I have an odd question...

  

If I fly in GPS mode and I do this:
It is a phantom 3 standard. I want to take off from a bluff/cliff over looking the beach/water.  I take off from the bluff and want to fly down to the beach which is about 100 feet below where I take off. My wondering is, doesn't the drone record the height of the takeoff spot. If so what happens when I dip below that height of original takeoff? I would think the drone's command would be to turn off the engines ....thus a free fall to the beach. Am I not understanding this correctly? This is a phantom 3 standard.

The starting point will be the altitude 0, when you fly below the cliff the marked altitude will be negative. If you are near the sand and activate the RTH the drone will rise to the altitude 0 + the height you have programmed for the RTH.

It would fly down just fine, provided the wind doesn't slam it into the cliffs.  The  height is relative to the home point so it would show up as a negative number.  

no , it will continue to fly ,   altitude will be recorded as a negative number , take off spot will be recorded as 0 feet , everything is relative to take off spot ,    not actual height above sea level ,   if RTH is activated from a negative altitude reading  your P3S will climb to take off height then continue to climb to  your RTH setting  altitude  then will come back down to take off spot .

kndll Posted at 2017-2-20 07:30
no , it will continue to fly ,   altitude will be recorded as a negative number , take off spot will be recorded as 0 feet , everything is relative to take off spot ,    not actual height above sea level ,   if RTH is activated from a negative altitude reading  your P3S will climb to take off height then continue to climb to  your RTH setting  altitude  then will come back down to take off spot .

Thank you for the reply. Its a bit confusing because when I land normally and hold the left stick down while on the ground it turns off props. I would think it would turn off props once it it the 0 altitude. Does the p3s sense the touch of the ground? I wouldn't think so.

Mark The Droner Posted at 2017-2-20 07:24
It would fly down just fine, provided the wind doesn't slam it into the cliffs.  The  height is relative to the home point so it would show up as a negative number.

I would make sure I am some distance from the bluff. I can't wrap my head around why it would not turn itself off though? Thank you

Da Lata Posted at 2017-2-20 07:24
The starting point will be the altitude 0, when you fly below the cliff the marked altitude will be negative. If you are near the sand and activate the RTH the drone will rise to the altitude 0 + the height you have programmed for the RTH.

Just sounds odd that it would not turn off the props once it hit at 0 or below. Thank you for the info.

The motors turn off when aircraft fails to descend more after about 3 seconds ...

The aircraft motors are running slow to descend. The aircraft lands. It stops descending even though motors are still running slow. Software detects altitude is not changing and after a few seconds switches of motors. It has nothing to do with displayed height.

I regularly fly low in my boat canal showing negative altitude. As another says if you hit RTH - aircraft will rise to positive altitude you set and then go to home point.

Nigel

No - altitude has nothing to do with the motors and vice versa.  What turns the motors off is holding the left stick down and then the AC's descent to stop - which would be landing.  The motors stop after several seconds, provided the AC landed and stayed mostly upright.  

Mark The Droner Posted at 2017-2-20 07:47
No - altitude has nothing to do with the motors and vice versa.  What turns the motors off is holding the left stick down and then the AC's descent to stop - which would be landing.  The motors stop after several seconds, provided the AC landed and stayed mostly upright.

................... mmmm didn't I say something like that ??



Cheers
Nigel

You posted while I was typing

Mark The Droner Posted at 2017-2-20 07:56
You posted while I was typing

Fair dos !!

No problem ... at least OP has a comprehensive number of replies all pointing to similar !!

Cheers
Nigel

solentlife Posted at 2017-2-20 07:49
The motors turn off when aircraft fails to descend more after about 3 seconds ...

The aircraft motors are running slow to descend. The aircraft lands. It stops descending even though motors are still running slow. Software detects altitude is not changing and after a few seconds switches of motors. It has nothing to do with displayed height.

The motors turn off when aircraft fails to descend more after about 3 seconds ...

Now that makes logical sense to me...I knew there was some reasoning with why it did what it did. Thank you all for the replys.

I say 3 secs ... could be slightly less or more ... but that's the principle ... it detects aircraft stops descending even though motors are slowed to descend.

BUT there is one slight change to its normal landing when landing BELOW Home Point height. (This does not apply to the more advanced Phantoms with VOS systems for height combined with barometrics).

The aircraft descends and as it nears zero altitude - it slows so it doesn't land hard. I have noticed that landing out below Home point height - the aircraft can take longer in that negative altitude as it expects to land and not continue descent ! It basically slows as it nears zero height .. and stays slow all way to ground ...

Its one of the reasons I usually manually land after using RTH to fly back.

