Zero height recorded for Home Point and what if landing not at Home Point

[solentlife]

An interesting point was raised on another forum and it was answered by various. I then added to the answer to explain some anomalies that you can experience when landing out at different heights to what is recorded as Zero for Home Point.

I know regular flyers will probably already know this - but new owners may not realise the significance. This is ONLY for the Standard Phantom as it does not have VOS ...

OK ...

Standard auto land at Home Point :

The aircraft descends based only on barometric pressure and its recording of zero height. As it descends it nears the ground and just before touch down it slows to not land hard. This can be about 50cms to about 1m of ground depending on barometer ..

Auto land at point LOWER than Home Point :

Aircraft descends exactly same and slows as it approaches zero height. It is expecting to touch down but it doesn't. It continues to descend ... but it will continue to do it slow as it cannot change its zero setting and expectation to touch down. The fact it now has negative display of height means nothing to it.

Auto land at point HIGHER than Home Point :

Again we have same descend and aircraft expecting to approach ground zero and slow but if the new landing point is higher by similar or more than the original home point - the aircraft will not slow before touch down. Without the VOS system of the other Phantom models - it has no way to know ground is higher ... barometric pressure does not alter for this.

The last one of higher than home point is one I regularly encounter because of the rising ground to my house from the river bank.
I prefer to manually land anyway so its a technical point for me ... but worth passing on I think. New owners may not be aware of the limitation of the landing system in this, especially in the higher height point.

Cheers
Nigel

 

[Philltayl]

Here we go again........the barometric sensor is quite good, it senses a change in the surrounding air pressure, therefore slows the decent as it nears the ground......


It neither knows or cares what the air pressure was at the home point. simple arithmetic.....

 

[shanek]

Here we go again........the barometric sensor is quite good, it senses a change in the surrounding air pressure, therefore slows the decent as it nears the ground......


It neither knows or cares what the air pressure was at the home point. simple arithmetic.....

Is that not what nigel said?

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[solentlife]

Here we go again........the barometric sensor is quite good, it senses a change in the surrounding air pressure, therefore slows the decent as it nears the ground......


It neither knows or cares what the air pressure was at the home point. simple arithmetic.....

Can you please read the post properly.

In fact the reference is the barometric pressure at take off and this signals ground zero to the model. The barometric pressure has a standard rate of reduction used for increase of altitude and is used not only by the Phantom but also by all aircraft Altimeters based on barometric pressure. That is why the Pilot adjusts it on the ground before take-of to prevailing pressure at that time. The Phantom records it automatically.

Just using "it senses a change in the surrounding air pressure" has no reference or point of interest to relate to on its own. But it is true when referenced to the take off points barometric pressure recorded.
You must have a reference point and that reference point is the pressure at take off and ground Zero.

Nigel

 

[RodPad]

An interesting point was raised on another forum and it was answered by various. I then added to the answer to explain some anomalies that you can experience when landing out at different heights to what is recorded as Zero for Home Point.

I know regular flyers will probably already know this - but new owners may not realise the significance. This is ONLY for the Standard Phantom as it does not have VOS ...

OK ...

Standard auto land at Home Point :

The aircraft descends based only on barometric pressure and its recording of zero height. As it descends it nears the ground and just before touch down it slows to not land hard. This can be about 50cms to about 1m of ground depending on barometer ..

Auto land at point LOWER than Home Point :

Aircraft descends exactly same and slows as it approaches zero height. It is expecting to touch down but it doesn't. It continues to descend ... but it will continue to do it slow as it cannot change its zero setting and expectation to touch down. The fact it now has negative display of height means nothing to it.

Auto land at point HIGHER than Home Point :

Again we have same descend and aircraft expecting to approach ground zero and slow but if the new landing point is higher by similar or more than the original home point - the aircraft will not slow before touch down. Without the VOS system of the other Phantom models - it has no way to know ground is higher ... barometric pressure does not alter for this.

The last one of higher than home point is one I regularly encounter because of the rising ground to my house from the river bank.
I prefer to manually land anyway so its a technical point for me ... but worth passing on I think. New owners may not be aware of the limitation of the landing system in this, especially in the higher height point.

