Single-Hertz Single-Antenna GPS-Driven Attitude Warning System

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Single-Hertz Single-Antenna GPS-Driven Attitude WarningSystemFinal Report16.621Fall 2001Author:Advisor:William BufordProf. HansmanPartner:11 DEC 20011Matthew Lockhart

Contents1 Abstract2 Introduction2.1 Motivation2.2 Prior Work2.3 Objectives3 Technical Approach3.1 Theory3.2 Hardware3.3 Software4 Experimental Procedure4.1 Recovery Testing4.2 Divergence Testing5 Results6 Conclustions7 Acknowledgements111245567101112131620List of Figures1 Traditional attitude indicator juxtaposed with 3a computer generated pseudo-attitudedisplay.2 Test Rig: View of the system as flown.73 Flowchart of Data Through the System: GPS8supplies velocity and time. The computercalculates, acceleration, pseudo-attitude,warning state. It then plays the appropriatewarning message if any. All data is thenarchived to a text file.4 Sample Output: Flight path angle, pseudo-roll 9angle, velocity and acceleration (in NEDcoordinates), warning state, date and timeare all logged.5 Test Coordinators: Hard at work taking data.106 Flight path and pseudo-roll angles for the15divergence test case with informationalwarnings.7 Flight path and pseudo-roll angles for the16divergence test case with no warnings8 Pseudo-roll angles for both divergence cases.189 Plot of all data points collected during19flight. Dashed lines indicate the warninglimits for the recovery test cases.2

List of Tables1 Recovery Testing Maneuvers.2Recovery maneuvers performed and associatedtimes, averages, and standard deviations.31214

1 AbstractThis paper describes the implementation and testing of anaircraft attitude warning system based upon pseudo-attitude.Pseudo-attitude consists of flight path and pseudo-roll anglescalculated from GPS velocity measurements.A computer was usedto monitor pseudo-attitude and provide a verbal warning to thepilot in case of dangerous flight conditions.In flight testing,the warnings did not affect pilot reaction times in recoveringfrom divergent attitudes but were effective in preventingdivergence in the absence of other attitude information.2 Introduction2.1 MotivationAircraft accidents continue to happen on a disturbingly regularbasis.A significant number of these accidents are attributed tothe pilot's lack of situational awareness.limitation of man or machine is reached.ignorant of the danger to his aircraft.1No particularThe pilot is simplyA potential solution for

this problem is the use of artificial awareness aids, oftenprovided through the use of computers.A computer driven warningsystem for aircraft attitude could increase pilot situationalawareness of dangerous situations; however, traditional aircraftattitude indicators are entirely analog devices and thus not wellsuited for digital monitoring.2.2 Prior WorkDuring PhD research in the Department of Aeronautics andAstronautics at MIT, Richard Kornfeld developed a system known aspseudo-attitude.Pseudo-attitude utilizes aircraft velocityinformation from a GPS receiver to calculate flight path andpseudo-roll angles, which are analogs of pitch and roll asmeasured by conventional attitude indicators.2

Figure 1.Traditional attitude indicator juxtaposed with acomputer generated pseudo-attitude display.Flight testing in the course of Kornfeld's research showed thatpseudo-attitude was sufficiently accurate to completely replaceconventional attitude indicators.Many applications for adigital attitude system exist, but the system as originallyimplemented is not particularly well suited for many of theseapplications.The original system employed a receiver with a 10Hz update rate, the rapid update rate being necessary to avoidlags in pilot input.In addition, the original software includedfiltering routines not strictly necessary for the determinationof pseudo-attitude.3

2.3 ObjectivesThe objectives of this project were to implement a pseudoattitude system utilizing a commercial off-the-shelf GPS receiverand use this system to test the utility of a verbal attitudewarning system.Much of the impetus for this project arose from the vast array ofpotential applications for pseudo-attitude systems and a desireto make these applications more straightforward.It was hopedthat a simple implementation could serve as a jump off point forfurther research or encourage others to explore further uses.Ideas for the attitude warning system sprang from one ofKornfeld's suggestion that pseudo-attitude could be used tosupplement existing cockpit instrumentation.Verbal warningscould serve to alert the pilot to a dangerous situation of whichhe would otherwise be unaware, or act as an additional vote incases of ambiguity with the instruments.The use of verbal audiowarnings also suggested an architecture that could be readilyintegrated into existing aircraft, many of which already have a 1Hz GPS receiver for navigation.The incorporation of pseudo-attitude warnings could potentially increase pilot situationalawareness without crowding instrument panel real estate orrequiring extensive modifications to flight hardware.4

