But even when maps go awry, we tend to take the technology for granted. Here’s an explanation of how GPS navigation works — so you can appreciate it a little more the next time it gets you where you need to be.
We refer to our mapping ability as “GPS,” but the Global Positioning System is actually the term for a constellation of satellites — 24 of them — that orbit the earth. The first GPS satellite was launched in 1978, and the full constellation of 24 came to be in December 1993, and became operational in 1995. The system cost $10-12 billion to build, and the yearly costs of upkeep were estimated at $400 million. Each satellite weighs approximately 2,000 pounds, and they’re solar powered, built to last approximately 10 years. Each satellite takes 11 hours and 58 minutes to orbit the earth, meaning each satellite makes two orbits per day, at an altitude of 10,600 miles above earth.
The satellites were originally intended for use by the U.S. military to deploy weapons — the system was known as NAVSTAR. But in 2000, U.S. President Bill Clinton opened the GPS system to the world at large, citing its global utility. Clearly, the system’s utility has been proven time and again.
The satellites are positioned so that there are at least four, but up to 12, satellites visible to your GPS-enabled device at any given time — the satellite positioning and orbital routes is known as a “birdcage” (see above). Your GPS device can tell you your precise location your position in a process called trilateration. It communicates with three satellites in sight — using high-frequency, low-power radio signals that travel at the speed of light — and then calculates the distance between those satellites and your device. Since the satellites have a fixed orbital pattern and are synced with atomic clocks from the U.S. Naval Observatory, this process tends to yield an accurate location reading. But to improve accuracy, GPS devices typically seek data from four or more satellites, especially for determining altitude.
Once your GPS knows where you are, it can determine map routes, speeds, and other location-specific information, such as sunrise and sunset. GPS receivers log locations of satellites in an almanac, and though the orbits can be affected by the moon and sun, the United States Department of Defense monitors the satellites’ exact positions and sends updates to GPS receivers.
Knowing your location, speed and altitude — and knowing someone else’s — is an immensely powerful tool, with implications far beyond military use. Perhaps its most pedestrian use, at this point, is helping people navigate journeys, whether it’s by foot, by car or by boat. But GPS is also incredibly important for aircraft pilots, who use it to avoid mid-flight collisions and to land.
GPS navigation is now offered in a bevy of apps, and city governments are using technology and sharing data in an effort to ameliorate traffic problems in urban environs. Plus, there are third-party companies — Inrix, whose Smart Driver Network aggregates traffic information from millions of GPS-enabled road sensors, devices and vehicles (it has partnered with Ford), TomTom, which has sold 65 million personal navigation devices and uses more than 100 million probes to source traffic data, and Waze, the crowdsourced app that offers route options based on data transmitted by the app’s 20+ million users to help drivers “outsmart traffic” — that offer real-time traffic information, using your GPS positioning. Without these tools, many of us would be stuck in bumper-to-bumper traffic on I-95, not realizing what other routes are available.
Why Your GPS Is Wrong Sometimes
While GPS provides us with great utility, it’s not perfect. GPS determines location by figuring out how long it takes for the signal to reach your device, since it’s traveling at a constant speed, 186,000 miles per second. So if you’re wondering why your GPS is sometimes telling you you’re a few blocks from where you actually are, there are a few explanations. First, the signal can get delayed as it passes through various densities in atmosphere. Second, the radio signal can bounce off large buildings or have a hard time penetrating dense foliage, thus delaying the arrival of the signal in your device (and it likely won’t even work if you’re underground).
Third, if the clock in the receiver is off — it’s supposed to be synced with the atomic clocks, but there could be slight variability — then the distance perceived by your receiver could be wrong. Above, we said that receivers typically look to communicate with four or more satellites for maximum accuracy; so if your device can only communicate with three of them, your “location” could be a little off. Satellite positioning also plays a role — trilateration accuracy can be affected if the satellites are close together or in a line.