GPS signals are faint radio transmissions broadcast by satellites roughly 20,200 kilometers above Earth, carrying precise timing data that a receiver on the ground uses to calculate its position. Each satellite sends a coded signal at the speed of light, and because radio waves slow down, bend, and bounce on their way through the atmosphere and the built environment, turning those signals into an accurate location fix is far more complicated than simply measuring distance. The physics involved ranges from Einstein’s relativity to the behavior of charged particles in the upper atmosphere, and the ways those signals can be disrupted, corrected, and even faked have real consequences for everything from your phone’s map app to power grids and financial networks.
How a GPS Signal Becomes a Location
A GPS satellite continuously broadcasts a signal that contains two essential pieces of information: the exact time the signal left the satellite (stamped by an onboard atomic clock) and the satellite’s precise orbital position at that moment. Your receiver picks up these signals from multiple satellites simultaneously and measures how long each signal took to arrive. Since the signals travel at the speed of light, that travel time translates directly into a distance. With distance measurements from at least four satellites, the receiver can solve for three spatial coordinates plus an adjustment for its own clock error, producing a latitude, longitude, and altitude.
The signals themselves are broadcast on several radio frequencies in the L-band, centered around 1.2 and 1.5 gigahertz. These frequencies were chosen because they pass through the atmosphere reasonably well and can be picked up by small, low-power receivers. Each satellite’s signal is extremely weak by the time it reaches the ground, typically far below the level of background radio noise, but the receiver uses the known code pattern to pull the signal out of the noise, somewhat like recognizing a familiar voice in a crowded room.
Why Relativity Is Not Optional
One of the less intuitive facts about GPS is that it would fail without corrections drawn from Einstein’s theories of relativity. The atomic clocks aboard GPS satellites experience two competing relativistic effects. Because the satellites move at roughly 14,000 kilometers per hour relative to the ground, special relativity causes their clocks to tick slightly slower than identical clocks on Earth’s surface. At the same time, because the satellites orbit high above Earth where gravity is weaker, general relativity causes their clocks to tick slightly faster. The gravitational effect wins out, and without any correction the satellite clocks would gain roughly 38 microseconds per day relative to ground clocks. That sounds trivial, but light travels about 11 kilometers in 38 microseconds, so the accumulated error would make civilian GPS useless within minutes.
These shifts are so large that, without carefully accounting for numerous relativistic effects, the system would not work at all.1PubMed Central. Relativity in the Global Positioning System The fix is built into the hardware before launch: each satellite’s clock is deliberately set to tick at a slightly different rate so that once in orbit, its output matches ground-based time. Additional software corrections handle smaller relativistic effects caused by the slight eccentricity of each satellite’s orbit. This interplay between fundamental physics and engineering is one of the more remarkable features of the system, and it means every time you check your location on your phone, you are relying on corrections predicted by general relativity more than a century ago.
What Happens to the Signal in the Atmosphere
Between the satellite and your receiver, GPS signals pass through two distinct layers of the atmosphere that each distort the signal in different ways.
The ionosphere, extending from about 60 to 1,000 kilometers above Earth’s surface, is filled with electrically charged particles created by solar radiation. When a GPS signal passes through this region, it slows down, which makes the satellite appear farther away than it actually is. The size of this ionospheric delay depends on how many charged particles the signal encounters, which in turn depends on the time of day, the season, the latitude, and the current level of solar activity. Dual-frequency receivers can largely cancel out ionospheric delay by comparing how the signal behaves at two different frequencies, since the charged particles affect each frequency by a different amount.2International Hydrographic Review. The implementation of dual frequency positioning for ionospheric effects correction Cheaper single-frequency receivers rely on mathematical models of the ionosphere that are broadcast as part of the GPS signal itself, but these models are less accurate, especially during geomagnetic storms.
Below the ionosphere, the troposphere introduces its own delays. Dry air, water vapor, rain, and even airborne particulates like dust and volcanic ash all slow GPS signals slightly. Water vapor is the trickiest of these because its distribution is highly variable and difficult to predict. Unlike ionospheric delay, tropospheric delay affects all frequencies equally, so the dual-frequency trick does not help. Instead, receivers use atmospheric models or real-time weather data to estimate and subtract the tropospheric contribution.3Journal of Geophysical Research: Atmospheres. Propagation delays induced in GPS signals by dry air, water vapor, hydrometeors, and other particulates
Multipath and Urban Signal Problems
In cities and other built-up environments, GPS signals bounce off buildings, vehicles, and pavement before reaching the receiver. This creates what engineers call multipath error: the receiver picks up the same signal via several different routes, some direct and some reflected, and the reflected copies arrive slightly later. Because the receiver calculates distance from arrival time, these delayed reflections bias the distance measurement and shift the computed position. Multipath is one of the most persistent challenges in GPS positioning, particularly for applications that need high accuracy in dense urban areas.4Alexandria Engineering Journal. Field procedure and data collection tool: Towards controlling GPS multipath error for mapping within urban semi-closed areas
Tall buildings also block satellites from view entirely, reducing the number of signals the receiver can use and weakening the geometry of the position fix. A receiver that can only see satellites clustered in one part of the sky produces a much less accurate position than one with satellites spread evenly overhead. This is why your phone’s GPS tends to wander when you are walking between skyscrapers or under highway overpasses. Modern phones partly compensate by combining GPS with Wi-Fi positioning, cell-tower triangulation, and motion sensors, but in deep urban canyons the raw satellite fix still suffers.