3 Technical Approach3.1 TheoryPseudo-attitude has at its core two vector equations derived byKornfeld.Flight path angle is the difference between the velocity vectorand the local horizontal reference, and is given by(1)Flight path angle is not a measure of pitch but a directindication of the path of travel of the aircraft.It is offsetfrom pitch by the angle of attack of the wings.Pseudo-roll is derived by taking the compliment of the anglebetween the components of the lift and gravity vectors that areperpendicular to the velocity vector.(2)The l, p, g, a, and v represent lift, the local horizontalreference, gravity, acceleration, and velocity respectively.a further derivation, see Ref [1].5For

3.2 HardwareVelocity information was provided by a Garmin eTrex GPS receiver.This unit is a single Hz system intended for recreational use bybackpackers and outdoorsmen.for less than \ 120.It is widely available and retailsVelocity measurements are rated to beaccurate to within 10 cm/sec.The unit's ASCII text-out outputmode was utilized in conjunction with a serial cable adaptor toexport velocity data to the computer.An Intel Pentium based laptop running the Linux operating systemwas used to perform the pseudo-attitude calculations andimplement the warning system.A standard RS-232 serial cable connected the GPS to the laptop.The system as tested is shown in figure 2.6

Figure 2. Test Rig:View of the system as flown.3.3 SoftwareThe system software was implemented in approximately 250 lines ofPERL.A full cycle of the code ran in significantly less thanone second, resulting in generation of pseudo-attitude nearlyinstantaneously, albeit only once per second.This meant thatthere were no significant sensing and processing delays, althoughit would be possible for high frequency oscillations to escapethe notice of the system.Most high frequency oscillations inattitude are very small, and thus the system's limitations didnot hamper the testing we performed.The GPS receiver in its text-out mode automatically transmitsposition, velocity (in North, East, Down coordinates), and timeinformation over the serial cable to the computer at a rate of 17

Hz.The incoming data is parsed to extract the velocity and timeinformation.The two most recent velocity measurements arebackdifferenced to calculate aircraft acceleration, and thecurrent velocity and acceleration are used to calculate pseudoattitude as discussed above.The pseudo-attitude is thencompared to limits established when the script is run.Shouldthe pseudo-attitude exceed the limits, an appropriate warningmessage is selected and played.Figure 3. Flowchart of Data Through the System:and time.GPS supplies velocityThe computer calculates, acceleration, pseudo-attitude,warning state.It then plays the appropriate warning message if any.All data is then archived to a text file.Warning messages were stored as .au files.There were atotal of eight warning messages, corresponding to the eightpossible warning states: climb, dive, left turn, right turn, and8

combinations of these.Each of the warning messages isapproximately three seconds in length.When a warning message istriggered, the warning envelope logic is disabled for threeseconds to prohibit overlapping audio messages.Velocity, acceleration, flight path angle, pseudo-rollangle, warning state, and time were all written into a text fileas they were generated.Figure 4. Sample Output: Flight path angle, pseudo-roll angle, velocityacceleration (in NED coordinates), warning state, date and time are alllogged.In addition to the system's automatic data logging, thecapacity also existed to insert manual time hacks into a separatefile for use in timing the various phases of flight testing.9

4 Experimental ProcedurePreliminary system verification was conducted in a GMCJimmy on the streets of Boston and Cambridge.This was done toassure that data was flowing through the entire system, and thecar was able to generate velocities sufficient to activate thewarnings.The system responded as anticipated, issuing correctwarnings for sloping roads and turns, logging data, and acceptingmanual time hacks.Flight testing was conducted in a Piper Arrow four-seataircraft, with a safety pilot and test subject in the front seatsat the controls, and two test coordinators in the rear seats tooperate the laptop, coordinate the test runs, and take data.Figure 5. Test Coordinators:10Hard at work taking data.

