How GPS Satellite Navigation Works?
Let’s Get Lost is the title of a 1940s jazz song, famously recorded by singer and trumpeter Chet Baker. Back then, getting lost was not just a romantic idea but still a realistic one. Today, it’s almost impossible to get lost, no matter how hard you try. Whether you’re haring down the freeway or scrabbling up Mount Everest, you’re always in sight of satellites spinning through space that can tell you exactly where you are. Walking round with a smartphone in your pocket, you’ll have ready access to a GPS (Global Positioning System) receiver that can pinpoint your position, on a good day, to just a few meters. Take a wrong turn in your car, and a determined voice—also powered by GPS—will insist you “Take the next left,” “Turn right,” or “Go straight ahead” until you’re confidently back on track. Even riding on a bus or train, it’s barely possible to get off in the wrong place. Handy display boards scroll the name of the stop you want long before you need to rise from your seat. Apart from helping us reach our destination, satellite navigation can do all kinds of other things, from tracking parcels and growing crops to finding lost children and guiding the blind. But how exactly does GPS satellite navigation work? Let’s take a closer look!
What is Satellite Navigation?
Satellite navigation (“satnav”) means using a portable radio receiver to pick up speed-of-light signals from orbiting satellites (sometimes technically referred to as space vehicles or SVs) so you can figure out your position, speed, and local time. It’s generally much more accurate than other forms of navigation, which have to contend with pesky problems like accurate timekeeping and bad weather. Because it’s a broadcast system based on radio signals that reach all parts of our planet, any number of people can use it at once, anywhere they happen to be.
The best-known satnav system, the Navstar Global Positioning System (GPS), uses about 24 active satellites (including backups). Day and night, 365 days a year, they whiz round Earth once every 12 hours on orbital planes inclined at 55 degrees to the equator. Wherever you are, you’re usually in sight of at least half a dozen of them, but you need signals from only three or four to determine your position to an accuracy of just a few meters.
GPS was kick-started by the US military in 1973 and its original satellites were designed to last about 7.5 years, but the latest generation are expected to survive twice as long. In total, around 60 Navstar satellites have been launched altogether, in three distinct generations and several separate groups called blocks, though many of them have now retired. At the time of writing, the last Navstar launch (the first satellite of a third-generation design) was satellite GPSIII SV01 on December 23, 2018.
GPS has three major components, technically known as “segments”: there’s one part in space, one part on the ground, and one part in your pocket. The 24 satellites form what’s known as the “space segment” of GPS, but the system also relies on an intricate ground-control network of antennas, monitors, and control stations (the “control segment”), centered on a Master Control Station (MCS) at Shriever Air Force Base in Colorado, USA (with a backup at Vandenberg Air Force Base in California). Apart from the space and control segments, the other essential part of satellite navigation is the “user segment”—an electronic receiver you hold in your hand or carry in your vehicle.
Finding your position using GPS satellite navigation signals is a hi-tech version of an age-old navigator’s trick that goes by the name triangulation. Suppose you’re walking through the woods, on completely flat ground, but you don’t know where you are. If you can see a landmark through the trees (maybe a distant hill), and you can guess how far away it is, you can look at a map and figure out that you must be somewhere on a circle whose radius (distance from the hill) is the distance you’ve guessed. One landmark alone can’t narrow your position any more than this. But what if you suddenly see a second landmark in another direction. Now you can repeat the process: you must be a certain distance from that object too, somewhere on a second circle. Put these two bits of information together and you know you must be somewhere where the two circles meet—one of either two places on the ground. With a third landmark, you can narrow your position to a single point. And that’s the essence of simple triangulation (you’ll find a longer introduction at Compass Dude). Triangulation works with line-of-sight and a bit of guesswork, with a compass and a map, and with fancier methods like radio signals, and radar. And it also works, in a more sophisticated way, using space satellites.
Sir Francis Drake (c.1540–1596) was the second person to circumnavigate the globe, finishing in 1580. The first was Ferdinand Magellan (1480–1521), a Portugese explorer who sailed the globe with great skill and braveness. Between 1519 and 1521, Magellan and his crew became the first to circumnavigate the planet, proving that the “flat Earth” was, in fact, more or less spherical. It’s tempting to imagine how much easier Magellan’s life would have been with satellite navigation, but that gets the logic of things the wrong way round. Without Magellan’s insight, we wouldn’t have satellite navigation technology at all: to build it and get it working, we had to know that we lived on a round Earth to begin with!
With GPS satellite navigation, your navigational “landmarks” are space satellites whizzing through the sky above your head. Because they’re about 20,000km (12,600 miles) away, well beyond Earth’s atmosphere, and because they’re constantly moving (not stationary, like Earth-bound landmarks), finding your position from them is a bit more tricky. If you pick up a signal from one satellite and you know it’s 20,000km away, you must be somewhere on a sphere (not a circle) of radius 20,000km, centered on that satellite. With two signals, from two different satellites, you must be somewhere where two spheres meet (somewhere in a circle of overlap). Three signals puts you at one of two points on that circle—and that’s usually enough to figure out where you are, because one of the points might be up in the air or in the middle of the ocean. But with four signals, you know your position precisely. Finding your location this way is called trilateration.