Cheers
Nigel

Thanks for the post, had not really thought about "Auto land higher than home point", so when would this happen?
I can only think it would be if you reset your home point, which I have never done.

Rod

 

[solentlife]

Its not when reset home point - but yes you would have similar. My post was based on deciding to land at different point than home position and it being different elevation.

You have option to auto land where aircraft is ... as well as RTH landing.

If you change home position for RTH - then yes of course because aircraft cannot reset barometric reference to match new positions elevation - it will assume it is same as original.

This is another reason why the other P3 models with VOS allied to barometric altitude sensing are better at this. The VOS system will detect ground as it nears touch down.

Nigel

 

[RodPad]

Go it, I never used Auto-Land ether.
Just RTH, and now Litchi, but usually hand catch it.

Rod

 

[Travisimo]

Thanks for the info. After about 30 flights since getting my P3S several weeks ago, I have never used auto takeoff or auto land, but I do use RTH quite often. So basically, you are saying to be careful if you try to use auto land in a spot that is at a higher altitude than your takeoff point because the P3S will not gently land? I assume it will land harder, but is this a risk that it will get damaged or tip over if you try to auto land at a higher point than home?

 

[solentlife]

As to how hard it would land to a higher point than home point - all I can suggest is watch how fast it descends to land during auto-land - cancelling before touch down to land manually.
Because of my sloping land where I usually fly - I prefer to manually land.
Whether it would tip over if allowed to complete the auto land and touch down ? But I think it might bounce a bit ! Its not something I would want to test !

Nigel

 

[toegema]

Is there a chance that the Standard (and the other models) also can "feel" when they are just above ground due to the ground effect giving a different load on the motors and still reduce speed on those last centimeters/inches even if you are on higher ground than take off?
Auto land and RTH is initially quite slow anyways.


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[solentlife]

Is there a chance that the Standard (and the other models) also can "feel" when they are just above ground due to the ground effect giving a different load on the motors and still reduce speed on those last centimeters/inches even if you are on higher ground than take off?
Auto land and RTH is initially quite slow anyways.


Sent from my iPhone using PhantomPilots

I doubt it very much as that would take some sophisticated algorithm to utilise.

If you think about it - Ground Effect should create a buffer that slows descent ... various aircraft over year have been built as Ground Effect vehicles ... Russians had a fast attack 'skimmer' aircraft run the Back Sea years ago at extreme high speed using ground effect. The Germans during the war investigated it.

But Ground Effect only really comes into it with our models at very low height - so low in fact it would only be the last few cms having real effect.

With Helicopters and LARGE rotor blades - we usually consider height of 1 to 1.5x blade length as ground effect area. But that's with vastly different blades and actions.

Nigel

 

[geoffN]

It seems incredible that the drone can detect a change in barometric pressure during a descent as little as a couple of feet.......I expect you're correct though.
I originally guessed that as it got within a few feet of the ground, there would be a sudden increase in pressure due to the force of the props forcing air down onto the ground and then it "bouncing" back up again, so increasing the pressure to the barometric sensor? Is that possible? I'm probably talking rubbish here!! I'll try landing on a higher spot later today.... very carefully!!

 

[geoffN]

Interesting thread some months ago on here....... P3S landing question ............................ possibly backing up my theory above? Certainly, the height indicator on my drone is not accurate to within a few centimetres that would be needed for a "barometric landing" where it slows some 30 cms above the ground.

Thinking about it, though this was some time ago, Barnes Wallis and his bouncing bomb were unable to use barometric pressure to gauge the height of their aircraft above the reservoirs...... technology has improved though I guess............... they used a different system involving 2 beacons of light from the front/rear of the plane, producing a single spot of light on the water surface at the required altitude. Now that's my history lesson for today you'll be pleased to here lol!!!

 

[With The Birds]

Interesting thread some months ago on here....... P3S landing question ............................ possibly backing up my theory above?

