t seems like a mystery. Everything knows exactly where it is in the world: our phones, watches, navigation systems, heck, even my dog’s collar knows its location. We know satellites are involved, but how can satellites tell me how to get to a campsite I’ve never been to?
We’re going to explain what GPS is, how it works, who its competitors are and how to get as accurate as possible when you use it to track yourself. Let’s solve this mystery.
What Is GPS?
The Global Positioning System (GPS) is, at its most basic, a bunch of satellites owned and maintained by the U.S. government to provide positioning, navigation and timing services, around the world, for free. (Well, nothing in life is free. It just doesn’t cost you anything to use and is paid for by the U.S. taxpayers. You’re welcome.)
There are 27 GPS satellites currently in use, plus four spare satellites also zipping along up there. You probably don’t care that they’re in medium Earth orbit (MEO), 12,550 miles up, and that each satellite circles Earth twice a day.
What you do care about is that these 27 MEO satellites mean that you can get a signal from at least four satellites at all times, just about everywhere on the planet. Everywhere.
So How Does GPS Work?
Most people don’t stop to think about how Google Maps knows where they are. Maybe your phone is talking to the satellites somehow? Maybe the satellites track the GPS chips in our devices? Maybe...ahh, who cares, as long as Google can get me to Aunt Edna’s for Thanksgiving dinner.
But the way GPS works is the most straightforward way you can imagine, which makes it one of the cleverest innovations of all time. Basically, each GPS satellite broadcasts a radio signal that includes the time and its location in space. You receive each satellite’s signal at a different time, so combining that difference (plus a little bit of geometry) with the time and location in the signal tells you where you are. This approach of transmitting a constant signal from the satellites and putting the math on our GPS devices means there’s never a capacity issue with the satellites and receivers can be cheap and ubiquitous.
Let’s use an analogy.
Say you have three friends who can all throw a baseball at the exact same velocity. The four of you are REALLY bored so you find an empty baseball field and have them stand on first, second and third bases. You then roam about the field, periodically telling them to throw baseballs at you at the same time.
Since you know their exact locations and they all throw the ball at the exact same time and at the same speed, unless you’re right in the middle of all three, the baseballs will hit you at different times, so you note the time you caught each one (I did say you were really bored).
You know where each ball started, when they were thrown, how fast they’re going and the time differences of when each hit you. Based on this, you can calculate how far you are from each thrower, which will give you your location. If they all hit you at the same time, you’re a few feet from the pitcher's mound, dead center of your buds. If you’re way off in left field, there’s going to be a big time difference between the first-base ball and that third-base throw.
It’s called trilateration and this is what your phone, handheld GPS, dog collar, etc. is doing. (Not to be confused with triangulation. Triangulation is based on calculating angles while trilateration uses distances.)
Each GPS satellite has its own super-accurate atomic clock that is kept synchronized with the other satellites’ clocks and those in ground stations. Each satellite is just going about its monotonous business, 12,550 miles up, constantly broadcasting the current time of that internal clock, along with its location in space. When your GPS device receives these broadcasts, it knows the same data you do in our baseball example:
- Where each satellite is in space
- When the signal was sent
- How fast that signal was going (it's a radio signal, so speed of light)
- The time difference between when each satellite's signal was received
Some quick math tells you how far you are from each of those satellites and Google Maps shows you that you’re standing in front of Starbucks. It’s a little trickier than our baseball analogy since it’s 3D versus 2D, but it works the same. The signals from three satellites will tell you where you are on the planet at sea level and a fourth will give you altitude and better accuracy.
How Does My GPS Device Receive the Signal?
Remember that the GPS broadcast is just a radio signal, a relatively weak one. Say you’re in your car, jonesing for some Oldies, so you tune the radio to FM 104.5. You’re actually controlling a radio wave receiver, tuning it to pick up whatever is being broadcast at 104.5 MHz, which in this case is some Otis Redding (sweet!).
The GPS satellites broadcast at a few different frequencies, but if your car radio went up to 1227.60 MHz, you’d be able to listen to the GPS signals! (We prefer Otis.)
You’d think you need some crazy-complex antenna to pick up satellite transmissions, but as with so many modern innovations, we can thank the smartphone revolution for making GPS radio receivers so small and inexpensive that you can now buy some off Alibaba for a couple bucks.
Total sidebar, but it is mind blowing how small these radio receivers have gotten. I mean, you can buy a Garmin fēnix 6 that not only has GPS but also Galileo, GLONASS, Wi-Fi and Bluetooth. In a watch, for cryin’ out loud!
How Accurate Does GPS Track and What Is the L5 Band?
Accuracy depends on how old your GPS receiver is, where you are on the planet and whether your receiver can augment the satellites’ signals. Not to mention if signals are blocked or reflected by buildings, bridges, what have you.
Prior to 2000, GPS signals were intentionally degraded for civilian use and the tracking accuracy was pretty bad. This was called “selective availability” and President Clinton signed a law in May of 2000 to end this practice. At that point, GPS accuracy was about 16 feet (5 m).
Without going too far down the rabbit hole, The U.S. government has been steadily upgrading both the GPS satellites and their capabilities, including broadcasting on multiple frequencies.
The 1227.60 MHz mentioned above is in what’s called the L2 band of frequencies (don’t ask why, it doesn’t matter for our purposes). The U.S. launched the first GPS satellite with an L5 transmitter in 2010 and started broadcasting on it for civil navigation in 2014. L5 is another band of frequencies and some GPS satellites broadcast a signal at 1176.45 MHz, which is in that band. This L5 transmission is higher power and has greater bandwidth than L2-band frequencies, which yields better accuracy and more resistance to jamming.