It also affects if you want to land HIGHER than Home point. My estate has rising ground from the river and if I land out nearer the house - I can be several metres higher. The aircraft if left to auto land can in fact not slow before touchdown ... so again I usually change to manual landing to avoid this. Why ? The aircraft usually slows about 1m or so of the ground before touchdown IF at its Home point and zero height. If you are now auto landing at a point that has height higher by more than that 1m or so - the aircraft does not know. The Standard does not have the VOS system the other P3 versions have. It relies on that zero Home setting and barometer.

My opinion is it is better to manually land whenever possible.

Nigel

solentlife Posted at 2017-2-20 08:19
I say 3 secs ... could be slightly less or more ... but that's the principle ... it detects aircraft stops descending even though motors are slowed to descend.

BUT there is one slight change to its normal landing when landing BELOW Home Point height. (This does not apply to the more advanced Phantoms with VOS systems for height combined with barometrics).

"I have noticed that landing out below Home point height - the aircraft can take longer in that negative altitude as it expects to land and not continue descent !"

This doesn't make any sense.
Just like a real plane, the Phantom doesn't have to land at the place it was launched.
It can land anywhere, higher or lower than launch point and the recorded home point is only lat/long with no altitude information.  
The Phantom does not "expect" to find solid ground anywhere.

I know the altitude is limited to +500m but is there a lower limit if you descend?

Cabansail Posted at 2017-2-20 15:34
I know the altitude is limited to +500m but is there a lower limit if you descend?

I've seen people say there is but never seen a report from anyone that actually experienced it.

Yeah I think I recall something about minus 100 meters ?  Who knows...

Edit:  I think the smart money's on msinger

chainmon Posted at 2017-2-20 07:42
Just sounds odd that it would not turn off the props once it hit at 0 or below. Thank you for the info.

It does not because many people fly from areas where they launch from a higher point and fly below that.

I've flown this valley 50m below my take off point.   Actually sometimes when flying in clear air space it's easier to spot your drone at distance when it comes back in below your launch height.

Labroides Posted at 2017-2-20 15:19
"I have noticed that landing out below Home point height - the aircraft can take longer in that negative altitude as it expects to land and not continue descent !"

This doesn't make any sense.

Actually sorry to say this but wrong.
The AC barometric sensor takes the pressure at the home point zero altitude as its reference. It then uses this for cakculation of height during the flight based on the standard pressure change per increase / decrease of altitude - same as aircraft pressure altimeter.

When you auto-land the pressure change tells the AC how high of the ground it is using that original pressure at home point as its reference for zero height. As the AC calculates pressure and believes its near touch down it slows descent to 'safe' speed.

If this is not true - then please explain how the AC knows to slow its descent --- what does it base this on ? Being the Standard - it only has the pressure sensor to use.

Basically in the auto land - the pressure change is controlling the descent allowing quick descent until near ground when it then slows it.

This then means that a landing point lower than home point will have AC descending past that slow down point at slow rate as AC expects to land out. It has nothing to do with AC whether it detects ground - which it cannot - its all to do with the zero altitude reference at take-off.

Blimey - its not hard to work out why !

Nigel

solentlife Posted at 2017-2-20 22:24
Actually sorry to say this but wrong.
The AC barometric sensor takes the pressure at the home point zero altitude as its reference. It then uses this for cakculation of height during the flight based on the standard pressure change per increase / decrease of altitude - same as aircraft pressure altimeter.

And Nigel ... I'd say that you are wrong (again).
You've assumed that DJI designers intend the Phantom to only land where it was launched.
But why would they do that?  
And if they did, wouldn't they explain this in the manual?

The stored home point data contains no altitude information and with changing air pressure over the time of a flight and aircraft heating up, it's very common for altitude measurements to be out significantly at the end of a flight.

Your idea "that a landing point lower than home point will have AC descending past  that slow down point at slow rate as AC expects to land" doesn't make any sense.
You say "its not hard to work out why".
Well I can't see why.  Perhaps you can enlighten me.
You will need to consider that there's an equal chance that a Phantom landing away from ground zero may be landing higher than launch point.
If it was important enough to program things to allow for landing lower than launch, why not for higher too?

Labroides Posted at 2017-2-20 23:38
And Nigel ... I'd say that you are wrong (again).
You've assumed that DJI designers intend the Phantom to only land where it was launched.
But why would they do that?  

Barometric pressure change over a flight time of average 15 - 20 mins ? Are you serious ?

Unless you are taking off when a SERIOUS frontal system is passing through - which is highly unlikely as the winds would tell you not to .. or a hurricane or tornado hits during your flight - your reasoning is flawed.

Please  explain how the AC knows to slow its landing just above ground when returning to Home Point ?

Please explain how the AC knows to report height during the flight ?

Please explain how the AC can report back to App negative height ?

