Short Term Conflict Alert (STCA) is a ground-based safety net intended to assist the controller in preventing collision between aircraft by generating, in a timely manner, an alert of a potential or actual infringement of separation minima. Generally, it is more difficult to optimise Short Term Conflict Alert (STCA) for Terminal Control Areas (TMAs) than for en-route airspace. This is because the nature of TMA operations makes it hard to tune the look-ahead parameters used by STCA to predict potential conflicts.
The reason for this is two-fold:
- TMA traffic is more closely spaced than traffic in en-route airspace; and
- TMA traffic undergoes far more turns in comparison to en-route traffic – often for much shorter periods of time and at higher rates of turn.
These two factors can result in relatively poor warning time performance and a relatively high number of STCA nuisance alerts in the TMA.
With respect to the use of STCA in TMA airspace, the following avenues can be explored:
- improve the prediction law (e.g. reducing linear prediction parameters and use of standard turning prediction)
- optimise the filtering processes (e.g. implementing prediction filters which ‘know’ the traffic patterns associated with approach procedures)
- optimise parameters and sensitivity, possibly under the control of the user (e.g. defining STCA volumes at recognised hotspots, with specific parameters).
The key to a STCA system that performs well is to apply the conflict thresholds and prediction times that are most appropriate to each volume of airspace. This might mean defining some quite small STCA volumes in the TMA where very specific parameters will apply. For example, aircraft in stacks (holding patterns) rarely fly straight for more than a minute. Therefore, a linear prediction time set at two minutes is entirely inappropriate for holding aircraft and is, in fact, generally inappropriate for most of the TMA.
STCA volumes for different parts of the TMA should use different parameters. For example, an outer TMA zone can provide for a gradual change in the STCA parameters between en-route airspace and the busiest part of the TMA. The outer TMA zone provides a buffer between the en-route corridor and the busy inner TMA. Further consideration may also be given to setting up specific STCA volumes for lower parts of the airspace, for example, to address potential nuisance alerts between Instrument Flight Rules (IFR) and Visual Flight Rules (VFR) traffic.
In some parts of airspace the future course of an aircraft is not predictable without specific additional information that only the controller or the pilot could disclose. However, there are some segments of certain flights when the aircraft trajectory is predictable based upon the approach procedures.
The most common STCA prediction filter is the linear prediction filter, which makes a straight-line prediction of the aircraft’s trajectory. In the TMA, where turns are common (sometimes at high rates), the linear prediction assumption can be very inaccurate. If one aircraft starts to manoeuvre towards another the linear prediction may not detect a conflict in time. As a result, in addition to the usual linear prediction, some STCA systems use a turning prediction which activates when an aircraft is detected as turning by the tracker.