What are flights SSM1 and SSM2

[Melburnian1]

On FR 24, 'Sydney site monitoring' flights SSM1 and SSM2 are displaying as at 0850 hours on Saturday 5 November.

Are these drones, or balloons? Apologies: I'm sure there's a simple answer: I've not seen these before.

Do they check runways for debris (which I had thought was the function of airport staff driving a ute up and down), or is there some other purpose?

 

[oz_mark]

As far as I know they are ground based transponders at a fixed (and accurately known) position, and used for calibration.

 

[Quickstatus]

I think its part of the overall aircraft tracking suite.

AirServices Australia explains this here:
Our technology | Airservices

The SSM (secondary surveillance monitor). I think is a long range surveillance system that links aircraft transponders to a ground station. Secondary because Primary Radar which essentially pings objects (in the classic way) has shorter range.