Precision Guided Airdrop System flight test results

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

A97-31291

AIAA-97-1468

PRECISION GUIDED AIRDROP SYSTEM FLIGHT TEST RESULTS Dr. Philip D. Hattis*, Dr. Brent D. Appleby**, Thomas J. Fill*** C. S. Draper Laboratory, Inc., Cambridge, MA 02139

Downloaded by Philip Hattis on May 10, 2018 | http://arc.aiaa.org | DOI: 10.2514/6.1997-1468

Richard Benneyt, U.S. Army Soldier Systems Command, Natick Research, Development, and Engineering Center, Natick, MA 01760 ABSTRACT Airdrop technology is a vital Department of Defense (DoD) capability that supports rapid deployment of war fighters and supplies. Consequently, the Army has sponsored development of gliding, steerable airdrop systems that can be deployed from high altitudes, with large offset, carrying small through large payloads. The goal was to enable payload delivery within 100 meters of the target. Under this effort, Draper Laboratory developed modular guidance, navigation, and control (GN&C) software to precision guide ram-air parafoils using a combination of Global Positioning System (GPS) and inertial navigation system (INS) data. A high fidelity simulator was constructed to evaluate the expected performance of the Draper software. Also, in conjunction with NASA, flight tests with an 88 sq. ft. parafoil and a 170 pound payload were performed to evaluate the GN&C system performance under real flight conditions. A number of GN&C system design refinements were formulated after review of initial flight test results that ultimately enabled a payload delivery accuracy of about 50 meters. This paper summarizes the motivation for precision guided airdrop systems, reviews the Draper GPS/INS based GN&C for ram-air parafoils, and presents both simulation and flight test results. INTRODUCTION Airdrop technology is a vital DoD capability for rapid deployment of war fighters and supplies. Its use for humanitarian relief efforts is also increasingly needed. Furthermore, such technology helps compensate for the significant reduction in the level of U.S forces strategically placed around the world by supporting a rapid flexible force projection capability. Historically, conventional airdrop missions have consisted of "round" parachute systems with little or no ability to be steered once dropped from a delivery Principal Staff, Guidance, Navigation & Control Directorate, AIAA Associate Fellow ** Senior Staff, Guidance, Navigation & Control Directorate *** Principal Staff, Guidance, Navigation & Control Directorate t Aerospace Engineer, Mobility Directorate, AIAA Member Copyright © 1997 by C. S. Draper Laboratory, Inc. Published by The American Institute of Aeronautics and Astronautics, Inc. with permission.

platform, The delivery accuracy of nonsteerable parachute systems is primarily a function of deployment altitude and the winds encountered during descent. Aircraft used as delivery platforms are less vulnerable to hostile threats when traveling below 1000 feet or near/above 25,000 feet, but are much more vulnerable in between these two extremes. Nonsteerable parachutes have poor touch down accuracy when deployed from high altitudes. The accuracy could be improved if the winds were known accurately in advance of deployment, but this is not generally the case. Even with accurate wind estimation techniques, high altitude airdrop delivery aircraft would be required to deploy within a small envelope and missions would require larger than typical drop zones. To enable accurate delivery of troops and supplies, airdrop missions with round parachutes require that the delivery aircraft fly at low altitudes (tree tops in some cases) and then "pop up" over the drop zone to deliver their cargo. This requires a corridor for the delivery aircraft to get to and from the drop zone. The pop up maneuver is needed because airdrop systems require a minimum altitude for deployment and stabilization of the system before ground impact in order to assure load survival. Also, drop zones for airdrops must be large enough to account for inaccuracies in actual aircraft location, drop zone winds, the time required to empty the cargo from the delivery aircraft, and for operations done in darkness, release point errors that result from drop zone identification difficulties. The U.S. Army Natick Research, Development and Engineering Center's (Army/Natick) role as materiel developer of airdrop systems for the Army has helped push airdrop capabilities. While round parachute systems continue to be made more capable at Natick, a variety of steerable parachute systems are also being developed to expand the capabilities available in the field. The major advantage of steerable parachutes (gliding wings) over round parachutes is their ability to control horizontal motion (glide). Ram-air parafoils are a gliding wing implementation with many years of developmental and application experience. Their liftto-drag glide ratio (LID) values typically range from 1.5:1 to 4:1. These systems usually have two "control lines" which are used for steering. The control lines are connected to the outer sides of the trailing edge of the parafoil canopy. Turning is accomplished by pulling on 158