If you have a newer (2018+) GPS receiver, it may be able to receive signals in the L5 band. If so, you could see accuracy better than 12 inches (30 cm)! Unfortunately, not many devices have L5 GPS radios yet, but we have this to look forward to. (The other GNSS systems (explained below) are also expanding into the L5 band.)
Another way to improve your positioning accuracy is to augment the GPS signal, most typically accomplished with Wide Area Augmentation System (WAAS), another U.S. government system. WAAS is a network of ground-based stations in North America and Hawaii that measure GPS signals, apply some corrections and then send them up to seven WAAS satellites over North America. WAAS-enabled GPS receivers are pretty common and will see accuracy of about 8-10 feet (2.5-3 m).
You’ll also hear Assisted GPS (A-GPS) mentioned. These are devices that can download data to help figure out GPS satellite locations faster or more accurately than just from the satellites’ signals alone. This can help with satellite acquisition, both indoors and out.
An example would be starting up your GPS handheld in downtown Chicago. GPS signals are going to be bouncing off buildings all over the place, so satellites will seem farther away when that happens. A standalone GPS can work through that confusion and improve on its initial accuracy, but it could take 10+ minutes. A phone that is pulling GPS signal data from a nearby cell tower or the Internet can get cranking much quicker.
Finally, Differential GPS (DGPS) is a technology that can give you insane tracking accuracy, as tight as 0.39 inches (1 cm), but it’s almost impossible you’ll ever use it on an adventure. The reason is because DGPS works by having a fixed-location base station pull in the GPS signals itself, calculate the difference between what it knows to be its own very accurate location and what the GPS calculations are saying its location is. It broadcasts this differential to a receiving unit, which uses these corrections to improve the accuracy of the GPS signals it's getting.
It’s sort of like how WAAS works, but these are smaller receivers, transmitting their corrections locally. The technology is most frequently used in surveying. A surveyor sets up a base unit, the location for which he determines quite accurately using standard surveying techniques. Then, when he walks around the land he’s surveying, the base station’s broadcasted corrections are put to use by his carry-around GPS receiver to pinpoint exact locations.
What Are Galileo, GLONASS and BeiDou?
Just like “Kleenex” has become the generic term for “tissue,” people use “GPS” to refer to any satellite-based navigation system. But GPS is just one example of what’s called a Global Navigation Satellite System (GNSS). To launch and maintain a truly global system takes tremendous resources, which is why all GNSSs are government owned, with four currently at a global scale:
- Global Positioning System (GPS) - United States
- Galileo - European Union
- GLONASS - Russia
- BeiDou (BDS) - China
Galileo, GLONASS and BeiDou operate similarly to GPS, although not necessarily on the same frequencies. As of January 2021, the numbers of active satellites and the systems’ accuracies are:
- GPS: 31 satellites with 27 in use; accuracy of 8-10 feet (2.5-3 m), with L5-band accuracy less than 1 foot (30 cm)
- Galileo: 26 satellites with 22 in use; optimal unencrypted accuracy of less than 3 feet (1 m)
- GLONASS: 27 satellites with 24 in use; optimal accuracy is about 9 feet (2.8 m)
- BeiDou: 35 satellites with 30 providing coverage (5 are for compatibility with their older system); free-service (as opposed to military) accuracy is about 3 feet (1 m)
There are even separate augmentation systems. Like WAAS for GPS in North America, the European Union built the European Geostationary Navigation Overlay Service (EGNOS) to augment GPS in Europe, before they built Galileo.
There are also a couple regional satellite systems:
- India Regional Navigational Satellite System (IRNSS) - India, with plans to grow it to global coverage in the future
- Quasi-Zenith Satellite System (QZSS) - four Japanese satellites augmenting GPS satellites over Japan and Asia-Oceania region (scheduled for navigation separate from GPS in 2023)
What to Look for in a GPS Device
We’re using “GPS” as the generic term for a GNSS device since anything you buy will at least have a GPS receiver in it.
First, look for a unit that has multiple GNSSs since the more systems your phone or device can receive, the better it can accurately calculate your location and track your speed. Debating which GNSS is superior is great fun for some folks, but it does depend on where you are on the planet, if you’re downtown vs an open field, how good the chip is in your device, and on and on. Suffice to say that ensuring you can receive multiple GNSSs (along with WAAS, EGNOS, etc. to help the GPS signal) is a great way to improve your accuracy.
But the best way to optimize the accuracy is to get a device that can receive signals in the L5 band from the GNSS satellites. There are only 10 GPS satellites currently capable of transmitting the L5 signal, but there will be a full 24 by 2027, and that L5 signal will give you better than 1 foot (30 cm) accuracy. Also, the other GNSSs are deploying L5-band satellites, so their accuracies will also improve.
You’ll find these marketed as multi-band/frequency or dual-band/frequency GPS receivers and phones. Here are some currently on the market:
- Garmin GPSMAP 65
- Garmin GPSMAP 65s
- Garmin GPSMAP 66sr
- Some Android phones, although it's rare, so look closely at the spec.
(If you want to keep track, Galileo has a nice list of compatible phones that stays pretty current and lists both single- and dual-frequency capabilities. If a phone or other GPS device supports dual-frequency Galileo, the odds are really good it supports dual-frequency GPS as well.)
Adventure Monkey's Closing Thoughts
Ultimately, it doesn’t matter how accurate your GPS (or, more accurately, GNSS) device is if you don’t know how to use it and get the most out of it.
Stay tuned to Adventure Monkey because we have a full slate of how-tos coming to help you navigate off road, get where you want to go, have an epic adventure and get home safely.