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3D RoboticsPermalink Reply by Chris Anderson on November 15, 2010 at 5:29am

What a "QNH"?

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JDronesPermalink Reply by Jani Hirvinen on November 15, 2010 at 5:53am

Chris, it's mostly used term on aviation for pressure

Three references for barometric pressure are in common usage: QNH, QFE and Standard Pressure (also referred to as QNE).

With QNH set, an aircraft altimeter indicates height above mean sea level. The QNH may be the pressure observed at the airfield, or the lowest pressure observed throughout a specified geographical area

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Permalink Reply by Mike Bakula on November 15, 2010 at 6:18am

QNH is also called the "altimeter setting" in civil aviation. It is the local barometric pressure, which provides an offset to the indicated altitude. This offset allows the altimeter to show the actual altitude of the ground on the altimeter, compared to mean sea level (MSL). Often this is set in the "Kollsman window" of the altimeter. A civil aviation technique is to do it backwards, adjusting the altimeter setting until the altimeter reads the field altitude (which is published).

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Permalink Reply by Jon Keller on November 15, 2010 at 11:35am

Hi Chris,

As the others said, its basically the ambient air pressure at sea level for a given area. Generally read in hectopascals. On a perfect "standard" day, the sea level air pressure should be 1013 (29.92ish inches mh) hectopacals, and 15 degrees C.

Obviously things are often not standard, and as weather systems change, ambient air pressure changes. 1 hectopascal counts for about 30 feet of altitude, and qnh can generally range from 900 to 1040 hectopascals depending on where you live, and this gives us a range of error of around 1500 feet. Quite a large margin, so being able to set the qnh would be handy to have.

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3D RoboticsPermalink Reply by Chris Anderson on November 15, 2010 at 3:23pm

Got it. We've been talking a lot about the best altitude strategy in APM. Right now you can set the GPS/Baro mixing in the config file. Right now we default to 100% GPS, but we've been really impressed with the onboard (temp compensated) pressure sensor, so we may change that default to 100% baro.

One question is whether we can sample the GPS alt long enough to get a pretty good absolute figure after launch and then use that to calibrate the pressure sensor, using baro entirely from there on. Not sure if that addresses your question, but it's what we're currently thinking about.

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DeveloperPermalink Reply by Doug Weibel on November 15, 2010 at 7:44pm

Jon,

The equation we are using is based on the pressure difference between the ground pressure and the pressure at altitude. We measure and record the ground pressure at startup. This is equivalent to dialing the current QNH pressure into an altimeter. We also record the ground MSL altitude as reported by the gps. Using the two values recorded at startup we can produce MSL from the pressure reading at altitude.

Of course we are mostly interested in AGL altitudes and in having the system as automated as possible, so the approach of just measuring the ground pressure at startup seems best. The most critical use of altitude is in landing and the vast majority of our users will be trying to land at their launch location versus elsewhere. By basing everything on the measured ground pressure at startup we get the most repeatable and accurate AGL data for the launch location. Also, since mission time is typically short, say a hour or less, we have not seen it as a requirement to be able to change the ambient pressure during a mission.

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Permalink Reply by Jon Keller on November 15, 2010 at 9:29pm

Hi Chris, Doug

That does indeed sound like the best way to do it, AGL is surely what everyone is after. Can you point me towards the code that does this? I guess the reason i dove into the code to begin with was my altitude (agl or amsl) has been quite a ways off when using the pressure sensor, so I'd like to do some debugging.

I'm assuming then that at startup, the telemetry should report altitude as zero? currently what I'm getting is anywhere from 200-1200ft.

I'll also take this chance to say that the ardupilot hardware and software is great. Everyone involved in creating this little device and it's software has done a great job. Getting back into my uav hobby has been very enjoyable and much less frustrating than it was in the past with other hardware.

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