What does ERR Subsys=9 mean in ArduPilot?

Subsystem 9 is FAILSAFE_FENCE. The ECode names which fence type breached: altitude ceiling, circular horizontal, polygon, or altitude floor — corresponding to the bits in the FENCE_TYPE parameter. The matching MSG line usually carries the human-readable type name too.

How do I enable multiple fence types at once?

FENCE_TYPE is a bitmask. To enable altitude ceiling (1) and polygon (4), set FENCE_TYPE = 5. To enable all four — ceiling, circle, polygon, floor — set FENCE_TYPE = 15. The values combine arithmetically because each is a different bit.

Why does my fence keep tripping right at the boundary?

Position estimation has uncertainty; in real wind the drone holds an oval around the commanded point, not a single dot. Add FENCE_MARGIN to give yourself a buffer (the autopilot warns when you cross the margin), and tune the position controller so the drone holds tighter at the fence edge.

How can I avoid altitude floor fence tripping during landing?

Set FENCE_AUTOENABLE = 2 — the fence disables automatically when the autopilot enters LAND mode and re-enables on the next arm. Alternatively, run with the floor fence disabled (don't set bit 8 of FENCE_TYPE) and rely on the ceiling and horizontal fences only.

Does a fence breach immediately switch the drone to RTL?

Only if FENCE_ACTION is set to do so. On Copter the default is 1 (RTL or Land), but you can configure 0 (Report Only) for testing, 2 (Always Land) for immediate descent, or 4 (Brake or Land) for static-fence applications where you don't want a return-to-home action.

Can a GPS glitch make the autopilot think the fence was breached?

Yes. The fence check uses the EKF's position estimate, which is fed by the GPS. A GPS glitch that pushes the estimated position outside the boundary triggers the breach even though the drone didn't physically move. Cross-check GPS.HDop and GPS.NSats at the breach timestamp before blaming the operator.

Understanding ArduPilot Fence Breach Events in .bin Log Files

TL;DR: Fence breach events in ArduPilot log as ERR Subsys=9 with an ECode that names which fence type tripped. The FENCE_TYPE parameter is a bitmask of four fence kinds: altitude ceiling (1), circular horizontal (2), polygon (4), and altitude floor (8). When the vehicle crosses any enabled fence, the autopilot writes the error and executes FENCE_ACTION — usually RTL on Copter. Diagnose by reading the breach time, the active fence types, the actual POS and BARO.Alt at that moment, and the resulting MODE transition.

What ArduPilot's fence system actually checks

The FENCE_TYPE parameter is a bitmask. Each bit enables a different boundary check:

1 — Altitude ceiling. Triggered when altitude exceeds FENCE_ALT_MAX.
2 — Circular horizontal. Triggered when horizontal distance from home exceeds FENCE_RADIUS.
4 — Polygon. Triggered when position crosses an arbitrary user-defined polygonal boundary uploaded via Mission Planner or QGC.
8 — Altitude floor. Triggered when altitude drops below FENCE_ALT_MIN (commonly used to prevent flying below terrain).

To enable all four checks, set FENCE_TYPE = 15. To enable only the circle and altitude ceiling, FENCE_TYPE = 3. The bits combine arithmetically.

What happens when a breach fires

The autopilot writes ERR Subsys=9 ECode=N where the ECode names the type that breached, and executes the action set in FENCE_ACTION. On Copter the action values are:

0 — Report Only (logged, but no autonomous action).
1 — RTL, or Land if RTL is not possible. The default.
2 — Always Land (immediate descent).
3 — SmartRTL if possible, otherwise RTL or Land.
4 — Brake or Land (good for static-fence applications where you want to stop, not return).
5 — SmartRTL if possible, otherwise Land.

On Plane the action values are different (1=RTL, 6=Guided, 7=GuidedThrottlePass, 8=AUTOLAND if possible else RTL).

Reading the breach in Mission Planner

Open the .bin log and:

Look in the Errors tab for FAILSAFE_FENCE entries (Subsys 9). The timestamp and ECode tell you which fence type triggered.
Plot POS.Lat, POS.Lng for horizontal breaches; plot BARO.Alt for altitude breaches.
The MODE message just after the breach shows the action that fired — Mode=6 (RTL) with Rsn=10 (FENCE_BREACHED) is the most common chain.
The MSG stream usually contains a textual note such as "Fence: Polygon breached" naming the type for human readers.

Confirming it in MAVExplorer

MAV> messages ERR
MAV> graph POS.Lat POS.Lng BARO.Alt
MAV> graph MODE.ModeNum MODE.Rsn

Filter the ERR messages to subsystem 9 and you have the breach timeline. Overlaying mode against position shows whether the response was the configured action and whether the drone returned successfully.

Why the breach happened — ranked

Wind drift in Loiter. The drone wasn't holding position precisely enough at the fence edge; gusts pushed it across the polygon line. Verify the wind reading and consider FENCE_MARGIN — the buffer distance before the fence that the autopilot warns at.
Altitude overshoot on takeoff. The drone climbed past FENCE_ALT_MAX during the initial throttle burst. Either lower the throttle expo or raise the ceiling to account for the climb settling.
Operator-uploaded polygon doesn't match the GCS view. The drone is exactly where you wanted it — the polygon you uploaded just doesn't cover it. Re-export the fence to KML and verify visually.
Home position set wrong. Circle fence uses FENCE_RADIUS around home. If the EKF acquired home at the wrong position, the radius is measured from there. Cross-check the home-set MSG line.
Altitude floor triggered during landing. FENCE_TYPE with bit 8 set will trigger as you descend. Enable FENCE_AUTOENABLE = 1 or 2 so the floor disables during land mode.

Configuring fences that don't trip themselves

FENCE_MARGIN — warning buffer (metres) before actual breach. Useful for operator awareness without triggering action.
FENCE_AUTOENABLE — 1 enables the fence on first arming, 2 enables on takeoff and disables before land. The latter avoids floor-fence problems during touchdown.
FENCE_ALT_MAX / FENCE_ALT_MIN — ceiling and floor in metres above home altitude.
FENCE_RADIUS — circle radius in metres from home.
FENCE_RET_ALT — altitude to maintain during fence-triggered RTL. Important if the breach was an altitude one and you don't want to immediately re-breach on the return.

When this isn't really a fence problem

GPS glitch fakes a breach. The reported POS.Lat/POS.Lng jumped outside the fence because of a GPS error, not because the drone moved. Cross-check GPS.HDop and GPS.NSats at the breach time. See our GPS glitch post.
Wrong home set. If home is on the wrong point because the EKF didn't acquire a 3D fix before arming, the entire fence geometry is offset. Look for the home-set MSG line and confirm the coordinates.
Polygon altitude not configured. Polygon fences default to no altitude bound; if you wanted a 3D polygon, you need the altitude ceiling bit (1) enabled separately.

When LogHat helps — and when it doesn't

LogHat surfaces the Subsys=9 breach time, draws the breached fence type on the 3D replay, and shows the matching MODE transition with Rsn=10. What we can't tell you is whether the polygon you uploaded was actually the intended boundary — that needs you to verify the coordinates against your operational plan.