by Joan Conover, SSCA Cruising Station Coordinator & HAL Maritime Safety Program
In late October 2025, several cruisers in the Southern Caribbean reported sudden GPS losses and erratic positions. For some, chartplotters froze; for others, boats appeared to jump a half‑mile sideways. At the same time, satellites, Starlink links, and HF radios kept working normally. That mix of symptoms is the giveaway—true GPS jamming behaves differently from a simple ground‑station outage.
Jamming vs Ground‑Station Outage
These are two distinct failure modes. A jammer blocks or corrupts the radio signal itself. A ground‑station outage interrupts the network that maintains precision timing and corrections. Both can confuse navigators, but the fingerprints differ.
Starlink Still Works — Here’s Why
Starlink satellites use their own GNSS receivers and atomic‑clock timing for orbit control. Even if a regional ground network or GOES uplink goes down, Starlink maintains timing through inter‑satellite laser links. That means you can lose GPS navigation accuracy while your internet connection stays rock‑solid.
Recognizing the Pattern
If your GPS position suddenly jumps, the chartplotter drops to “NO FIX,” or your AIS targets vanish while other systems seem fine, you are likely inside a jamming footprint. If everything looks normal but your fixes slowly drift off the track line, you are probably seeing a control‑segment outage.
Southern Caribbean Example – Oct–Nov 2025
Monitoring groups and open‑source reports confirmed GNSS interference starting October 29 2025 off Venezuela and Trinidad. Both Defence Blog (Oct 31 2025) and Gate Checked (Oct 31 2025) mapped strong GPS blocking, while Sky News (Nov 5 2025) noted expert opinion that U.S. counter‑interference testing might be responsible. By early November, ships and aircraft were again reporting stable signals.
False Alarms — When It’s Not Jamming
Sometimes what looks like jamming isn’t deliberate at all. After a hurricane, damaged ground antennas, flooded GNSS reference stations, and debris‑laden power networks can scatter or weaken signals, producing sudden “no‑fix” errors identical to jamming footprints. Vessels departing a storm‑hit harbor may see satellites vanish or positions drift wildly as infrastructure comes back online in fragments.
Solar events can do the same. A strong coronal mass ejection (CME) or geomagnetic storm disturbs the ionosphere—the thin layer GPS signals pass through—creating delays and refraction that confuse receivers. During Level 3 or 4 solar storms, a boat’s chartplotter may show offsets of several miles even in open water.
Knowing when the Sun is active or when nearby land systems are damaged helps separate true jamming from environmental interference.
What Mariners Can Do
Cross-check your position using radar ranges, depth, and visual bearings. Log time, location, and receiver behavior. Report anomalies to your national maritime authority or SSCA host network. Even a short log helps identify patterns the next time signals go silent.
The takeaway: jamming blinds your receiver; outages simply starve it of updates. Knowing the difference—and the natural look alikes—can keep you safe when satellites misbehave.
