It’s taken 400 years of scientific discoveries to make it possible for anyone to find his location anywhere on the globe using GPS. //

With the letters GPS, we instantly recognize an innovation that has revolutionized our lives. The concept was born half a century ago in a sweltering room at the Pentagon over Labor Day weekend in 1973.

That’s the genesis of the concept for a constellation of platforms orbiting the Earth, transmitting radio signals to determine location. Many years of calculation, experiment, and miniaturization led to the Navigation Signal Timing and Ranging (NAVSTAR) satellites that became known as the Global Positioning System (GPS). //

Our society has been blessed with rare and precious genius that has combined across centuries to yield the civilizational achievements we enjoy today. Orbital mechanics originated from careful geometric analysis by Johannes Kepler in the 17th century. Two centuries later, electromagnetism was empirically measured by Michael Faraday and mathematically characterized by James Clerk Maxwell.

Atomic oscillation arose from the quantized radiation law Max Planck discovered, while Albert Einstein discovered relativistic effects, both in the early 20th century. These were the giants on whose shoulders later scientists and engineers stood to build their guideposts in the heavens.

While only a tiny fraction of the electorate understands the enormity of government waste, fraud, and abuse, now and then we learn of some extraordinary achievements underwritten with your tax dollars. GPS is one of them.

• Galileo/GPS/BeiDou/Glonass open source monitor
• Live observer map
• Status (coverage, DOP) map

This is a quick writeup of a lesson on how GPS (or in general, satellite based navigation) works. I’d like to thank Jasper Vos and Michel Dingen of OBS De Notenkraker for the opportunity & the very useful feedback on the lessons!
Context

10-11 year old children mostly have phones here, and almost all of them turn out to have location sharing turned on with their parents. They also use their phones while on holiday to figure out where they are.

A smattering of kids here knew that phones use satellites for precise positioning, and one or two even know about GPS. So that is good!

It also turns out there is healthy interest in discovering how it actually works.

Late July 2019, Galileo, “the European GPS” suffered from a week long outage. I’m a proud European, and I think we should have our own well-functioning navigation system, so I tried to figure out what was going on. Surely someone was monitoring this stuff in public? I come from the internet where we monitor all the things, if someone asked for it or not.

This led me on a journey to monitor Galileo, but quickly also GPS, the Russian GLONASS and Chinese BeiDou systems. Along the way, I found out out how positioning satellites really work. This also helped me understand what went wrong with Galileo, more about which later.

In this post, I want to share what I learned, firstly because it is fascinating, but secondly because it serves as documentation of what the monitoring website “galmon.eu” is actually showing.

Galmon, which we should really rename to Navmon, is a lot like the RIPE Atlas Probes, but then for space. Based on a network of volunteers literally around the world, we monitor the output of each and every navigation satellite and make the results openly available as a pretty website, JSON but also as raw data (messages). Galmon is GPL licensed open source and lives on GitHub. //

How do navigation satellites work?

Let’s imagine we launch a bunch of metronomes, musical devices that tick at a precise frequency. We’ll make them tick precisely once a second and, unlike a regular metronome, we’ll also make them tick exactly on “whole seconds”. So they emit a tick at 0 seconds past the hour, 1 second past the hour etc, one tick every second, on the second.

We put the metronomes in different orbits around the earth, so at any time, some of them are further away than others. Then, we listen to their ticks, which are conveniently transmitted over radio.

Because of the speed of light, a metronome that is 30000 km away will send out its tick on a whole second, but it will arrive with us 100 milliseconds later. A metronome that is closer to us, say 25000 km, will have its tick arrive slightly earlier. These differences are large enough that if we would put the ticks on a loudspeaker we could hear the difference.

Because we can measure the precise delay, we can tell exactly how far away each satellite is from us. In itself however, this does not help us determine our location, because we don’t know where the metronomes are! //

In two dimensions, we only need two satellites to do this, assuming we already had an accurate clock that also ticks once a second, on the second. In three dimensions, we need three satellites ticking away at us.

