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Tutorial Series · Part 22 of 27

GPS Outages: Tunnels, Cities and Jamming

Explain what happens when GNSS disappears or degrades and how INS and motion constraints help bridge outages.

By

  • gnss
  • outages
  • resilience
Series Context

Navigation from First Principles

This article is part of an ordered course. Use the previous and next links to stay in sequence.

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GNSS can be blocked underground, degraded near buildings and affected by interference or jamming. GPS.gov lists signal blockage, indoor and underground use, reflected signals, radio interference and jamming among causes of degraded GPS positioning.1 Inertial sensors help bridge outages, but their errors grow with time.2

What an outage really means

A car enters a tunnel. GNSS measurements stop. The navigation system can still propagate using inertial sensors and possibly wheel speed, map matching and motion constraints. But uncertainty should grow until external information returns.

Prediction without measurements

During GNSS availability:

xk=predict with INS+correct with GNSS\mathbf{x}_k = \text{predict with INS} + \text{correct with GNSS}

During outage:

xk=predict with INS only\mathbf{x}_k = \text{predict with INS only}

The covariance grows:

Pk+1=FPkFT+Q\mathbf{P}_{k+1}=\mathbf{F}\mathbf{P}_k\mathbf{F}^T+\mathbf{Q}

Without correction, the process noise and sensor biases dominate.

A zero-velocity update is a useful constraint when the system is stationary:

zZUPT=0=v+vnoise\mathbf{z}_{ZUPT}=\mathbf{0}=\mathbf{v}+\mathbf{v}_{noise}

Groves’ inertial-sensor tutorial covers zero updates, motion constraints and fault detection in the inertial navigation context.2

Interactive demo: outage resilience simulator

This demo reduces the problem to a simple route with open-sky, urban, and outage segments. The fused track includes wheel speed, map constraints, and stop events so the effect stays visible instead of disappearing into filter detail.

Interactive Demo

Outage Resilience Simulator

The route shows truth, GNSS, inertial drift, and the fused path through open sky, city blockage, and a full outage. Use the toggles to see which extra aids slow the drift and narrow the uncertainty band.

Open sky City Tunnel / jammed Recovery

Controls

Readouts

GNSS recoveryreacquired
Worst fused drift0.0 m
Uncertainty bandlow
What to noticeGNSS returns and pulls the fused estimate back toward truth.

What to watch

  • Position uncertainty expands through the outage section.
  • Better IMU presets drift more slowly.
  • Wheel speed, map constraints, and ZUPT help, but none of them make the outage disappear.
  • Reacquisition can visibly pull the solution back toward the true path.

Try this

Use the phone-grade IMU with a long outage and no aiding. Then turn wheel-speed aiding and ZUPT back on. The comparison shows why outage-bridging sources are practical necessities rather than optional polish.

Limits of the demo

Jamming and spoofing detection are much larger topics than this post. The demo intentionally stays on safe ground: availability loss, degraded updates, inertial coasting and recovery. It is about resilience, not interference tactics.

Footnotes

  1. GPS.gov, “GPS Accuracy”. https://www.gps.gov/gps-accuracy - Official GPS accuracy FAQ covering satellite geometry, signal blockage, atmospheric conditions, receiver design, multipath, jamming and smartphone accuracy. Accessed 2026-06-11. ↩

  2. Paul D. Groves, “Navigation Using Inertial Sensors”, IEEE Aerospace and Electronic Systems Magazine, 2015. https://ieee-aess.org/media/navigation-using-inertial-sensors - Tutorial covering inertial sensor technology, strapdown navigation, alignment, zero updates, motion constraints, pedestrian dead reckoning and fault detection. Accessed 2026-06-11. ↩ ↩2