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Autonomous Formation FlightAutonomous Formation Flight

Capabilities and Future Applicationsof the NASA Autonomous Formation

Flight (AFF) Aircraft

Capabilities and Future Applicationsof the NASA Autonomous Formation

Flight (AFF) Aircraft

Brent Cobleigh

NASA Dryden Flight Research Center

Brent Cobleigh

NASA Dryden Flight Research Center

1st AIAA Unmanned Aerospace Vehicles, Systems, Technologiesand Operations Conference, May 2002

AIAA-2002-3443

Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. NoCopyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has aroyalty-free license to exercise all rights under the copyright claimed herein for GovernmentalPurposes. All other rights are reserved by the copyright owner.

AIAA's 1st Technical Conference and Workshop on Unmanned Aerospace Vehicles, S20-23 May 2002, Portsmouth, Virginia

AIAA 2002-3443

Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code.The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes.All other rights are reserved by the copyright owner.

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Autonomous Formation FlightProject Overview

Autonomous Formation FlightProject Overview

¥ Objective:Ð Achieve at least 10% drag reduction during autonomous

formation flight of two F-18 aircraft

¥ Primary TechnologiesÐ Vortex aerodynamicsÐ Formation autopilot guidance and control (3.3 ft, 2s accuracy)

Ð Robust datalink (wireless modem)

Ð Highly accurate relative position measurement system¥ Uses moving base station, carrier-phase, differential GPS/IMU¥ Accuracy (1s):

Ð 1 foot required; 4 inch desired

The Autonomous Formation Flight (AFF) Project was competitively fundedunder NASAÕs Revolutionary Concepts (RevCon) Program. A minimum 10%sustained drag reduction was selected by the project as the success criteria.Analytical studies and piloted flight test have shown that 15 to 20% dragreduction is possible when a trailing aircraft positions its wing in the wing tipvortex of a leading aircraft. Many bird species form ÒVÓ patterns duringmigrations to take advantage of this aerodynamic effect. Four key technologieswere required to make automatic formation flight feasible: a highly accuraterelative navigation system, a robust datalink, the formation guidance and controlsystem, and vortex-induced drag reduction.

The project team was led by NASA Dryden Flight Research Center, whoseroles included control law design, Independent Separation Measurement System(ISMS) design, system integration, simulation testing, aircraft integration andground test, and flight test operations. Boeing developed the formation controllaw, Formation Flight Control System (FFCS) and Formation FlightInstrumentation System (FFIS) hardware, Production Support Flight ControlComputer (PSFCC) software, and FFCS software. UCLA developed the FFIShardware prototype and led the carrier-phase differential GPS/InertialMeasurement Unit system software and ground testing deve lopment. Analyticalvortex aerodynamic predictions were accomplished by Boeing and NASA AmesResearch Center.

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AFF Project OverviewAFF Project Overview

Phase 0

Phase 1 RR

Phase 1

Phase 2

Completed 2/01

Completed 11/01

09/02 (On hold)

9/03 (Planning)

AFF StationKeeping

AFF StationKeeping

Piloted Vortex Mapping,Control Reqs Definition,

FFIS Eval,ISMS Eval

Piloted Vortex Mapping,Control Reqs Definition,

FFIS Eval,ISMS Eval

AFF DragReduction Demo

AFF DragReduction Demo

AutonomousRefueling

AutonomousRefueling

ProgramRedirection

The AFF project was built in multiple phases, stressing a buildup approach tothe key technologies. Phase 0, which was in progress prior to the RevConaward, demonstrated a station keeping control law that maintained the positionof a trailing aircraft with respect to the lead to an accuracy of 5 feet (control andGPS errors included), 25% of the 20 foot accuracy requirement. The phase 0control law was not designed to fly in the vortex of the lead aircraft and thesystems were prototype in nature. Prior to the implementation of the full AFFsystem described in this paper, a risk reduction flight test phase was conductedto accelerate the understanding of several key items. The effect of the leadaircraftÕs vortex on the trailing aircraft was mapped at several flight conditionsfor use in the control law validation and simulation accuracy. The FFISnavigation system was also tested as a ride-along system, to develop andvalidate its algorithms and accuracy prior to using it in the flight control system.The ISMS relative position algorithms were also flight tested to assure theiraccuracy. The 25-flight risk reduction phase assured the project team that theAFF system was ready to go to final verification testing and flight test. Recentevents have resulted in the project redirecting its focus. A redefined phase 2 ofthe project, which may be conducted prior to the phase 1 effort, is being pl annedto use the hardware and software developed for formation flight to conductautonomous refueling research and development.

