Navigation without GNSS: practical exercises on radar, bearings, “gyrovector”, depth control and actions in case of suspected spoofing

When satellite positioning disappears or “lies,” discipline wins on the bridge: DR/EP, radar, bearings, depths, ranges. None of this is “old tricks” — it’s the core resilience of a modern navigator, especially in areas where jamming (signal blocking) and spoofing (signal substitution) of GNSS/AIS are reported. Current guidance explicitly recommends planning voyages with the risk of GNSS loss in mind and practicing alternative plotting methods.

I. RADAR plotting: quick drills that genuinely keep you on track

Parallel Indexing (PI)

What it is: a line on the RADAR parallel to the planned course, drawn through a radar-conspicuous mark at the required offset (offset/CIR). If the mark’s echo runs along the line — you’re on track; if it deviates — correct. How to train: preselect 2–3 marks on different legs; at each scale check VRM/EBL. Practice PI in sea- and ground-stabilized modes, remembering a nuance: in true motion, the PI must “move” with the ship and cannot be fixed to the display, while ground stabilization helps separate stationary objects.

RADAR fixes

Combine two ranges (arcs of position), range + bearing, and tangent bearings to headlands/islands. Use Radar Image Overlay on ECDIS as a “sanity check”: if the coastline on RADAR doesn’t match the ENC, verify GNSS.

II. Bearings: cross bearing and the running fix

Cross bearings. Take 2–3 well-separated marks (target ~90° between LOPs), apply variation/deviation as needed, obtain the fix.

Running Fix. When only one mark is visible, take successive bearings at time intervals, then transfer the previous LOP by the traversed “gyro vector” (your course and distance over Δt). The classic approach is described in “Bowditch” (The American Practical Navigator) and modern notes: positioning by a transferred LOP instead of three simultaneous LOPs.

III. The “gyro vector” and ARPA: how to verify the traffic picture

The gyro vector is your vector using gyrocompass course and speed (log) over the chosen interval. You need it to:

transfer an LOP in a running fix;
construct velocity triangles in manual CPA/TCPA calculations;
determine set & drift.

On ARPA, don’t confuse modes and vectors: relative/true motion, head-up/course-up/north-up, true/relative vectors — different representations of the same situation. Regulatory definitions (eCFR 33 CFR 164.38) clearly explain the differences; confirm you’ve selected the mode required for the task.

IV. Contour navigation by depths — and how to read it

“By-contour” practice

In coastal navigation the echo sounder is a second eye. Use series of soundings to cross-check isobaths and the “safe depth contour”: if under-keel clearance is less than planned — adjust EP/course. Remember sound speed in water and draft/dynamic draft affect indications — by up to 5% or more depending on temperature/salinity; calibration and corrections are mandatory.

CATZOC in ENC (IHO S-67)

Not all depths are equally accurate. CATZOC (A1…D/U) indicates positional/depth accuracy and survey completeness. In B zones, isolated uncharted dangers may exist; in C/D — proceed with extra caution and increase UKC/XTD. Don’t over-zoom the ENC — use recommended scales to avoid the “illusion of accuracy.”

V. Ranges and transits: “without a compass and without GNSS”

A pair of lights/marks in range gives a line of position with very high accuracy: keeping them “in line,” you steer along the channel centerline or control the wheel-over point. It’s a universal tool for entering narrow fairways and for verifying the EP.

VI. Bridge drill set (60–90 minutes on a simulator or at sea)

PI RADAR monitoring. Select 2–3 PI lines at different ranges; on each leg keep the mark’s echo on the index. Any deviation > one cable — order a correction and make a brief logbook entry.
Fixes every 3–6 minutes. A stream of RADAR fixes: “two ranges” → “range + bearing” → tangent bearings. Add visual LOPs when the coast is visible.
Bearings and the running fix. Do cross bearings twice; once — a running fix with correct LOP transfer using the gyro vector.
Depths + ENC. A series of soundings with corrections for draft/sound speed; compare with isobaths and CATZOC; explain why you increase UKC in a B/C area.
Ranges/transits. Choose natural or lit ranges; practice holding the channel axis without a compass (by keeping them “in line” only).

VII. Suspected spoofing: how to act without panic

Indicators: GNSS position “jumps,” COG/SOG are illogical; AIS targets “walk on land”; RADAR overlay doesn’t match the ENC; DR/EP doesn’t agree with the receiver’s “fix.” Action checklist:

Switch the position source or set ECDIS to DR/EP mode, work off PI/RADAR and visual/RADAR fixes; reduce speed in confined waters.
Compare depths with the ENC and CATZOC; don’t rely on AIS for maneuvering — treat it as a reference layer.
Record the event (logbook, VDR), notify shore authorities; official guidance explicitly requires having a response plan and practicing “anti-spoofing” drills. In the U.S., MARAD/NAVCEN provide reporting forms for GPS disruptions; similar recommendations apply elsewhere.

VIII. Small habits that save you in big moments

Keep DR/EP running. Even with “perfect” GNSS, maintain DR — discrepancies become obvious immediately.
PI “always on” in coastal navigation: your continuous control of XTD and wheel-over points.
Understand ARPA modes. Relative/true motion, screen orientations, and vector types matter — the wrong mode yields the wrong conclusions.
CATZOC ≠ “decoration.” Plan UKC/XTD based on data quality; in B/C/D — take larger margins.
The echo sounder isn’t gospel. Correct for draft and sound speed; beware of false returns.

Sources: UKHO on jamming/spoofing (2025); IHO S-67 on CATZOC (2020); MCA MGN 379 (rev. 2024); eCFR 33 CFR 164.38 (ARPA); NOAA U.S. Coast Pilot 2 on echo sounders; MARAD/NAVCEN on incident reporting.