years ago the industry was offered the option of MLS as an answer to freq'congestion,fm interference and installation (terrain)concerns .It had been developed for the military,and was free of interfernce.Several installations around the country(Canada) proved their potential in Mountainous areas
UNfortunately the transition to MLS usage required the industry to equipe their fleets with both.side by side,until the whole of the industry was converted.Alongside this was the main selling point of cost(the MLS was to be far less than the ILS installation)this did not prove to be the case.So the industry retained ILS(with FM shielding) to the CAT3 standard that you see today.with the US gov' releasing the use of GPS to the industry,MLS was forgotten(several installations remain in the US)...