How do satnavs calculate distance from time?
Suppose you’re carrying a GPS-enabled cellphone or satnav in your car. How does it know the exact distance to the three or four satellites it uses to compute your position? Every satellite constantly beams out signals that are, in effect, time-stamped records of its position at that time. Since they’re carried by radio waves, the signals must be traveling at the speed of light (300,000km or 186,000 miles per second). Theoretically, then, if a receiver picks up the signals some time later, and has a clock of its own, it knows how long the signals have taken to get from the satellite, and how far they’ve traveled (because distance = speed × time). That sounds like a nice, simple solution, but it introduces two further problems.
First, how long does the signal take to travel? Haven’t we just swapped one problem for another (time for distance)? The solution to this involves a hi-tech version of “synchronizing watches”: each satellite carries four extremely precise atomic clocks (two cesium and two rubidium, typically accurate to something like one second in 100,000 years), while the receivers (which have less accurate clocks of their own) receive their signals and compensate for the time it takes for them to travel down from space. That means each receiver can figure out how long each signal has taken to reach it and therefore how far it’s traveled.
Second, although radio waves do indeed travel at the speed of light, they only do so in a vacuum (in completely empty space). Radio signals beaming down to us from space satellites aren’t traveling through empty space but through Earth’s atmosphere, including the ionosphere (the upper region of Earth’s atmosphere, containing charged particles, which help radio waves to travel) and the troposphere (the turbulent, uncharged region of the atmosphere, where weather happens, which extends about 50km or 30 miles above Earth’s surface). The ionosphere and troposphere distort and delay satellite signals in quite complex ways, for quite different reasons that we won’t go into here, and GPS receivers have to compensate to ensure they can make accurate measurements of distance.
Are Military and Civilian GPS any Different?
Photo: Satellite-guided missiles and drones use the military-grade PPS version of GPS, which is theoretically more accurate than civilian GPS. Photo by Nicholas Messina courtesy of US Navy.
GPS was originally conceived as a military invention that would give US forces an advantage over other nations, but its inventors soon realized the system would be just as useful to civilians. The only trouble was, if civilians (or rival forces) could pick up the same signals, where would that leave their military advantage? For that reason, they developed two different “flavors” of GPS: a highly accurate military-grade, known as Precise Positioning Service (PPS), and a somewhat degraded civilian version called Standard Positioning Service (SPS). While PPS-enabled receivers could originally locate things to an accuracy of about 22m meters (72ft), SPS receivers were deliberately made about five times less accurate (to within the length of a football field, or about 100m) using a tweak called Selective Availability (SA). That was switched off by order of US President Bill Clinton in May 2000, greatly improving accuracy for civilian users, which is largely why GPS has taken off so readily ever since. Even civilian SPS receivers are now officially accurate to within “13 meters (95 percent) horizontally and 22 meters (95 percent) vertically”, though a variety of different errors (caused by the atmosphere, obstructions blocking line of sight to satellites, signal reflections, atmospheric delays, and so on) can compound to make them very much less accurate at times.
Theoretically, military and civilian GPS satellite navigation systems could be as accurate as one another if we didn’t have to worry about them traveling through Earth’s atmosphere. According to the official website GPS.gov: “The accuracy of the GPS signal in space is actually the same for both the civilian GPS service (SPS) and the military GPS service (PPS).” In practice, while SPS signals are broadcast using only one frequency, PPS uses two. Comparing the two frequencies allows military grade GPS receivers to calculate precise corrections for radio delays and distortions caused by transmission through the atmosphere, and that still gives military GPS an edge over civilian systems. In time, civilian GPS will become increasingly accurate, especially as more satellites (and more different satellite systems) are added, but it’s likely that military systems will always have an advantage, for one reason or another.
GPS Satellite Signals
Navstar satellites constantly broadcast the two different flavors of GPS, PPS and SPS, on two different radio frequencies (carrier waves) known as L1 (1575.42MHz) and L2 (1227.6MHz). L1 carries the civilian SPS code signal (also known as the C/A code or Coarse Acquisition code), which is relatively short and broadcast about 1000 times a second, and what’s known as the navigation data message, which includes the date and time, satellite orbit details, and other essential data. L2 carries the military PPS code, also known as P-code (Precision code), which is very long and precise and takes an entire week to transmit. It’s encrypted to form what’s known as the Y-code, partly so that only authorized users can access it, and partly (because encryption is a form of signing things to confirm they’re authentic) to help prevent things like “spoofing” (where third parties broadcast fake, disruptive signals purporting to be from GPS satellites). Military-grade GPS receivers pick up both frequencies, and compare them to correct for the effects of the ionosphere. Civilian receivers pick up only one frequency and have to use mathematical models to correct for the ionosphere instead.