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AFF Key System RequirementsAFF Key System Requirements

¥ Allow engagement of a research formation autopilot system

¥ Allow the autopilot system to replace pilot inputs¥ Maintain +/- 3.3 ft (2s) vertical and lateral in vortex; +/-10 ft

nose-to-tail position

¥ Provide feedback to flight crew on system status

¥ Ability for flight crew to change gain sets, relative positioncommand, and slew rate

¥ Provide accurate relative position (1 foot, 1s) and velocitymeasurements to the RFCS

¥ Automatically disengage after detected and some undetectedsystem failures, aircraft transient, or pilot override

¥ Redundant disengages for formation position and/or close rate

¥ Allow change in software constants (gains, filters,É) without asoftware recompile

The AFF system requirements were derived from the program objective andflight test experience that suggested a maximum of system adaptability whileminimizing the amount of testing required. Ground and flight safety were alsokey requirements drivers. The program objective required the ability toautonomously control the aircraft while maintaining a precise relative position toa lead aircraft. The position requirement was derived from analytical studies ofthe vortex interaction effect, which suggested that approximately 90% of thedrag benefit could be achieved if the autopilot could hold lateral and verticalposition to an accuracy of +/- 3.3 ft. Since the drag benefit is less effected bynose-to-tail spacing a larger, 10 ft, requirement was defined. This controlrequirement led to the navigation sensing requirements. It was desired to have asensing accuracy that was one order of magnitude better than the requiredcontrol accuracy (4 in). Operational issues led to requirements for a flight crewinterface that allows changes to the relative position command, relative positioncommand rate, and the ability to change the current gain set. The interface alsoprovides the flight crew an ability to monitor the status and health of theresearch systems. Safety concerns drove requirements for automatic disengagesdue to aircraft faults, detected and some undetected hardware and softwarefailures, pilot override, excessive aircraft dynamics, and unsafe nose-to-tailclearance or closure rate.

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Safety Concerns & MitigationsSafety Concerns & Mitigations

Mid-Air Collision

¥ Design / Automatic:Ð FFCS & ISMS independently

disengage if separation or closurerate minimums exceeded

Ð Simulation and flight test validationof separation calculations

¥ Warnings:Ð Minimum 55 ft nose-to-tail

¥ Training:Ð Pilots proficient at formation

Ð Simulation used to train for nominaland failure cases

Ð Control room monitor of separationÐ Buildup test approach

Structural Overstress

¥ Design / Automatic:Ð PSFCC disengage at 2 gÕs

Ð Majority of test points in non-criticalenvelope

Ð V&V testing of softwareÐ Pilot override of system

¥ Warnings:Ð N/A

¥ Training:Ð Simulation used to train for failure

cases

Two major concerns for autonomous, close formation flight

Although the formation separation required for drag reduction is larger thanmany standard formation tasks, the added workload and distractions presentduring a research mission and the possible disturbances introduced after a failurein the single-string research flight control system make mid-air collision aconcern. Although the test pilots are ultimately responsible for maintainingadequate separation, several automatic features were included in the systemdesign. The FFCS and ISMS systems can command the PSFCC to revert to astandard F-18 should the separation drop below the predefined minimum. Thesesystems use different algorithms (both GPS based) to calculate the separation. Aminimum nose-to-tail clearance of 50 feet, during engaged flight, will give thepilot adequate warning of a mid-air collision potential. The pilot can disengagethe system using several cockpit switches or by moving the control stick.

