Watching Bruce's new DIY 5.8 Ghz FPV videos may well answer this question in the fullness of time.
I'm a big fan of the FrSky modules and have fitted a number of them, both one way and telemetry, to various transmitters including Futaba FC-16 and FC-18. I use the small FrSky 4 channel receivers in most of my foamies and parkflyers.
I also have one of the slightly older (last year) HK OSV modules with the $17 camera and the HK/Foxtech 200mW 5.8 TX/RX set. The OSV does not support Return to Launch but has an RSSI input which should give me some indication of impending range issues.
Since I am currently building my first FPV based on the above kit this has become a pressing issue. I am going to fit an additional FrSky DF4R Telemetry RX as a signal monitor and use the RSSI pulse train from that.
The generic problem is this - some RSSI RX outputs are an analogue voltage and some are a variable duty cycle pulse train 100% = lots of signal 0% = no signal - or maybe the other way around. The OSD I am using has an analogue RSSI input and I am looking for a circuit to convert pulse trains to analogue voltage.
Grateful for any pointers.