Could this GPS tarmac transmitter be a solution for noisy flight path over SFO?

For years, neighbors who live along the flight path to San Francisco International Airport have suffered — and complained — about airplane noise. Now the airport is testing a new way to battle the nuisance that is growing worse in many nearby cities: Precision navigation.

A powerful GPS transmitter on the airport’s tarmac will guide incoming planes with more accuracy than satellites in the sky alone, so pilots can fly higher over cities or soar out into the Bay, potentially reducing noise over Peninsula communities like Palo Alto and Menlo Park.

The $11 million project — the first in the West — improves a plane’s three-dimensional positional accuracy from many hundreds of feet to an exacting 30 to 50 feet.

In the tight airspace of the Bay Area, “this enables landing patterns that wouldn’t be possible using less precise navigational aids,” said SFO spokesman Doug Yakel. “The broadcasted messages are giving ‘digital breadcrumbs’ to aircraft to follow a pathway into SFO.”

The goal is to enable steeper glide paths or even curved or offset approaches to keep the noise of incoming air traffic farther away from the Peninsula’s most-populated neighborhoods. Descent routes haven’t changed yet, but several new approaches are awaiting federal approval.

“We’re hoping that this is a system that can offer some relief to nearby communities,” Yakel said.

While community groups who monitor airplane noise welcome the new technology, called a Ground-Based Augmentation System (GBAS), they say it has many shortcomings, including that its use is limited to the final 10 miles of a plane’s approach. Also, it won’t spread out air traffic around the region, so some cities could still experience the roar of “speed brakes.”  And they’re frustrated that the Federal Aviation Administration, which oversees all flight procedures, isn’t fast-tracking or funding the effort. 

“The potential is there. But we’re not achieving that potential,” said Marie-Jo Fremont of Palo Alto, where dozens of flights roar overhead every day.

As passenger levels have grown over the years, the national airspace has been under increasing strain to both keep up and do it safely. Until recently, most airports relied on the traditional instrument landing system to determine whether an aircraft is off-center or approaching the runway too high or too low.

Then, in 2014, the FAA’s $35.6 billion modernization effort put GPS navigational satellites in orbit to make airplanes more fuel efficient and increase the capacity of routes in the sky.

That changed the flight paths in 12 metropolitan areas, including San Francisco. Air traffic that had been dispersed was suddenly focused on a single corridor. Homes were bombarded with noise, said Palo Alto Councilman Greg Tanaka.

To prove there was a problem, Palo Alto’s Jennifer Landesmann and her group Sky Posse filed a massive FAA data request under the Freedom of Information Act. They confirmed the sudden concentration of flights — and a significant drop in altitudes. Planes also have changed their braking techniques, which adds to the noise problem, the group found.

“Now you have a lot more planes, doing a lot more things, that are closer to you,” said Landesmann. “It makes a very big difference.”

For Karen Schilling Gould, a 30-year resident of Palo Alto, “the noise is incessant. I can’t enjoy my backyard the way I used to. It interferes with sleep.”

“There have been times where the planes were so low and so strong that I once woke up thinking there was an earthquake, because my house is shaking,” she said.

But it’s risky to reroute planes using today’s constellation of GPS satellites. With three international airports, the Bay Area has crowded skies, said Yakel. While satellite-based GPS is accurate enough for everyday use, it has minor errors due to atmospheric conditions, satellite distribution and other issues, he said.

“For the precision that we need for aviation – it’s not good enough,” he said.

The new tool — a transmitter at a ground station, near the intersection of four runways on the edge of the bay — helps solve this problem by sending out location information that provides corrections to satellite-based GPS signals.  It is already being used at Newark and Houston airports.

“This transmitter talks to nearby satellites. It’s saying: ‘I know exactly where I am, where do you think I am?’,” said Yakel. “And from that minor difference, it will do calculations and it can send out corrected messages to aircraft.”

That makes it possible for airplanes to safely hold higher altitudes on approach, flying at 5,400 or 5,500 feet, well above the current recommended altitude of 4,000 feet, then make a steeper descent. Planes could also fly more focused routes over San Francisco Bay, not over towns.

The routes haven’t changed yet. The next step is for aircraft to test several new approaches. One would allow planes to fly higher before landing, then shift the glide angle descent from 3.0 to 3.2 degrees. Another would allow planes to fly farther out over the bay. Those plans have been submitted to the FAA for approval, which is expected by the end of the year.