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a control line to deform (pull down) a trailing edge of the wing which adds drag to one side of the wing and produces a moment on the system. Under an Army/Natick sponsored development program, steerable airdrop recovery weights have now reached 36,000 pounds. Meanwhile, the development of the GPS and new deployment technologies have attracted the Army's attention and investment in order to provide new airdrop capabilities for the war fighter. The Mobility Directorate (Natick) in conjunction with, the Early Entry, Lethality and Survivability (EELS) Battle Lab, the U.S. Army Missile Command, NASA, and a host of contractors are working on a family of steerable airdrop systems, known as the Advanced Precision Airborne Delivery System (APADS), which incorporates "smart" technology to deliver supplies and equipment at various weights and performance characteristics. These systems can be deployed from high altitudes, with gliding performance that provides large offset capabilities. APADS will provide the war fighter with a capability to autonomously deliver payloads accurately (within 100 meters of the target) from up to 25,000 feet above mean sea level, in adverse weather conditions, with air release offsets up to 12 miles. This will enhance the survivability of the delivery aircraft by taking it away from most small arms fire, light antiaircraft artillery and man-portable missiles. Utilizing GPS-based guidance technology, the APADS delivery accuracy allows for smaller drop zones and reduced load dispersion at the drop zone. The development of the APADS family of systems is supported by an Army Mission Need Statement, the Army Science and Technology Modernization Plans, an Air Force precision airdrop capability Mission Need Statement, and a variety of Battle Lab Operational Capability Requirements (OCRs). In support of the APADS program, Draper Laboratory developed precision guided airdrop software (PGAS) to enable autonomous ram-air parafoil airdrops

utilizing combined GPS and inertial navigation systems. The PGAS was designed to be modular with an open architecture, and can be made compatible for use with varied sensor packages and for a spectrum of airdrop systems. To support development and evaluation of the PGAS, a high fidelity six-degree-of-freedom (6 DOF) engineering simulation was constructed that includes dynamics models of small and large ram-air parafoils, the PGAS GN&C flight software, environment models, sensor models, and sophisticated user interfaces. Also, in collaboration with the NASA Dryden Flight Research Center (DFRC), a highly instrumented ram-air parafoil airdrop test platform was assembled and flight tested using PGAS. The following sections review the PGAS objectives, its simulation evaluation and the test program results. More details on the PGAS GN&C

design implementation are provided in a previous paper1. PGAS OBJECTIVES The PGAS development effort was initiated to address APADS needs while providing a governmentowned, modular software package with sufficient design flexibility to handle a variety of airdrop platforms. The following specific objectives were established at the beginning of the program: • Develop a prototype autonomous, precision airdrop system GN&C package usable during variable wind and weather conditions. • Enable landing accuracy of 100 meters (with a 50 meter goal).

• Provide means to maintain precision guidance within high jamming environments. • Implement a hardware interface and software architecture that is applicable to a family of parachutes/parafoils. • Apply an existing GPS/INS package for initial prototype flight test. • Exploit existing DFRC staff airdrop expertise and their previously existing airdrop system components to minimize the cost and time required to flight test PGAS.

PGAS DESIGN CHALLENGES The PGAS development effort faced design challenges both as a consequence of the nature of airdrop system dynamics and due to the specific performance objectives being applied. The following subsections provide significant examples. Limited Parafoil Control Authority The L/D of ram-air parafoils changes very little with angle of attack. Consequently, there is little ability to change the flight path angle except during turns. Jamming Protection GPS provides a high value source of precision navigation data, but is vulnerable to signal spoofing and jamming. PGAS included use of an INS to provide full navigation coverage despite external interference. Whenever GPS data were available, the INS accuracy could be maintained by using GPS data for INS platform drift corrections. Off-the-Shelf Sensors An existing, non-missionized embedded GPS/INS (EGI) package was procured for PGAS flight test navigation. Use of the EGI necessitated that the planned flight test platform be designed with interfaces compatible with the EGI, and that the GN&C software utilize the already filtered navigation outputs of the EGI. Engineering Simulation The PGAS software needed a simulation environment for development and test that included 159




suitable parafoil dynamics models. An extensive Draper Laboratory effort went into acquiring any relevant, raw parafoil data (provided by the Army and NASA) then formulating and simulation implementing the data into 6 DOF canopy models. Included early in the program was consultative help in parafoil modeling theory provided by Systems Technology, Inc. GN&C Applicability to a Family of Parafoils The goal of demonstrating the applicability of the GN&C to parafoils with varied payload capacities required that the simulation development include implementation of both small and large canopy models. Accommodation of Winds Parafoil flight results in vehicle exposure to unpredicted winds that can be a substantial fraction of the wing air speed. Consequently, there was a need to do on-board estimation of current winds, and guidance had to accommodate their effects.

