A recent post expressed concern (serious concern, in fact, to anyone tracing¬†through the references cited therein), regarding future air traffic. Those¬†concerns, serious as they are, all stemmed from events that ¬†preceded¬†developments¬†adding further disquiet. Consider this “triple-whammy” —
* prospect for tripling of air traffic by 2025
* addition of UAVs into NAS
* ADSB methodology plans
Now add all implications of this headline:¬†¬†GAO Faults DHS, DoT for GPS Interference/Backup Effort ¬†in the 2013 year-end issue of InsideGNSS. ¬†In that year-end description a friend of mine (Terry McGurn) is quoted, saying¬†“I don’t know who’s in charge.” That thoroughly credible diagnosis, combined with subsequent observations by father-of-GPS Brad Parkinson, crystallizes in¬†many minds an all-too-familiar scenario with a familiar prospective outcome: The steps they’ve correctly prescribed “can’t” be implemented now. It’s “too late” to change the plan —¬†until “too late” acquires a new meaning (i.e., too-late-to-undo-major-damage).
Historically this writer has exerted considerable effort to avoid dramatizing¬†implications of the status quo. While straining to continue that effort, I¬†feel compelled to highlight those implications related to air safety.
OK there will be no naming names here, but many with influence and authority do¬†“not believe that the minimum performance required by the ADS-B rule presents a¬†significant risk to the operation of the National Airspace.” ¬†Here’s my worry:
* That minimum required performance allows ten meters/second¬†velocity error
* Existing efforts to provide precise position therefore become ineffective¬†within an extremely short time; what translates into collision avoidance¬†isn’t accuracy in present but in future relative¬†position
* Consequently the accuracy that matters isn’t in position but in velocity relative to every potentially conflicting object
* Each of those relative velocities can be in error by 10 m/sec or more
* Guidance decisions made for collision avoidance must be prepared in advance
* Even TCAS, with its sudden climb/dive maneuvers (which recently produced a¬†news story about screaming passengers), plans a minute and a half ahead
* With 10 m/sec velocity error that gives 900 meters uncertainty in projected¬†miss distance — no higher math, just simple arithmetic
* That simple calculation is very¬†far from being a containment limit
* Only containment limits based on conservative or realistic statistics¬†can provide confidence for safety
* containment limit can’t just be 3 sigma; for low enough collision probability¬†with non-gaussian distributions, ten¬†sigma could still be insufficient.
Combined weight of these factors calls for assessment of prospects with future¬†(tripled) air traffic plus UAVs. It has been asserted that, rather than ten¬†m/sec, one m/sec is much more likely. As noted earlier, “much more likely” is¬†far from sufficient reassurance. Furthermore, multiplying time-to-closest-approach by 1 m/sec, and amplifying that result by enough to produce a credible¬†containment limit, still produces unacceptably large uncertainty in projected¬†miss distance. So: expect¬†(1) either¬†frequent TCAS climb/dives or (2) guidance¬†commands generated for safe separation resulting in enormous deviations from¬†what’s really needed. Forget closer spacing.
Add to that runway incursions, well over a thousand per year and increasing —¬†also with everything said about needs for avoiding other objects applicable to¬†maritime (shoals) as well as airborne conflicts; Amoco-Cadiz, Exxon-Valdez, … ¬†.
These points are covered by presentations and documentation from decades back. ¬†The best possible sense-&-avoid strategy is incomplete without precise relative¬†(not¬†absolute) future¬†(i.e., at closest approach, not¬†current) position —¬†which translates into velocity accuracies expressed in cm/sec (not meters/sec). ¬†The industry has much to do before that becomes familiar — let along the norm¬†that containment¬†requires. ¬†A step in this direction is offered by flight¬†results (pages 221-234, Fall 2013 issue of Institute-of-Navigation Journal).