Two sets of tests were conducted.The first examinedrecovery from unusual flight attitudes, and the second looked atthe pilot's ability to control the aircraft based solely onverbal warnings from the system.All flight tests were performed with the test subject'svision restricted to the instrument panel through the use of ahood.4.1 Recovery TestingFor the recovery tests, the test subject was asked torecover from an unusual attitude to steady, level flight.Thesetests conducted both with and without the benefit ofinformational warnings from the system.The system was set toissue warnings at /- 30 degrees pseudo-roll and /- 10 degreesflight path angle.For each run, the test pilot looked down at his lap as thesafety pilot performed a brief slewing maneuver to disorient thetest subject.The safety pilot then placed the aircraft into thespecified attitude from the test card (shown in Tables 1 and 2),transferring control to the test pilot when the system registereda warning state, at which point a warning statement would beissued.For the first series of tests, the warning consistedonly of the spoken word "warning," while the second series of11

tests employed warning statements that provided information as tothe nature of the unusual attitude such as "Right turn."Thetest subject then looked up, assumed control of the aircraft, andrecovered as quickly as possible to level flight.The length oftime from the initial registering of the warning state and thereturn to level flight was recorded for each run.Tables 1. Recovery Testing Maneuvers.Maneuver Sequence 11. CLIMB2. DIVE & ROLL RIGHT3. CLIMB & ROLL LEFT4. DIVE & ROLL LEFT5. ROLL LEFT – CONSTANT ALTITUDE6. CLIMB & ROLL RIGHT7. ROLL RIGHT – CONSTANT ALTITUDE8. DIVEManeuver Sequence 21. DIVE2. ROLL LEFT – CONSTANT ALTITUDE3. DIVE & ROLL LEFT4. CLIMB & ROLL RIGHT5. CLIMB & ROLL LEFT6. ROLL RIGHT – CONSTANT ALTITUDE7. DIVE & ROLL RIGHT8. CLIMB4.2 Divergence TestingThe second series of testing, the test subject would beasked to maintain steady, level flight in the absence of attitudeinformation except for the system warnings.be carried out in two runs.This testing wouldFor both test runs, the instrumentpanel was covered, and the test subject was essentially flyingblind.This condition would simulate failure of cockpit12

instrumentation or distraction of the pilot.defined aspath angle.Divergence was /- 30 degrees pseudo-roll or /- 10 degrees flightFor the test case with informational warnings, thesewarnings would be issued at half the failure criteria, /- 15degrees pseudo-roll or /- 5 degrees flight path angleFor the duration of flight testing, the system archivingfunction was active and recorded flight path and pseudo-rollangles, velocity, acceleration, warning state, and time.Additional time hacks were logged to assist in post-processing ofthe data.5 ResultsDue to scheduling constraints and mechanical issues withthe aircraft, testing was performed with only one test subjectrather than the planned four.Additionally, an incompatibilitywith the power conservation function caused the laptop to dropits audio drivers, thus necessitating the issuance of warningsover the intercom by one of the test coordinators.This added alag to the delivery of warnings to the pilot of somewhere between0 and 1 seconds.Throughout the flight test, the system's calculatedpseudo-attitude tracked very closely with observed attitude,which is consistent with Kornfeld's original findings.13

Flying unusual attitudes with restricted vision proved tobe more physiologically demanding than originally anticipated,and the recovery testing was cut short for the comfort of thetest subject.Table 2 shows the performed maneuvers, therecovery times for each maneuver, and the average time andstandard deviation for each run.Table 2.Recovery maneuvers performed and associated times, averages,and standard deviations.The test subject wavered between the 15 degree rollwarning limits, but avoided the test's failure limits.Each timethe subject received a warning, he was applied corrective actionto return to a flight attitude closer to level.Figure 6 showsthe flight path angle and pseudo-roll angle over the two and ahalf minutes of testing.14

Figure 6. Flight path and pseudo-roll angles for the divergence testcase with informational warnings.The second divergence test case was conducted with nowarnings of any kind.The test subject's only source of attitudewas his innate ability to sense the motion of the aircraft viathe inner ear.Under these conditions, the aircraft steadilyincreased in right bank over the course of the test, ultimatelyreaching the 30 degree failure criterion 63 seconds into thetest.Figure 4 shows the flight path angle and pseudo-roll angleover the 63 seconds of testing.15

Figure 7.Flight path and pseudo-roll angles for the divergence testcase with no warnings.6 ConclusionsThe 1 Hz pseudo-attitude system was successfullyimplemented.The computer-calculated pseudo-attitude trackedvery closely with the aircraft's attitude indicator.The flightpath angle showed an offset from pitch, corresponding to theangle of attack of the aircraft's wings, and this confirmsKornfeld's findings in this regard.The pseudo-roll angle didnot differ from actual roll to any observable degree.The use of the verbal warnings did not produce any improvementsin the recovery time from unusual attitudes.16Two factors may