DFRC COLLABORATION Both extensive, overall flight test experience and a prior parafoil research program2'3 made it valuable for DFRC staff to collaborate with Draper Laboratory in PGAS flight demonstrations. Under a memorandum of agreement with the Army, the DFRC prepared an airdrop flight test platform with a customized flight test computer and data recording system. Draper Laboratory procured and provided to the DFRC two EGI navigation boxes to be integrated into the airdrop

platform in combination with the PGAS code for flight testing. All PGAS program flight hardware integration was made a DFRC responsibility while software integration into the flight test platform was made a joint DFRC/Draper Laboratory responsibility. PGAS GN&C OVERVIEW The PGAS GN&C was specialized to deal with the ram-air parafoil airdrop requirements but was modularized to permit easy accommodation to a wide variety of ram-air parafoils and similarly controlled airdrop systems through a limited number of parameter revisions. The following subsections provide a brief overview of the PGAS GN&C algorithm features. More detail regarding the algorithm characteristics are in separate references1'4"6. Navigation The EGI outputs three filtered solutions based on

combined and separate use of the GPS and INS data. The PGAS navigation algorithm could choose to use data from any of the EGI filtered state sets. Based on the EGI supplied data, the PGAS navigation algorithm provided the GN&C system with the vehicle's target relative position and velocity, the heading angle and angular rate, the altitude above ground level, the estimated current wind conditions, as well as the

vehicle glide slope and sink rate. Note that the vehicle heading angle and angular rate data required INS data.

Winds were determined by PGAS using a seven state Kalman filter that accounted for persistent and transient wind components as well as air-relative vehicle path states.

Guidance The following sequentially executed modes were executed during PGAS flights: • Maneuver to a way point. • A holding pattern mode, which maintained a near

circular path around the current way point. • Maneuver to final approach mode, which managed a final turn onto a landing zone approach path. • Final approach mode, which held the parafoil path on a line to the touchdown point after coming out of the final turn. • Flare mode, which initiated and executed the flare maneuver that was needed to reduce vehicle touchdown speeds. Control The PGAS control logic produced differential control line deflection commands (rudder) to achieve the command heading angle. Figure 1 illustrates the control line locations, with the deflection of the control lines accomplished by digitally commanded electromechanical actuators that were inside the flight test payload but could also be atop the payload in some airdrop systems. The control variables are able to be selected based on just a few key parafoil parameters, enabling rapid reconfiguration of the control algorithm to new parafoil airdrop systems. THE ENGINEERING SIMULATION TESTBED A workstation based engineering simulation of ram-air parafoils was developed by Draper as a testbed that supported the PGAS development and evaluation. The simulation contained two ram-air parafoil canopy models: 1) An 88 sq. ft. canopy that was utilized on the DFRC airdrop platform to flight test the PGAS; 2) A 3600 sq. ft. canopy that was representative of the medium airdrop systems developed and tested by the Army as part of the APADS initiative. The parafoil models accommodated variation of payloads and control system/actuator bandwidth. To enable realistic representation of flight dispersions, the simulation contained a variety of moderate and severe wind profiles provided by the Army for candidate airdrop zones. The engineering simulation was used to evaluate the expected flight performance capabilities of the PGAS on the DFRC flight platform, to evaluate phenomena experienced during test flights, and to assess the adaptability of the PGAS design to large airdrop systems. The following subsections discuss wind models used in the Draper ram-air parafoil engineering simulation and PGAS performance results derived from it.

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models and without a priori knowledge of expected inflight winds. For all wind profiles and release conditions (the Figure 2 legend identifies tags for different wind models), the simulated 88 sq. ft. parafoil guidance was able to navigate to within 50 meters of the target. Simulation tests during development of the PGAS algorithm indicated that, given the good navigation state knowledge and effective guidance and control during descent, most of the touchdown accuracy dispersion occurred during the terminal flight phases when within the surface wind boundary layer. The worst case accuracy errors seemed to be reduced when no a priori assumptions about the near surface winds were made, especially under circumstances where the a priori near surface wind data was incorrect. Similar cases were run on the simulator using the 3600 sq. ft. parafoil with a 13,088 pound payload, and a parafoil L/D of 1.5. Results indicated strong landing accuracy performance sensitivity to control system/actuator bandwidth, and actuator time response. More details of these results are also provided in a previous paper1.


