Why Can GPS Jammers Cause Strange Speed Readings Before GPS Signal Loss?
A phone is sitting on a table, completely still, yet it suddenly shows 72 km/h. A few seconds later, the location starts drifting. Then the GPS signal disappears completely.
At first, this might look like a simple software glitch. But when GPS reception gradually deteriorates, a similar pattern can occur with GPS jamming or other forms of GNSS interference.
GPS blockers do not necessarily make a phone lose its location the moment they become effective. As satellite reception becomes weaker or more unstable, the receiver may continue calculating a position from increasingly unreliable data.
That is when an incorrect GPS speed reading can appear before the signal is lost altogether. As we mentioned in our previous article, How to Detect GPS Interference Before the Location Disappears ?.
Why can GPS jammers cause incorrect speed readings?
GPS speed is not determined simply by comparing two locations. A receiver also analyzes changes in satellite signals to estimate movement and speed.
When GPS jammers interfere with those signals, the receiver may have difficulty maintaining a reliable calculation. The reported location can still look approximately correct, while the calculated speed suddenly becomes unrealistic.
You might see a sequence like this:
0 km/h → 68 km/h while stationary → unstable position → GPS signal lost
This is why searches such as "GPS shows speed when I'm not moving" or "GPS says I'm moving while standing still" do not necessarily mean that the navigation app itself has malfunctioned.
What happens just before GPS signal loss?
The most useful clue is often not the moment when the GPS disappears, but what happens in the few seconds beforehand.
A typical sequence associated with deteriorating GPS signal quality may look like:
normal position → unusual speed → declining accuracy → drifting location → GPS signal loss
At this point, the receiver may already be struggling to maintain a stable satellite lock.
However, an important distinction needs to be made: GPS jammers are not the only possible cause.
Poor satellite visibility, buildings, signal reflections, multipath effects, atmospheric conditions, or a change in the phone's positioning source can produce similar GPS anomalies.
An unusual speed reading on its own therefore does not prove that GPS jammers are present.
The situation becomes more noteworthy when an impossible speed appears immediately before the device loses its GPS connection.
An impossible speed can be the first visible sign
Before a phone displays "GPS signal lost," the first noticeable symptom of GPS interference may be surprisingly subtle.
The phone can still appear to be in roughly the correct location, yet suddenly report 40, 80, or even 100 km/h while sitting completely still.
Then the position begins to drift.
Finally, the GPS signal disappears.
This is why GPS jamming is not always immediately obvious. The first sign may not be a warning message or an empty map. It can simply be a speed reading that makes no sense.
When investigating possible GPS interference from GPS jammers, the few seconds before the loss of location can therefore be just as revealing as the moment when the GPS stops working.
