18 was only rated for non-precision approaches, which includes GPS.
GPS alone doesn’t give you accurate enough altitudes, iirc.
It just gets you to the runway.
They were using the Localizer as far as I know, and not GPS.
IIRC the runway had REILs. Big ass strobes to identify the runway.
http://en.wikipedia.org/wiki/Runway_end_identifier_lights