What is GNSS Interference?
GNSS interference is the deliberate or accidental disruption of satellite navigation signals (GPS, Galileo, GLONASS, BeiDou), causing receivers to compute no position and time, or the wrong ones. Most people think of navigation, but in OT the real issue is time: GNSS receivers are the reference clock behind PTP grandmasters, NTP servers, protection relays and mobile networks. Disturb satellite time and you disturb the order of events in a substation or plant.
🧠 What is the difference between jamming, spoofing and meaconing?
GNSS signals are extremely weak: a GPS signal arrives at the Earth’s surface at around −130 dBm, below the thermal noise floor. Even a small transmitter can drown them out or imitate them.
| Type | What happens? | Effect on the receiver | Detection |
|---|---|---|---|
| Jamming | Noise or a carrier on the GNSS frequencies (e.g. 1575.42 MHz) | Loss of reception, no position or time | Fairly easy: the signal-to-noise ratio collapses |
| Spoofing | Counterfeit satellite signals stronger than the real ones | Receiver follows a false position or time without raising an error | Hard: everything looks normal |
| Meaconing | Genuine signals are captured and rebroadcast with a delay | Shifted time and position despite authentic content | Hard; authentication alone does not always help |
Jamming is the loudest, but spoofing is more dangerous for OT. A clock that loses its signal switches to holdover and raises an alarm; a clock that is spoofed drifts silently and drags the whole network with it.
🛰️ Which satellite systems are there?
- GPS (United States) — the oldest and most widely used system; the default time source in industrial clocks
- Galileo (European Union) — under civilian control; since 24 July 2025 OSNMA (Open Service Navigation Message Authentication) has been officially operational: a free digital signature in the navigation message on E1-B that lets a receiver verify the data really comes from Galileo
- GLONASS (Russia) and BeiDou (China) — global systems that multi-constellation receivers use as additional sources
OSNMA makes Galileo the first global satellite system to offer authentication in its open, civilian service. It protects against forged navigation data, but not against jamming, and only partly against meaconing, because a delayed replay still carries a valid signature.
🏭 Why does OT depend on GNSS time?
For many organisations GNSS is the cheapest route to UTC. A rooftop receiver feeds a stratum-1 NTP server or a PTP grandmaster, which then distributes time across the network.
| Application | Typical accuracy requirement | Consequence of a time error |
|---|---|---|
| Sampled values and differential protection (IEC 61850) | approx. 1 µs | Incorrect current comparison, spurious or missed tripping |
| Phasor measurement units (PMUs) for grid monitoring | approx. 1 µs | Wrong phase angles, misleading view of the grid |
| 5G TDD base stations | ±1.5 µs | Inter-cell interference, loss of 5G services |
| Algorithmic trading (MiFID II, RTS 25) | ±100 µs to ±1 ms | Regulatory non-compliance |
| SCADA events, logging and SIEM | milliseconds to seconds | Wrong order of events |
The PTP Power Utility Profile, IEC/IEEE 61850-9-3, targets 1 µs accuracy after up to fifteen network switches and allows the grandmaster itself only 250 ns of error. That makes the GNSS receiver in a digital substation a critical component, just as critical as the IEDs that depend on its time.
📡 How common is GNSS interference?
Since 2022, the year of Russia’s full-scale invasion of Ukraine, GNSS interference originating from Russia and Belarus has been a persistent feature of the Baltic Sea region, with a sharp increase from August 2024. In April 2024 two Finnair flights to Tartu had to return to Helsinki because GPS interference prevented a landing. In October 2025 Lithuania reported hundreds of interference events a week, roughly twenty times the 2024 level.
On 6 June 2025 thirteen EU member states, including the Netherlands, asked the European Commission for joint action and for resilient alternatives to satellite positioning and timing. On 26 March 2026 EASA and EUROCONTROL published a joint action plan for safe aviation operations during GNSS interference.
Interference need not be malicious. On 26 January 2016, after a ground-segment software error, fifteen GPS satellites broadcast a UTC correction that was about 13 µs off for several hours, and time servers around the world jumped. Galileo was out of service for almost a week in July 2019 after a failure in its ground segment. In the Netherlands, the aerospace centre NLR detects dozens of jammers passing its Amsterdam site every day, often in vehicles whose drivers want to evade tracking. Using, selling and advertising jammers is prohibited; the Dutch Authority for Digital Infrastructure (RDI, formerly Agentschap Telecom) enforces the ban and commissioned NLR research into GNSS spoofing in 2019.
🔐 What are the consequences of a time error in OT?