(Note that in the figure above, we could also be in a second position where the circles intersect - we can rule out that solution by assuming we are not actually in space ourselves).

But, at a very elementary level, this is how GNSS works: satellites tell us where they are, and they send out a ‘tick’ exactly every second (and on the second), and by timing how late that tick is in arriving at our location, we know how far away the satellite is. And by drawing some circles (actually spheres), we can discover where we are. This technique is called multilateration.

What if we don’t have an accurate clock?

We want to use GNSS to figure out where we are, even if we aren’t dragging an accurate clock with us. The speed of light means that for every nanosecond that we get our clocks wrong, our position will be off by 30 centimeters. Nanosecond accurate clocks are delicate machines that do not fit in phones.

Luckily, through some clever math, it is possible to use the satellites themselves as an accurate clock - to do so, we do need an additional satellite and a guess of the time. Such a guess could be derived by taking the average of all GNSS clocks received combined with a rough knowledge how how far away such a satellite could be. Armed with this rough guess, we might end up with:

Note that the three circles do not all cross in a single point. Because our rough clock estimate was wrong (let’s say it runs late a bit), all satellites appear to be a little bit further away than they actually are. Or in other words, all the circles are a little bit too big, causing them not to intersect in one point.

A receiver can based on this observation adjust its internal clock until all circles intersect in one point:

Once this happens, we know that the correct time has been derived.

Leap seconds are controversial things. Since the Earth does not rotate at a steady rate, over time the Earth could get ahead or behind “atomic time”. Whatever solution you propose for this, someone is going to be unhappy.

I take no position on what the best thing to do is here, except that one day I would like to do the math on the “great leap second gyroscopes” that we could mount near the poles to steady the Earth’s rotation, so we can stop talking about this. We may occasionally have to desaturate these gyroscopes with huge rockets also.

Anyhow, some new minor leap second drama is coming up, and for once we can’t blame astronomers, geologists or the International Earth Rotation Service. Imagine if they ever went on strike, by the way!

The system is essential but also vulnerable. We need a backup.

According to a study last year commissioned by the National Institute of Standards and Technology, GPS has about $1 billion a day in economic impact in the US. Its reach is, simply, mind-blowing.

"Gauging the overall value of GPS is nearly impossible," writes Greg Milner in Pinpoint, a 2016 book about how the space-based system came to be and the effect it's having on the world. "It has become difficult to untangle the worth of GPS from the worth of everything." //

"The timing aspect of this is probably more widely used than the where-are-you aspect," says Goward. //

The origins of GPS stretch back to secret work by the Department of Defense in the 1970s, in a quest for precision targeting. As Milner recounts it, GPS chief architect Brad Parkinson summed up that goal in the phrase "Drop five bombs in the same hole." //

In 1983, after a Korean Air Lines passenger jet strayed into Soviet airspace and got shot down, killing 269 people, President Ronald Reagan declassified GPS to give civilian aircraft access to the navigation signals. Almost a decade later, GPS famously earned its stripes as a military resource during Operation Desert Storm, when it helped guide US and allied forces across desert expanses to a swift victory over Iraq during the Gulf War. //

Though the funding to keep things running goes through the Pentagon -- the Space Force GPS program had a 2020 fiscal year budget of $1.71 billion -- there's civilian oversight as well. The Defense Department and the Transportation Department co-chair the US government's National Executive Committee for Space-Based Positioning, Navigation and Timing, which coordinates GPS-related matters across federal agencies and includes representatives from Boeing, Garmin, Google, Ohio State and Stanford.

Note the keywords in that committee name: positioning, navigation and timing, or PNT. Where you are, where you're going, and when the signals hit a receiver. It's a term that's inescapable when you're talking with folks who live and breathe GPS.

Huge disruptions are messing with entire portions of the globe. What's less clear? Where it's coming from.

