[azalemeth]

I'm sorry to hear that you witness a mid-air collision -- I hope it wasn't completely fatal. In the UK, all glider pilots wear parachutes and the overwhelming majority have standardised on a collision avoidance system known as FLARM that is "glider friendly" (unlike GNSS) and can differentiate thermalling from colliding.

It's just an internal 16 channel GPS receiver with an external antenna and an altimeter that predicts the flight path and then transmits it - including a unique identifier - as low-power digital burst signals at one-second intervals. Other aircraft also equipped with FLARM receive that, compare it with their own flight path prediction, and also check for collision information with known data on obstacles, including electric power lines, radio masts and cable cars, etc. If a proximity warning is generated to one or more aircraft or obstacles, it bleeps like anything and generates bright LEDs that point in the direction of the threat. The display also gives indication of the threat level, plus the horizontal and vertical bearing to the threat -- and there are three warnings (iirc ~30s, ~15s, ~6s) -- it warns by time and not distance.

I remember the thing going off a few times; it's quite helpful and draws your attention to a region of sky immediately, including behind, above and below you. It's also dirt cheap† and is a battery-powered self-contained box with (I suspect) a microcontroller and glorified smartphone innards inside.

†(by aviation standards)