While today GPS is one of the most commonly used technologies, a system used by millions worldwide for navigation and location sharing, few of us know its origins. In this text, we will try to provide info on GPS history, on the technology behind the service and will also talk a bit about current alternative services.
GPS History and Origins
GPS was born in the U.S army, as is the case with most other advanced technology. It is simple; although today we have powerful technology companies such as Facebook, Google, and Apple, who are ready to spend millions of dollars on developing new, still unproven technological advancements, back when GPS was just an idea (back in the 60's) companies weren’t as powerful and they didn’t have as much cash as today.
Back then, the US army was immensely powerful and had enough financial assets to invest in such colossal project. Because back then, launching satellites into orbit was immensely more expensive than today, with Space X’s reusable rockets and a much smaller size and weight of most satellites.
The basis of GPS system is the phenomenon known as Doppler Effect, “the change in frequency or wavelength of a wave for an observer who is moving relative to the wave source.” For instance, when you wait for a train at the station, an approaching train’s siren will have a subjectively higher pitch that a siren of a train leaving the station. In other words, if an object emits waves, they will have a higher frequency when the object is approaching, and lower wavelength when the said object is moving away from a random observer.
The idea about a global positioning system based on satellites orbiting the Earth started with the launch of the first human-made satellite, Sputnik. American scientists discovered they could track the Russian satellite by measuring the change in its radio signals, thanks to the Doppler Effect. This is how the idea was born.
The phenomena is the basis of the systems that preceded GPS and was used back in the 60’s to track satellites by measuring shifts in their radio signals (and radio signals are also waves). On the basis of satellite tracking, the U.S Navy constructed a satellite system capable of tracking nuclear missile-equipped submarines. The system, called TRANSIT, consisted out of six satellites in total, orbiting around Earth’s poles. Submarines were able to find out their location in minutes by measuring Doppler Effect of satellites. Instead of seconds, they needed minutes, sometimes hours to calculate the location.
The experiment proved successful, and beck in the 60’s it was an unparalleled navigation technology, much more advanced than anything else available, so the DoD (Department of Defense) got interested in it, and decided to finance the next project. The project was in development during 70’s with the first satellite launching in 1974. The new system was called NAVSTAR, and it was the true beginning of the GPS.
By 1985 there were 11 satellites orbiting the Earth, each carrying a highly precise atomic clock, ensuring for extremely accurate measurement of transmission times. The system was opened for use by civil sector in 1983 after the Russians shot down Korean passenger jet because the plane wandered off course and entered the soviet airspace. GPS was first used by aircrafts in order to fix their position, preventing them from straying from their routes (and preventing similar disasters like the shooting of the Korean jet) and improving their navigation.
The first proper GPS satellite – called Block II - was launched in 1989. During the next five years, the U.S military launched other 23 satellites, completing their first GPS network made out of 24 satellites in total. At the moment there are 31 satellites in total orbiting the Earth, which is the GPS system we know today. But, how it all works? Let’s find out.
Tech Behind GPS
The tech behind GPS is quite complex, but the basic principle is simple. The main part of the complex mechanism of the GPS is called trilateration. Yes, trilateration, not triangulation. The former is used for measuring distances, while triangulation is used for measuring angles.
So, each GPS satellite (and there are 31 of them in total) emits a signal at all times, and they are placed in a system that makes at least six of them visible from any point in the world at all times, but for calculating the exact location, only three are enough.
So, when you want to find out your location, you turn on the GPS (either on your phone, on a GPS navigation system, or on a GPS receiver) and it starts receiving signals from satellites. Each satellite, aside from GPS signal, transmits the exact time signal the signal has been sent. Then the GPS receiver subtracts time the signal was transmitted from the time the signal has been received, calculating the distance from that satellite.
Image Source: NASA
By combining three satellites, GPS receiver can calculate distance from each of them, but since it can’t calculate the angle, just the distance, receiver doesn’t have the exact location, just a distance from the satellite shown as a radius of a circle, with the exact location being anywhere on the circle radius. But, when you combine data from a second satellite, and then from third (this is the trilateration part), you can pinpoint the location as a point where three circles intersect each other. This gives the GPS receiver a two-dimensional position – north, and east.
Since the time passed between sending a signal from a satellite and receiving it on the GPS receiver is used to calculate distance from a satellite, getting signals from three satellites gives you a location but with a presumption that you are on mean seal level, because there’s a fourth dimension that’s pretty important, time. And without the precise time, you will just get a 2D fix on your location.
Since receivers aren’t equipped with atomic clocks (like GPS satellites are), they cannot know the precise time, thus preventing them from solving the equation and providing the time dimension for the location. So, a fourth satellite is needed for knowing the exact location (except if you’re on sea level if not your location won’t be accurate, with sometimes the margin of error being measured in hundreds of meters), because it provides the exact time from its atomic clock. Now, with the info from four satellites, the receiver is able to pinpoint your location in three dimensions – adding altitude to the north, and east.
So, that’s about it, but modern smartphones are equipped with a solution that gives them the power to calculate location in seconds (standalone GPS receivers can take minutes) called A-GPS (assisted GPS). It is simple – A-GPS uses information from cell towers, and data connection (3G or 4G) to quickly calculate location, without needing to get a fix from GPS satellites. But, when you’re out of cell tower coverage, the GPS receiver inside your smartphone will have to use standard technique of connecting to GPS satellites, thus taking much more time for fixing your position.
While GPS is the most popular global positioning system in the world, there are alternatives. Since GPS is owned by the US Government, only the US armed forces, government agencies and selected allied armed forces and agencies can use Precise Positioning Service (PPS), while civilians along with other countries’ armies are restricted to Standard Positioning Service (SPS).
You see, GPS satellites send data on two frequencies – L1 and L2. While SPS uses just L1 frequency, the PPS uses both of them, providing much more precise location data. That’s why some countries are working on (or already have) their own solutions. Let us check them out.
There are actually a lot of alternatives to GPS. As expected Russia has the most advanced positioning system (btw, GPS is a brand name, so every other similar tech has its own name) since, you know, if you have the second most powerful army in the world you probably don’t want to use technology from your biggest rival (although the US used Russian Baikonur Cosmodrome for launching their rockets, but that was before all this mess that escalated in the past couple of years). The same goes for China, another country that has pretty advanced GPS alternative. But let us start from the beginning.
GLONASS is a Russian global positioning solution. It consists out of twenty-nine satellites and is almost as accurate as GPS. Although Russian satellites take a bit lower orbit than those used for GPS, giving them less accuracy in theory, they are better suited for operating at high latitudes (and Russian territory is mostly located at high latitudes), since the satellite fleet is oriented at an angle of 65 degrees to Earth’s equator (compared to GPS’s 66 degrees inclination).
COMPASS is the name of China’s global positioning system. The fleet counts sixteen satellites (that hover over China and the Pacific regions), and when it becomes fully operational in 2020, it should have a full global coverage and a fleet made out of thirty five satellites.
Galileo is a project developed by the EU, and it is still in development. There are five satellites in orbit at the moment, but the plan is to build a thirty satellite fleet that will have a higher accuracy than GPS (one meter opposed to GPS’s fifteen meters) because Galileo satellites are placed on a higher orbit, allowing for better satellite lock for GPS receivers.
Other projects worth noting are French DORIS, a system similar to GPS that is used for research purposes; IRNSS, an Indian positioning system that covers India but won’t be available worldwide; and we have QZSS, a system developed by Japan used for better GPS accuracy over Japan territory, especially in the country’s huge cities.