How GPS Can Give a False Position

How does a receiver turn time into a position?#

GPS satellites broadcast timing information and data describing their orbits. A receiver uses those data to work out where each satellite was when its signal left. It compares the signal's transmission time with its reception time to estimate the journey's duration.

Radio waves travel at the speed of light. Multiplying travel time by that speed converts seconds into distance. Several distances to known satellite positions constrain the receiver's location. This is a ranging problem; the satellites do not send a personalized message telling your phone which street it is on.

The receiver does not need to transmit a request to the satellites for this calculation. Once it has coordinates, mapping software places them on a map. The measurement and the map are separate parts of the apparent answer.

Sources: [1][3]

Why does the receiver have to solve its clock too?#

The receiver's clock is not perfectly synchronized with GPS time. Its initial distance estimates therefore contain a timing offset. They are called pseudoranges: apparent ranges that include errors, rather than exact geometric distances.

For a simplified model, an apparent range contains the true satellite-to-receiver distance plus the receiver clock offset multiplied by the speed of light. Actual processing also accounts for satellite clock errors, atmospheric delays and other effects. The receiver clock term is shared across measurements made at the same epoch; the geometric distances differ from satellite to satellite.

That leaves four basic unknowns: three position coordinates and one receiver clock offset. A normal unconstrained three-dimensional solution uses at least four satellites to estimate them together. More measurements can help check consistency and improve the estimate; four satellites are the basic counting argument, not a guarantee of accuracy under every sky.

The scale is unforgiving. A microsecond is one millionth of a second. Light covers roughly 300 metres in that time, so a one-microsecond timing discrepancy corresponds to about 300 metres of apparent range. That is a distance equivalent, not a prediction that every timing error moves the map dot by 300 metres. A common offset can be absorbed into the clock estimate; position error depends on the pattern of errors and satellite geometry.

Sources: [2][1]

How can false signals produce a believable answer?#

Spoofing introduces counterfeit satellite signals that deceive a receiver into generating incorrect position, navigation or timing information. At the measurement level, the problem is that the receiver can treat misleading timing observations as evidence of distance. Its position-and-clock calculation may then fit a false answer.

A set of measurements can agree with one another while disagreeing with reality. Internal consistency alone does not establish that their radio source is genuine. This explains how a receiver might keep supplying coordinates instead of simply declaring that navigation is unavailable.

The displayed position has changed; the vehicle has not teleported. A physical course change requires a further link: a person or an automated system must act on the misleading information. How far bad data propagate depends on the receiving equipment and the systems connected to it. Deceiving a navigation input does not by itself establish control of an aircraft.

Sources: [4][2][5]

Has misleading a position ever changed a real course?#

In June 2013, University of Texas at Austin researchers conducted an invited experiment aboard the yacht White Rose of Drachs in the Mediterranean. The university's contemporary account describes a chart showing the vessel on its intended line while course corrections responding to misleading GPS information moved the real yacht onto a parallel track hundreds of metres away.

The causal step matters: the crew's corrections turned false position information into actual motion. The demonstration showed that this chain could occur in that vessel and test arrangement. It did not establish that every receiver accepts counterfeit signals, that every vehicle reacts in the same way, or that a remote stranger can steer any aircraft.

The university reported that the yacht's navigation equipment did not raise an alarm during the experiment. That observation belongs to this demonstration. It should not become a universal claim that spoofing is always invisible.

Sources: [5]

Does a strange map position prove spoofing?#

No. GPS accuracy also depends on signal blockage, atmospheric conditions, satellite geometry and receiver quality. Reflections from buildings can give signals extra paths, an effect called multipath. Mapping software can mislabel a road or address even when the underlying GPS hardware works correctly.

Jamming degrades or prevents reception of genuine satellite signals; spoofing supplies deceptive signals. Equipment, software or network failures can also interrupt the delivery or display of navigation information. The symptom on a screen alone does not distinguish all those causes, much less identify a perpetrator.

EASA lists position discrepancies, clock shifts and spurious terrain warnings among aviation interference symptoms. These are possible effects, not a checklist that every affected aircraft will display. Investigators need evidence beyond an alarming screenshot to establish what happened.

Sources: [6][4][3]

What makes a useful cross-check?#

A useful comparison brings in information that does not share the suspected failure. EASA's aviation guidance emphasizes non-GNSS navigation, crew training, manufacturer instructions, communication with air traffic control and reporting. The available alternatives depend on the aircraft and situation; these principles are not cockpit instructions.

Two displays are not necessarily two independent witnesses. If both take their position from the same compromised source, agreement can repeat the same error. That follows from the data flow, rather than from the number of screens.

For a civilian reader, the practical lesson is to notice disagreement between a navigation answer and independent evidence of location or motion. Confidence in a calculation is useful only when the inputs deserve confidence too.

Sources: [4]