Because the research flight control system is single string and the software isnot fully qualified to flight critical standards, there is a potential for a failure tointroduce large control commands. Unchecked, these commands could causeoverstress of the aircraft structure under some situations. Automaticdisengages were added to the redundant PSFCC software at 2gÕs to minimizeany transient. In addition, much of the flight test program will be conducted in anon-critical flight envelope (failure of the research system will not result in lossof aircraft or crew), defined as the flight conditions that will not result in aircraftoverstress regardless of the control deflections. The pilot can also move thecontrol stick during a transient which will disengage the research flight controlsand fade in the pilot command over 1 second.

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AFF System ArchitectureAFF System Architecture

AMUX

AMUX

AMUX

Modem

1553

AIMS

TM

PushButtonDisplay

SyncPulse

PSFCC

PSFCC

CCDL

Actuators

Actuators

BUS 1

BUS 2

FFIS

PCM AircraftInstrumentation

Z-12

ISMS

AIMS

FFCS

PSFCC: Production Support Flight Control Computers- A combination of astandard F-18 flight control computer and a research processor that, whenengaged, can replace the standard flight control with a research flight controllaw. Data is shared with the standard flight control through shared memory.

FFCS: Formation Flight Control System- A VME based system that housesthe research control system, outputting analog pitch stick, roll stick, and ruddercommands to the PSFCC. The FFCS also receives navigation data from theFFIS via 1553 bus, monitors aircraft information from the F-18 1553 bus, andinterfaces with the flight crew through the push button display.

FFIS: Formation Flight Instrumentation System- PC-104 based system thatcalculates high rate relative position and state using carrier phase differentialGPS combined with an inertial measurement unit. FFIS provides data to FFCSover a 1553 databus and timing synchronization through a digital sync pulse.

AMUX: Analog Multiplex Filter Card- Hardware card that filters analogsignals between FFCS and PSFCC, limits PSFCC voltage input levels, andcauses PSFCC disengagement when watchdog timers signals from the FFCS orISMS are interrupted.

ISMS: Independent Separation Measurement System- Calculates relativeposition and closure rate between aircraft using basic GPS differencing. GPSdata is shared through wirless modem. ISMS interrupts watchdog timer toAMUX card if separation or closure rate violate predefined limits.

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AFF System ArchitectureAFF System Architecture

FFIS

FFCS

Trail Aircraft

Lead Aircraft

FFIS

FFCS PSFCCAMUX

Differential Carrier Phase GPS & WLAN Datalink

Independent SeparationMeasurement System

Outer-LoopGuidance and Control Multiplex / Filter

Inner-Loop ControlEnvelope MonitoringFlight Crew

Interface

Wireless LANDatalink

PBD

WirelessModemDatalink

ISMS

ISMS

The Formation Flight Instrumentation System computes and outputs theaircraftÕs local and relative state along with a timing sync pulse to each of theFormation Flight Control Systems as long as they have intership communication.In addition the ISMS independently calculates the aircraft separation and closurerate, starting a periodic watchdog timer signal that signals the AMUX that theaircraft ready for engagement. The formation autopilot is controlled by the trailaircraft flight crew through the Push Button Display (PBD). Menu driven displaysallow the selection of gain sets, the relative position target, time to ramp betweenchanges in the relative position target, etc. Once configured, the researchautopilot, housed in the FFCS, can be enabled, assuming that various systemhealth monitors and setup conditions are satisfied. The outputs from the FFCSformation autopilot (pitch stick, roll stick, rudder pedals, and throttle commands)go to the AMUX card which filters the analog signals. If the FFCS and ISMSwatchdog timers are operating, the AMUX allows the commands to pass the theProduction Support Flight Control Computers. When the pilot engages thePSFCCs, the pilot inputs are replaced with the FFCS generated commands.

The current implementation only allows the trail aircraft to be controlled by theFFCS, however, the system could easily be upgraded to allow cooperative,autonomous control of both aircraft.

