The satellites in view give too imprecise a position

What ArduPilot says:

• PreArm: High GPS HDOP

HDOP (horizontal dilution of precision) says how well the satellites in view are spread across the sky: the lower, the more exact the position. ArduPilot wants it at or below GPS_HDOP_GOOD (140 by default, which is an HDOP of 1.4) whenever the flight mode, a circular or polygon fence, or Super Simple needs a position. Until the receiver has reported a value, the check fails.

What to do, most likely first

1. The GPS has only just got its fix and is still finding satellites.

   CheckThe ground station shows HDOP falling and the satellite count rising as you wait.

   FixWait a minute or two with the copter still and in the open.

2. Part of the sky is hidden: buildings, trees, a wall, or you are indoors.

   FixMove to a place with open sky all round.

3. The antenna is covered or badly placed: under a carbon plate, a battery or wires, or the module lies on its side.

   FixMount the module flat, antenna up, above the frame, with nothing conductive over it.

4. The copter's own electronics disturb the receiver: a video transmitter, a camera, a companion computer.

   CheckHDOP rises and satellites drop out when that device is switched on.

   FixMove the GPS away from it, on a mast if need be.

A higher GPS_HDOP_GOOD lets the copter arm with a less exact position; Loiter and RTL then wander by as much. Flying without GPS is done in modes that need no position, such as Stabilize and AltHold.

Parameters to look at

Where to do it:Setup & Tuning GuideSetup & Tuning Guide