How are altitude limits sensed by the DJI Naza hardware?

[Gary Seven]

Hello everyone. I'm pretty sure I know the answer to this, but would like someone to confirm. Is the altitude read by the Naza-M V.2 hardware done by a simple barometric pressure sensor? Is that how the Naza knows how to react when you set Altitude Limits in the Assistant software? If so, then the altitude sensed by the sensor must be ASL (above sea level), correct?

I ask because I live here on the Mediterranean (in Spain) and up to now all my flights have taken place at MSL. I'm preparing to take my F450 to my vineyards which lie at about 350m above sea level. As I have my limits current set to 75m I assume that my little quadcopter won't even want to take off, right?

Yeah yeah, I know I can go into the Assistant and either disable "Limits" or change the value to something like 400m, which in reality would limit my aircraft to about 100m AGL, but I was just curious how the altitude was sensed in the first place.

Cheers!

 

[Cbergen]

This altitude is based on GPS, not the baro sensor. So you should be fine.

 

[dazzab]

This altitude is based on GPS, not the baro sensor. So you should be fine.

Altitude measurements in GPS are not even close to the accuracy achieved with the baro. I'm sure the limits would be applied to a reading of what the altitude at takeoff point is so moving to a different location will make no difference. That's why you can take off from a hill, fly in to a valley and have a negative reading on altitude.

 

[Ronan]

I think the altitude is set at take off. That's why people are able to fly them everywhere in the world, even with a limit set.

I know i have had my limit set to 50m, fly next to the sea and then to a small mountain near where i live. The limit worked fine at both places, based on my IOSD MK II.

 

[Old Man]

If you have a lock on 5 or more satellites and a decent GPS unit the altitude measurement will be within a meter or so of actual altitude. GPS is far more accurate a measurement than barometric. Baro altitude is influenced by pressure AND temperature and can be hundreds of feet off of true altitude, which is why pilots have a saying that "if flying from high to a low, look out below". Altitude Hold and Loiter function best when GPS has a more that 5 to 8 satellites in view. If GPS goes down those functions become considerably less stable.

Depending upon what equipment you have altitude can be established with either a baro sensor or GPS. If you have both and a good 3D satellite lock then GPS becomes the default primary. I don't fly DJI products but the system I use shows the GPS altitude above sea level when the bird is sitting on the ground. Altitude limits are established by the number of feet above the initial take off altitude. Since I also use a distance limit as a failsafe feature that distance is also measured from the take off point, or "Home" location.

 

[Gary Seven]

Thanks for the replies, everyone. After I posted my question I started thinking about it more and realized it was silliness to think a pressure sensor could control the altitude of this vehicle so smartly. And thanks to [MENTION=12931]oldman[/MENTION]; for that adage which I completely forgot about (haven't flown a powered aircraft in over 20 years now): "if flying from high to a low, look out below."

I went back and reread the Naza-M guide, and this it what it has to say about limits:

"The flight limits function is default enabled in the NAZA-M Flight control system, it’s aimed to restrict the flying height and distance of the aircraft. The Max Height restricts the vertical distance between the aircraft and the
Home point, the Max Radius restricts the horizontal distance between the aircraft and the Home point. The default Max Height is 2000m and Max Radius is 2000m. Users can write the values of the Max Height and Max
Radius in the Assistant software, the range of the Max Height is 10m-100000m, the range of the Max Radius is the same. So that the aircraft will fly in the entered range, which is a cylinder space above the Home point."

So you're all right. The Naza-M takes it reading when establishing Home Point, and that in effect sets the starting (ie, ground zero) altitude. So I guess that clears it up. Personally, I am amazed this DJI Naza hardware (and software) works so well.

Thanks everyone!

 

[Carapau]

Actually barometric altimeters tend to be far more accurate than GPS especially over a short time frame such as the flight times we deal with when flying MRs. The way the MR deals with your elevation is through hight not altitude i.e. it is relevant to its take off point. As a result, the air pressure and temperature as well as the altitude of the aircraft at the time it is powered up is irrelevant as it sets this as zero and then flies relevant to the take off point. With all that said I suspect that most flight controllers use a bit of a mix of both sources to ascertain the current altitude.

 

[Old Man]

GPS uses time to compute signal bounce between sats and ground stations to factor distance. A good GPS system using military grade accuracy is accurate to within about an inch. All computed within seconds. Civilian GPS is detuned from military accuracy ( the government owns GPS) so accuracy is typically within a meter or so if you have a 5 to 8 satellite fix. The only thing more accurate than GPS for civilians is an inertial navigation system, or INS.

