Definition
A mode of GPS navigation operation in which the receiver suspends automatic sequencing through a flight plan and instead provides course guidance to or from a single selected waypoint along a manually chosen course. The pilot selects the desired course (bearing) using the OBS knob or input, and the GPS displays course deviation relative to that bearing through the active waypoint, behaving like a traditional VOR receiver tied to that waypoint.
Plain English
A setting on the GPS that locks it onto one waypoint and lets the pilot choose any course line running through it, instead of letting the GPS automatically move to the next point in the flight plan.
Context Anchor
Seen on GPS navigators and panel navigation displays, especially when selecting a specific course to a waypoint or when automatic waypoint sequencing is not desired.
Derivation
OBS comes from VOR navigation, where the Omni Bearing Selector is the knob used to choose which radial to fly. GPS units borrowed the name because the function is similar: pick a course through a fixed point and fly to or from it.
Why Pilots Care
Allows flying a chosen course that is not necessarily the shortest route to the waypoint.
Intuition Check
OBS does not mean “observe” or “observation” here. In this context, it refers to selecting a bearing, or course line, for navigation.
Example Sentence 1
Approaching the holding fix, the pilot pressed OBS and set the inbound course so the GPS would not sequence to the next waypoint during the hold.
Example Sentence 2
In OBS mode the GPS shows deviation from the selected direction rather than a direct path.