Dead Reckoning

What Is Dead Reckoning?

Dead reckoning navigation involves plotting your position based on a previously established fix, then projecting your course using heading, airspeed, time, and wind estimates. The process starts from a known point and advances the position calculation forward, accounting for wind drift and speed changes, but without external position updates.

How Dead Reckoning Works

• Start from a fix: Use a confirmed position (visual or radio fix) as your starting point.
• Apply heading and speed: Maintain a precise heading and constant airspeed.
• Time tracking: Use elapsed time to calculate distance traveled.
• Wind correction: Adjust for wind using a calculated wind correction angle (WCA). The WCA is found by dividing the crosswind component by the speed factor (SF), where SF = speed/60.
• Plot position: Mark your estimated position on the chart at each time interval.

Dead Reckoning Calculation Example

Suppose your last fix was at 1200 UTC, heading 090°, TAS 120 kt, with a forecast crosswind of 12 kt from the north. The speed factor is 120/60 = 2. The WCA is 12/2 = 6°, so you steer 096° to maintain an eastward track. After 15 minutes, you expect to be 30 NM east of your last fix.

Difference Between Dead Reckoning and Estimated Position

• Dead reckoning position (DR): Calculated solely from heading, speed, time, and wind, without reference to external observations.
• Estimated position (EP): Adjusts the DR position based on observed drift or visual cues, but not a full fix.

Causes of Dead Reckoning Error

• Inaccurate starting fix
• Heading or compass errors
• Incorrect airspeed
• Wind forecast inaccuracies
• Timekeeping mistakes All errors accumulate over time, so the longer you go without a fix, the greater the uncertainty.

Visual Navigation and Map-to-Ground Correlation

When dead reckoning, pilots search for ground features near the DR position to confirm or update their location. This requires careful map-to-ground comparison, avoiding assumptions based on expectation rather than evidence.

When to Use Dead Reckoning

Dead reckoning is essential when radio aids or GPS are unavailable, or in areas with limited navigation infrastructure. It's also the foundation of visual navigation, especially in basic or emergency scenarios.