filmen:
From the
TCAS II V7.1 intro booklet:
Originally Posted by TCAS II V7.1 intro booklet
The horizontal dimensions of the protected airspace are not based on distance, but on tau. plus an estimate of the protected horizontal miss distance. Thus, the size of the protected volume depends on the speed and heading of the aircraft involved in the encounter. The horizontal miss distance filter seeks to constrain the volume in order to exclude RAs for aircraft with sufficient lateral separation, and uses both range and bearing information to accomplish this.
Tau is a measure of the
time to intercept, thus distance will depend upon rate of closure. There is no provision mentioned in any documentation I am aware of for the protective TCAS "bubble" to expand in oceanic airspace. For a fuller understanding of TCAS concepts, I think you'll find the above linked FAA booklet quite informative.