GPS Jamming: A Growing Issue?

September 15, 2010By: Tony Murfin

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Professional OEM Newsletter, September 2010

I’m sure every Blackberry owner/user has suffered at some time from an intermittent annoying buzzing on their car radio. I was reminded again last week when, with my Blackberry safely stowed in the middle front cup-holder, and its little red transmission light blinking, I got that characteristic buzz on the car radio. I know it’s a prime example of Radio Frequency Interference (RFI) — and not to adversely criticize RIM — the web insists that most cell-phones and PDAs will interact with electronics from time to time. Humidity might even affect how often or how long this could happen — really?

But seriously folks, GPS-jamming is becoming a real area of interest for some people. As the proliferation of jamming devices you can mail order from the web increases, the likelihood of someone using these electronic nightmares increases.

I remember back in the days of Microwave Landing Systems (MLS), the airline industry was questioning why the world was going to replace all the perfectly good low-frequency Instrument Landing Systems (ILS) for landing aircraft. Now, ILS has been around since WWII, and if your airline is anything like interested in keeping its schedules during winter in foggy Europe, you’ve got to have ILS to get into Heathrow or Amsterdam on certain days. But ILS and your run-of-the-mill TV transmissions can sometimes get tangled — and a "buzz" in the cockpit at the wrong moment can really mess up your day. So microwave frequency MLS came along, but took so long to implement that GPS overtook it, and now we’re all flying GPS approaches, all around the world.

All well and good until a satellite TV dish on someone’s roof creates a localized buzz in the GPS signal, or a cigarette-package-sized device completely shuts down signals inside the M-25 Motorway around London. BIG PROBLEM for Heathrow, Gatwick, or even Stansted airports — big enough to have a temporary effect on air traffic — something which short-term could, on a more limited scale, look like the recent effects of Icelandic Volcanic Ash. I say temporary and limited scale because aviation authorities can find GPS jammers pretty quickly and minimize disruptions, which have not been a major operational issue to date.

But GPS doesn’t use just a weak signal, its virtually lost in ambient electronic noise, and its real easy to jam with very low levels of jamming signal. The U.S. and other military users have known about and addressed this issue for a very long time.

If you know what direction the jammer is coming from, you could perhaps blank out that direction and prevent losing your entire navigation solution. So change your single-element omni-directional antenna into an antenna with several elements and throw a mass-of-signal processing at the incoming signals from those separate elements, and you might find one segment is being jammed. Then you can remove those distorted signals and only pass on the "clean" ones to your navigation processor. You’ll loose some satellites, of course, but you’ll see the others and you can probably compute a navigation solution.

The trick is in the signal processing you throw at this problem, and it's taken some very clever engineering to solve that problem, then be able to package it in something smaller than a Cray supercomputer to make it mobile. I would anticipate that most "high-value assets" in today’s conflict zones carry some form of anti-jam protection based on the use of phased array antennas. As for the lower value assets — like your average Humvee — its hard to stick electronics on it with a price tag higher than the cost of an entire fleet of vehicles, so work is underway to get the price of protection down so most everything can be protected.

So what do the rest of us do (who don’t fly an F-18 or a Blackhawk) and who need our Garmin or TomTom to show us how to get to the Mall of America when visiting Minneapolis? Well, its not really a problem yet, folks, and we don’t need to rush to the military surplus store just yet. Emission and susceptibility standards were created to protect the consumer, and we pretty quickly find those "buzzing" TV dish antennas that are having a bad day. But the GPS infrastructure on the ground — that which helps with integrity and improved accuracy for aircraft, and civil aircraft themselves — this is where you would expect some of that reduced cost protection from the military to eventually end up.

It will be interesting to see at this year’s Institute of Navigation (ION) convention in Portland if there will be suppliers perhaps showing the promise of military/commercial reduced size and cost anti-jam systems.

Tony Murfin
GNSS Aerospace LLC