They ALL reference to the barometric pressure detected at Take Off at the Home Point. That is taken as GROUND ZERO.

The only time barometric pressure does not fully control height stability is when IMU detects rise and fall acceleration of the AC ...

The Laws of Physics cannot be ignored to save your argument against what is obvious to anyone who cares to consider HOW the AC works out altitude.
I have explained why a reference point is used to determine altitude .. I have explained that increase and decrease of altitude is detected by a standard calculation used by anyone involved in barometric computations including Altimeters.

DJI have designed a model that will perform AVERAGED flight patterns. It is impossible for a budget machine to have built in compensation to cover all eventualities. The landing descent speed is reasonably low at height to cover most situations. But there are exceptions and I was illustrating those.

The Advanced and Pro have VOS to compensate for lack of GPS and also errors in Barometric system. Why do you think DJI would bother to go to that trouble ? Just to give indoor flying without GPS ? Of course not. Its to increase the capability beyond the ability of Barometric Pressure as well as lack of the GPS. Why do you think there are reports of switching of VOS when flying over water ?

It now befalls you to explain how they cannot be used. Instead of just saying I'm wrong - try getting some phsycial evidence / proof that I'm wrong.  I think I will be waiting a very long time before that happens ..

Nigel

Quote : "The stored home point data contains no altitude information"

That cannot be true as how does AC know when its zero altitude ? It must have stored a reference figure.

Nigel

solentlife Posted at 2017-2-21 01:47
Barometric pressure change over a flight time of average 15 - 20 mins ? Are you serious ?

Unless you are taking off when a SERIOUS frontal system is passing through - which is highly unlikely as the winds would tell you not to .. or a hurricane or tornado hits during your flight - your reasoning is flawed.

No ... you've decided that it's important enough for DJI's designers to have programmed it specially.
But there's no reason they should, particularly when it would only work half the time if the did.
The Advanced and Pro work just the same whether they have VPS (it's not VOS) switched on or off.
In landing (wherever they land) the Phantom just descends until it can descend no more.
And the idea of switching it off when close over water is because the reflective surface confuses it.

You've locked your thinking into the silly idea that your Phantom has to land at the launch point when it can land just as easily anywhere.
There's nothing special about the level of the launch point.

quote : "No ... you've decided that it's important enough for DJI's designers to have programmed it specially."

?? Where do you get that from ? I actually said they designed an average machine to a budget ... don't twist my words.

quote"The Advanced and Pro work just the same whether they have VPS (it's not VOS) switched on or off."

To the observed - but it is using its extra sensory units VPS if you prefer the term - as well as barometrics.

quote"You've locked your thinking into the silly idea that your Phantom has to land at the launch point when it can land just as easily anywhere."

That is a ridiculous statement to be honest. Where have I said that it has to land at home point ? I am illustrating why the landing can differ when not at home point. Of course the AC can land anywhere you choose or it if it failsafes etc. etc. Again you are selectively twisting words and taking out of context.

quote "There's nothing special about the level of the launch point."

The only two points about Home point (launch point as you put it ) is Geographical position in reference to GPS and Elevation as reference for altitude. If the AC did not record those two ... a) it could not RTH and b) it could not give you altitude during flight etc.

I give up with you to be honest as you obviously have no intention of taking the 'blinkers of'.  You've failed to even answer ONE question ... you've failed to provide any data to support your argument. I'm tired of repeating Physical Reasoning for altitude computation by barometric Pressure to you.

I wish you great flying and videos ... I know once the weather improves I shall be out there enjoying flying my P3S ... and landing it at varying elevations ...

Nigel

solentlife Posted at 2017-2-21 02:24
quote : "No ... you've decided that it's important enough for DJI's designers to have programmed it specially."

?? Where do you get that from ? I actually said they designed an average machine to a budget ... don't twist my words.

There are errors in your reasoning but there's no need to get tangled up with them.
The single point where your whole idea comes apart is this.

You've started with an assumption that DJI would go to the trouble of making the Phantom want to or expect to land at the same level as it's home point.
And you've gone from that to build your convoluted hypothesis.

But why would they bother when it wouldn't work at all in cases where the Phantom landed at a higher point ... and the Phantom can land quite well enough at a higher point?

This guy flew nearly 8km from top of a mountain and landed 1930m below take off point with his Phantom

Hope this ends any arguments.

Tum de tum de tum ... horses to water .....

Hi BC ... nice one - answers the negative nicely.

I have to admit that I wondered if like many electronic systems a max +ve setting may be mirrored at -ve ... but it appears not the case here. That's good.

Nigel

solentlife Posted at 2017-2-21 03:50
Tum de tum de tum ... horses to water .....

