Guided Airdrop Systems Research Articles

In-canopy sensors for state estimation of precision guided airdrop systems

Guided airdrop systems have traditionally used position and velocity information from a GPS receiver as their only source of feedback. The use of additional sensors in the guidance units is challenging because sensors in the guidance unit are in close proximity to powerful electric motors. Furthermore, there is a large amount of relative motion between the guidance unit and the parachute as they are coupled by a flexible network of rigging lines. By placing sensors in the parachute itself, it is possible to obtain accurate estimates of the canopy motion and orientation with low-cost sensors requiring minimal calibration. Specialized in-canopy sensor pods were developed to provide distributed sensing throughout a parachute canopy and a sensor fusion algorithm was developed to combine the raw data from these sensor pods into useful canopy state estimates. The effectiveness of this approach is demonstrated first in simulation, and then with flight test results on full-scale airdrop systems. The rich feedback signal available from in-canopy sensors can provide improved datasets for more detailed system identification as well as enabling novel guidance, navigation and control approaches which will lead directly to improved landing accuracy.

Human-in-the-loop control of guided airdrop systems

Advances in guided airdrop technology including guidance, navigation, and control algorithms, novel control mechanisms and wind sensing algorithms have led to significant improvements over unguided airdrop systems. Guided systems are autonomously controlled with an embedded microprocessor using position and velocity feedback. While capable of highly accurate landing, these systems struggle to overcome deviations from expected flight dynamics due to canopy damage or cargo imbalance, complex terrain at the drop zone, and loss of sensor feedback. Human operators are intelligent, highly adaptive, and can innately judge the flight vehicle and environment to steer the vehicle to the desired impact point provided sufficient information. This work experimentally explores operators' abilities to accurately land an airdrop system using different sensing modalities. Human operator landing results are compared with a state of the art fully autonomous airdrop system. Across the methods analyzed, human operators attained up to a 40% increase in landing accuracy over the fully autonomous control algorithm.

Adaptive Control of Precision Guided Airdrop Systems with Highly Uncertain Dynamics

The bulk of research in the field of precision guided airdrop systems has focused on improving landing accuracy in the presence of atmospheric winds that can exceed vehicle airspeed. One important challenge of parafoil systems is their highly uncertain flight dynamic behavior and control response, which can result from canopy degradation or an offnominal inflation event. This significantly impacts the ability to reach the target and can often lead to very large miss distances. This work addresses guided airdrop system model uncertainty with a novel combined direct and indirect adaptive control strategy to quickly characterize vehicle dynamics and lateral control sensitivity in flight. Extensive simulation and experimental flight testing indicate that the proposed adaptive algorithm is capable of high-accuracy landing in a large variety of degraded conditions, including unknown nonlinear changes in control sensitivity as well as control reversals. In comparison, current industry standard algorithms experience over an order of magnitude decrease in accuracy when tested under identical scenarios.

Autonomous Airdrop Systems Employing Ground Wind Measurements for Improved Landing Accuracy

Aerial cargo delivery, also known as airdrop, systems are heavily affected by atmospheric wind conditions. Guided airdrop systems typically employ onboard wind velocity estimation methods to predict the wind in real time as the systems descend, but these methods provide no foresight of the winds near the ground. Unexpected ground winds can result in large errors in landing location, and they can even lead to damage or complete loss of the cargo if the system impacts the ground while traveling downwind. This paper reports on a ground-based mechatronic system consisting of a cup and vane anemometer coupled to a guided airdrop system through a wireless transceiver. The guidance logic running on the airdrop system's onboard autopilot is modified to integrate the anemometer measurements at ground level near the intended landing zone with onboard wind estimates to provide an improved, real-time estimate of the wind profile. The concept was first developed in the framework of a rigorous simulation model and then validated in the flight test. Both simulation and subsequent flight tests with the prototype system demonstrate reductions in the landing position error by more than 30% as well as a complete elimination of potentially dangerous downwind landings.

