# Why there is a terrible drift in my GPS? **URL:** \ **Category:** Flight Testing & Log Analysis\ **Created:** [March 18, 2017, 7:56am UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798 "2017-03-18T07:56:12Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Boss\_Kwei](https://discuss.px4.io/letter_avatar_proxy/v4/letter/b/8c91f0/32.png) [@Boss\_Kwei](https://discuss.px4.io/u/Boss_Kwei)\ **Post date:** [March 18, 2017, 7:56am UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798/1 "2017-03-18T07:56:13Z")
Hi, All Something really strange with my quadrotor, I need some help. My log file shown as bellow: crash:(arm- \> test RC -\> switch to AUTO.TAKEOFF -\> everything is fine -\> fall down -\> kill) [http://logs.uaventure.com/view/usPigkFbcBkbUviVG9FGmJ](http://logs.uaventure.com/view/usPigkFbcBkbUviVG9FGmJ) success: [http://logs.uaventure.com/view/rfZQksKdVfmuKFZ3AxWG6P](http://logs.uaventure.com/view/rfZQksKdVfmuKFZ3AxWG6P) What I did is, first arming the throttle, second waiting GPS ready for a few seconds, then I switch to TAKEOFF mode, everything is OK but suddenly my quadrotor **fall down** so I have to push KILL switch. My quadrotor did not fly away, but when I check log files, the GPS status looks very strange. Why there is a terrible drift in my GPS, and is there any wrong in my configuration? Any advice will be appreciated, thanks… ---
**Author:** ![clovett](https://discuss.px4.io/user_avatar/discuss.px4.io/clovett/32/3458_2.png) [@clovett](https://discuss.px4.io/u/clovett)\ **Post date:** [March 18, 2017, 9:15am UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798/2 "2017-03-18T09:15:41Z")
based on the large acceleration jolt it looks like you hit the ground at 16:46, which is when the GPS drift started happening. This is common to see GPS go a bit nuts on a hard landing. Also if it landed under trees then that can also mess with the GPS. ---
**Author:** ![Boss\_Kwei](https://discuss.px4.io/letter_avatar_proxy/v4/letter/b/8c91f0/32.png) [@Boss\_Kwei](https://discuss.px4.io/u/Boss_Kwei)\ **Post date:** [March 18, 2017, 10:24am UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798/3 "2017-03-18T10:24:59Z")
@clovett Thank you, I will try again later… ---
**Author:** ![hamid-m](https://discuss.px4.io/letter_avatar_proxy/v4/letter/h/e19b73/32.png) [@hamid-m](https://discuss.px4.io/u/hamid-m)\ **Post date:** [March 21, 2017, 3:55pm UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798/4 "2017-03-21T15:55:27Z")
@Boss_Kwei, Some observations on your flight: - **Altitude Estimate** between _16:46:43_ and _16:46:48_ the `GPOS.Alt` snaps down after a seeming runaway - Q: What is the difference between `GPOS.Alt` and `GPS.Alt`? - **Roll Angular Rate:** At around that same time, the roll angle begins to oscillate. The oscillations grow, attenuate, then grow until the vehicle crashes. I would suspect an unexpected measurement caused instability in a x-gyro bias estimate and the closed-loop control finally lead to a crash. ---
**Author:** ![CarlOlsson](https://discuss.px4.io/user_avatar/discuss.px4.io/carlolsson/32/4175_2.png) [@CarlOlsson](https://discuss.px4.io/u/CarlOlsson)\ **Post date:** [March 23, 2017, 9:33pm UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798/5 "2017-03-23T21:33:12Z")
@hamid-m GPOS.Alt is the estimated altitude from the EKF GPS.Alt is the measured altitude from the GPS sensor ---
**Author:** ![hamid-m](https://discuss.px4.io/letter_avatar_proxy/v4/letter/h/e19b73/32.png) [@hamid-m](https://discuss.px4.io/u/hamid-m)\ **Post date:** [March 23, 2017, 10:22pm UTC](https://discuss.px4.io/t/why-there-is-a-terrible-drift-in-my-gps/2798/6 "2017-03-23T22:22:47Z")
Thanks @CarlOlsson. The EKF altitude snapping back is highly suspect and it seems things did not recover after that.