One never has to update a GPS position fix as long as they maintain good satellite locks. Baro altitude has to be continually updated via the user inputting new pressure data obtained from nearby certified weather reporting stations.


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[ovdt]

Interesting answers. Altitude is not based on GPS, it's based on barometer. For instance, on MK electronics maximum elevation is 5000m which can be sensed by the barometer. Over this elevation, the AH function will not work.

This the same with DJI FCs, but I think only DJI can provide the data.

 

[pfarmer]

I also believe altitude is based on the altimeter based on an external altimeter and gps telemetry. Max altitude in my experience can vary by about 20 feet at 400 due to temperature, wind and so on. It also appears to be influenced as expected by canopy being in place. I find I can reach a slightly higher altitude by a faster ascent and then hold there once the sticks are released. The problem with the altimeters is location in a non static pressure area which changes due local conditions at altitude and loading of the copter which requires a higher throttle setting with a resulting increase in air flow.

 

[Co_Rotor]

If you have a lock on 5 or more satellites and a decent GPS unit the altitude measurement will be within a meter or so of actual altitude. GPS is far more accurate a measurement than barometric. Baro altitude is influenced by pressure AND temperature and can be hundreds of feet off of true altitude, which is why pilots have a saying that "if flying from high to a low, look out below". Altitude Hold and Loiter function best when GPS has a more that 5 to 8 satellites in view. If GPS goes down those functions become considerably less stable.

Depending upon what equipment you have altitude can be established with either a baro sensor or GPS. If you have both and a good 3D satellite lock then GPS becomes the default primary. I don't fly DJI products but the system I use shows the GPS altitude above sea level when the bird is sitting on the ground. Altitude limits are established by the number of feet above the initial take off altitude. Since I also use a distance limit as a failsafe feature that distance is also measured from the take off point, or "Home" location.

I'm sorry, but for the GPS units we're using, this is just incorrect. Most RTK (real-time kinematic), or survey grade GPS units can barely obtain sub-meter accuracy in the horizontal (altitude). A Naza GPS measuring "within a meter or so of actual altitude" is just not going to happen. Barometers can though, absolutely.

Also, Selective Availability, which is what I presume you're alluding to when you say "civilian GPS is detuned from military accuracy", has not been in use for nearly 15 years. Civilian GPS units can be just as accurate as military models today. Effective accuracy of consumer level GPS units (like the NAZA) are in the sub-decameter range, or about 30 feet. That means that if the GPS reports your position in a given x,y location, your actual location could be anywhere within about a 30ft radius of your originally reported location.

The reason a DJI Naza controlled UAV is able to return to it's takeoff location fairly accurately is because the system combined both GPS location and inertial measurements from the onboard accelerometers to calculate it's position. Non-RTK GPS is simply not accurate enough to allow that kind of precision.

 

[pfarmer]

I'm sorry, but for the GPS units we're using, this is just incorrect. Most RTK (real-time kinematic), or survey grade GPS units can barely obtain sub-meter accuracy in the horizontal (altitude). A Naza GPS measuring "within a meter or so of actual altitude" is just not going to happen. Barometers can though, absolutely.

Also, Selective Availability, which is what I presume you're alluding to when you say "civilian GPS is detuned from military accuracy", has not been in use for nearly 15 years. Civilian GPS units can be just as accurate as military models today. Effective accuracy of consumer level GPS units (like the NAZA) are in the sub-decameter range, or about 30 feet. That means that if the GPS reports your position in a given x,y location, your actual location could be anywhere within about a 30ft radius of your originally reported location.

The reason a DJI Naza controlled UAV is able to return to it's takeoff location fairly accurately is because the system combined both GPS location and inertial measurements from the onboard accelerometers to calculate it's position. Non-RTK GPS is simply not accurate enough to allow that kind of precision.

Do you have a reference document for the Naza for the use of inertia for rth? From what I see the GPS is more accurate than 10 meters but suffers from update rate. I think an example of the concept are vehicles. The GPS signal will be lost inside of a tunnel but approximate position will be shown with the real position updated on exit, which if you watch is noticable. The vehicle has an advantage of using the speed signal as well. I test the rth frequently and most of the time rth will over fly takeoff, once it sits in a hover then it corrects and lands very close to takeoff. I am sure for altitude that the barometer is use but accuracy varies by load and if the canopy is in place. I have a secondary altimeter and it compares favorably with the Naza setpoint but only after I dampened it by placing it inside of a film canister with a vent supplied by a antenna tube.