Combined Lateral and Longitudinal Control of Parafoils Using Upper-Surface Canopy Spoilers

Precision placement of guided airdrop systems necessarily requires some mechanism enabling effective directional control of the vehicle. Often this mechanism is realized through asymmetric deflection of the parafoil canopy trailing-edge brakes. In contrast to conventional trailing-edge deflection used primarily for lateral steering, upper-surface bleed air spoilers have been shown to be extremely effective for both lateral and longitudinal (i.e., glide slope) control of parafoil and payload systems. Bleed air spoilers operate by opening and closing several spanwise slits in the upper surface of the parafoil canopy, thus creating a virtual spoiler from the stream of expelled ram air. The work reported here considers the autonomous landing performance of a small-scale parafoil and payload system using upper-surface bleed air spoilers exclusively for both lateral steering and glide slope control. Landing accuracy statistics computed from a series of Monte Carlo simulations in a variety of atmospheric conditions and experimental flight tests were found to be in good agreement. Median miss distances for the combined lateral and longitudinal control logic are on the order of 13 m, indicating an improvement in landing accuracy of nearly 50% over similar systems employing only lateral steering control.

Parafoil Control Using Payload Weight Shift

Autonomous guided airdrop systems based on steerable ram-air parafoils rely on differential deflection of the canopy trailing edge for lateral control with very limited longitudinal control. This work explores an alternative method that achieves both lateral and longitudinal control by shifting the center of gravity of the payload relative to the canopy rigging attachments. A multibody simulation model is required to capture the complex motion associated with the flexible nature of the parafoil, payload, and rigging system and simulate the effects of both longitudinal and lateral shifting of the center of gravity. Simulation results show that longitudinal weight shift can be an effective means of providing airspeed and glide slope control, and lateral weight shift can be an effective means of providing turn rate control. Results demonstrate that, for both lateral and longitudinal control, spreading the attachment points of the rigging to the payload will cause the aircraft to be more sensitive to shifts in the payload center of gravity. It was also found that, for both lateral and longitudinal control, decreasing the vertical distance from the rigging attachments to the center of gravity on the payload causes the aircraft to be more sensitive to shifts in the payload center of gravity. Finally, dynamic results demonstrate that the forces and moments required to induce a weight shift in the payload will often cause an initial response in the opposite direction of the final steady-state response.

Glide Slope Control Authority for Parafoil Canopies with Variable Incidence Angle

The addition of glide slope control to guided airdrop systems has the potential to provide dramatic improvements in landing accuracy. Dynamic rigging incidence control has been demonstrated in flight tests and shown to provide effective glide slope control. The current work presents flight-test results exploring the range of glide slope control that can be achieved with high-performance parafoil canopies of two different aspect ratios. The low- and medium-aspect-ratio canopies have peak glide ratios of 4.4 and 4.9, respectively. By varying the incidence angle in flight, the glide slope can be reduced from these peak values to a glide slope near two for both canopies. Flight tests investigating the relationship between symmetric brake and incidence control demonstrate that symmetric braking produces effective airspeed control with little effect on glide slope, the incidence angle produces significant effects on both glide slope and airspeed, the rapid variation of the incidence angle in flight can lead to significant long period oscillations in both glide slope and airspeed, and the coupled effect of the control mechanisms of incidence angle and symmetric brake can produce dramatic changes in glide slope in any wind condition.

Adaptive Glide Slope Control for Parafoil and Payload Aircraft

The invention of the steerable, gliding, ram-air parafoil enabled the possibility of precision, autonomous aerial payload delivery. Research and development work on guided airdrop systems has focused primarily on evolutionary improvements to the guidance algorithm, although the navigation and control algorithms have changed little since the initial autonomous systems were developed. Recent work has demonstrated the potential for dramatic improvements in landing accuracy through the incorporation of canopy incidence variation to achieve glide slope control for parafoils. The current work presents the development of a control law to implement glide slope control on an autonomous airdrop system. Autonomous landings with the control law in both simulation and flight test demonstrate an improvement in landing accuracy by a factor of two, though the improvement can be even greater in especially windy conditions. Finally, the ability to perform in-flight system identification to adapt internal control mappings to match flight data and provide dramatic improvements in landing accuracy when there is a significant discrepancy between the assumed and actual flight characteristics is demonstrated in both simulation and flight test.