The Offshore Voyaging Reference Site

The Secret Life Of Your GPS

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Raise your hand if you don’t rely on GPS. Any takers? No?

Now raise your hand if you understand what your GPS is doing when it concludes “You are HERE”. I don’t see many hands this time, either.

We have reached an interesting situation, where many (if not most) sailors entrust the safe navigation of their ship to a little black box whose operating principle is known only by a rare few.

You don’t need to understand how GPS works in order to use it. Nevertheless, it is undeniably interesting—and a bit humbling—to delve into the inner workings of our favourite navigation tool.


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Dick Stevenson

Matt,
I appreciated the report.and the research that I suspect went into it. In this age where we assume that we will not understand the tools we use daily, even hourly (smart phones), it feels helpful to have the “workings” drawn out in broad strokes. It contributes to an overall experience of having a firm(er) foundation.
Thanks, Dick Stevenson, l/v Alchemy

Matt

Not only do we not fully understand the tools we use daily – in many cases, nobody does.

Take your smartphone, for example. You’d need six years of graduate school and another six in industry to fully understand the architecture of its CPU. You’d need that much training, again, to know the manufacturing process for that CPU. Repeat for the cellular radio. And the battery. And the display. Now look at the software – the OS kernel is the product of 20 years of work by at least a thousand people. On top of that, you add man-centuries of effort put into a network stack, runtime environments, rendering engines and several dozen more core components – each of which has a one- to three-year learning curve before you can understand its workings well enough to make meaningful improvements.

There is not one single person alive who can completely comprehend the entirety of the device. Most of the technology we take for granted is the product of transnational efforts encompassing hundreds or thousands of people, each of whom only knows their piece of the puzzle and how it fits into the general architecture of the whole.

Richard Dykiel

Very informative! Is this behind the pay wall? I would have liked to share it with my family.

John Harries

Hi Richard,

Yes, it’s behind the paywall. Articles like this take a huge amount of work, which Matt, (and us) need to be compensated for.

Richard Dykiel

Understood, and well deserved too!

Marc Dacey

A valuable and cogent article that underlines for me not the simplistic statement that “GPS can be wrong” but that “our understanding of the limitation of GPS can be deficient”. If we don’t have the basic understanding of how the system works, we can be prone to errors like chart datum mismatch or not knowing why a four-satellite fix is more or less the minimum. And yet at last week’s Boat Show, I saw salesdrones blithely demonstrating the “power of GPS to guide your autopilot to a waypoint, like a buoy or a seawall”. We live in interesting times when the greatest joy of boating for some is the ability to delegate the boating part to GPS/AP combos.

ChrisW

Matt, having had the great good fortune to be part of the senior management of the USAF GPS Program from 1989 to 1992, I’ve seen a lot of tutorials. Wish we had had this one back then to share with folks. One side car. More people use GPS for timing than use it for positioning (or they did before the smartphone explosion). It is the time standard behind the world banking system, and its extreme accuracy allows the time value of money in the market to be calculated for less than 1 sec intervals. Also the people who came up with the Time standard for GPS were awarded the Nobel for their atomic clock work. At least one of that team is a sailing cruiser.

ben garvey

Nicely done Matt.

I teach GPS courses to volunteer search and rescue teams, police, firefighters, outdoor groups and other first responders a few times a year. When I started this (15 years ago!) I had a heck of a time finding good graphics and descriptions… so I made them up! I’ve been using variants of the same presentation for the intervening years. I am relieved to see my data and summaries agree with yours (phew, I really did understand it after all…) – but your graphics are better and your language more accessible and concise.

You have found a very nice balance between the technical content and practical implications of same.

One thing I take care to let my students know about is the low power with which GPS signals are sent. Jamming (intentional and unintentional) happens more frequently than most folks know.

In my courses, we spend 2 hours going thru the theory above, and 4 or 5 running exercises navigating to and from surveyed points in the thick woods. Most folks get the theory pretty well, but the issues always come down to the same things: knowing how to relate the data shown on the gps to and from a map; and confidently knowing what buttons to press on the device to get meaningful data back from it. It’s the old “how do I program my VCR?” problem… slowly but surely though, the interfaces are getting more intuitive and the devices are becoming more ubiquitous.

again – nice work.

bg