Fox News ran a story today on GPS jammers, saying that:

An electronic device small enough to fit in a shirt pocket and big enough to conceivably bring down an airplane can be easily purchased over the Internet. All a terrorist needs is a credit card and $49.

Unfortunately, News Limited’s website “news.com.au” regurgitated the story with this sensationalist headline:

Internet jammer can be bought for $55, can bring down plane, say experts.

Now, in the immortal words of the ABC’s Hungry Beast programme, this sounds “a little bit bullshit”.  Here’s why.

The GPS system was designed many years ago by the United States military. It is a network of up to 32 satellites in Medium Earth Orbit, and each of these satellites broadcasts a unique signal. When a terrestrial receiver can “see” or “hear” four or more satellites, the receiver can determine its latitude, longitude and altitude with some accuracy.

An early model Navman GPS receiver

The system was used exclusively by the military up until the early-1990s. The emergence of domestic GPS receivers saw civilian access being allowed to an “intentionally degraded” signal, accurate to within about 15 metres, while the “military” signal provided accuracy to within a few metres. Because it was, and still is, a military navigation system provided free of charge to the masses, the old adage “caveat emptor” applied to early adopters of the technology.

By way of illustration, I was working for a government agency in the early-1990s, installing GPS receivers in a fleet of vehicles. These receivers were accompanied by a data transmitter which sent the vehicles’ locations back to a central controller. One morning in 1994 (?), when the United States commenced military action in Kuwait, our GPS system suddenly went haywire, with blatantly incorrect locations being reported by our vehicles. It took us some time to establish that there was nothing wrong with our network, rather the United States military had made changes to the GPS system ahead of large-scale troop movements in the Middle East.

At the time we used a differential GPS system to give us more accuracy; that is, we had fixed GPS bases (known locations) and compared the GPS signals received at these bases with their precise, known locations. This allowed us to calculate the error which was being introduced into the civilian signal, and get a much more accurate fix on our vehicles. The degradation of the civilian signal has since been significantly improved, if not completely removed, by the US military.

While the manufacture of “civilian” GPS receivers has now become a multi-million dollar industry, the facts remain that the GPS network is still owned and operated by the US military; it has errors which are intentionally introduced by the US military; and it’s provided to civilian users free-of-charge with no service guarantee whatsoever.

For these reasons, the aviation industry has never relied upon GPS as a primary system for navigation.

This may come as a surprise to the editors of Fox News and News Limited, but pilots in domestic aircraft were navigating their way around the world for many, many years prior to the availability of GPS receivers.

Pilots use a very stringent set of rules, systems and equipment in order to navigate. Pilots use transponders, Automatic Direction Finders (which include ground-based radio beacons, VHF omni-range systems) and other equipment to establish their location. They also talk to Air Traffic Controllers, and receive and comply with instructions which assist with navigation and help regulate air traffic.

Pilots use an altimeter to establish their altitude, which compares air pressure with that on the ground – not a GPS receiver.  Pilots use (surprise, surprise) a compass to determine their heading – not a GPS receiver.

Sometimes, planes might be fitted with a GPS receiver, which can be used to corroborate information provided by these other systems. Now I’m not a pilot, but I’ll go out on a limb here and say that if ever there was a discrepancy between a GPS receiver and a traditional navigation system, the GPS would largely be ignored.

If some joker actually managed to get a working “internet jammer” aboard an aircraft in the first place, a navigational discrepancy would probably be looked upon by a pilot simply as an interesting anomaly. Visions of planes falling out of the sky, as painted by News Limited’s headline, are both unfounded and unnecessary.

To say that an “Internet jammer can … bring down [a] plane”, well … it’s a little bit bullshit.

gps · jammer · navigation · plane