Alternative PNT — navigating without GPS

Alt-PNT replaces or backs up GNSS using ground-based networks, inertial hardware, and environmental sensors, so a system keeps a usable Position, Navigation & Timing solution during jamming, spoofing, or outages. The winning idea is not one silver-bullet sensor but a layered architecture of independent sources that degrades gracefully.

Symbols used on this page are defined in the Nomenclature → frames, position & motion.

Why GNSS needs a backup

GNSS is ubiquitous and cheap, but its signals are:

• Weak (~ −160 dBW at the receiver) → trivially jammed by cheap transmitters.
• Unauthenticated (civil signals) → spoofable with counterfeit signals.
• Line-of-sight → unavailable indoors, underground, underwater, deep urban canyons.

The cascading architecture

No single technology matches GNSS's accuracy + coverage + cost, so resilient PNT is an architecture: diverse, independent layers arranged so that when a higher-quality source drops, the system falls through to the next-best, always backstopped by a self-contained INS + stable clock.

The INS always runs and drifts; every layer above exists to bound that drift by injecting a fix. Toggle sources off (jam GNSS, occlude RF…) and watch the estimator fall through — and the error snap down whenever any source is available:

This is the same drift-bounding idea as the IMU page's periodic-fix sim, generalized to a stack of heterogeneous PNT sources.

The alt-PNT technologies

Inertial as the backbone

The INS is the always-available core: fully self-contained, immune to jamming/spoofing, works everywhere. Its weakness is drift — see the IMU page for the physics (error growing like $t^2$ from accelerometer bias, $t^3$ from gyro bias) and the grades of IMU that set how long it can coast between fixes.

Passive aiding — MagNav & celestial

When RF is denied entirely, emission-free sensors bound the drift covertly: MagNav map-matching and a star tracker both give an absolute reference without radiating anything — ideal for contested or stealth operations.

Fusion & integrity

All sources feed one estimator (an EKF/UKF or factor graph): the IMU propagates the state at high rate; each external source is a measurement update weighted by its covariance. An integrity monitor cross-checks the sources' innovations and rejects spoofed/faulty measurements — the primary spoofing defense — while a stable clock keeps everything time-aligned for TDOA ranging.

Comparison matrix

The whole argument for a layered architecture is visible in one table — no single row is strong everywhere:

✅ strong · ⚠️ moderate/conditional · ❌ weak or none. Note the pattern: the passive, unjammable sources (INS, MagNav, celestial) mostly cannot provide timing, and the ones that give excellent timing are either radiated or local — which is exactly why the cascade needs both a clock and an inertial backbone.

Design principles

1. Diversity of physics — layers must rest on different physical phenomena (RF ranging, inertial, magnetic, optical/celestial, atomic time) so no single attack or environment defeats them all.
2. Graceful degradation — losing a source must reduce accuracy, never produce a discontinuity or a total loss; the cascade falls through automatically.
3. Integrity monitoring — continuously cross-check sources to detect and reject spoofed or faulty measurements before they corrupt the state.
4. Sensor fusion — one central estimator (EKF/UKF or factor graph) weights every measurement by its current uncertainty rather than hard-switching between sources.
5. Timing coherence — a stable clock keeps all sensors time-aligned; TDOA-style ranging and coherent fusion are impossible without it.

Design trade-offs (SWaP-C)

Every deployment balances Size, Weight, Power and Cost:

The right mix is set by platform value and denial duration. A submarine (long GPS-denied, high value) justifies a navigation-grade INS + atomic clock + gravimetric/MagNav aiding; a small UAV may only carry a MEMS INS plus an SoOP receiver and accept metre-class drift. See the IMU grade table for what each tier actually costs you in drift.

References

• Belfer Center — Navigating Without GPS: A Cascading Alt-PNT Architecture
• Safran — How Resilient PNT Works in GPS-Denied Environments
• PNI Sensor — PNT Alternatives for Defense Applications
• Northrop Grumman — Positioning, Navigation, Timing

Related: GNSS (the primary source and its failure modes) · IMU (the backbone) · EKF (the fusion engine) · Star Tracker (celestial aiding).