Simulated Wind Models Winds are a key factor in the trajectory management and payload delivery accuracy performance of airdrop systems. The Draper engineering ram-air parafoil simulation testbed incorporated measured wind profiles from past flight test programs which characterized the variability of the wind velocity and direction as a function of airdrop vehicle altitude. In addition to the measured wind profiles, a stochastic wind gust model was also included. All the mean wind profiles allowed treatment of horizontal winds, but no vertical wind components were included due to a lack of available vertical component profile data. More details about the wind models are provided in a previous paper1.

Aerodynamic Center

Control By Warping Trailing Edges

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Figure 1. The parafoil/payload configuration, with the control lines running from the payload to the parafoil trailing edges. Example Simulated PGAS Performance Results Many PGAS performance evaluation runs wen executed using the Draper ram-air parafoil engineerinj

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simulation. Assessment of the PGAS performance will the 88 sq. ft. parafoil was performed prior to and durinj the flight test program. Also, the expected PGAJ performance with a 3600 sq. ft. parafoil model wa; determined. Figure 2 provides the pre-flight-test simulated touchdown accuracy scatter data for airdrops of the DFRC 88 sq. ft. parafoil test platform with a 175 pound payload. This parafoil was modeled to have a L/D of about 2.5, and a PGAS control system/actuator bandwidth of about 1.5 Hz. The plotted results show simulated airdrops from 10,000 ft above ground level (AGL) with release point offsets from the landing target varied randomly, but averaging 12,000 ft. Note that the middle point of the random distribution utilized for the release point determination was located at an offset from the landing target based on the particular simulation run case's seasonal average winds. All runs were made with representative GPS and INS data noise

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Figure 2. Pre-flight-test simulated landing accuracy data for the 88 sq. ft. parafoil It is important to note that the 6 DOF simulation did not treat the relative motion of the airdrop system payload and ram-air parafoil canopy. Treatment of this added dynamical effect requires additional simulation DOFs. If the relative motion can be excited to high amplitude, and could induce an oscillation within or near the controller bandwidth, then additional dispersion of the touchdown accuracy performance is possible. These effects are likely to be of greatest consequence for large parafoils which can produce high amplitude/low frequency relative motions. Treatment of these effects was left for future study.

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THE PGAS FLIGHT TEST PROGRAM

The purpose of the PGAS flight test program was to validate the ram-air parafoil models and PGAS GN&C performance against the flight simulation models and to identify design and performance issues not able to be determined by simulation. The flight test program had Draper Laboratory working with the DFRC to conduct 34 airdrops that initially evaluated the test platform characteristics and subsequently evaluated the operation and precision of the autonomous guidance and control system. The flight test payload fully integrated the EGI, flight computer, control actuators, research sensors, and power systems into a small research package. Figure 3 is a photo of the resulting interior of the research payload. System mass, including a backup parachute and ballast, was about 175 pounds, providing the desired parafoil wing loading factor of about 2. In order to minimize flight test cost and turn-around time between flights, the tests were conducted at a commercial sky dive facility in California City, near Edwards Air Force Base. The carrier vehicle was a general aviation aircraft that transported the flight test vehicle to various release altitudes and offsets from the target area. Release point offset directions were selected to bias the likely vehicle flight path away from inhabited facilities and structures. The PGAS design also included way points that could direct the vehicle on paths offset from the desired touchdown target until altitudes well below the release point were reached so as to minimize the chance of vehicle excursion toward structures if vehicle control was disabled in mid-flight. Furthermore, a separately powered radio-controlled, manual-override capability was included to direct the vehicle through a safe flight in the event that the autonomous GN&C system experienced anomalies during flight. A typical flight profile is depicted in Figure 4.

provided light emitting diode (LED) indications of the condition of the system. If the LEDs indicated proper configuration of the system when the drop point was reached, then the payload was released. The parafoil canopy was deployed using a 15 foot static line. On autonomous flights, 30 seconds was allowed to elapse prior to initiation of autonomous guidance for the parafoil to complete deployment, for the airdrop vehicle orientation to stabilize, and for the control actuators to be released from their fixed pre-deployment position. Flight data were recorded in a solid state ring buffer memory, which was activated before aircraft takeoff. The buffer contained sufficient memory to record desired test parameters for an entire descent from high altitude but would record over prior data from the carrier aircraft flight phase if the time from takeoff to airdrop touchdown exceeded the buffer capacity. The flight data recording was automatically deactivated shortly after touchdown to preclude overwriting the drop phase data. The payload retrieval crew utilized the payload ethernet port to recover the flight data in the field with the same portable workstation as was applied to the flight computer initialization.