account for this.As previously noted, the test subject becameincreasingly airsick as the testing continued; and the testingwith informational warnings occured after the testing with simplewarnings.It seems more likely though, that no amount ofadditional testing would produce different results given thecurrent test protocol, which called for the instruments to remainuncovered.Thus, as soon as the test subject assumed control ofthe aircraft, he had full use of the instruments including theattitude indicator.For a pilot with any degree of experience,the time needed to process the visual information of the attitudeindicator is likely to be less than or equal to the three secondsit takes the system to issue the verbal warnings.This wascorroborated by test pilot's subjective opinion of the system'sutility.The divergence cases indicate that a pseudo-attitudesystem can indeed be used to prevent divergence in cases ofinstrument failure or pilot distraction.In both divergencecases, pseudo-roll oscillated sinusoidally with a period ofroughly 60 seconds.This corresponds to the fugoid mode of theaircraft, and is not believed to have been the result of any testsubject control inputs.More interesting is the roll angle.Inthe case without warnings, the aircraft diverged steadily overthe entire test run.For the case with warnings, the testsubject was issued situational warnings at 60 and 130 seconds,following which he took corrective action to return the aircraft17

to a wings level attitude.In neither case did he have anyinternal indication of the divergence.too subtle for him to detect.The aircraft's motion wasFigure 8 shows a comparison ofpseudo-roll angle for the two runs.Figure 8. Pseudo-roll angles for both divergence cases.These data demonstrate that the use of pseudo-attitude as abackup indicator for aircraft (a use proposed by Kornfeld) is avery viable application.The test subject also expressed adesire for a backup visual system in addition to the audio.Figure 9 shows a graph of all data points taken over thecourse of approximately 35 minutes of flight time.Also shown indashed lines are the divergence limits used for most of the18

testing.Recorded pseudo-attitude exceeded these limits for lessthan 1% of the time in flight path angle and less than 5% of thetime in pseudo-roll.The unusual attitude testing accounts for asignificant portion of this time outside the limits.Figure 9. Plot of all data points collected during flight.Dashedlines indicate the warning limits for the recovery test cases.In order to bring the warning system up to fulloperational readiness, further work must be done to refine theconcept of what a safe flight attitude is.The possibility ofhaving varying levels of warnings, differing with phase offlight, particular pilots, visibility conditions, and altitudeall could be investigated further.19

Perhaps more importantly, this system was implemented witha minimum of hardware and software.Low cost, low power, lowmass pseudo-attitude systems such as the one implemented for thisresearch make available position, velocity, acceleration, andattitude from a single source.This implementation demonstratesthe feasibility of a multitude of other applications, from thebackup instrumentation tested here to primary navigation,autopilot, or autonomous guidance systems.7 AcknowledgementsThe author would like to thank Richard Kornfeld whose developmentof the pseudo-attitude system allowed this research to beattempted in the first place.The author would also like to thank his lab partner MattLockhart, advisor Prof. R. J. Hansman, test subject, and the .62xstaff for their concern and guidance.Thank you to Rodin Entchev for the use of the GPS, Sarah Dagenfor the loan of her laptop, and Jennie Cooper for the use of hercar.Thank you also to Veronica Sara Weiner for recording thevoice warnings.20

References1.Kornfeld, R.P., Hansman, R.J., and Deyst, J.J., “The Impactof GPS Velocity Based Flight Control on FlightInstrumentation Architecture,” Report No. ICAT-99-5, June1999.2.Kornfeld, R.P., Hansman, R.J., and Deyst, J.J.,“Preliminary Flight Tests of Pseudo-Attitude Using SingleAntenna GPS Sensing,” 17th Digital Avionics SystemsConference, Seattle, WA, Oct. 31 - Nov. 6, 1998.3.Kornfeld, R.P., Hansman, R.J., and Deyst, J.J., “SingleAntenna GPS-Based Aircraft Attitude Determination,”Navigation,, Vol. 45, No. 1, Spring 1998, pp. 51-60.4.Kornfeld, R.P., Hansman, R.J., and Deyst, J.J., “SingleAntenna GPS Information Based Aircraft AttitudeRedundancy,” 1999 American Control Conference, San Diego,CA, June 2-4, 1999.5.Sanders, M.S. and McCormick, E.J. Human Factors inEngineering and Design, McGraw-Hill, Inc. New York, 1993.6.Stokes, A., Wickens, C., and Kite, K., Display Technology:HumanFactors Concepts, Society of Automotive Engineers,Inc., Pennsylvania, 1990.21

attitude system utilizing a commercial off-the-shelf GPS receiver and use this system to test the utility of a verbal attitude warning system. Much of the impetus for this project arose from the vast array of potential applications for pseudo-attitude systems and a desire to make these applications more straightforward. It was hoped

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