You took the words out of my mouth.
Looks like the concept is just too hard for you to take in.
I've put it in front of you multiple times and won't waste any more efforton it.

solentlife Posted at 2017-2-21 01:52
Quote : "The stored home point data contains no altitude information"

That cannot be true as how does AC know when its zero altitude ? It must have stored a reference figure.

I missed this earlier.

The stored home point used for RTH has lat/long data but no altitude data.
Of course the altitude is stored for reference purposes for telemetry heights but it is not used for RTH landings - because the Phantom doesn't care what level it is going to land at.
You could dig a 5 metre hole or put a 5 metre tower at home point while flying and the Phantom would have no problem with that.
It just descends until it feels the ground below its feet - at whatever level it lands - home or anywhere else.

Funnily enough, there was another discussion on a Phantom forum discussing a similar matter which makes interesting reading.  http://www.phantompilots.com/thr ... 07460/#post-1107455            I must admit, I tended to agree with Labroides here in that the drone can in fact "sense" the ground through the increase in air pressure as the "thrust" from the props comes off the ground.  Did a bit of reading about "precision barometers" that Nigel suggested could detect air pressure differences of a couple of feet.................. certainly the ones I looked at (costing £1000+)  did not have sufficient accuracy to achieve this.  Even if one was capable of such minute variations, I can quite see air pressure changing by this  minute amount over a short period of time... even the slightest breeze could throw this off and certainly the wind from the props would make it totally inaccurate for landing.  Certainly, the barometer on my drone is usually off by several feet and to try landing with it would be a certain recipe for disaster and a new set of props!!

GeoffN Posted at 2017-2-21 10:10
Funnily enough, there was another discussion on a Phantom forum discussing a similar matter which makes interesting reading.  http://www.phantompilots.com/threads/zero-height-recorded-for-home-point-and-what-if-landing-not-at-home-point.107460/#post-1107455            I must admit, I tended to agree with Labroides here in that the drone can in fact "sense" the ground through the increase in air pressure as the "thrust" from the props comes off the ground.  Did a bit of reading about "precision barometers" that Nigel suggested could detect air pressure differences of a couple of feet.................. certainly the ones I looked at (costing £1000+)  did not have sufficient accuracy to achieve this.  Even if one was capable of such minute variations, I can quite see air pressure changing by this  minute amount over a short period of time... even the slightest breeze could throw this off and certainly the wind from the props would make it totally inaccurate for landing.  Certainly, the barometer on my drone is usually off by several feet and to try landing with it would be a certain recipe for disaster and a new set of props!!

Funnily enough?
It's just the same person we have here spouting his unscientific opinion based on poor observation and testing to a different audience.

You are correct about the variation in indicated altitude over the duration of a flight and that's one point that Solentlife doesn't consider because it shoots a big hole in his theory.
That's just one of a number of reasons why DJI could not and would not need to program anything like he has suggested.

ps.     I didn't suggest the Phantom senses the ground by anything other than accelerometers sensing no descent despite the left stick being held down.

I would think that the aircraft senses the ground effect near the ground using its accelerometers and attempts to slow the aircraft down. This would appear to be why the aircraft appears to bounce at a couple of feet off the ground when brought down too fast. Given the innacuracy of the barometer, it could not rely on that to slow the aircraft on descent. And that is easily proven when you see the false readings it often displays.

I suspect that just using an accelerometer, it would cause problems whenever the drone is descending very slowly from greater heights?   Interestingly, the "precision aneroid barometers" that Solentlife mentioned for detecting the last couple of feet from the ground.......... these only have an accuracy of about .015 at the best ............. I would have thought this totally unsuitable for slowing things down a couple of feet from the ground.

GeoffN Posted at 2017-2-22 01:10
I suspect that just using an accelerometer, it would cause problems whenever the drone is descending very slowly from greater heights?   Interestingly, the "precision aneroid barometers" that Solentlife mentioned for detecting the last couple of feet from the ground.......... these only have an accuracy of about .015 at the best ............. I would have thought this totally unsuitable for slowing things down a couple of feet from the ground.

Probably ground effect only from prop wash slows it then?

bugbee Posted at 2017-2-24 12:27
Probably ground effect only from prop wash slows it then?

Ok, post #33

For anyone interested .... video will be uploaded later once processed.

I flew my P3S and after doing a range test decided to land in a different spot of known elevation. My grounds have been twice Topographed due to building works and required by local authority.
The landing point being noted on the plans as 4m lower than the takeoff point.

Litchi app displayed -4m as it touched down .... but interestingly once motors switched off - the altitude reset to zero.

I'm going to be trying other combinations out of pure interest .... just to see results.

Nigel

Yes once motors switched off if you take off again you create a new home point so the altitude latitude longitude are different