As an alternative to TCAS in air and ASDE on ground, all facets of collision avoidance (see 9-minute video) can be supplanted with vast improvement:
- INTEGRATION – one system for both 2-D (runway incursions) and 3-D (in-air)
- AUTONOMY – no ground station corrections required
- COMMUNICATION – interrogation/response replaced by ModeS squitter operation
- COORDINATION – coordinated squitter scheduling eliminates garble
- TRACKING – all tracks maintained with GPS pseudoranges in data packets
- DYNAMICS – tracks provide optimally estimated velocity as well as position
- TIMELINESS – history of dynamics with position counteracts latency
- MULTITARGET HANDLING – every participant can track every other participant
- CONTROL – collisions avoided by deceleration rather than climb/dive
My previous investigations (publication #61 and #66, combined with publication #85 as well as Chapter 9 of GNSS Aided Navigation and Tracking) provided in-depth analyses for all but the last of these items. The control aspect of the problem is addressed here. This introductory discussion involves only two participants, initially on a coaltitude collision course. One (the “intruder”) continues with his path unchanged (so that the method could remain applicable for encounters between a participant and a non-participant tracked by radar or optical sensors). The other (“evader”) decelerates to change projected miss distance to a chosen design value. This simplest-of-all scenarios can readily be extended to encounters at different altitudes and, by reapplying the method to all users wherever projected miss distance falls below a designated threshold, to multiple-participant cases.
Considered here are simple scenarios with aircraft initially on a collision course at angles from 30 to 130 degrees between their velocity vectors. Those limits can of course be changed but, the closer the paths are to collinear the more deceleration is required to prevent a collision (in the limit – direct head-on – no amount of deceleration can suffice; turns are required instead). Turns can be addressed in the future; here we briefly discuss the 30-to-130 degree span.
In Coordinates Magazine and again as applied to UAVs it was shown that, over a wide combination of intruder speed, evader speed, and angles (within the 30-to-130 degree span just noted), the required amount of evader speed reduction is modest. A linearized approximation can be derived intuitively from scenario parameter values. The speeds and the angle determine a closing range rate, while closest approach time is near the initial time-to-go (ratio of initial distance to closing rate) though deceleration produces a difference. The projection of evader speed reduction along the relative velocity vector direction has approximately that much time to build up 500 to 1000 meters of accumulated horizontal separation. Initiation of the speed change that far in advance allows the dynamics to be gradual, in marked contrast to the sudden TCAS maneuver. To avoid a wake problem, the evader’s aim point can be directed to a few hundred feet above the original coaltitude. Continuous tracking of the intruder allows the evader to perform repetitive trim adjustments.
A program with results illustrating this scheme will not fit on a one-page summary, but it comes as no surprise that, with accurate tracks established well in advance (a minute or two prior to closest approach time), a modest deceleration can successfully avert collisions.