GPS has quietly become the backbone of modern aviation. Most pilots don’t actively think about it anymore. Position, groundspeed, moving maps, ADS-B, instrument approaches—it all just works. The system has been so reliable for so long that it’s easy to treat it as a constant.
That assumption is worth examining more closely.
At its core, GPS is a radio frequency (RF) system. In that sense, it is not fundamentally different from a VOR or any other navigation aid that relies on electromagnetic wave propagation. The difference is in scale. A VOR transmitter might be tens of miles away. A GPS satellite is over 12,000 miles away. By the time those signals reach your aircraft, they are extraordinarily weak—so weak, in fact, that they are typically below the ambient noise floor seen by the receiver.
This is important to understand.
Your GPS receiver is not simply “picking up” signals in the conventional sense. It is reconstructing them. Each satellite transmits a pseudo-random code (a unique, noise-like identifier for each satellite that the receiver searches for), and the receiver continuously searches for that code in the presence of noise. When it finds a match, it locks onto the signal and begins extracting timing information. That timing information, when combined across multiple satellites, becomes your position solution.
As long as the RF environment behaves, this process is extremely robust. The receiver can tolerate a surprising amount of noise and still maintain lock. But there is a limit.
What happens when the noise floor rises?
This is where GPS interference enters the picture.
Interference, in its simplest form, is anything that disrupts the receiver’s ability to distinguish the satellite signals from the surrounding noise. This can occur intentionally, as in the case of military GPS jamming exercises, or unintentionally through other RF emissions operating in nearby frequency bands. In either case, the effect is the same: the receiver’s job becomes harder.
It is useful to think of this in terms of signal-to-noise ratio (a measure of how strong the desired signal is compared to the background noise). The satellite signals are already weak. If the noise level increases, the effective signal-to-noise ratio decreases. As this ratio drops, the receiver begins to lose confidence in what it is seeing. It may first lose lock on one satellite, then another. The remaining satellites may still provide a position solution, but the geometry (the relative positioning of the satellites in view) degrades and accuracy begins to suffer.
From the cockpit, none of this is immediately obvious.
This is another important point.
GPS failures are rarely abrupt in the way pilots expect. There is usually no single moment where the system simply “turns off.” Instead, the degradation occurs gradually inside the receiver. Position updates may continue. The moving map may still appear stable. In some cases, the system will even smooth over the degradation using filtering techniques (mathematical smoothing based on past position and motion), creating the impression that everything is functioning normally.
Under the surface, however, the system is running out of margin.
Eventually, that margin is exhausted. When it is, the receiver can no longer guarantee the integrity (a measure of trustworthiness, not just accuracy) of the position solution. At that point, the avionics system is required to flag the data as unreliable. This is when pilots begin to see annunciations such as loss of integrity, degraded GPS, or a reversion in navigation mode.
By the time those indications appear, the failure has already occurred.
To understand why this matters, it helps to look at how GPS feeds the rest of the avionics system.
From a system standpoint, GPS is not just another input—it is the root of the entire navigation chain in most modern aircraft. The flight management system depends on it for lateral navigation. WAAS (Wide Area Augmentation System) approaches depend on it for vertical guidance. ADS-B Out depends on it for position reporting. Even basic situational awareness tools such as moving maps, terrain overlays, and traffic displays rely on a valid GPS solution.
When GPS begins to degrade, these systems do not all fail at the same time or in the same way. Some will continue operating using filtered or predicted data. Others will drop functionality as soon as integrity thresholds are exceeded. The result is not a clean failure, but a cascade of partial failures that can feel inconsistent from the cockpit.
This is why GPS interference can be difficult to recognize in real time.
In April 2026, the Federal Aviation Administration issued updated advisories highlighting recurring GPS interference events across parts of the United States. These events were associated in part with military testing, but reports from pilots indicated that the effects were not always confined to the published areas. Aircraft operating well outside the expected regions experienced degraded GPS performance, in some cases without clear warning.
This behavior is not as surprising as it might seem.
RF propagation does not follow clean geographic boundaries. Interference can extend beyond the intended area depending on altitude, terrain, and atmospheric conditions. Two aircraft in the same general region may experience very different signal environments. One may maintain a stable GPS solution, while another begins to lose satellites and accuracy.
From the pilot’s perspective, this can feel unpredictable. From a systems perspective, it is entirely consistent with how RF systems behave.
The more important takeaway is that GPS is no longer a guaranteed input. It is a conditional one.
Modern avionics are designed with this reality in mind. They include integrity monitoring, fallback modes, and clear criteria for when GPS data can no longer be trusted. What has changed is the environment in which these systems operate. Interference—both intentional and unintentional—is becoming more common, and the margin that once made GPS feel “invisible” is not always there.
This sets the stage for one of the more critical failure modes in modern instrument flying.
In the next part of this series, we will look at how WAAS determines whether an approach can continue, and why an LPV approach that appears perfectly stable can be removed without warning—sometimes at the worst possible moment.