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Sunday, February 5, 2012

Interference

I keep my Bonanza at a small, private field in McKinney TX, called AeroCountry (T31).  It has one runway, a strange combination of grass and concrete.  The Northern end is 1,300 feet of grass, the Southern end is 3,000 feet of concrete, for a total length 4,300 feet.  The join is pretty well done - I've landed on the grass and taxied onto the concrete and there's just a small bump.

No instrument approaches, however.  Many of the aircraft there are smaller experimental types, and some exotic aerobatic aircraft (the hangar next to mine holds a Yak 26).  Until recently, the runway was too narrow to meet the FAA requirements to have an IAP (Instrument Approach Procedure), but when a developer wanted to buy the East side of the airport and put up luxury condos and attached private hangars, part of the deal was that he would pay to re-develop the runway, making it wider.  Now we can have an IAP developed, and I'm pushing the owners council to ask the FAA to make a GPS IAP for us.  Otherwise in bad weather I have to land at Collin County Regional (KTKI), about 8 miles away.

Of course in order to fly a GPS approach, the aircraft must have a working GPS unit, certified for IFR operations.  My Bonanza has a Garmin GNS 430W, certified for precision approaches down to 300 feet AGL (or down to 200 in ILS mode), an Apollo panel mounted GPS without WAAS (for navigation and non-precision approaches, I usually just leave it turned off), and a handheld Garmin 496 in a mount, which is used for navigation, backup, and Nexrad weather radar display.

The trouble is, that they all have to receive satellite signals, and these signals come in on a variety of microwave frequencies, around 1.5 GHz.  Paradoxically, the most capable of the 3 GPS units in my Bonanza is also the most sensitive to interference - the 430W needs 5 working satellite signals to be able to support a precision approach.  The 496 only needs 3, but it can't be used for approaches.  And somewhere near T31 is an interferer that is blanking out all GPS use.

Garmin GNS 430 W


Some of the pilots thought that someone had put up a 1.5GHz base station on a nearby water tower that doubles as a cell site, having read about the issues with Lightsquared and the FCC.  Others suspected a nearby radar test range owned by Raytheon.  And a few suspected a microwave relay tower furtehr to the southwest owned by AT&T.  Since I work in wireless telecommunications and am an FAA licensed flight instructor, I know how to work with both the FAA and the FCC and have the contacts to make something happen.

Eventually the FCC agreed this was their baby, and sent out a technician to track down the source.  He drove around where the pilots said we all lost GPS, and found nothing.  So he called me, and I agreed to take up my airplane and try to narrow down the source.  I took off one Saturday with a friend in the right hand seat, and we flew all around the field at 600 to 700 feet AGL (the FAA requires at least 500 feet clearance above or laterally from any obstacle, building, person or vehicle).  We flew various legs around the field, and proved that the source was not the cell tower, the radar range, or the communications tower.  It appeared to be a circle centered near a road intersection.


Based on where we lost GPS lock, the FCC drew up a map showing the 2 circular patterns that best fit the data, and concluded that interferer is near the intersection of Westridge and Independence roads, in a newly built neighborhood.  They asked me to do some more runs from the West and South to confirm the circle's radius, but now suddenly the interferer has gone, after 4 months.  Vanished. Poof!

The FCC's best guess is that it's a bad HDTV adapter in someone's home.  It seems that some can fail in a way that radiates in the GPS bands - at least one model has been recalled, but there are still some out there.  I wish we had found it - it worries me that the GPS blank spot could come back at any time.  Unless the perpetrator knew what he was doing and realized that someone in an airplane was trying to track him or her down - Chinese spy satellite radio, anybody?  Although the usage pattern is closer to that of the TV adapter, staying on for days and weeks at a time - maybe they went out and bought an HDTV and threw out the old analog one.  Still, before we can get a GPS IAP, it needs to be resolved.

This is why your instrument instructor told you to monitor the GPS unit's satellite lock throughout an approach - it can just vanish.  Then what will you do?

2 comments:

Cedarglen said...

Thanks. Great Post. I too wish that you had identified the source. As you note, without that information, the folks at T31 won't be enjoying 'legal' GPS approaches. The terminal gear's capabilities vary by brand and type, but I'd have doubts about one that required FIVE 'lites to operate properly. Best wishes. Did the FAA or FCC contribute to your fuel expenses or the safety pilot? (Cheap Bastards!!)

D.B. said...

Actually, I rather enjoyed it. When else do you get to fly low and slow, with a mission?

Technically GPS only requires 3, but the FAA decided that wasn't good enough, and they would need 4 for instrument navigation. And then another for WAAS precision approaches. Overkill really.