@ian16th...
Problem was that coverage was not worldwide IIRC
HH
This will always be so with any 'secondary' radar. The definition of secondary radar means that it is dependent upon a second piece of kit. In some cases on the ground, in others in the a/c.
Look at the MH370 thread, many people have been educated as to how the civil authorities are almost totally dependent upon the a/c side of these systems being serviceable and used correctly. No 'squawk' and the a/c was invisible!
Just about all of the primary ground based radars are in the hands of the military.
Sorry but Loran? Secondary Radar?? Hardly!
Loran in either its A or C variants was / is a LF Hyperbolic Radio System. Primary or secondary it had absolutely nothing to do with radar.
Prior to rejoining the civvy world it was my dubious pleasure to be one of the very few trained 2nd / 3rd line techs at RAF Sealand 30MU responsible for the servicing and repair of the ADL-21 Airborne receivers... when it was good, it was very very good... but a rogue receiver (and s/n 222 comes to mind... temperature sensitive motherboard) could be a real pig.
More info about Loran A and C available here...
http://en.wikipedia.org/wiki/Radio_navigation#LORAN-C
Superceded generally by Omega and then later by GPS...