Test Procedures

All PGAS flight tests required initialization of the EGI and flight computer. The EGI utilized the precision, encrypted GPS data code (P/Y code), with a 7-day key that required weekly reentry. Mission specific data such as way points were entered into the flight computer through a portable workstation temporarily connected through an ethernet port on the airdrop platform. The GPS receiver was initialized upon power up by reading GPS satellite data for about 12 minutes to obtain almanac ephemeris data for the constellation of GPS satellites. The INS within the EGI also required an initial alignment upon power up. For the PGAS flight test program, all these activities were performed on the ground prior to carrier aircraft takeoff. The flight test procedure had the aircraft fly to the prescribed drop altitude in the vicinity of the designated offset location. A panel in the back of the payload

Figure 3. The interior of the airdrop research

payload The overall flight test program plan was executed in tandem with preflight and postflight test analysis on the PGAS engineering simulation. All flight test scripts were simulated before flight tests were executed to

screen the specific test procedures and to establish a basis for assessment of any in-flight performance discrepancies. Flight data collected in the field were returned by the internet to Draper Laboratory for post-

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Llne-of-landlng

coordinates Vehicle files a descending pattern upwind of the landing coordinates

Vehicle navigates to


landing coordinates

Vehicle starts landing

Vehicle touches down near the landing coordinates

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Figure 4. A typical PGAS flight test profile (derived from a figure in Reference 2) the avionics components for emissions was made, and a variety of emission reduction design modifications were instituted. Initial test flights showed that the carrier airplane's location transponder beacon was also providing electromagnetic emissions that affected both the EGI GPS antenna and the vehicle flight control computer. Additional airdrop test rig wiring shielding was installed and flight procedures to limit vehicle exposure to the beacon during payload airdrop release

flight evaluation. The program plan accommodated

GN&C algorithm updates, as required, over the course of the flight test sequence based on the individual postflight performance analyses. However, the flight GN&C algorithm code was always maintained at Draper Laboratory under a configuration control protocol to assure that test flight code loads were exactly consistent with the latest PGAS design baseline. Vehicle Issues Identified During the Ground and Flight Test Program The following list provides some examples of the

were implemented to overcome this problem.

vehicle issues identified during flight-test-preparatory

Use of the EGI in the flight test vehicle with its asdelivered software release resulted in protracted

ground tests and initial flight tests. • Ground tests showed that internal electromagnetic interference generated by vehicle avionics components affected the vehicle's ability to receive a signal from the manual-override control loop at the maximum airdrop altitudes despite use of a 10 watt uplink signal transmission tower. A careful sweep of

alignment or failure to align problem when the INS

alignment location latitude and longitude were not entered upon initialization with very high accuracy. This problem prevented successful vehicle initialization before some scheduled test flights. An

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upgrade to the EGI software by its manufacturer was necessary to reduce the impact of the problem. Autonomous Flight Test Results A series of 14 autonomous airdrop tests were performed with the DFRC airdrop flight test platform (known as Wedge 3) in its final configuration. A number of GN&C system design issues were identified during these flight tests, and modifications were successfully developed to address them. The following subsections address the sequence of autonomous flights, the nature of the significant design issues that were identified during the test program, and the improvements in performance that were achieved by the end of the program. Flight Test Sequence Overview Table 1 provides a summary of the last 14 flight tests in the PGAS program. These flight tests were all performed autonomously from aircraft release until close to touchdown. Some flights did have a manual takeover right before touchdown to assure successful execution of a flare maneuver that protected the avionics by reducing the touchdown velocity (which is discussed more in the next subsection). Only on flights 24-25 was the manual takeover so early as to affect the touchdown accuracy. The first 18 flights of the PGAS test program evaluated the vehicle dynamics in manually flown missions and assessed the autonomous mode capabilities in portions of some of the airdrops. Avionics design issues, including electromagnetic interference sensitivity, were wrung out during these flights. Flight 19 had a parafoil deployment malfunction that resulted in an uncontrolled vehicle "spiral dive" that resulted in significant test platform damage. After several weeks of repair work, flight 20 executed a manual flight profile that recertified the test platform for subsequent autonomous tests. More details about the first 20 flights are documented in project reports5"6. The PGAS implementation established a set of assignable design parameters whose performance impact could be evaluated in flight and whose values could be revised as necessary without code changes. These parameters included software limits on control actuator usage and controller gains affecting bandwidth. As indicated in Table 1, some of these parameters were adjusted during the flight test program to improve vehicle performance. Some GN&C software design changes were also introduced on flight 28, including some additional adjustable design parameters, to provide a new inner control loop around the control line actuators. The motivation for these changes and the resulting performance impacts are discussed in the following subsections.

