How does GPS work?

The Global Positioning System, or GPS, is defined as a constellation of satellites operated by the U.S. Space Force, that broadcast radio signals from orbit back to Earth to support highly accurate positioning, navigation and timing (PNT) measurements. It is available worldwide and provides critical positioning information to military, civilian and commercial users.

Let's take a closer look at how GPS works.

GPS hardware

First, let's start with the elements that make up the system.

Satellites are continually broadcasting their orbital position and exact time at that position. These signals are received by antennas and then processed in a GPS receiver to compute a user’s location. Like many other Global Navigation Satellite Systems (GNSS) constellations, GPS includes three main segments: the space segment, control segment and user segment.

Space segment

The GPS space segment includes over 30 satellites in orbit. These satellites broadcast radio signals to control and monitoring stations on Earth and directly to users requiring highly precise satellite positioning.

Control segment

The U.S. Space Force also oversees the GPS control segment. It includes master control and backup control stations, dedicated ground antennas and several monitor stations located worldwide. These stations work to ensure GPS satellites are healthy, orbiting in the correct locations and have accurate atomic clocks on board. These stations are integral to the overall health and accuracy of the GPS constellation.

User segment

The user segment includes everyone relying upon GPS satellites for PNT measurements. The user segment requires both an antenna and a receiver. From a mobile phone providing directions to autonomous vehicles requiring lane-level positioning accuracy; from a farmer tracking planting and harvesting routes year-over-year to a UAV mapping a rainforest, many applications use GPS for high precision positioning and accuracy around the world.

GPS signals

GNSS radio signals are quite complex. GNSS operates at frequencies that are higher than FM radio but lower than a microwave oven. GPS satellites transmit information on the L1, L2 and L5 radio frequencies.

GPS works the way it does because of the transmission scheme it uses, which is called code division multiple access (CDMA). CDMA is a form of spread spectrum (meaning the signal is deliberately spread in the frequency domain resulting in a signal with a wider bandwidth). GPS satellite signals, although they are on the same frequency, are modulated by a unique pseudorandom digital code (PRN code). Receivers know the PRN code for each satellite, because each satellite uses a different code. This allows receivers to correlate with the CDMA signal for a particular satellite. CDMA signals are received at a very low power level, but through this code correlation, the receiver is able to recover the signals and the information they contain.

GPS math

Now, let's take a look at the math behind how GPS works. GPS uses trilateration to determine user position, speed and elevation. Essentially, if you know the location of three satellites and your distance from them, you can determine your position. Additional satellites further enhance the precision of locating a user's position. Today's GPS receivers routinely track four to seven satellites simultaneously.

Conclusion

The determination of position is made quite a bit more complicated by several factors:

• Satellites are moving
• Signals from the satellites are very weak by the time they reach Earth
• Atmospheric conditions, like ionospheric activity, interfere with the transmission of radio signals
• Cost and sophistication of user equipment

If you would like to take a deep dive into understanding how Global Navigation Satellite Systems, like GPS, work, head on over to our resource library for An Introduction to GNSS.