8% over AKF. Furthermore, the height estimate from HCAKF is smoother than those from traditional EKF and AKF.

With the ever-increasing number of small satellites launched into space, on-board orbit determination is becoming more and more important. Precise satellite positions are needed for constellation maintenance, formation flying, communication optimization, collision avoidance and Earth observation. In addition, accurate time information is required for the functioning of the instruments on board. Low-cost off-the-shelf Global Navigation Satellite System (GNSS) receivers and antennas, adapted to the small size and weight of the satellite, allow precise on-board orbit determination and time synchronization for even the smallest satellites.We analysed more than one year of on-board navigation solutions (NAVSOL) from 8 Astrocast CubeSats, which are equipped with our multi-GNSS single-frequency payload. Based on the residuals from a dynamic orbit fitting, we found that the quality of the NAVSOL from the Astrocast satellites varies strongly both over time and between satellites, with an RMS ranging from a few meters to several tens of meters.In this study, we focus on three main effects that influence the quality of the NAVSOL from the Astrocast satellites: (1) The orbit-fit residuals show a positive radial offset in the range of about 6 m to 15 m, which is caused by ionospheric refraction. Our data show that GNSS signals at negative elevations are very strongly affected by the ionospheric refraction and thus account for the major part of the radial offset. (2) Discrepancies between the NAVSOL positions and velocities, and simulations with a GNSS signal generator show a once-per-revolution periodicity in the out-of-plane component, with amplitudes of 2 m and 5 cm/s in position and velocity, respectively. This effect is probably due to an inappropriate dynamic model used in the real-time navigation algorithm of the GNSS receiver. (3) Depending on the location of the GNSS antenna on board the satellite, the noise level varies by a multiple. This is assumed to be due to the antenna orientations, interference of the GNSS signals with electromagnetic waves coming from the satellite platform, signal attenuation during the transmission from the antenna to the receiver and differences in the soldering or tuning of the individual GNSS antennas.These findings help us refine hardware configurations and firmware settings in preparation for future Astrocast satellites or other nanosatellite missions. By eliminating or minimizing the mentioned effects, we expect to consistently achieve an accuracy on the level of a few meters for the real-time navigation solution of commercial off-the-shelf single-frequency GNSS receivers in space.

The PRISMA project constitutes an in-orbit test bed for Guidance, Navigation and Control (GNC) algorithms and sensors for advanced formation flying and rendezvous operations. Several experiments regarding manoeuvring and sensor experiments will be performed, sorted in 4 main categories – Autonomous Formation Flying, Homing and Rendezvous, Proximity Operations and Final Approach and Recede. The project consists of two satellites, one highly manoeuvrable Main satellite and one Target satellite without orbit control. The spacecraft masses are in the order of 150 and 50 kg respectively. The project is carried out by Swedish Space Corporation (SSC) in close cooperation with the German Aerospace Centre (DLR), CNES and the Danish Technical University. The project will also demonstrate 2 new propulsion technologies, the High Performance Green Propellant motor system and a MEMS technology microthruster cold-gas system milli-Newton thrust range. The launch is planned for the spring of 2009. The mission will be controlled from SSC facilities in Stockholm and Kiruna.

The Earth is surrounded by a swarm of satellites and associated debris known as Resident Space Objects (RSOs). All RSOs will orbit the Earth until they reentry into Earth’s atmosphere. There are three main RSO categories: Low Earth Orbit (LEO), when the satellite orbits at an altitude below 1 500 km; a Medium Earth Orbit (MEO) for Global Navigation Satellite Systems (GNSS) at an altitude of around 20 000 km, and a Geostationary Earth Orbit (GEO) (also sometimes called the Clarke orbit), for geostationary satellites, at an altitude of 36 000 km. The Geostationary Earth Orbits and the orbits of higher altitude are also known as High Earth Orbits (HEO). Crucial for keeping an eye on RSOs, the Surveillance of Space (SofS) comprises detection, tracking, propagation of orbital parameters, cataloguing and analysis of these objects. This paper presents a large baseline stereovision based approach for detection and ranging of RSO orbiting at medium to high altitudes. Two identical observation systems, consisting of camera, telescope, control computer and GPS receiver are located 37 km apart, and set to observe the same region of the sky. The telescopes are placed on equatorial mounts able to compensate for the Earth’s rotation, so that the stars appear stationary in the acquired images, and the satellites will appear as linear streaks. The two cameras are triggered simultaneously. The satellite streaks are detected in each image of the stereo pair using its streak-like appearance against point-like stars, the motion of the streaks between successive frames, and the stereo disparity. The detected satellite pixels are then put into correspondence using the epipolar geometry, and the 3D position of the satellite in the Earth Center, Earth Fixed (ECEF) reference frame is computed using stereo triangulation. Preliminary tests have been performed, for both MEO and HEO orbits. The preliminary results indicate a very high detection rate for MEO orbits, and good detection rate for HEO orbits, dependent on the satellite’s rotation.