- Protection relays — differential and busbar protection compare measurements by timestamp; a shift of microseconds can cause a spurious trip
- Sequence of events — after a fault you want to know which relay operated first; with skewed clocks the sequence of events cannot be trusted
- Forensics — correlating syslog, firewall logs and historian data in a SIEM fails when sources are minutes apart
- Certificates and authentication — a clock that runs too far ahead or behind rejects valid certificates or accepts expired ones; see certificate management
- Batch and energy records — time-stamped records, from production batches to metering data for grid congestion agreements, become open to dispute
🛠️ How do you make your timing resilient to GNSS interference?
- Take inventory — map every GNSS receiver, grandmaster and NTP server, and which processes depend on them. Include this in your risk assessment.
- Choose holdover deliberately — in holdover an OCXO typically drifts 1–10 µs per day, a rubidium oscillator a few hundred nanoseconds. Decide how long your application must stay within tolerance without GNSS.
- Use multi-constellation and OSNMA — receivers that cross-check GPS, Galileo and other systems and verify Galileo authentication detect spoofing faster.
- Site antennas carefully — as high and unobstructed as possible, out of sight of public roads; consider anti-jam antennas (CRPA) at critical sites.
- Add a terrestrial source — time over fibre (for example White Rabbit, incorporated into IEEE 1588-2019) or a UTC service traceable to UTC(VSL), the Dutch national time scale. AMS-IX offers such a terrestrial NTP service.
- Monitor and compare sources — let grandmasters check each other and the terrestrial source, alarm on jumps or loss of satellites, and feed those alerts into your monitoring and SIEM.
- Design for redundancy — at least two independent grandmasters, separate antennas and a tested failover, as part of defence in depth.
🇪🇺 What policy applies in the Netherlands and the EU?
Reliable timing is part of the duty of care under NIS2 and the Dutch Cybersecurity Act (Cyberbeveiligingswet), and of the physical resilience that the Wwke demands of critical entities. At European level the Commission is working on complementary PNT services (timing and positioning) alongside Galileo. In the Netherlands, the Accelerating Network for Resilience in the Physical Environment, representing more than fifty organisations from energy, telecoms, water, defence and other sectors, presented a report on 25 June 2026 recommending a national GNSS-independent time network, with VSL as its foundation.
❓ Frequently asked questions
Is GPS jamming illegal in the Netherlands?
Yes, using, selling and advertising GPS jammers is prohibited in the Netherlands. The Dutch Authority for Digital Infrastructure (RDI) enforces the ban and confiscates jammers. GNSS interference from a jammer affects not only its user but also time servers and antennas nearby.
Can GNSS spoofing disrupt a substation?
Yes, GNSS spoofing can silently shift the time of a PTP grandmaster, after which IEDs in an IEC 61850 substation work with wrong timestamps. Research on phasor measurement units has shown that forged GPS time can produce a false picture of the grid and even spurious trips. Holdover, source comparison and monitoring reduce that risk.
Does Galileo OSNMA protect against all GNSS interference?
No, Galileo OSNMA authenticates the navigation message and so protects against forged data, but not against jamming. Meaconing, the delayed replay of genuine signals, is not fully caught either. OSNMA is therefore one layer in a broader defence against GNSS interference.
How long can a time server run without GNSS?
That depends on the oscillator and the accuracy required. During GNSS interference an OCXO often stays within 1.5 µs for a few hours, while a rubidium clock typically manages a day or more. For millisecond-level logging an OCXO will last for weeks, whereas a cheap crystal can drift out of tolerance within an hour.
Is internet NTP a good alternative to GPS?
No, public internet NTP is not a good alternative to GPS for OT: it is usually unauthenticated and requires a connection between OT and the internet. A private terrestrial NTP service or a dedicated fibre link, on the other hand, is a useful complement during GNSS interference for millisecond-level applications. For microsecond applications, PTP with a local grandmaster remains necessary.
How do you detect GNSS spoofing?
You recognise GNSS spoofing by sudden jumps in time or position, implausibly high signal strength, and disagreement between constellations or between GNSS and a terrestrial reference. Modern timing receivers raise alarms for this that you can forward to your SIEM. Comparison with an independent clock is the most reliable check.
📌 In summary
For OT, GNSS interference is above all a timing problem: blind trust in satellite time risks wrong protection actions, unusable logs and rejected certificates. Combine holdover oscillators, multi-constellation receivers with Galileo OSNMA, a terrestrial time source and active monitoring, so your clocks keep running when the satellites fall silent or start lying.