Issues Identified During Flight Tests A number of GN&C performance issues were identified during the autonomous Wedge 3 flight tests that were mitigated by parameter changes, software updates, or procedure changes. The following items review the most significant of these issues. • Integrator gain and limit adjustments. Flight 21 showed that the vehicle was susceptible to control authority saturation if the "rudder" (differential actuator control) trim position was poorly known and the amount of allowance for rudder motion was too small. Flight 22 provided evidence that the trim integrator gain selection could also affect vehicle stability. Improved initial input data of the trim position and adjustments in the integrator gains and limits were shown on flights 23 and 24 to add control margin, thereby resolving the vehicle flight stability problems. • Actuator command tracking improvements. The PGAS used pulse code modulated actuators to manage the Wedge 3 control lines. Flight tests 26 and 27 began to show evidence of degradation of the precision with which the actuator output followed the PGAS control commands. Figure 5 illustrates the command/response mismatch from the left actuator on flight 26. While the command tracking error was never more than about 0.2 inches, this was enough to significantly degrade the precision of the final turn into target maneuver, which, in turn, compromised the vehicle touchdown accuracy. Microscopic inspection of the actuator gears indicated that very slight gear-tooth scuffing might be at fault. To overcome the actuator tracking problem, a software change was devised to incorporate inner-control-loop closure around the actuators. Since no manufacturersupplied model of the actuator dynamics was available, a ground test program was performed to identify actuator resonances in order to assure an appropriate bandwidth/filter structure for the new inner control loop. In addition, a maneuver-lead term was incorporated in the PGAS to compensate for the lag effects introduced by the rest of the new innercontrol-loop logic. The new software feature was introduced on flight 28, but was not fully and properly implemented until flight 31. The resulting improved control actuator command/response characteristics are shown in figure 6, which is derived from flight 33 data. • Controller bandwidth assignment. Flight tests prior to those detailed in Table 1 had indicated that the PGAS performance, including landing errors, was sensitive to the controller bandwidth. A high bandwidth was found to facilitate excitation of a Wedge 3 Dutch roll oscillation mode. A "medium" 164




bandwidth set of control gains, which avoided the Dutch roll excitation, was assigned for most of the autonomous flight tests. Flight 25 explored an even lower controller bandwidth, but the sluggishness of the vehicle control response jeopardized proper execution of the final turn into the target, necessitating manual takeover by the ground pilot to assure proper turn-in to the landing zone.

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Figure 5. Open-loop command following of left servo during flight 26

Figure 6. Closed-loop command following of the left servo during flight 33 Ground relative altitude determination problems. Experience using the EGI box with encrypted GPS

code showed that the 1-sigma altitude determination accuracy was about 10 meters, while the desired flare altitude for the airdrop flight test platform was about 8 meters. Flared landings were important as a means of reducing landing loads to protect the Wedge 3

avionics. However, the uncertainty in the groundrelative altitude made it likely that when flare was attempted autonomously, it would not occur before touchdown, or would occur too high with resulting parafoil stall. To protect the vehicle, procedures were instituted to take command manually of the vehicle at about 15 meters altitude, and to initiate the flare using the manual control. Since this procedure was initiated at the end of the final approach phase (well after the final turn into the landing zone), it had insignificant effect on the Wedge 3 landing accuracy. Only on flights 24 and 25 did manual takeover occur early enough to affect the landing location, with the pilot's actions motivated by other test range issues. Accuracy Trends Derived from the Evolving GN&C Design PGAS flight tests 25-34 assessed GN&C design changes motivated by desired improvement in the landing accuracy. The following factors were the major contributors to the observed accuracy trend. • Use of a "medium" controller bandwidth that was selected based on applying a bandwidth that was as high as possible while retaining reasonable Dutch roll oscillation stability margins. These controller gains enabled an expected landing accuracy of about 100 meters. • Addition of an actuator inner control loop. The resulting enhancement in the final turn precision improved the expected landing accuracy to about 50 meters. • Sensitivity to wind effects. The aerodynamics of ram-air parafoils preclude much control of the flight path angle. Consequently, any unanticipated wind effects on the vehicle during the final approach to the landing zone (after the final turn-in maneuver) are not correctable. This makes the feasible landing accuracy a function of available information regarding the near-surface winds, which can vary substantially from winds at altitude. The large landing errors on flight 27 showed the effect that strong, unexpected winds can have after the final turn-in maneuver. Only vehicle access and account for actual near-surface wind measurement data taken close to the time of an airdrop can fully mitigate the effects of near surface winds. • Impact of GPS altitude uncertainty. The Wedge 3/parafoil system had a glide ratio of about 2.5. This means that the 1-sigma GPS altitude uncertainty of 10 meters is equivalent to an intrinsic 1-sigma landing error contribution of about 25 meters. This landing accuracy error source cannot be overcome without use of ground relative altitude sensors that provide accurate data prior to the final turn-in to the target (at over 1000 feet altitude). 165




