measuring altitude with pressure/gps

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measuring altitude with pressure/gps

Martin Mueller
Hi,

we are looking into meteo data that we collected using Parrot Bebop2 and were wondering if there is an altitude parameter which is already kalman filtered based on GPS and pressure measurements. So far we have been relying on the ROTORCRAFT_FP:up parameter and applied a linear detrending routine to bring the start and landing altitude to 0m (accounting to pressure changes).

However we just realized that the difference between the GPS and the other parameters is generally increasing with height so we wonder a bit whether the other parameters are purely pressure based and there is a problem with the hydrostatic equation. Maybe the assumed temperature profile doesn't fit the conditions we were facing (much colder), but we are not sure how much of an error to expect from this.

We are generally wondering if there is one parameter that is already kalman filtered. It doesn't look like it since ROTORCRAFT_FP and INS are almost identical (except for the sign convention and sampling rate) and look very similar to the AIR_DATA:amsl_baro (seems to be pressure based). None of them has features like the jumps one can see in the GPS data.

Can we use the paparazzi Kalman filter to easily postprocess a "fused" altitude from GPS and pressure?

Martin

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Re: measuring altitude with pressure/gps

Kirk Scheper
Hey Martin,
As far as I know the result in the INS and the ROTORCRAFT_FP messages
contain the result of the Kalman filter combing the GPS and the Baro
when using the vertical float filter (vff). When using the extended
vertical float filter (vff_entended), the baro offset is estimated
(using the gps and/or sonar if active).

Without any additional information, I am not sure what you are referring
to with 'the difference between the GPS and the other parameters is
generally increasing with height', so I can't help you there.

Please note that you have to define INS_USE_GPS_ALT in your airframe for
it to be considered in the filter. Additionally, consider the values for
INS_VFF_R_GPS and VFF_MEAS_NOISE as they will change the noise model
used for the GPS and baro respectfully, changing the confidence on each
parameter in the filter.

 From what you described, I would have to guess you didn't define
INS_USE_GPS_ALT so the GPS is not being used in the vertical filter.

I noticed that INS_USE_GPS_ALT and INS_USE_GPS_ALT_SPEED are not
mentioned in the documentation for the INS so I will update this at my
next opportunity.

Yours,
Kirk

On 20/08/18 10:28, Martin Mueller wrote:

> Hi,
>
> we are looking into meteo data that we collected using Parrot Bebop2 and were wondering if there is an altitude parameter which is already kalman filtered based on GPS and pressure measurements. So far we have been relying on the ROTORCRAFT_FP:up parameter and applied a linear detrending routine to bring the start and landing altitude to 0m (accounting to pressure changes).
>
> However we just realized that the difference between the GPS and the other parameters is generally increasing with height so we wonder a bit whether the other parameters are purely pressure based and there is a problem with the hydrostatic equation. Maybe the assumed temperature profile doesn't fit the conditions we were facing (much colder), but we are not sure how much of an error to expect from this.
>
> We are generally wondering if there is one parameter that is already kalman filtered. It doesn't look like it since ROTORCRAFT_FP and INS are almost identical (except for the sign convention and sampling rate) and look very similar to the AIR_DATA:amsl_baro (seems to be pressure based). None of them has features like the jumps one can see in the GPS data.
>
> Can we use the paparazzi Kalman filter to easily postprocess a "fused" altitude from GPS and pressure?
>
> Martin
>
> _______________________________________________
> Paparazzi-devel mailing list
> [hidden email]
> https://lists.nongnu.org/mailman/listinfo/paparazzi-devel


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