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WHAT IS GPS?

The Global Positioning System (GPS) is a U.S. government satellite-based navigation system that currently consists of at least 24 operational satellites. GPS works in any weather conditions, anywhere in the world, 24 hours a day, with no subscription fees or setup charges. The U.S. Department of Defense (USDOD) originally put the satellites into orbit for military use, but they were made available for civilian use in the 1980s.

HOW GPS WORKS

GPS satellites circle the Earth twice a day in a precise orbit. Each satellite transmits a unique signal and orbital parameters that allow GPS devices to decode and compute the precise location of the satellite. GPS receivers use this information and trilateration to calculate a user's exact location. Essentially, the GPS receiver measures the distance to each satellite by the amount of time it takes to receive a transmitted signal. With distance measurements from a few more satellites, the receiver can determine a user's position and display it electronically to measure your running route, map a golf course, find a way home or adventure anywhere.

Today, GPS is built in to all types of devices, such as smartwatches, satellite communicators, automobiles, boats and more. To calculate your 2D position (latitude and longitude) and track movement, a GPS receiver must be locked onto the signal of at least three satellites. With four or more satellites in view, the receiver can determine your 3D position (latitude, longitude and altitude). Generally, a GPS receiver will track eight or more satellites, but that depends on the time of day and where you are on the Earth. Some devices can do all of that from your wrist.

Once your position has been determined, the GPS unit can calculate other information, such as:

  • Speed
  • Bearing
  • Track
  • Trip distance
  • Distance to destination
  • Sunrise and sunset times
  • And more
Garmin smartwatches

HOW ACCURATE IS GPS?

Today's GPS receivers are extremely accurate, thanks to their parallel multichannel designs. Our receivers are quick to lock onto satellites when first turned on. They maintain tracking locks in dense tree cover or in urban settings with tall buildings. Certain atmospheric factors and other error sources can affect the accuracy of GPS receivers. Garmin GPS receivers are typically accurate to within 10 meters. Accuracy is even better on the water because there are no obstructions to interfere with the signal.

A Garmin GPS receiver’s accuracy is improved when using the Wide Area Augmentation System (WAAS). This capability can improve accuracy to better than 3 meters by providing corrections to the atmosphere and satellite positions. No additional equipment or fees are required to take advantage of WAAS satellites.

OTHER GPS SYSTEMS

There are other systems similar to GPS in the world, which are all classified as global navigation satellite systems (GNSS). GNSS is a generic term referring to all satellite navigation systems. Most Garmin receivers track GPS, GLONASS and Galileo, and some regional variations even track BeiDou and QZSS. These are sometimes referred to as multiconstellation receivers since they track and utilize multiple satellite constellations. Generally, you can expect a more reliable solution when you track more satellites. You could be tracking nearly 20 or 30 satellites with newer Garmin products.

Hikers on a mountain near the water

THE GPS SATELLITE SYSTEM

The 31 satellites that currently make up the GPS space segment are orbiting the Earth about 12,000 miles above us. These satellites are constantly moving, making two complete orbits in less than 24 hours. They travel at speeds of roughly 7,000 miles per hour. Small rocket boosters keep each satellite flying on the correct path.

Here are some other interesting facts about the GPS satellites:

  • The official USDOD name for GPS is NAVSTAR.
  • The first GPS satellite was launched in 1978.
  • A full constellation of 24 satellites was achieved in 1994.
  • Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
  • A GPS satellite weighs approximately 2,000 pounds and is about 17' across with the solar panels extended.
  • GPS satellites are powered by solar energy, but they have backup batteries on board in case of a solar eclipse.
  • Transmitter power is only 50 watts or less.
  • GPS is currently deploying Block III, it's third-generation satellite.

WHAT'S THE SIGNAL

GPS satellite signals are incredibly weak by the time they reach the Earth’s surface. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects, such as buildings and mountains. However, modern receivers are more sensitive and can usually track through houses.

In recent years, GPS has added an additional frequency, named L5, to satellites being launched. This advanced signal has more power and better tracking characteristics over the original L1 signal. Recent Garmin GPS receivers are now using L5 to improve accuracy and reliability. This multi-band solution (i.e., both L1 and L5) provides improvements under trees or in urban canyons.

A GPS signal contains three different types of information:

  • Pseudorandom code is an I.D. code that identifies which satellite is transmitting information. You can see which satellites you are getting signals from on your device's satellite page.
  • Ephemeris data is needed to determine a satellite's position and gives important information about the health of a satellite, current date and time.
  • Almanac data tells the GPS receiver approximately where each GPS satellite should be at any time throughout several months and shows the orbital information for that satellite and every other satellite in the system.

GPS SIGNAL ERROR SOURCES

Factors that can affect GPS signal and accuracy include the following:

  • Ionosphere and troposphere delays: Satellite signals slow as they pass through the atmosphere. The GPS system uses a built-in model to partially correct for this type of error.
  • Signal multipath: The GPS signal may reflect off objects, such as tall buildings or large rock surfaces, before it reaches the receiver, which will increase the travel time of the signal and cause errors. The L5 signal improves the receiver’s ability to sort out which are reflections and which are line of sight.
  • Receiver clock errors: A receiver's built-in clock may have slight timing errors because it is less accurate than the atomic clocks on GPS satellites.
  • Orbital errors: The satellite's reported location may not be accurate.
  • Number of satellites visible: The more satellites a GPS receiver can "see," the better the accuracy. When a signal is blocked, you may get position errors or possibly no position reading at all. GPS units typically will not work underwater or underground, but high-sensitivity receivers can track some signals when inside buildings or under tree cover.
  • Satellite geometry/shading: Satellite signals are more effective when satellites are located at wide angles relative to each other, rather than in a line or tight grouping. This is generally why altitude isn’t as accurate as horizontal position.
  • Selective Availability (SA): The USDOD once applied SA to satellites, making signals less accurate in order to keep "enemies" from using highly accurate GPS signals. The government turned off SA in May 2000, which improved the accuracy of civilian GPS receivers.