AIM ¶ 1-1-16 — Doppler Radar Navigation

Doppler Radar is a semiautomatic, self-contained dead reckoning navigation system consisting of a radar sensor plus a computer. Unlike VOR or GPS, it does not continuously depend on ground-based or external aids to determine position — making it useful in remote areas or over oceans where ground stations aren't available.

Here's how it works operationally:

• The system transmits radar signals toward the ground and uses the Doppler shift of the returns to measure ground speed and drift angle.
• It uses the aircraft's compass system as its directional (heading) reference.
• The computer integrates this data over time to dead reckon the aircraft's position.

Key limitations to understand:

• Doppler is less accurate than INS (Inertial Navigation System).
• On long-range flights, errors accumulate, so periodic updates from an external reference (such as a ground-based navaid or position fix) are required to maintain acceptable position accuracy.

For pilots, this means Doppler is a capable backup or primary system in some installations, but you must understand its dependency on the compass and the need to cross-check position to avoid drift errors over long distances.