Reference

Vehicle Navigation in Interference Zones

Certain route segments exhibit consistent electromagnetic interference that renders onboard navigation systems unreliable or nonfunctional. Compass deviation may exceed 20°. Position data from electronic systems may be erratic or absent. These conditions are documented in the affected route descriptions. This reference provides procedures for maintaining accurate vehicle navigation through interference zones using methods that do not depend on electronic systems.

Identifying interference

Interference zones have been observed at consistent positions across multiple traversals. The onset is typically abrupt—systems transition from functional to unreliable within 200–500 m. Indicators of interference onset include erratic compass heading without corresponding vehicle movement, position data drift exceeding the vehicle's actual displacement, loss of fix on electronic positioning systems, and anomalous readings from any electromagnetic sensor.

The source of interference in documented zones has not been identified. The zones do not correspond to known geological magnetic anomalies. The interference does not appear to be weather-dependent. Zone boundaries are stable to within 200 m across observations.

Before entering a known interference zone, record the current position and heading from instruments while they are functional. This position serves as the entry fix for dead reckoning through the zone.

Compass deviation correction

In zones where the compass is affected but still provides a reading, deviation can be estimated and corrected. This requires at least two known landmarks visible from the vehicle's position.

Two-landmark deviation check

  1. Identify two landmarks at known positions. Landmarks should be separated by at least 30° of bearing from the observation point.
  2. Take a compass bearing to each landmark.
  3. Compare the observed bearings to the true bearings calculated from the known positions.
  4. The difference between observed and true bearing is the deviation at that heading. If the deviation is consistent for both landmarks (±2°), apply it as a correction to all compass readings in the zone.
  5. If the deviation differs between the two landmarks by more than 4°, the magnetic field is non-uniform. Compass navigation is unreliable. Use landmark triangulation or dead reckoning instead.
Documented interference zones: Basin and Range km 1,140–1,160 (deviation 15–18°, reasonably uniform); Divide Transit km 460–480 (deviation exceeds 20°, non-uniform). Compass correction is applicable in the Basin and Range zone. It is not reliable in the Divide Transit zone.
Landmark triangulation

When compass deviation is non-uniform, position can be established by visual triangulation from known landmarks. This requires at least three visible landmarks at known positions and a method of measuring the angle between them.

Three-landmark fix

  1. Identify three landmarks at known positions. Document landmark positions before entering the interference zone while instruments are functional. Photographs with bearing annotations are effective records.
  2. Measure the angle between each pair of landmarks from the observation point. A sighting device with angular graduation is required; compass bearing is not reliable.
  3. The three angle measurements define the observation point's position relative to the three landmarks. Plot the position using the measured angles and the known landmark positions.
  4. Repeat at intervals to track progress along the route. The interval depends on vehicle speed and visibility conditions. In clear conditions with well-separated landmarks, a fix every 2–3 km is adequate.

Landmark selection criteria: the landmark must be visible from the route, positionally stable (not a vehicle, animal, or temporary feature), and identifiable without ambiguity. Rock formations, peaks, distinctive trees, and structural remnants are suitable. The documented route descriptions note specific landmarks for each interference zone.

Stellar navigation

When landmark visibility is limited (darkness, fog, terrain obstruction), stellar observation provides heading and position reference. Stellar navigation is not affected by the electromagnetic interference observed in documented zones.

Heading from stellar observation

  1. Identify the pole star or a reference star at a known azimuth for the current date and time.
  2. Measure the angle between the reference star and the vehicle's heading using a sighting device.
  3. The vehicle's true heading is the star's known azimuth minus the measured angle (adjusted for sign).
  4. Update the heading fix periodically, as stellar positions change with time. Update interval: every 30 minutes minimum.

Stellar navigation requires clear sky and knowledge of the current star positions. Overcast conditions render this method unavailable. In documented interference zones, if both landmarks and stars are unavailable, dead reckoning is the remaining option.

Dead reckoning

Dead reckoning estimates position from a known starting point using heading and distance traveled. Accuracy degrades with distance. Over a 20 km traverse, position error may accumulate to 500 m or more depending on terrain and heading accuracy.

Odometer and heading method

  1. At the last known-good position (the entry fix), record position, heading, and odometer reading.
  2. Maintain the documented heading for the interference zone segment. The heading for each documented zone is noted in the route description.
  3. Monitor the odometer continuously. At each kilometer, record the cumulative distance and any heading changes.
  4. Estimate the current position by applying the heading and distance from the entry fix. Plot positions at each kilometer mark.
  5. When instruments resume function (exit from interference zone), compare the dead-reckoned position with the instrument position. Note the accumulated error for future calibration.
The vehicle odometer is a mechanical or wheel-speed sensor and is not affected by electromagnetic interference. Heading is the primary source of error in dead reckoning. Where compass deviation is known and uniform, apply the correction. Where deviation is non-uniform, hold the wheel steady and rely on terrain features (road edges, visible track) to maintain heading.
Revision

Procedures tested in both documented interference zones. Compass deviation values measured on multiple traversals. Landmark positions verified by instrument fix outside the interference zones. Dead reckoning error estimates based on direct measurement.