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Production Support FlightControl Computers (PSFCC)Production Support Flight

Control Computers (PSFCC)

¥ Quad-redundantreplacement for basicF-18 Flight ControlComputers

¥ Research processor that communicates to the primaryprocessor through dual-port RAM

¥ Allows research control system to be engaged

¥ All failures & faults cause reversion to basic flight control

¥ Limited to 8 analog inputs

¥ Mature: Used on several previous programs

¥ Rapid prototyping and demonstration of research flightcontrols

The Production Support Flight Control Computers (PSFCC) are a quad-redundant replacement for the standard F-18 flight control computers. Theycontain an additional processor board and analog boards to receive externalinputs. The processor board communicates to the primary system through dualport RAM. This arrangement allows the research processor to use softwarecalculations, such as actuator commands, from the standard processor or replacethem with calculations from the research software. All standard input/outputsignal management remains intact. The system is designed to fade to thestandard F-18 in one second should any detected failure occur, the pilot select adisengagement, or any predefined limit (such as normal acceleration) beexceeded.

The PSFCCs allow new flight control designs to be rapidly demonstrated at asafe flight condition, while using the fully qualified F-18 flight control systemfor take-offs, landings, and to recover the aircraft in any type of out of controlsituation. One limitation of the PSFCCs is the number of analog inputs islimited to 8. Since there are more than 8 actuators on the F-18, the dual pitchstick, roll stick, rudder pedal, and throttle commands are sent to the PSFCCsfrom the FFCS to control the aircraft. In this way, the PSFCC allows the FFCSto replace the pilot during limited tests.

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Formation Flight ControlSystem (FFCS)

Formation Flight ControlSystem (FFCS)

¥ Ruggedized 6U Motorola 172VME-based architecture

¥ 8 MB Flash memory

¥ (8+) RS-422, (8) analog, (3) 1553

¥ 2 spare slots

¥ Runs C-code on WindRiverVxWorks OS©

¥ Design based on successfulPhase 0 system design

¥ SoftwareÐ Formation flight autopilot

Ð Push button display interface

Ð Aircraft 1553 bus monitor

Ð FFIS bus controller

Ð Analog command outputs

Ð Pilot display

The Formation Flight Control System (FFCS) is a 6U, 5-slot VME chassisthat includes a processor, 8MB of flash memory; RS-422, 1553, and analog I/O;integrated power supply; and 2 spare slots for future expansion. The researchprocessor uses a Wind River VxWorks © operating system and the currentsoftware is coded in ÒCÓ. Gains, limits, filters, calibrations, and other constantsare stored in flash memory that is read in after startup. The flash memory allowschanges to constants (gains, filters, moding, etc.) without recompiling oroverlaying the software, significantly reducing the amount of retesting requiredto qualify a new software load. The FFCS hardware has been ruggedized tomeet flight environment requirements. The FFCS is the heart of the AFFsystem. In addition to housing the formation flight guidance and control system,the FFCS interfaces with the flight crew through the push button display, readsdata from both F-18 1553 data buses, controls the 1553 data bus with the FFIS,and outputs the formation autopilot commands to the PSFCCs.

The FFCS is fully capable of hosting other types of guidance and controllaws that generate pilot command outputs to support autonomous and/orcooperative system research and demonstration. Spare slots can be used toprovide many different types of I/O or add an additional processor. The FFCScould also be configured to control other aircraft systems.

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Formation Flight InstrumentationSystem (FFIS)

Formation Flight InstrumentationSystem (FFIS)

¥ Uses embedded GPS and IMU to calculate position,velocities, accelerations and attitudes in absoluteand relative formats

Ð Carrier-phase differential GPS with moving base stationcoupled with IMU through Extended Kalman Filter

