GPS At Risk: Those Signals Are More Vulnerable Than You Realize

Over the years I've had several instances where the location reported by the GPS was totally crazy. A few years ago we were flying north from El Paso TX and for about five minutes our GPS position was supposedly over Hudson Bay! We were flying with 3 or four GPS, and all of them were anomalous.
Mark

I don't know if I am up to date, but,,,

GPS testing was (is?) conducted near White Sands. They sent out "notices to airman" (quaint language!) advising about periods when GPS might be unavailable, misleading, or crazy wrong.

We read quite often where someone following the GPS in the car will do something stupid.
Does the same apply to GPS in a plane or are those GPS units foolproof?
How do airliners navigate? Do they use GPS? Are those far more sophisticated GPS's?
Curious minds want to know.

My niece's husband is a pilot for Southwest. I've had several discussions with him on GPS related issues. He says the planes he flies use triple redundant military grade receivers. On several occasions however, he has had to manually correct courses plotted by GPS.

These GPS "hiccups" will not bode well for completely autonomous land based vehicles either.

I believe they were "an aid to navigation". Now, many people, myself included, take this information as gospel most of the time.

Any means of navigation has failure modes. So using them well means checking for integrity on single sources, and crosschecking with other references. Navigators have been doing this for approximately forever. Aviation use, both military and civil, has a huge advantage that multipath is ordinarily only a problem very near the ground, unlike our automotive usage where it is a constant threat. Unlike our dashboard Garmins. Both military and commercial aircraft use generally has a combined system that includes inertial navigation components. It is really helpful that the weaknesses of the two techniques are nicely complementary to each other. GPS is essentially immune to long-term drift, whereas inertial has a serious problem with that that only grows with time. On the other hand inertial is pretty much immune to suddenly deciding it is in another place entirely, which is something that GPS can do for a variety of reasons. Another point is that the GPS receiver itself in aviation use generally has far more integrity checking, so is much more likely to report itself out of service temporarily than our dashboard units which are quite happy to report a serious error now and again. In summary, aviation is far better using GPS than without it.

dont forget the sun burps once in a while and throws gps's off and its supposed to happen this week again

Funny you mention this. I had my car GPS and Waze both show me that I was driving in a river last Friday night. After about 10 minutes, they both corrected at the same time and were back on course. I thought I was seeing things.

Shouldn't be too common today. Early on it was sometimes not receivable as the military was figuring out how accurate to make it to the public. Commercial aircraft also have inertial nav systems which don't rely on any radio signals. GPS is mainly used today, though.

I presume that you are saying that we just had a coronal mass ejection. I speculate that that could cause a failure to receive GPS signals at all, and not cause data or computational errors.

Unless it's changed, GPS satellites use only 25 watts to transmit to terra. My 50 watt VHF radio in the vehicle can obliterate reception, as well as AM/FM radio signal. It walks right over it.

The GPS signals are between 1 and 2 GHz. The signals you mention are far, far out of band. If you are seeing reception problems with those interfering sources most likely the trouble is happening at an internal intermediate stage in your GPS receiver. One with a better design quite likely would find those to pose it no problem.

Since the free space propagation loss is proportional to the distance squared divided by the wavelength squared,the power at the GPS antenna received from a 150MHz VHS transmitter 10' away is approximately 160dB larger than the power received from a GPS satellite. The front end of the GPS would need to be very selective to remove that much out of band signal without saturating the circuitry. If the VHF radio generates significant harmonics at the same frequency as GPS, you're probably out of luck. Spread-Spectrum modulation can pull out signals approximately 40dB below the noise level, so the VHF transmitter would need to be -120dB down from the center frequency. Distance is your friend!
Mark