Altitude detection - GPS or barometer

[dgd3]

From what I gather the phantom uses a barometer to know it's altitude. It seems to me that GPS could give a much better indication of altitude. Any comments?

 

[RoyVa]

GPS system can do altitude in 3d but it is erroneous in it altitude calculations on moving objects. The GPS displays gemetric height above the geoid and its height AMSL is + or minus 45m depending on the conditions of the day. For this reason it is no very accurate. The density of the air in different locations also introduces further errors.
Therefore altitude is calculated by barometric altimeters for aircraft and these are calibrated for accuracy. This is the standard for the aviation industry.

 

[dgd3]

Which brand model instrument is used to calculate barometric pressure?

What are its accuracy and repeatability?

 

[SteveMann]

Manned aircraft using $10,000 GPS IFR-Certified units in their airplanes still rely on the barometric altitude for decision height. GPS altitude is notoriously inaccurate mostly because of the geometry of the satellites. GPS calculates position by measuring the time it takes for the signals from the satellites to arrive at the receiver. Calculating the X-Y position is fairly simple triangulation with great distances on the triangle axes and angles in degrees. But the Z axis is so tiny compared to the X-Y axes that vertical calculations would appear impossible. Ten feet in altitude might make a few femtoseconds difference in the GPS signal timing at the receiver.

 

[SteveMann]

Which brand model instrument is used to calculate barometric pressure?

What are its accuracy and repeatability?

I'm not sure of the question.
Here is the spec sheet for a typical barometric sensor chip. [link].
The only spec we care about is the linearity. The barometer chips are expected to work typically to 60,000 ft, so the linearity of a couple of hundred feet is going to be pretty good. When you set your home point the relative altitude is set to zero. The atmospheric pressure or temperature ares unlikely to change much over the typical short flight, so for our kind of flight, the altimeter chip is pretty darned accurate.

 

[Meta4]

From what I gather the phantom uses a barometer to know it's altitude. It seems to me that GPS could give a much better indication of altitude. Any comments?

Get a handheld GPS and experiment sometime.
It's good for horizontal position but is often 100 feet or more out in the vertical axis.

 

[jumanoc]

In addition, barometric unit is required for autolandings, it is the way phantom detects ground and safely lands. Not way to perform autolandigs with gps unit only.

 

[dgd3]

Thank you for the GPS vertical explanation. I've seen that the altitude indicated on my iPad seems to be out one or two or 3 m even sitting on the ground before I take off. Is the home elevation set when the propellers are first turned on? I took off from a higher elevation and sent my phantom down the hill expecting to see perhaps a negative altitude but that didn't happen. Perhaps I didn't send it down far enough it was only about 5 m.

 

[Ross]

Altitude at switch on will often be out as the barometer measures differences in pressure. The environmental pressure changes with the weather. A pilot using an altimeter will generally set it to zero before each take off (or the known height above sealevel of his location). Some of the third party apps offer this function which is GOOD.

In practical terms I have struggled with setting waypoints accurately for altitude if launching from an elevated position. For one thing you can't set a negative altitude for a waypoint, and secondly the results of altitude settings do seem to be unexpectedly affected by an elevated launch position.

 

[4thdpi]

I'm not sure of the question.
Here is the spec sheet for a typical barometric sensor chip. [link].
The only spec we care about is the linearity. The barometer chips are expected to work typically to 60,000 ft, so the linearity of a couple of hundred feet is going to be pretty good. When you set your home point the relative altitude is set to zero. The atmospheric pressure or temperature ares unlikely to change much over the typical short flight, so for our kind of flight, the altimeter chip is pretty darned accurate.

Not sure about barometric pressure at fl600. GPS needs 3 satellites to triangulate or more. That's why us in airliners turn our barometric pressures to 29.92 from fl 180 and above. Barometric pressure is useless above this height

 

[solentlife]

Thank you for the GPS vertical explanation. I've seen that the altitude indicated on my iPad seems to be out one or two or 3 m even sitting on the ground before I take off. Is the home elevation set when the propellers are first turned on? I took off from a higher elevation and sent my phantom down the hill expecting to see perhaps a negative altitude but that didn't happen. Perhaps I didn't send it down far enough it was only about 5 m.

I often have negative shown on my GO / Litchi display as my garden slopes about 6m from house to my boat channel. The water is then about 1.5 - 2m below that.

I can get reasonably accurate readings for those differences.