The Gun Counter

What Happens if GPS Fails?

H/T to National Reviews The Corner

I should probably upgrade from my old Scout/Silva compass. I should at the very least get one with a sighting mirror to get better readings.

I would think there would be enough cell phone towers to provide pretty good navigation for aircraft. At least, ones over the US.

BDK wrote:I would think there would be enough cell phone towers to provide pretty good navigation for aircraft. At least, ones over the US.

Cell towers are timed by GPS.

I'm actually curious what would happen to a cell network if GPS were offline and the cell sites couldn't get timing. I'm sure they have a local timing source in each shelter, but those can drift quite a bit if not properly disciplined, and can drift independently of each other.

BDK wrote:I would think there would be enough cell phone towers to provide pretty good navigation for aircraft. At least, ones over the US.

My understanding is that cell networks are programmed to basically compute the ground speed of the cellular device based on the speed of tower hopping and deny a connection above a certain speed. Otherwise, a single airliner of phones would suck a lot of network capacity switching towers so fast all at once.

As far as I know the tower signals are non-directional, so you would have to have at least three strong signals to triangulate a position, and who knows about accuracy. That's not such a great prospect for air navigation based on terrain, earth curvature, etc., compared to the visible GPS constellation.

Cell towers are timed by GPS.

I'm actually curious what would happen to a cell network if GPS were offline and the cell sites couldn't get timing. I'm sure they have a local timing source in each shelter, but those can drift quite a bit if not properly disciplined, and can drift independently of each other.

If they have internet connectivity, they could theoretically connect to one of the atomic clock time references online, and correct for observed ping times.

Cell sites are actually quite directional. Most new 4G/LTE installs are 8, 12, or 18 panel arrays, covering 45, 30, or 20 degrees of arc each. They then stack a couple of polarities in each antenna to get as much density as possible.

Each antenna is fed by a radio, and a set of radios is then controlled by a base station. From what I know leasing space to the carriers, if their NOC had a desire to know, they could trace you down to the individual antenna you're connected to, and probably have a pretty good idea of where you're at based on your signal level, without ever having to ping your phone for GPS.

The main reason you can't connect to a cell network on a plane is that they aim the RF down a couple of degrees to maximize base station density and performance. Your sub-watt transmitter doesn't have the oomph to reach out and sync to a base. There might be some logic in the base station network to blacklist a phone doing 500mph, but I doubt it gets invoked too often.

Huh, ok, thanks for the correction Rusty.