rotorfossil / S76 Heavy
As far as I am aware the Multilat system in the N Sea is capable of using ADS-B data from appropriately equipped aircraft. If the aircraft has no ADS-B then it needs to triangulate the transponder transmissions and therefore you need to be in range of several (3 I presume) Multilat receiver aerials. If you have ADS-B then only 1 antenna is needed to pick up your extended squitter info which includes your position and a measure of the integrity thereof.
I find it frustrating that operators are continuing to buy new aircraft without ADS-B. Having identified the issue a year or so ago, the last 6 or so EC225s that Bristow has taken delivery of have ADS-B though 4 of those are now in Australia (ie 2 in the N Sea).
To have ADS-B on the 225 is a doddle - you just need to ensure that EC fits the latest dash-number of the transponder box (TDR94D -409) and connects a twisted pair of wires between the GPS and the Transponder. Not that difficult really and minimal cost!
It has other advantages such as Skywatch not needing to direction-find your bearing with dodgy DF antenna, rather it can just read your gps position, it knows its own gps position and so your bearing and distance is calculated using geometry - much more accurate than DF.
Even to retrofit older aircraft normally just needs an SB applied to the transponder box to bring it up to date, that wire between the GPS and the TDR. Older GPSs had problems outputting the necessary parameters for NIC NAC and SIL to be calculated by the TDR, but even the trusty old Trimble / Freeflight 2101 now does it having had a software update. If I was in charge all offshore helis would be modified to have ADS-B!
I think the problem is that the powers that be, including those who order new aircraft, have no idea about this technology and how straightforward it is to implement. Frustrating!
HC