Ð Desired relative accuracy 4 in error (1-sigma), 1 footdemonstrated in flight

Ð PC-104 system including¥ 4 CPUÕs, timing board, ethernet, A/D board, 1553 boards, serial

bus, wireless PCMCIA mode, GPS board, power supplies

¥ Externally coupled to GPS antenna, modem antenna, IMU

¥ UCLA developed and tested S/W & prototype H/Wover several years

Ð Lab testingÐ Single and dual car testingÐ Initial hardware-in-the-loop testing

¥ Boeing completed system H/W ruggedization andbench testing

¥ Environmental and initial flight testing completed atNASA Dryden

The Formation Flight Instrumentation System (FFIS) was developed by theUCLA Mechanical and Aerospace Engineering DepartmentÕs AutonomousVehicles Systems Instrumentation Laboratory. The system relies on tightcoupling of GPS-based relative position solution with a commercial IMUthrough an extended Kalman filter, outputting 40hz local and 10hz relativeposition and state. The accuracy of the GPS-based relative position solution istied to its ability to generate carrier-phase, differential GPS level results withone of the formation aircraft acting as the moving base station. Hardware-in-the-loop simulations and ground tests using two systems in a single moving carhave shown that accuracies of less than 6 inches (1s) are possible with this typeof implementation. During development flight tests of the system, relativeaccuracies of 1 foot (1s) were demonstrated, though some tuning of the systemis expected to improve the result. Mapping of the drag benefits as a function ofposition has shown that controlling the F-18 within a 5 foot circle is adequate toobtain most of the benefit. Thus, the FFIS accuracy of 1 foot is acceptablenavigation accuracy for the formation control law.

Although the current implementation is focused on two aircraft, the FFIScould be adapted to calculate the relative state between many aircraft.

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Independent SeparationManagement System (ISMS)

Independent SeparationManagement System (ISMS)

¥ Mature, in-house, modulardesign

¥ Collects data frominstrumentation, aircraftbuses, and cockpit switches

¥ Passes data to trail aircraftusing wireless modem

¥ Passes data to ground usingstandard telemetry

¥ Differences GPS positionswhen satellites common

¥ Computes error from desired relative position (Phase 1 RiskReduction)

¥ Drives HUD needles with position error (Phase 1 Risk Reduction)

¥ Interrupts watchdog timer when limits are violated (Phase 1)

An independent, back-up method for automatically monitoring the nose-to-tail clearance between the formation aircraft was solved through the use of anexisting Airborne Instrumentation Management System (AIMS) module.Combining an off-the-shelf wireless modem and GPS receiver with this system,allowed for relative position estimate accurate enough for separation monitoring.Previous experience with simple differencing of the GPS measurements on bothaircraft have shown to obtain relative position errors less than 5 ft (2s) whencommon satellites are present. The reference frame of the relative positioncalculation is tied to the current aircraft heading or a heading fixed by the pilotusing a switch in the cockpit.

During the vortex mapping flights, the ISMS also drove a display in the trailaircraft that aided the pilot in finding a desired relative position. The pilotselected a target relative position by dialing two 8-position switches, giving atotal of 64 preprogrammed test points. After selecting a target, the ISMS drovevertical and horizontal needles on the Heads Up Display that, when centered,positioned the aircraft at the selected test point.

Once the formation autopilot is functional, the ISMSð will interrupt itswatchdog timer signal when the formation separation or closure rate violatespredefined limits. Stopping the watchdog timer causes the formation autopilotto disengage, reducing the chance of a mis-air collision.

The flexibility of the current ISMS design allows the automaticdisengagement of the research flight control system based on the status ofaircraft or research system states, measured aircraft dynamics, etc.

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Cockpit Highly AdaptableResearch Monitor (CHARM)Cockpit Highly Adaptable

Research Monitor (CHARM)

¥ Provides cockpit parameterdisplay capability to pilot andflight test engineer that can berapidly reconfigured to improveresearch efficiency

¥ Capable of monitoring anddisplaying all onboard data

¥ Provides control room displaycapability in the cockpit

¥ Provides wireless systeminterface to local and remoteFFIS for status, monitoring, andtroubleshooting

¥ Ruggedized for flightenvironment

Pilot Display

FTE Display

The Cockpit Highly Adaptable Research Monitor (CHARM) was designed toimprove situational awareness in the cockpit. The PC-based CHARM systemincludes a large panel display in the Flight Test Engineer station and a smallerdisplay in the pilot station to host the same types of control room monitoringpages as are currently available in the ground control room. The CHARM cangather data from the aircraft 1553 bus or any parameters available on the PCMstreams to drive the desired displays. Potential uses for the displays aremonitoring of the aircraft systems, monitoring of GPS status, remote control ofwireless systems, monitoring of aircraft trajectory, warnings or cautions, real-time maneuver feedback, instrumentation check-out, etc. These displays can bedeveloped and adapted very rapidly without having perform the costly and timeconsuming task of requalify the basic F-18 systems.

