The Vulnerabilities of GPS


The NY Times ran an article yesterday about the vulnerabilities of GPS. It’s here if you desire to read it.

It mostly reiterates what I’ve written in The Green Beret Preparation and Survival Guide. Bottom line: we are way too dependent on GPS — and its used for much than navigation. We use GPS signals for time-keeping. Thus the stock market and many other parts of our society are dependent on it. The one interesting thing the article covers that I don’t touch on is that it’s getting easier to spook a GPS signal. I often wonder if some of those Navy ship collissions occurred because the GPS navigation was spoofed? I hope all naval officers still know how to use compass and sextant? Anyone know?

GPS stands for Global Positioning System. A basic understanding of GPS is useful so we understand what it can and can’t do.

Let’s get a little geeky. The GPS receiver gets a signal from each satellite with the exact time it is sent. By subtracting the time the signal was sent from the time it was received the GPS receiver can calculate how far it is from the satellite. The receiver knows where the satellite is in orbit so it has a fix on that satellite. For our GPS receiver to work it needs to make contact and get a fix with at least 3 GPS satellites for a two dimensional fix (latitude and longitude) and 4 satellites for a three dimensional fix (adding in elevation). If you are only getting 3 satellites and aren’t at sea level, your actual location could be different from what the GPS is showing. If you’re up at a high altitude in the mountains, this can become significant. Usually, though, this isn’t a problem. Of the 31 active GPS satellites, there are usually 6 in range from most places on the Earth’s surface.

Ever notice that it takes your GPS varying amounts of time to get a fix? If the GPS hasn’t been on recently it could take as long as 30 seconds. Tall buildings or other obstructions can also make it take longer. Most GPS accuracy is to within 5 meters.

Cellphone GPS units act a bit differently incorporating Assisted-GPS to get a fix quickly. They use cell phone tower data to assist. Sometimes they can give you a fix without even accessing satellites. This only works though it you are in cellphone and Wifi coverage.

Another thing to consider is whether the map coverage you’re using is in your device’s memory or downloading. Ever have the GPS map become blank when you’re out of coverage? We should always download our local area tiles for whatever mapping GPS we use. When I plan trips, I download the map tiles into memory for the route and destination. This allows the GPS to work faster and gives me a map even if I can’t download it live. For your vehicle’s GPS, are the maps you’re using in the memory or downloading? Put them in the memory.

I’ve noticed when biking and using GPS that every so often it will tell me it has lost the signal. Some of these ‘dead spots’ are the same, but others seem random. Which brings me to this significant point: you can’t count on GPS!

There are other problems with GPS:

They need the satellites working. EMP — electro-magnetic pulse, whether natural (solar flare) or man-made (nuclear weapon) can wipe those satellites out.

The GPS receiver, whether in your vehicle, a cell phone or handheld GPS receiver, requires power to work. Cell phones and batteries can die. Commercial airplanes are required to have backup navigation to GPS. Just in case. We need to do the same.

Sadly, many people no longer carry paper maps in their car. Beyond that, many don’t know how to read a road map, never mind a topographical one.

When I was a brand new butter-bar second lieutenant in the First Cavalry Division, I was told succinctly that a platoon leader had to do two things well: Maintain communications on the radio and navigate. Failing either of those two and your time as leader was limited and your career in the Army over.

In a survival situation, especially moderate to extreme, it is highly likely you will have to move from point A to point B. It also possible you won’t have a GPS to do that with.

The Green Beret Preparation and Survival Guide

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