The document summarizes a proposal for specifying aircraft trajectories in 4 dimensions (latitude, longitude, altitude, time) to enable increased air traffic capacity through automated separation assurance. It describes representing trajectories as tubes defined by straight and turning path segments, with error tolerances defining bounding spaces. XML is proposed for specifying trajectories in a flexible, readable format. The goal is to establish standards to facilitate coordination and increase airspace usage as travel demand grows substantially in coming decades.

1. Trajectory Specification For High-Capacity Air Traffic Control Russ Paielli NASA Ames Research Center AIAA ATIO-06 Conference Wichita, KS, Sept 27, 2006 [paper available at http://RussP.us/publist.htm]

2. Outline Motivation Trajectory prediction Trajectory specification Error tolerances and bounding space Horizontal and vertical specifications Polynomial approximation XML Concluding remarks

3. Motivation Demand for domestic air travel expected to double or triple within ~20 years Airspace capacity currently limited by controller workload (~15 aircraft/sector max) Automated separation assurance can increase airspace capacity 4D trajectories can facilitate automated separation assurance No standard currently exists for specifying and communicating continuous 4D trajectories with error tolerances

4. Trajectory Prediction

5. Trajectory Specification

6. 4D Trajectory Specification Not just a series of 4D points! 3D fixed tube with position along tube as fourth dimension Groundtrack composed of straight (great circle) segments and constant-radius turns (2D) Altitude as function of along-track distance (third dimension) Along-track position as function of time (fourth dimension) Error tolerances determine bounding space around reference trajectory

7. Trajectory Error Tolerances Explicit along-track/cross-track/vertical tolerances Conformance required with high reliability Can vary with traffic situation Limited by navigation capability of aircraft Looser tolerances in light traffic Determine a precisely specified bounding space for each aircraft at each point in time Useful for automated separation assurance Disabled vertical and/or along-track bounds reduce dimension of specified trajectory could be useful for early implementation

8. Advantages of Explicit Bounding Space Enhanced fault tolerance Conflict-free trajectories can be guaranteed for given time horizon even if ground systems and/or datalink fail Maximize airspace capacity Particularly useful in weather-constrained areas Comparable to painting lane lines on roads

9. Capacity Enhancement In Weather-constrained Areas

10. Misunderstandings to Avoid About Trajectory Specification Does not imply centralized “control” But facilitates centralized coordination Can be used to downlink trajectory requests or uplink trajectory assignments Does not mandate “precise” tracking of 4D reference trajectory Precisely specifies bounds on aircraft position at any point in time Bounds can be large when appropriate

11. Horizontal Trajectory Specification Two segment types straight (greatcircle) turn (circular arc) Each segment defines own coordinate system Along-track/cross-track tolerances define bounding space Along-track updates compensate for wind modeling errors

12. Vertical Trajectory Specification

13. Problem With Altitude As Function Of Time

14. Leveloff Transition Tolerance

17. Why XML? Text format less error-prone and more flexible than binary format Directly readable by engineers/developers Flexible selection and ordering of data fields Replacing binary formats in many domains e.g., B2B, SVG, OpenDocument, MS Office Independent of computer platform and programming language Versatile, popular, standardized

18. XML Sample <segment number=&quot;1&quot; vtype=&quot;climb&quot; htype=&quot;straight&quot; stype=&quot;constCAS&quot;> <time start=&quot;0:08:42&quot; duration=&quot;7:42&quot;/> <begin lat=&quot;xxx.xxxx“ lon=&quot;xxx.xxxx&quot;/> <end lat=&quot;xxx.xxxx“ lon=&quot;xxx.xxxx&quot;/> <along coeffs=&quot;xxx.xxx xxx.xxx&quot; CAS=&quot;280&quot; length=&quot;27.815&quot;/> <alt coeffs=&quot;126.8 21.609 4.1417e-3&quot; thrust=&quot;90&quot; end=&quot;270&quot; /> </segment>

19. Concluding Remarks 4D trajectory specification 3D tube with position along tube as fourth dimension Error tolerances define bounding space at each point in time Facilitates automated separation assurance and resulting increased airspace capacity XML is a strong candidate for the job Versatile, popular, standardized Lead time for establishing and implementing standards is very long -- no time to waste!