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Re: [Paparazzi-devel] problem with ground altitude

From: Hector Garcia de Marina
Subject: Re: [Paparazzi-devel] problem with ground altitude
Date: Wed, 18 Sep 2013 08:23:59 +0200

Because it is a static error, just an offset. Ground level is just a relative reference, the important point for almost all applications is to know where the ground is wrt the plane, and not wrt the mean sea level (what you are asking for). If you need to know precisely the later, then indeed you need to do something else.

On 18 Sep 2013 07:33, "alonso acuña" <address@hidden> wrote:
I have learned a few things tonight:
- in order to not use SRTM data the files in data/srtm need to be deleted. Simply running the gcs without the -strm option is not enough as the data still gets used.
- the ground level as shown in GCS has no relation to ground level as known to the airborne code. To see or change airborne code's ground level add <dl_setting MAX="4000" MIN="0" STEP="1" VAR="ground_alt"/> to conf/settings/fixedwing_basic.xml or similar file you are using.
- there is no warning when the airborne ground level is very different to the SRTM or to GCS ground level. So for example when the aircraft goes into this part of the flight plan:
   <block name="final">
      <exception cond="ground_alt + 10 > GetPosAlt()" deroute="flare"/>
  it will be doing the flare at the wrong altitude.

If several people are telling me that GPS given altitude is not accurate then why is the airborne code ground level set from GPS at the beggining of the flight plan? Why not use the ground_level value from the flight plan?

On Tue, Sep 17, 2013 at 7:13 PM, Eduardo lavratti <address@hidden> wrote:
SRTM and google have the same error. GPS vertical error is 3 times greater than horizontal error.

You get correct altitude using precision gps or altimetric map + geoidal correction.

Date: Tue, 17 Sep 2013 18:26:27 -0600
From: address@hidden
To: address@hidden
Subject: Re: [Paparazzi-devel] problem with ground altitude

Hello. I have checked once again today. We had 3m GPS accuracy and double checked with a portable GPS and we continue to be standing about 10m above what SRTM data and Google Earth think is the ground level. I hope someone can help me to see what options I have and answer the questions in my original email. Will be much appreciated.

On Tue, Sep 17, 2013 at 12:58 AM, alonso acuña <address@hidden> wrote:

This is what I thought but the difference between SRTM and GPS has remained around 10 to 13m on the 4 days over the past weeks that we have been on the field. GCS indicates precision of about 5m. I suspect this is not going to change and might be an error in the SRTM data.

On Sep 17, 2013 12:47 AM, "Hector Garcia de Marina" <address@hidden> wrote:

Hi Alonso,

This behavior is completely normal. Your GPS has an static error. That depends on how is the signal (weather, solar activity, number of satellites, their position...) at the moment of the reception. Also it depends on the receiver itself.

The receiver uses to tell you how good is its estimation with the HDOP, VDOP and other information, check these acronyms in wikipedia for more information.

The same case is for the 2D position.


On 17 Sep 2013 07:29, "alonso acuña" <address@hidden> wrote:
Hello. It appears that there is a difference of about 10 meters in the reported ground altitude in SRTM date and google earth with the altitude reported by GPS at this location. So when the aircraft is on the ground it says AGL 10 and exported kml shows the path 10 meters above ground in Google Earth. 
I have read the code for NavSetGroundReferenceHere flight plan command and it seems it makes the aircraft take the current GPS altitude as ground level which seems a good thing. However this does not fix things in the GCS.
How is the AGL shown in GPS calculated? 
How can I make it not use SRTM data? 
How can I can I tell what the aircraft thinks the ground level is ?
And what is the ground_level parameter in flight plan for?

Thanks for the help.


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