Estimated Position

What is Estimated Position?

Estimated position is the best calculated location of an aircraft when a positive fix is not available. It is determined by projecting the aircraft's last known fix forward using the planned or actual heading, true airspeed, elapsed time, and estimated wind. This process is known as dead reckoning (DR).

Estimated Position vs Fix

A fix is a position established with certainty, using visual references, radio aids, or other reliable methods. An estimated position, by contrast, is derived from calculations and is always less reliable than a fix because it accumulates errors over time. The longer the time since the last fix, the greater the uncertainty in the estimated position.

Estimated Position Calculation

To calculate an estimated position:

• Start from the last known fix (latitude and longitude).
• Apply the aircraft's true heading and true airspeed.
• Factor in the elapsed time since the last fix.
• Adjust for estimated wind (direction and speed). This gives a new latitude and longitude, but with an error margin that grows with time and uncertainty in wind, speed, or heading.

Causes and Symptoms of Estimated Position Errors

Errors in estimated position can arise from:

• Inaccurate wind estimation
• Incorrect heading or speed
• Instrument errors
• Longer intervals between fixes Symptoms include increasing uncertainty about the aircraft's location, difficulty correlating ground features with the map, and discrepancies between expected and observed positions.

Operational Use

Pilots use estimated positions when fixes are not available, such as over water or featureless terrain. They must regularly seek fixes to limit the uncertainty and, if unsure of position, use systematic search procedures or request assistance from air traffic services.