Augmentation Systems That Sharpen the Fix
Standard GPS on its own delivers horizontal accuracy of a few meters for a civilian receiver under decent conditions. For applications that need better, a family of augmentation systems exists to tighten the fix by broadcasting correction data in real time.
Space-based augmentation systems, or SBAS, use networks of ground monitoring stations to measure the actual errors in GPS signals at known locations. Those measured errors are sent to geostationary satellites, which rebroadcast them to receivers as correction data for satellite orbit, clock, and ionospheric delay errors.5Remote Sensing. SBAS-Aided GPS Positioning with an Extended Ionosphere Map at the Boundaries of WAAS Service Area In North America, the FAA operates the Wide Area Augmentation System (WAAS) for this purpose. Europe has EGNOS, Japan has MSAS, and India has GAGAN. These systems improve accuracy to roughly one to two meters and, crucially, provide integrity monitoring: they can warn a pilot or driver within seconds if the GPS signal becomes unreliable.
For even higher accuracy, real-time kinematic (RTK) systems use a nearby ground-based reference station whose exact position is known. The reference station compares the GPS position it calculates with its true position and broadcasts the difference as a correction to nearby receivers. RTK can achieve centimeter-level accuracy, which is why it is standard in surveying, precision agriculture, and construction machine guidance. The tradeoff is that you need a reference station within a few tens of kilometers, and you need a data link to receive its corrections.
Solar Storms and Space Weather
The ionosphere is not a fixed obstacle. Its density of charged particles swings with the solar cycle, and during major solar events it can change dramatically in minutes. Solar flares blast the sunlit side of Earth with X-rays and ultraviolet radiation, temporarily inflating the ionosphere and spiking the total electron content that GPS signals have to traverse. The September 2017 solar storms illustrated this vividly: during the strongest flares, total electron content jumped by up to roughly 19 percent in some equatorial regions, high-frequency radio communication blacked out for 30 to 90 minutes, and GPS positioning errors spiked as the ionospheric models embedded in the signals could not keep up with the rapid changes.6Space Weather. Multi‐Instrument Investigation of the Impact of the Space Weather Events of 6–10 September 2017
Geomagnetic storms, which follow a day or two after a coronal mass ejection, create a different set of problems. They stir up irregularities in the ionosphere, especially near the equator and the poles, causing GPS signals to scintillate, a rapid flickering in signal strength and phase that can cause a receiver to lose lock on a satellite altogether. For most everyday users, these events cause a temporary dip in accuracy that passes within hours. For aviation, where safety-of-life decisions depend on guaranteed positioning accuracy, space weather is a genuine operational concern monitored around the clock.
GPS Spoofing and Signal Security
Civilian GPS signals are unencrypted. Any receiver can pick them up, but that also means anyone with the right hardware can generate fake GPS signals that mimic the real ones. This is called spoofing, and it tricks a receiver into calculating a false position or time. The equipment to do this has become cheaper and more accessible over the past decade, and documented spoofing incidents range from shipping vessels being misdirected in contested waters to drones being forced off course.
Detecting spoofing is an active area of research. One promising approach uses crowd-sourced information from connected vehicles: by comparing what a vehicle’s GPS receiver reports against independent data from Wi-Fi access points and cell towers along its route, a system can flag inconsistencies that point to a spoofing attack. Experimental testing showed that such a system can detect an attack in about six seconds using Wi-Fi data, or about 30 seconds using cellular data, with a very low false-alarm rate.7Computer Networks. GPS spoofing detection via crowd-sourced information for connected vehicles Military GPS signals use encrypted codes that are much harder to spoof, but civilian infrastructure overwhelmingly relies on the open signal.
Jamming is a cruder threat. A small, cheap device that broadcasts noise on GPS frequencies can drown out the satellite signals for receivers nearby. Truck drivers in some countries have been caught using GPS jammers to evade fleet tracking, and those jammers have inadvertently disrupted airport navigation systems and emergency services when the trucks passed near sensitive facilities. Both spoofing and jamming highlight a vulnerability that is growing more important as more systems depend on GPS.
