Dead Reckoning if GPS Lost

[mrk.di...@googlemail.com]

Hi folks,

I just have a question about the Dead Reckoning in APM Plane, in Mission Planner i have the choice to either activate it and stop using GPS or deactivate it and navigate with GPS,

Isn't there a possibility to activate DR if the GPS Signal gets bad, ex. less than 6 satellites?
Another noob question is: how accurate is DR without Airspeed sensor?

    // @Param: GPS_USE

    // @DisplayName: AHRS use GPS for navigation

    // @Description: This controls whether to use dead-reckoning or GPS based navigation. If set to 0 then the GPS won't be used for navigation, and only dead reckoning will be used. A value of zero should never be used for normal flight.

    // @Values: 0:Disabled,1:Enabled

    // @User: Advanced

    AP_GROUPINFO("GPS_USE",  3, AP_AHRS, _gps_use, 1),

I am asking for the case of using a Ardupilot 2.6, as far as i know activating the EKF -that only the Pixhawk can process- is in itself a form of dead reckoning that estimates the actual position even if the GPS Signal is lost or bad? Please correct me if i'm wrong.


Markus.

BTW: Nice to write to you guys, i'm new to the world of APM and it's really exciting!
My Gear: Fx61 with apm, i think i'll upgrade soon to a Pixhawk

[Tom Pittenger]

APM 2.6 only supports DCM while the Pixhawk supports DCM and the newer EKF. They both have similar Dead reckoning performance but EKF has much better noise rejection and is a smoother sensor fusion in general.

With GPS_USE=1 and GPS is lost in flight you are, by definition, dead reckoning. I don't think we've done any measurements on dead reckoning drift rates, testing needed! You have plenty of sensors to keep you flying so you're not going to drop out of the sky, it's just the guidance won't be very accurate compared to whatever GPS referenced mission you have loaded.

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[mrk.di...@googlemail.com]

Thanks a lot, 
by DCM you mean? 
The thing is, if you don't have line of Sight view of your plane, and the GPS is lost/bad, how could one make the Plane change into RTL with Dead Reckoning on?
As far as i know there is no Failsafe option if GPS Lost/Bad?

Please correct me if i'm wrong

[Tom Pittenger]

The DCM is a poor man's EKF, it's a sensor fusion mechanismthat can be performed in fixed-point math. The EKF has more "features" but is harder to compute because it requires floating point math.

I think you have a misconception of dead-reckoning. In a small scale, we are always dead-reckoning. Take for example a GPS that outputs 1Hz, but then the inertial sensors that update at 1000 Hz. For one second, every second, we are dead reckoning and only for that instant we get the GPS do we know, to within the accuracy of the GPS, where we are. So, in short time frames, we are dead-reckoning alllll the time. Now if the GPS 1Hz stops, we just keep dead reckoning but round-off and integration errors start to build up and thus the dead reckoning error grows. For a few seconds you won't notice anything. The error growth is dependent on many variables so it's hard to just say "it's good for X minutes or seconds". For example it grows faster when the errors are small due to round-off error such as when sitting still on a table but are easier to track when moving in an aircraft during flight. Also, in the air you have more info such as airspeed that help tame the errors. GPS is just one of many sensors in the aircraft.

The keyword you're looking for is "failsafe". Take a look at the RTL (Return to Launch) options in the failsafe configuration.

http://plane.ardupilot.com/wiki/arduplane-parameters/

In particular, search for "GPS Loss". Also, I see that the value AHRS_GPS_USE is a bit misleading. It says GPS or DR, but in reality it's GPS which falls back to DR, or DR-only.

-TomP