Table 1: Summary of 14 Autonomous PGAS Airdrop Flight Tests (Bold comments highlight GN&C changes introduced on specific flights) Touchdown Miss Altitude Comments Flight Distance Error (feet) (feet) 21 -22 • Medium bandwidth control gains 299 • Discrepancy in Rudder trim data load value led to saturation during (short) maneuvers 22 116 • Medium bandwidth control gains 729 (short) • Assigned trim integrator gains led to maneuver oscillation tendencies -40 23 155 • Medium bandwidth control gains (lateral) • Trim integrator gains readjusted 24 15 • Medium bandwidth control gains 115 (long) • Control integrator limit increased • Manual control takeover applied just prior to final maneuver to target 372 -17 • Low bandwidth control gains 25 (short) • Manual control takeover applied just prior to final maneuver to target 35 • Medium bandwidth control gains 26 412 • New software release to clean up recently identified implementation (short) discrepancies • Evidence of degrading servo response to control commands 27 • Medium bandwidth control gains 27 817 (lateral) • 35 ft/s wind shear experienced below 1000 ft (after maneuver to target initiated) • More evidence of degrading servo response to control commands -22 • Medium bandwidth control gains 28 150 (short) • Servo control loop closure introduced • Maneuver lead logic introduced, but with initial step • Maneuver lead value set at 20 seconds -38 • Medium bandwidth control gains 29 316 (long) • Maneuver lead value set at 30 seconds (still with step) +1 • Medium bandwidth control gains 30 315 (short) • Maneuver lead value set at 40 seconds (still with step) -2 31 115 • Medium bandwidth control gains (short) • Corrected, ramped maneuver lead logic introduced • Maneuver lead value set at 20 seconds 32 -6 81 • Medium bandwidth control gains (lateral) • Maneuver lead value set at 10 seconds 18 33 127 • Medium bandwidth control gains (long) • Maneuver lead value set at 5 seconds 34 -42 170 • Same gains as flight 33 • Recovered in-flight from a spiral dive (due to bad canopy deployment) • Experienced severe wind turbulence and sheer • Unusually large GPS derived altitude error a major contributor to the miss distance be taken to resolve some open issues and to enable maximum benefit of the PGAS for future applications: • Further development of parafoil simulation models to add additional relative payload/canopy DOFs. The added DOFs would enable consideration of low frequency relative motions that can adversely affect closed-loop control of large parafoils and payloads. With the added large parafoil model fidelity, it can be determined by simulation whether

OPEN ISSUES AND FUTURE DIRECTIONS The precision airdrop GN&C technology is directly applicable to a variety of capabilities/systems/scenarios being considered by the Army, Air Force, NASA and a host of other organizations. The PGAS technology discussed in this paper, with its modular architecture, is well suited and expandable to meet the technology needs for a multitude of systems. The following items are examples of additional developmental steps that can 166




and/or when these effects degrade payload delivery accuracy performance. With this added knowledge the PGAS design can be updated to treat these additional dynamics effects. Flight test of the PGAS on larger parafoils. A number of ram-air parafoil canopies are available for flight test consideration, ranging from 450 sq. ft to 7350 sq. ft., with payload capacities ranging from 700 to 38,000 pounds. Generalization of the PGAS design to accommodate other airdrop systems such as deployable semirigid wings. Development of a portable workstation software package that would permit carrier aircraft to support in-flight determination of PGAS software flight-specific parameters and would enable their loading from a single workstation onto a wide variety of airdrop packages. This mission planning software could be integrated into the delivery aircraft and used to download real-time target information just prior to airdrop system deployment. Communication with the airdrop system during flight from the ground, satellites, etc., could also be incorporated to change the mission at any time during descent. Incorporation of "learning" logic that would enable the system being flown to be optimized in real time during descent and to account for the effects of both minor fabrication differences between canopies and partial malfunction scenarios. Use of this technology for the delivery of multiple mini payloads from a single system container. The GN&C system integrated with a "smart container" ejection capability could be modified to dispense multiple low-weight payloads (i.e. sensors, wide area munitions, robots) during its descent trajectory with added logic to compensate for the changes in payload weight and center of gravity during the flight. Addition of features to accommodate airdrop systems with propulsion to provide more offset capability and control. Many new military capabilities could result from this class of system including surveillance and accurate placement of multiple small payloads over a wide battle area. Use of the PGAS to drive a heads-up or bodymounted display for warfighters deployed from high-altitudes and large offsets. The display could provide real time trajectory information, "suggestions" regarding control, target information, location of other jumpers, etc., for deep covert insertion of Special Operations Forces during night and/or adverse weather conditions.