Beyond Position: GPS Timing and Critical Infrastructure
Most people think of GPS as a positioning system, but its timing function is arguably even more consequential. Every GPS satellite carries atomic clocks synchronized to within billionths of a second, and that precise time signal has become a backbone for infrastructure that has nothing to do with navigation. Telecommunications networks use GPS timing to synchronize cell towers and data transmissions. Power grids use it to timestamp measurements across the grid so operators can detect and locate faults. Financial exchanges use GPS-derived time to sequence trades and comply with regulations that require timestamps accurate to microseconds.
A report from the National Institute of Standards and Technology examined the dependencies of U.S. critical infrastructure on GPS timing across the financial, telecommunications, and electric power sectors, and found that GPS is the primary synchronization source for a broad range of systems in all three.8National Institute of Standards and Technology. An Evaluation of Dependencies of Critical Infrastructure Timing Systems on the Global Positioning System (GPS) The concern is not that GPS will suddenly vanish but that a disruption, whether from a solar storm, jamming, or spoofing, could cascade through systems that were never designed with a backup timing source. Some operators maintain local atomic clocks or use fiber-optic timing networks as holdover sources, but many do not, and the gap between how much infrastructure relies on GPS timing and how much of that infrastructure has a resilient backup is still significant.
Multiple Constellations and Interoperability
GPS is the original satellite navigation system, but it is no longer the only one. Russia operates GLONASS, the European Union operates Galileo, and China operates BeiDou. Each system uses its own set of satellites, frequencies, and signal structures, and modern receivers increasingly use signals from all four constellations simultaneously. More satellites in view means better geometry, which translates directly into better accuracy and reliability, especially in challenging environments like cities or mountainous terrain.
Combining signals from different constellations introduces technical complications, though. Each system has its own reference frame and time scale, and the signals have different frequency structures, which creates biases in the receiver that must be accounted for in the positioning calculation. GLONASS adds an extra layer of complexity because each of its satellites broadcasts on a slightly different frequency, creating additional biases that vary from satellite to satellite.9Scientific Reports. Precise positioning with current multi-constellation Global Navigation Satellite Systems: GPS, GLONASS, Galileo and BeiDou Receiver manufacturers have gotten steadily better at handling these inter-system differences, and for the end user the result is simply a position fix that works in more places and fails less often. For high-precision users doing centimeter-level work, the combined constellation approach has noticeably shortened the time needed to reach a reliable fix.
Where GPS Signals Cannot Reach
GPS signals are extremely weak at ground level, and they cannot penetrate significant barriers. Water absorbs the L-band frequencies GPS uses within the first few centimeters of the surface, making satellite navigation impossible for submarines, underwater drones, or divers. Underground environments like tunnels, mines, and subway systems are similarly cut off. Even dense tree canopy or heavy building interiors can attenuate the signal enough to make a fix unreliable.
In these environments, alternative positioning technologies fill the gap. Underwater vehicles typically rely on acoustic positioning systems, where a network of transponders on the sea floor or mounted on surface vessels measures the time of arrival of sound pulses. Inertial navigation systems, which track acceleration and rotation to dead-reckon a position from a known starting point, work in any environment but accumulate drift over time and need periodic correction. Indoor positioning uses Wi-Fi, Bluetooth beacons, ultra-wideband radio, or even magnetic-field mapping of a building’s structural steel. None of these alternatives matches GPS for global, all-weather, continuous coverage, which is precisely why GPS remains dominant wherever it can reach.
How Animals Navigate Without Satellites
Long before humans launched satellites, migratory animals were solving their own version of the positioning problem. Many species navigate using Earth’s magnetic field, sensing both its direction and its intensity to determine where they are. Researchers studying animal migration now routinely fuse GPS tracking data from tagged animals with satellite-derived geomagnetic field measurements to understand what magnetic cues the animals might be responding to along their routes. One such study found that the average error in satellite-derived magnetic intensity measurements was about 22 nanotesla, a figure at the lower range of what animals can sense, meaning the satellite data is just barely fine-grained enough to investigate animal magnetic navigation.10Movement Ecology. Fusion of wildlife tracking and satellite geomagnetic data for the study of animal migration
The parallel is instructive. Both GPS and animal magnetoreception are systems that extract positional information from physical signals, one electromagnetic timing signals from artificial satellites, the other geomagnetic field lines generated by Earth’s core. Both are vulnerable to disruption from geomagnetic storms. And both achieve their accuracy not from the raw signal alone but from sophisticated processing: in one case, relativistic corrections and atmospheric modeling; in the other, a neural system that evolution has tuned over millions of years. The main advantage GPS has over a bar-tailed godwit’s built-in compass is that a GPS receiver can tell you exactly where you are in coordinates accurate to a few meters, but the godwit still manages to fly 11,000 kilometers nonstop from Alaska to New Zealand without ever needing to charge a battery.