As of October 27, 2004 SOIA has been implemented and is in use at SFIA. What follows is a description of how it works.

Simultaneous Offset Instrument Approach(SOIA) is a technique by which two planes can land on runways located closer than the current FAA specification -4300 feet for simultaneous landings. The FAA spec is a fuzzy thing violated by several(many?) airports and, in fact, Chicago O'Hare is proposing to build a new runway only 1300 feet from current runways.

What makes SOIA possible, technically, is Precision Runway Monitor(PRM) radar. Radar has been around since the 1940s and has limitations due to the way it works. Basically a pulse of radio frequency is sent out and a reflection is detected. One limitation is that the RF pulse spreads out as it gets further from the transmitter. Thus it becomes difficult to tell the difference between two planes that are far away from the transmitter.

The other limitation is the time lag between pulses. In conventional radar, that time is as much as eight to 12 seconds. That's a problem when controlling planes that are close together because planes landing may be moving at 150 miles/hour or more. You can see that the planes would move a large distance between pulses. (150 miles/hour is 220 feet/second so ten seconds would be 2200 feet.)

Of course, when planes are landing they (hopefully) would not be headed directly at each other. :-)

PRM decreases the sample time- the time between pulses- so the planes can be tracked more closely.

The next part, using PRM, is simply designing a landing pattern that takes advantage of the more accurate information. Currently, the rules at SFIA are that the pilots of planes landing at the same time must be able to see each other at 3500 feet altitude and greater than 3 nautical miles(3.41 statute miles)from the end of the runway. When those conditions are met, SFIA can handle two streams of planes landing. Here's the current operation(from a FAA presentation):

OK, but what does SOIA do for SFIA?