Many organizations are exploring additional applications for precision guided airdrop GN&C capabilities. These include the use of a gliding system for the recovery of the X-38 crew return vehicle by NASA, autonomous/over-rideable gliding wing recovery for the final stage of pilot ejection seats, and gliding systems for the resupply of small, widely dispersed Marine fighting teams from sea bases. CONCLUSIONS A modular, government-owned software package capable of enabling precision guidance of ram-air parafoils was developed at Draper Laboratory. The system was designed to utilize global positioning satellite signals and inertial navigation system data for navigation. It was simulation tested in a sophisticated six degree-of-freedom simulator against high-fidelity models of both an 88 sq. ft. parafoil with a 175 pound payload and a 3600 sq. ft. parafoil with a 13,088 pound payload. It also was put through a successful series of flight tests using a 88 sq. ft. parafoil and a 175 pound payload. The flight tests verified the simulation predictions that the 100 meter touchdown accuracy requirement was achievable under varied wind conditions and with significant release position offsets from the target. Given the high quality of the available in-flight inertial navigation data and the proven effectiveness of onboard wind estimation at altitude, the major sources of touchdown position error were found to depend on knowledge of the vehicle-to-landing-site relative altitude, uncertainty in winds near the surface, and the ability to provide inner-loop closure around the control line actuator positions. With the actuator loop closure, the 88 sq. ft. parafoil achieved touchdown accuracies of about 50 meters, with much of the landing error attributable to uncertainties in the GPSderived vehicle altitude. ACKNOWLEDGMENTS The Draper Laboratory work discussed in this paper was sponsored under U.S. Army contract DAAK60-94-C-0041. The results presented are the consequence of significant technical effort by staff at the Draper Laboratory, the NASA Dryden Flight Research Center, and the U.S. Army Soldier Systems Command, Natick Research, Development, and Engineering Center. Publication of this paper does not constitute approval by the U.S. Army or Charles Stark Draper Laboratory, Inc. of the findings or conclusions contained herein. The Draper Laboratory work reflected in this paper included participation by Dr. Timothy Barrows, Harvey Malchow, Terrence McAteer, Lawrence McGovern, and Robert Polutchko. Special thanks are due to NASA employees John Jarvis, James Murray, Dave Neufeld, and Alex Sim who enabled and executed the flight test program.

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REFERENCES Philip D. Hattis and Richard Benney, "Demonstration of Precision Guided Ram-Air Parafoil Airdrop Using GPS/INS Navigation," presented at the Institute of Navigation's FiftySecond Annual Meeting, Cambridge, Massachusetts, June 18-20, 1996. Alex G. Sim, James E. Murray, David C. Neufeld, and R. Dale Reed, "The Development and Flight Test of a Deployable Precision Landing System for Spacecraft Recovery," NASA Technical Memorandum 4525, September 1993. James E. Murray, Alex G. Sim, David C. Neufeld, Patrick K. Rennich, Stephen R. Norris, and Wesley R. Hughes, "Further Development and Flight Test of an Autonomous Precision Landing System Using a Parafoil," NASA Technical Memorandum 4599, July 1994. "Final Report: Development and Demonstration Test of a Ram-Air Parafoil Precision Guided Airdrop System, Volume 2: Engineering Simulation Models and Flight Code Design," Draper Laboratory Report CSDL-R-2752, October 1996. "Final Report: Development and Demonstration Test of a Ram-Air Parafoil Precision Guided Airdrop System, Volume 3: Simulation and Flight Test Results," Draper Laboratory Report CSDL-R-2752, October 1996. "Final Report: Development and Demonstration Test of a Ram-Air Parafoil Precision Guided Airdrop System, Addendum: Flight Test Results and Software Updates After Flight 19," Draper Laboratory Report CSDL-R-2771, December 1996.

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