Signal fading is the main disturbing phenomenon for Global Navigation Satellite System (GNSS) receivers in urban environment, which induces frequent loss of carrier lock in GNSS phase tracking loops. By taking advantage of the prevalent configuration of multi-frequency reception in emerging GNSS receivers, this paper proposes a Frequency Diversity combined GNSS carrier Tracking Loop (FDTL) for robust and centimeter-level positioning under harsh propagation conditions. Specifically, a bank of correlators is used to estimate the signal parameters of interest with a Maximum Likelihood Estimation (MLE) discriminator to avoid the need for a separate acquisition which normally requires the most processing time in GNSS receivers. The Doppler relationship between multifrequency signals offers an inherent constraint which is used to improve accuracy of MLE measurements. In the loop tuning process, a frequency diversity combined Adaptive Kalman Filter (AKF) is implemented for system dynamic propagation and measurement update. The primary advantages of the proposed FDTL algorithm include: 1) significantly improved carrier tracking robustness compared with traditional Phase Lock Loops (PLLs); 2) enhanced sensitivity obtained from the diversity combination. The effectiveness of the algorithm is demonstrated by simulated and real GNSS fading signals.

In recent years, autonomously guided parachute systems have been developed with the goal of increasing the landing accuracy of airdrop operations. These precision airdrop systems utilize an onboard global positioning system (GPS) coupled with a guidance, navigation and control (GNC) system to continuously monitor and correct the flight path of the parachute system during descent. A critical component of the GNC system is an algorithm that predicts the response of the system to various aerodynamic forces in real-time that are typically based on a low-order multi-degree of freedom (DOF) dynamics model. Reduced order modeling is the process of representing a complex dynamic system with many DOF with a model having relatively few DOF. The goal of this study is to examine the use of large scale finite element models to construct and calibrate Reduced Order Models (ROM) for parachute systems. The resulting ROM would provide parachute designers with a simple tool to examine parachute behavior under various operating conditions. The use of numerical simulations to calibrate the ROM would help to eliminate the number of physical tests needed to characterize a parachute system’s properties. In this study, a 6 degree of freedom ROM is adopted. The system dynamic properties for the ROM and the resultant forces and moments acting on the ROM are obtained from finite element simulations. The 6 DOF model originally proposed by Cockrell and Doherr and later used by Dobrokhodov is adopted for the ROM. The finite element (FE) code, TENSION, is used to perform the large scale simulations and extract the ROM dynamic properties and resultant forces. Several examples are performed that verify the ROM procedure by comparing ROM predictions to FE simulations for various parachute system trajectories. The performance of the ROM is shown to be excellent for in-plane motion under an oscillating wind field. It is shown, however, that the ROM procedure deteriorates as the FE model is refined by adding more DOF. It is demonstrated that, for highly deformable FE models with many DOF, more sophisticated ROM procedures are needed.

For their size and cost advantage, small or so called man-portable unmanned aerial vehicles (UAV) have gathered considerable interests over the past few years. Although these vehicles may be considered inexpensive and dispensable, the imposed mission requirements are very demanding and complex relying on fully autonomous and intelligent self decision making operations. Most of small UAV currently in service utilizes commercial-off-the-shelf (COTS) autopilot which often puts constraints on vehicle maneuverability and programming flexibility. The motivation for this study is to develop an integrated guidance, navigation and control (GNC) system that provides flexibility in various engineering aspects of small UAV, ranging from a new platform design to autopilot synthesis. This paper presents some results from the initial design and development effort; hardware integration of necessary sensors, a processor selection, and flight testing. Strap-down inertial navigation principle is considered. Flight test data are utilized for the post-flight numerical studies in order to understand the system limitations and characteristics, which is a crucial step for the software design.

Real-time high-precision baseline measurement of satellite formation flying based on GNSS

Global navigation satellite systems (GNSS) are widely used in low Earth orbit (LEO) satellite navigation; however, their availability is poor for users in medium Earth orbits (MEO), and high Earth orbits (HEO). With the increasing demand for navigation from MEO and HEO users, the inadequate coverage of GNSS has emerged. Inter-satellite links (ISLs) are used for ranging and communication between navigation satellites and can also serve space users that are outside the navigation constellation. This paper aims to summarize their application method and analyze their service performance. The mathematical model of visibility is proposed and then the availability of time division ISLs is analyzed based on global grid points. The BeiDou navigation constellation is used as an example for numerical simulation. Simulation results show that the availability can be enhanced by scheduling more satellites and larger beams, while the presence of more users lowers the availability. The availability of navigation signals will be strengthened when combined with the signals from the ISLs. ISLs can improve the space service volume (SSV) of navigation constellations, and are therefore a promising method for navigation in MEO/HEO spacecraft.

Relative Navigation for Satellite Formation Flying based on Radio Frequency Metrology

With the advantages of high velocity measurement accuracy and fast dynamic response, the laser Doppler velocimeter (LDV) is expected to replace the odometer to be combined with a strapdown inertial navigation system (SINS) to form a higher precision integrated navigation system. However, LDV scale factor error and misalignment angles between LDV and inertial measurement unit will affect the accuracy of navigation. Considering that not all global navigation satellite system (GNSS) receivers can directly provide velocity information and current mainstream calibration methods are sensitive to the measurement noise and outliers of velocity and position information, a robust calibration method aided by GNSS is proposed in this paper, which is based on position observation. Different from current popular calibration methods, the attitude information of the GNSS/SINS integrated navigation system obtained by an adaptive Kalman filter is used to construct the observation vector together with LDV velocity outputs and GNSS position outputs in this method. The LDV scale factor error and the misalignment angle are determined by the ratio of two observation vector modulus and the Davenport's q-method method, respectively. The accuracy and robustness of the calibration method are verified by one vehicle test with normal GNSS signals and one vehicle test with GNSS signals with outliers. And the horizontal position error of dead reckoning of the calibrated LDV/SINS integrated system are less than 0.0314% and 0.1033% of the mileage, respectively.