SPLICE Safe and Precise Landing - Integrated Capabilities Evolution

The SPLICE project is developing, maturing, demonstrating, and infusing precision landing and hazard avoidance (PL&HA) technologies for NASA and potential commercial spaceflight missions. Near-term development includes high precision and accuracy velocimetry with ranging (via the NDL), high-resolution real-time mapping and hazard detection with ranging (via the HDL), lunar terrain relative navigation (TRN), and the requisite high performance computing capability. These technologies are initially intended to provide PL&HA for the moon, but are extensible to any planetary body. Long-term, the goal is to make these capabilities available to government and commercial entities and to license technology to commercial entities for production

PRECISE LANDING OF VTOL UAVS USING A TETHER

Unmanned Aerial Vehicles (UAVs), also known as drones, are often considered the solution to complex robotics problems. The significant freedom to explore an environment is a major reason why UAVs are a popular choice for automated solutions. UAVs, however, have a very limited flight time due to the low capacity and weight ratio of current batteries. One way to extend the vehicles\u27 flight time is to use a tether to provide power from external batteries, generators on the ground, or another vehicle. Attaching a tether to a vehicle may constrain its navigation but it may also create some opportunities for improvement of some tasks, such as landing. A tethered UAV can still explore an environment, but with some additional limitations: the tether can become wrapped around or bent by an obstacle, stopping the drone from traveling further and requiring backtracking to undo; the tether can fall loose and get caught while dragging on the ground; or the base of the tether could be mobile and the UAV needs to have a way to return to it. Most issues, like those listed above, could be solved with a vision system and various kinds of markers, but this approach could not work in situations of low light, where cameras are no longer effective. In this project, a state machine was developed to land a tethered, vertical take-off and landing (VTOL) UAV using only angles taken from both ends of the tether, the tension in the tether, and the height of the UAV. The main scenarios focused on in this project were normal operation, obstacle interference, loose tether, and a moving base. Normal operation is essentially tether guidance using the tether as a direction back to the base. The obstacle case has to determine the best action for untangling the tether. The loose tether case has to handle the loss of information given by the angle sensors, as the tether direction is no longer available. This case is performed as a last-ditched effort to find the landing pad with only a moderate chance for success. Lastly, the moving base case uses the change in the angles over time to determine the speed needed to reach the base. The software was not the only focus of this project. Two hardware components of this project were a landing platform and a matching landing gear to support the landing process. These two components were designed to aid in the precision of the landed location and to ensure that the UAV was secured in position once landed. The landing platform was designed as a passive funnel-type positioning mechanism with a depression in the center that the landing gear was designed to match. The tension of the tether is used to further lock the UAV into place when in motion. While some of this project remained theoretical, particularly the moving base case, there was flight testing performed for validation of most states of the proposed state machine. The normal operation state was effective at guiding the UAV onto the landing pad. The loose tether case was also able to land within reasonable expectations. This case was not always successful at finding the landing pad. Particular methods of increasing the likelihood of success are discussed in Future Work. The Obstacle Case was also able to be detected, but the response action has yet to be tested in full. The prior testing of velocity following can be used as proof of concept due to its simplicity. In conclusion, this project successfully developed a state machine for precisely landing a tethered UAV with no environmental knowledge or localization. Further development is necessary to improve the likelihood of landing in problematic scenarios and more testing is necessary for the system as a whole. More landing scenarios could also be researched and added as cases to the state machine to increase the robustness of the landing process. However, each current subsystem achieved some level of validation and is to be improved with future developments

MicNest: Long-Range Instant Acoustic Localization of Drones in Precise Landing

We present MicNest: an acoustic localization system enabling precise landing of aerial drones. Drone landing is a crucial step in a drone's operation, especially as high-bandwidth wireless networks, such as 5G, enable beyond-line-of-sight operation in a shared airspace and applications such as instant asset delivery with drones gain traction. In MicNest, multiple microphones are deployed on a landing platform in carefully devised configurations. The drone carries a speaker transmitting purposefully-designed acoustic pulses. The drone may be localized as long as the pulses are correctly detected. Doing so is challenging: i) because of limited transmission power, propagation attenuation, background noise, and propeller interference, the Signal-to-Noise Ratio (SNR) of received pulses is intrinsically low; ii) the pulses experience non-linear Doppler distortion due to the physical drone dynamics while airborne; iii) as location information is to be used during landing, the processing latency must be reduced to effectively feed the flight control loop. To tackle these issues, we design a novel pulse detector, Matched Filter Tree (MFT), whose idea is to convert pulse detection to a tree search problem. We further present three practical methods to accelerate tree search jointly. Our real-world experiments show that MicNest is able to localize a drone 120 m away with 0.53% relative localization error at 20 Hz location update frequency

Development of a Controlled Dynamics Simulator for Reusable Launcher Descent and Precise Landing

This paper introduces a Reusable Launch Vehicle (RLV) descent dynamics simulator coupled with closed-loop guidance and control (G&C) integration. The studied vehicle's first-stage booster, evolving in the terrestrial atmosphere, is steered by a Thrust Vector Control (TVC) system and planar fins through gain-scheduled Proportional-Integral-Derivative controllers, correcting the trajectory deviations until precise landing from the reference profile computed in real time by a successive convex optimisation algorithm. Environmental and aerodynamic models that reproduce realistic atmospheric conditions are integrated into the simulator for enhanced assessment. Comparative performance results were achieved in terms of control configuration (TVC-only, fins-only, and both) for nominal conditions as well as with external disturbances such as wind gusts or multiple uncertainties through a Monte Carlo analysis to assess the G&C system. These studies demonstrated that the configuration combining TVC and steerable planar fins has sufficient control authority to provide stable flight and adequate uncertainties and disturbance rejection. The developed simulator provides a preliminary assessment of G&C techniques for the RLV descent and landing phase, along with examining the interactions that occur. In particular, it paves the way towards the development and assessment of more advanced and robust algorithms

Multiple-Sliding-Surface Guidance and Control for Terminal Atmospheric Reentry and Precise Landing

The development of an effective guidance and attitude control architecture for terminal descent and landing represents a crucial issue for the design of reusable vehicles capable of performing a safe atmospheric planetary entry. The sliding mode control represents a nonlinear technique able to generate an effective real-time closed-loop guidance law, even in the presence of challenging contingencies. This work proposes a multiple sliding-surface guidance control law that is able to drive a lifting vehicle toward safe landing conditions, associated with a desired downrange, crossrange, runway heading, and final vertical velocity at touchdown, even starting from challenging initial conditions. The time derivatives of the lift coefficient and the bank angle are used as the control inputs, whereas the sliding surfaces are defined so that these two inputs are involved simultaneously in the lateral and the vertical guidance. The commanded attitude is pursued by the attitude control system, which employs a feedback nonlinear control law that enjoys quasi-global stability properties. Effectiveness and accuracy of the guidance and control strategy at hand are proven numerically by means of a Monte Carlo campaign, in the presence of stochastic wind and large dispersions on the initial conditions

Next Generation NASA Hazard Detection System Development

The SPLICE project is continuing NASAs efforts to develop precision landing GN&C technologies for future lander missions. One of those technologies is the next generation Hazard Detection (HD) System, which consists of a new HD Lidar and HD Algorithms. The HD System is a modular system that will be adapted to meet specific mission needs in the future. This paper presents the design approach, the nominal concept of operations for which the first prototype is being designed, and the expected performance of the system