Applications of GPS Satellite Navigation
Most of us use GPS satellite navigation for driving to places we’ve never been before—but that’s a relatively trivial application. Once you can pinpoint your precise position on Earth, much more interesting things become possible. Roll time forward a few decades to the point where all cars have onboard satnav and can drive themselves automatically. Theoretically, if a car knows where it is at all times, and can transmit that information to some sort of centralized monitoring system, we could solve problems like urban congestion, finding parking places, and even auto theft at a stroke. If every car knows its location, and knows where nearby cars are too, highway driving could become both faster and safer; it will no longer rely on the vigilance of error-prone human drivers, too easily confused by tiredness and bad weather, so cars will be able to travel at much higher densities. The same goes for airplanes, where GPS is finally set to become an integral part of air traffic control—gradually reducing our historic overdependence on radar—over the next decade.
And it’s not just cars and planes that will benefit from pinpoint precision. For emergency services and search and rescue workers, navigating to remote, sometimes uncharted locations, in a hurry, makes all the difference between life and death. Farmers have been using GPS systems in tractors, combines, and crop-dusters to map, plant, manage, and harvest their crops with efficiency and precision. According to an industry body called the GPS Alliance, high-precision satellite navigation boosted US crop yields by almost $20 billion from 2007 to 2010 and is now used in 95 percent of crop dusting. Meanwhile, farm animals, pets, and rare wildlife are easier than ever to track using GPS-enabled collars and backpacks. Blind people, traditionally guided by seeing-eye dogs or the elbows of friends and family, can finally gain true independence equipped with talking handheld GPS satellite navigation systems, such as Trekker Breeze, that can announce street names or read spoken directions from A to B. Needless to say, a system conceived by the military still enjoys many military applications, from guiding so-called “smart bombs” to their targets with pinpoint accuracy to helping troops navigate through unfamiliar terrain. GPS is as standard a part of modern military equipment as maps and compasses were 100 years ago.
Rival Satellite Navigation Systems
In the United States, GPS is universally used as a synonym for any and every kind of satellite navigation; in other countries, such as the UK, “satnav” is a more familiar generic term. In fact, GPS is only one of several global satnav systems. The Soviet Union launched a rival system called GLONASS in 1982 (also using 24 satellites) and Russia continues to operate it today. Europe has been slowly building its own, more accurate 30-satellite system called Galileo, which is expected to be completed around 2020, and China is developing a global system known as Compass. The preferred umbrella term for world-spanning satnav systems is GNSS (Global Navigation Satellite Systems). Apart from the four big global systems, there are also a few smaller regional rivals, including China’s BeiDou and India’s IRNSS.
Although a given satellite receiver is typically designed to use only one of the global systems, there’s no reason why it can’t use signals from two or more at once. Theoretically, combining signals from GPS, GLONASS, and Galileo could give satnav devices something like a 10-fold increase in precision, especially in urban areas where tall buildings can block or distort signals, reducing the accuracy of any one system used alone. Using multiple systems also promises to make satellite navigation much faster: if more satellites are “in view,” the so-called Time-to-First-Fix (TTFF)—the initial delay before your satnav locks onto satellites, downloads the data it needs, and is ready to start calculating your position—is reduced. Since TTFF typically varies from about 30 seconds to several minutes, it makes a big difference to casual GPS users (and is one of the first features people compare when they look at buying a new satnav receiver).
Challenges and Issues
Knowing the absolute position of anything, anytime, anywhere brings obvious benefits in a globalized world that relies on swift, safe, and reliable transportation. But it raises issues too. If civilian transportation systems are designed to rely on satellite systems provided by the US or Russian military, doesn’t that make us too vulnerable to the sudden twists of international politics, especially in times of war? Although the US military no longer routinely degrades the quality of GPS signals, and announced in September 2007 that it would be removing Selective Availability altogether from future versions of GPS satellites, currently it can still nobble the system anytime it pleases. Could a future world of driverless cars, hyper-efficient parcel shipping, and automated air-traffic control be plunged into chaos purely at the whim of the superpowers? The European Galileo project is entirely a civilian system, which should eliminate possible military interference in time. But for the moment, it remains a concern.
Fast-disappearing privacy is the flipside of the same coin. If your car and your cellphone are both equipped with satnav, and you’re always using one or the other (or both), your movements can be tracked at all times. That raises obvious privacy issues, especially in repressive states. But every new technology brings its pros and cons, from internal combustion engines to submachine guns, and nuclear power plants to antibiotics. Progress involves making a tradeoff between benefits and costs, in the hope of doing things better than we ever could before. Satellite navigation is no different, swapping safe and unreliable navigation for efficient and effective transportation, albeit at a cost in privacy and (for the time being) continued dependence on military infrastructure.
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