From what I can see it's all supposition. That thread is if no help.

 

[solentlife]

It seems incredible that the drone can detect a change in barometric pressure during a descent as little as a couple of feet.......I expect you're correct though.
I originally guessed that as it got within a few feet of the ground, there would be a sudden increase in pressure due to the force of the props forcing air down onto the ground and then it "bouncing" back up again, so increasing the pressure to the barometric sensor? Is that possible? I'm probably talking rubbish here!! I'll try landing on a higher spot later today.... very carefully!!

Interesting ...

I tried out the auto land and found the change in speed to be there but not excessive - so leaving DJI to land out fully should be without incident. As the change in speed is small - the effects of change of elevation would likely be minimal - but they are still there. For practical purposes if you let the DJi system land out wherever it would be fine.

My intention with starting this thread was information for those who wish to understand - it doesn't alter significantly the way we fly the P3S .. but its nice to know more about it. DJI have produced a model that basically holds the hand of the owner - mmmmm yes well we will ignore the fiasco of FW and Go over the last months !! So flying it can be literally no knowledge or as much as you wish to take in.

Note I say Practical purposes. Which at end of day is all DJI can cater for in such budget machine.

Note also - this only applies to significant elevation change - such as take off from a hill top and land at foot of hill or vice versa.

Note also this only applies to Full Auto Land ....

I agree with "With the Birds" that unfortunately speculation does come into a lot of DJI functions because DJI chooses to not answer questions consistently to have exact data. This leaves people such as myself to go out and fly-test to see what happens. I also fall back on my previous careers training as Navigator where such matters as barometric computation is taught and important.

Nigel

 

[solentlife]

For anyone interested in accuracy of barometers ... you may be surprised to know that a Precision Aneroid Barometer can in fact detect change from moving to / from a desk to the floor.

VIRTUAL TOUR OF THE WEATHER STATION

as an example ... there are even more accurate units than this ...

Barometers are not only those domestic type with RAIN .. DRY ... etc. and a needle that may move if it gets enough shove !!

RC Glider boys use an electronic version in their gliders which likely is similar to DJI use in the P3 - to register height etc.

Eagle Tree and various other companies produce such items and accuracy is actually high.

Quick google will confirm the units mentioned here ...

Nigel

 

[geoffN]

For anyone interested in accuracy of barometers ... you may be surprised to know that a Precision Aneroid Barometer can in fact detect change from moving to / from a desk to the floor.

Not wishing to doubt you here......i'm sure you're more of an expert than me. Googled these barometers .......... The one I was looking at....... it wasn't a cheap one at over £1000!!........... " Range: 900mb to 1060mb x 0.5mb and 670mm to 790mm x 0.5mmHg with an accuracy of +/-0.7mb". I can't see that with that accuracy, it could detect a change in air pressure over a couple of feet?. Can't also see Dji sticking a £1000 barometer in their £400 drone either!!!

 

[solentlife]

Precision Aneroid can be in dial form or electronic to interpret the data from the capsules.

Its a daft situation actually - I own a Laboratory and I have to pay thousands for test gear. Even a DVM costs me more than any of the DJI drones - why ? Certification and Testing of the apparatus to allow it to be used to produce results. But stupid thing is - a cheap $5 DVM can often provide same result.

I was only illustrating that Barometers have moved on a lot since the days of the Tap Tap Dry - Rain jobs most people have hanging on the wall. (In fact you shouldn't TAP TAP a barometer ... nudge it ok .. but not hard tap)

Plus the electronic versions produced for Glider boys and other hobbys are actually surprisingly accurate and small in size ... easily achieve what DJI would need in the P3 and cheap enough.

When I took my Aural Examinations for Licences - the electronic Precision Barometer was common question to operate and read ... believe me - that CAN read the change from desk to floor and was demonstrated many times to myself and other examinees.

Here is the 'simple' one of years ago ... still used by Weather Stations to calibrate others but compared to todays updated units less accurate.

Nigel

 

[solentlife]

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

 

[solentlife]

Watch the numbers at bottom of screen ..

Nigel