According to a non-peer-reviewed paper, Humphreys and Peter Iannucci of the Radionavigation Laboratory state they have determined the Starlink satellites operating in low Earth orbit could provide an 'unjammable' alternative to Global Navigation Services, GPS. "Anticipation is building for commercial broadband Internet services provided by mega-constellations of satellites in low Earth orbit. Such services’ global reach, low latency, and wide bandwith situate them to revolutionize broadband communications. This paper seeks to establish a less-obvious assertion: In addition to broadband service, these constellations could revolutionize satellite-based positioning, navigation, and timing," the paper reads. "Their space vehicles are far nearer and more numerous than those of traditional global navigation satellite systems in medium Earth orbit or geostationary orbit, and their communications transponders have both exceedingly high gain and access to a vast allocation of spectrum."

The paper describes in great technical detail, a system that utilizes the Starlink satellites working alongside traditional GPS signals to deliver precise location signals more accurate and faster than current GPS. Read the research paper published by the University of Texas at Austin: Fused Low Earth Orbit Global Navigation Satellite Systems

Orbiting the earth every 11 hours and 58 minutes, GPS satellites are the atomic clocks that synchronize the ...

What would happen if GPS - the Global Positioning System - stopped working?

For a start, we would all have to engage our brains and pay attention to the world around us when getting from A to B. Perhaps this would be no bad thing: we'd be less likely to drive into rivers or over cliffs through misplaced trust in our navigation devices.
With no GPS, emergency services would start struggling: operators wouldn't be able to locate callers from their phone signal, or identify the nearest ambulance or police car.

There would be snarl-ups at ports: container cranes need GPS to unload ships.

Gaps could appear on supermarket shelves as "just-in-time" logistics systems judder to a halt. Factories could stand idle because their inputs didn't arrive just in time either.

Farming, construction, fishing, surveying - these are other industries mentioned by a UK government report that pegs the cost of GPS going down at about $1bn (£820m) a day for the first five days.

If it lasted much longer, we might start worrying about the resilience of a whole load of other systems that might not have occurred to you if you think of GPS as a location service.

Consider phone networks: your calls share space with others through a technique called multiplexing - data gets time stamped, scrambled up, and unscrambled at the other end.

A glitch of just a 100,000th of a second can cause problems. Bank payments, stock markets, power grids, digital television, cloud computing - all depend on different locations agreeing on the time.

If GPS were to fail, how well, and how widely, and for how long would backup systems keep these various shows on the road? The not very reassuring answer is that nobody really seems to know.

No wonder GPS is sometimes called the "invisible utility".
Trying to put a dollar value on it has become almost impossible. As the author Greg Milner puts it in Pinpoint: How GPS is Changing Our World, you may as well ask: "How much is oxygen worth to the human respiratory system?" It's a remarkable story for an invention that first won support in the US military because it could help with bombing people - and even it was far from sure it needed it. One typical response was: "I know where I am, why do I need a damn satellite to tell me where I am?"

it wasn't until the first Gulf War, in 1990, that the sceptics came around.

As Operation Desert Storm ran into a literal desert storm, with swirling sand reducing visibility to 5m (16ft), GPS let soldiers mark the location of mines, find their way back to water sources, and avoid getting in each other's way.

It was so obviously lifesaving, and the military had so few receivers to go around, soldiers asked their families in America to spend their own money shipping over $1,000 (£820) commercially available devices.

The American taxpayer puts up the billion-odd dollars a year it takes to keep GPS going, and that's very kind of them. But is it wise for the rest of the world to rely on their continued largesse?

In fact, GPS isn't the only global navigational satellite system.
There's a Russian one, too, called Glonass - although it isn't as good. China and the European Union have their own well advanced projects, called Beidou and Galileo respectively. Japan and India are working on systems too. These alternative satellites might help us ride out problems specific to GPS - but they might also make tempting military targets in any future conflict, and you can imagine a space war knocking everyone offline. A big enough solar storm could also do the job.