The ILS wasn't working, but the GPS was. All major and most minor airports have approach plates/terminal procedures. Calculate the desired glide path to touchdown on the runway. Display on a MFD (multi function display) plane, glide path, runway. Proper glide path? Green backlight. Outside of margin glidepath? Yellow backlight. Fall within dangerous limits near the ground? Red backlight, engage autopilot (to go to hold position indicated by approach plates), go to full power, and disable pilot input until above safe altitude (props if you notify control via ADS-B of go-around due to human error).
http://en.wikipedia.org/wiki/Approach_plate
"Approach plates are essential for an airplane to make a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow. They provide specific waypoints and altitudes necessary to line an aircraft up with a designated runway for landing, as well as important navigational information, such as radio frequencies of navigational aids and required course headings, and the prescribed minimum visibility requirements to execute the approach."
http://flightaware.com/resources/airport/KSFO/procedures
Runway 28L approach plate:
http://flightaware.com/resources/airport/KSFO/IAP/ILS+RWY+28...