The system hosts displays developed from the same software as the NASADryden control room. The CHARM system has been environmentally tested tomeet vibration, altitude, and temperature levels appropriate for the F-18 cockpit.

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Aircraft CapabilitiesAircraft Capabilities

¥ Aircraft instrumentationÐ Both aircraft: PCM system, aircraft 1553 monitors, Z-12 GPS system,

accelerometer & rate package, signal conditioning, time code generator,AIMS

Ð NASA 845 only: afterburner pressure rakes, fuel temp, fuel flows, datarecorder

¥ Special systemsÐ HUD camera, Data recorder (NASA 847), Smokewinder

Pallet847

E-Bay

Smokewinder / HUD panel

Both F-18 aircraft have been instrumented to support flight research. Bothaircraft have systems to gather data from basic aircraft and research systems,format the data, and transmit the information to the ground using telemetry.This system is very flexible and can receive data in many formats from newaircraft systems. A GPS receiver on both aircraft record GPS data in flight and abase-station is available to provide post flight differential corrections to the data.A research accelerometer and angular rate package supplements the aircraftmeasurements. Both aircraft cameras that can transmit real-time video of theaircraftÕs Heads Up Display. A wingtip mounted smoke generation system canalso be installed and operated from the cockpit. All data available on theaircraftÕs 1553 buses, including flight control, navigation, engine, and subsystemcalculations and status can be made available for transmission to the ground.

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Western Aeronautical Test RangeWestern Aeronautical Test Range

Controlled Airspace

Modern Control Facilities

Adaptable Display Tools

The Western Aeronautical Test Range is a network of facilities used to supportresearch and development of experimental aircraft as well as launch, landing,and on orbit support of the Space Shuttle and other NASA systems.Available data products include:

¥ Telemetry tracking, recording, and archive

¥ Time space positioning

¥ Video and voice communication

¥ Data processing, display, and analysis

For the AFF project the following tools have been implemented:

¥ 3 Telemetry Streams on 2 Aircraft

¥ RADAR and GPS Tracking

¥ 2 or 3 real-time video streams (2 HUD & Chase)

¥ 4 UHF channels (primary, secondary, 2 hot microphone)

¥ VHF channels (for ground crew support)

¥ Merged data streams into one stream for real-time display, analysis, andarchival

¥ Parameter Display Software and PAGE display types

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Simulation FacilitySimulation Facility

¥ Simulation FeaturesÐ Dual F-18 fixed base cockpit simulations

Ð Production avionics, 1553 buses, mission computers

Ð One standard and one wide angle graphics display

Ð Real-time, non-linear, 6 DOF, oblate earth

Ð GPS satellite simulator for dual aircraft testing

Ð Autonomous formation flight hardware (FFIS, FFCS)

Ð Actuator options: software, analog, or ironbird

Ð Control system options: software or flight hardware

¥Simulation UsesÐVerification and validation of flightsoftware/hardware

ÐFailure modes and effects tests

ÐPilot / control room staff training

ÐFlight test technique development

ÐControl system design and analysis

ÐSystem integration

ÐFlight data prediction and comparison

A dual cockpit hardware-in-the-loop simulation has been developed forhardware and software integration and software verification and validationtesting of the AFF systems. One of the two fixed-based cockpits has a 170degree field of view projection system. Both cockpits are driven by flighthardware including Heads Up Display, system status display, and MissionComputers and one of the simulations can be driven by flight control computerhardware (both can use software models). An F-18 ironbird is connected to thesimulation for testing with the production hydraulic and actuator systems active.The AFF hardware (FFCS, FFIS, ISMS) is also integrated into both simulations.A GPS constellation simulator provides RF signals to both aircraftÕs GPSreceivers based on trajectory information fed to it from the simulation. Theprimary uses for the simulation are:

ÐVerification and validation of flight software/hardware

ÐFailure modes and effects testing

ÐPilot and control room staff training

ÐFlight test technique development

ÐControl system design and analysis

ÐSystem integration

ÐFlight data prediction and comparison

An adaptable scramnet network links the various simulation systems.

