Negative relative altitude at launch

Hi there,

I’ve been slowly building and tuning my first quad and have noticed a small anomaly I can’t figure out.

For some reason, my GPS’ relative altitude is showing a negative value both before takeoff and after landing. The value is not consistent across flights nor is the value the same before takeoff and after landing.

I’ve attached a few graphs which show relalt on the left axis and HDOP/NSATS on the right.

The baroalt tracks almost identically with the relalt tracks shown above.

I can appreciate that if I had poor GPS coverage that the 3D position may be out a little (even if I think the HDOP is probably acceptable) but it doesn’t explain why the position starts in negative territory.

Can you suggest a cause or method of further investigation?

Many thanks.

Is there any more information I can provide that would help consider a cause?

I appreciate it’s a curly issue but can anyone suggest where to start or any other information I can provide?

How much vibration are you getting? Is the board to close to wires or electrical interference? I had a similar problem when I had a bearing to close to the APM which cause the baro to go in the negative. Helicopter of course.