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:
- Trip distance
- Distance to destination
- Sunrise and sunset times
- And more
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.