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).