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Future ApplicationsFuture Applications

¥ Autonomous Aerial RefuelingÐ Great benefit for autonomous aircraft, large aircraft, and all weather

capability

Ð Natural extension of existing systems

Ð At most minor changes to control laws

Ð May require additional sensor for positioning due to GPS blanking

Ð ÒNavyÓ refueling more difficult

A natural extension of the Autonomous Formation Flight system design isautonomous aerial refueling. Installing the lead aircraftÕs station-keepingequipment on the tanker immediately creates the capability to fly in formationwith the tanker. In general, the control problem--maintaining a relative positionwhile the tanker boom is attached--is easier than the difficult task of flying in thehighly nonlinear wing-tip vortex wake. Thus it is expected that the AFF controlwould be suitable for the task with minor changes. One thought on mechanizingthe design is to give the boon operator the ability to change the position of theautonomously controlled aircraft. A ÒtrimÓ switch added to the boom controlscould be sent over the datalink along with the tankerÕs GPS information, givingthe boom operator the ability to position the aircraft in the best location. Sincethe tanker operation requires the aircraft to be in closer proximity to one another,there is potential for mid-air collision following a failure in the single-stringresearch autopilot. This may require that some redundancy be added to thesystem that was designed for AFF. Other issue include the potential for thetanker to obscure the reception of GPS signals or the possibility of GPSjamming. To solve this, additional sensors, such as laser or optical systems, mayneed to be added to make autonomous refueling more robust. Once autonomousrefueling using a boom is solved, the NavyÕs drogue system could beinvestigated. Some method to track the drogue location would be required.Whether this requires a system in the tanker which sends the drogue position tothe refueling aircraft or some type of sensor on the refueling aircraft itself, is notclear.

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Future ApplicationsFuture Applications

¥ Formation Flight of dissimilar aircraftÐ E.g. Multiple UCAVs or fighters in vortex wake of C-17

Ð Reduced need for aerial refueling / extended range

Formation Flight of Dissimilar Aircraft: Another application of formationflight involves dissimilar aircraft. Transport or tanker missions could be used toextend the range of other, smaller aircraft such as fighters and UAVÕs. Thelarger vortex energy could potentially provide significant drag reduction forfollower aircraft. The possibili ty of positioning more than one aircraft in thepath of a single vortex from the transport could be investigated. The vortexproduced by a large aircraft may have the ability to provide benefits to severalaircraft. In addition to extending the range of aircraft, the number of tankermissions could be reduced.

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Other Possible ApplicationsOther Possible Applications

¥ Remotely Piloted Vehicle control (single aircraft)Ð Uplink received by FFCS flys the aircraft

¥ Validate new remotely piloted hardware/software

¥ Develop/validate new RPV ground control station concepts

¥ UAV swarmingÐ Autonomous or RPV leader

¥ Advanced controls (single aircraft)Ð Rapid flight test of new control methods

¥ Neural networks

¥ H¥

Remotely Piloted Vehicle: The AFF system could also host an uplink receiverto allow a ground-based pilot to fly the aircraft. This configuration could beused to develop and validate new RPV hardware or software or develop andvalidate new RPV ground control station concepts. This investigation wouldonly require one of the AFF aircraft.

UAV swarming: Techniques for UAV swarming guidance or control could beinvestigated using AFF technology. Although only 2 aircraft are available, anautonomous or RPV leader would be tracked and followed by the secondaircraft.

Advanced Controls: The AFF aircraft provide a safe method to rapidlyprototype new control concepts. Some possible concepts include neural networkor H¥ controls. The AFF operational concept would allow minimally testedcontrol methods to conduct exploratory flight test early in their development.