Lost mine today, RTH failed.

[JonEQuest]

I am a total newbie and this is my first post. I just got my P3A 2 weeks ago and have only flown it about 10 times. What I quickly realized is that my RTH altitude needs to be a good bit higher than ANYTHING in my area. For example I live near a church with a ~100' tall steeple. I thought, "what if I use RTH or lose signal and my bird come straight for that steeple?" I need to be higher, 110' is not enough because it is not perfect at judging altitude all the time so set my RTH altitude at about 40m or 130' vs. the stock 30m setting. I am asking the more experienced fliers, is there anything else I should take into consideration for safer RTH?

 

[WetDog]

You're mostly there. You've made the connection that most newbies fail to make. The Fine Manual is confusing - DJI's technical writing staff could well use some improvement (as can many, many other manufacturers) but making sure your RTH height is appropriate is the big one. Another RTH gotcha is the three different TYPES of RTH. (RTFM).

The only other RTH gotcha is that DJI is a card carrying member of the Flat Earth Society. Altitude is RELATIVE TO YOUR HOME POINT. So, if the church steeple is 100 feet high, but the church ground level is 60 feet higher than your home point, you need an RTH of at least 160 feet to clear it (plus a fudge factor as you are doing).

 

[JonEQuest]

Funny you mentioned that. I was just flying and the Phantom said it was at 86' but it was a little higher than the steeple. Me and my friend thought the steeple looked taller than 85' so to be well above it at 86' seemed off. So when it is telling me altitude in telemetry that is relative to the height where I took off and not where it is now? So if I understand you correctly, say I am 1 mile away and the craft is actually 150' high it may say only 50' because that area is 100' lower than where I am at?

 

[impilot51]

Your correct. The altitude shown in telemetry is relative to the recorded home point recorded at take off. That is zero feet. It has nothing to do with the actual above ground altitude at its current location.


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[skyeboysteve]

I'm sure I'm betraying a bit of ignorance with this question but here goes. Why is compass calibration relevant when flying in GPS mode? I haven't looked at the compass in my boat since the day I got GPS. Wouldn't it just follow the shortest path to the logged GPS point (after ascending to RTH altitude) regardless of what the compass reads? The only reason I did the recalibration, aside from thinking I was playing it safe by following the app's suggestion, was because the app wouldn't allow the motors to start until I did a successful recalibration, which many are concluding was the cause of my loss. Snerd says, "end of story" but I remain unconvinced.

I appreciate all of the informed opinions here but if someone could steer me to a resource for deeper understanding than is provided by DJI videos and manual, I would appreciate it. I haven't yet decided to buy another one but I know I need to learn more before burning another thousand+ dollars. Thanks.

I used to wonder the same thing, as navigation from one point to another in a straight line is simple with GPS and requires no compass. So why is a compass so important to a Phantom?

The phantom has no idea which (and how much) each of its four motors to adjust to move from point A to point B unless it knows where's its motors are in 3D space. With an airplane or boat, you know where your propulsion is pointed relative to the horizon and your destination bearing and can direct it accordingly by referencing the horizon outside (or attitude indicator inside) thanks also in part to having a "pointy-end". Combine that with GPS and you simply align your ground track line with course line and any crab angles needed are automatically inherited.

As Ianwood referenced before, the Phantom uses an AHRS to know where its motors are pointing where they are relative to each other and earth. AHRS uses the magnetic field in all THREE dimensions... Not just knowing where north is, but it uses the magnetic lines of flux in all three dimensions to determine its attitude on all three axis. It can't rely simply on an array of accelerometers to determine which way is up, (like what you have in your smart phones; or something like a bubble level) due to errors incurred by its own acceleration as it is constantly adjusting itself to compensate for the environment and control inputs. Some systems can calibrate themselves simply by cross-referencing a current database of geo-coordinates with their known magnetic lines of flux in that vicinity. For example, at magnetic north, the magnetic lines of flux are almost perpendicular to the horizon, and thus one's AHRS needs to be calibrated accordingly (there's of course variations between magnetic north and true north [that GPS is based on] depending on your locale--but that goes without mentioning other than to say that's also calibrated of course). Near the equator, the magnetic lines of flux are generally parallel with the horizon. Instead of using a giant database, DJI has opted to just have us operators calibrate locally to get a general sense of what earth's magnetic lines of flux are doing in your vicinity.

 

[skyeboysteve]

The GPS knows the current position of the P3 and the home position.The shortest path to the home position is distance and direction. The GPS doesn't understand azimuth information, i.e. it doesn't know which way the P3 is pointing. The compass is required to fly the P3 in a particular direction.

The GPS on your boat probably computes a direction from successive location updates coming from the GPS. Given two locations it's possible to compute the distance and direction. The navigation controller on the P3 can't work this way because the tolerances are much smaller than when controlling your boat.

Yes, GPS receivers compute your track by how you're trending from its frequent updates. On a boat, you inherently know your orientation in space once you've identified your relative position to the bow (usually somewhere behind it) and that you're still floating right side up. Navigation requires not only knowing your own orientation (which is often not even considered since it's usually so inherently obvious--especially on surface vehicles), but of course also knowing the difference between where you are and where you want to be.

 

[ianwood]

The simplest way to understand why GPS alone is not enough is to understand the difference between course over ground (GPS) and heading (compass).

 

[dhsprogis]

I'm sorry for your loss.
I have had three instances of signal loss and RTH. The first two were easy and while I was freaking out, the drone did come home. I assumed control for the landing. The third was troubling. I had been flying at about 100 ft with no issues. When I returned home, I had 50% battery so decided to climb for a look around. I was only at 300 ft when my drone began drifting backward - I think I was in a stiff wind current. I lost signal, the drone must have gone into RTH, dropped to 100 ft and started home. Fortunately, I got the signal back and saw that it was coming back. Unfortunately, it did not make it before it hit 3% battery or whatever the low threshold is and landed where it was. Unfortunately, it landed in a tall tree. Fortunately, it fell out of the tree and landed in a bush below. It was undamaged. My heart might have been another matter!
I have witnessed home-point issues. I have heard the app signal that the home point is updated but the GO app does not represent it properly. Existing camera mode and returning into it seems to sync it.

 

[Meta4]

guys, I had a few days ago more or less the same issue : after 10 minutes flying I hovered the Phantom to think what I could manouvre next. Suddenly the Phantom started slowly landing.
Battery at 16%, distance 10 meters, hight 5 meters. Enough for 2 times RTH. The App showed me : communication loss. Took about 10 minutes to recover a communication. Around 200 Meters circle were no metall or radio equipments. The day before I did a RTH manually and automaticaly, as I am a newbee and have to train and get used to these things.
Nothing in my mind I made wrong. But that the Phantom did not made a RTH this time broke my trust.
I am living on an island. So I will fly very often over water. If the Phantom would have landed in water.... Landing right there, automatically could be fatal. So why not hovering until it gets communication again ? Yes crashing in a worse place after the battery is empty could happen, but there is a bigger chance to get communication back !

I don't quite understand your post but it looks like you activated RTH close to you.
This highlighted section of the manual explains this.
I recommend experimenting with RTH more to be comfortable with its use.

If you were at 16% battery, that may also have been an issue.
16% is very low and it is not safe to be flying at such a low level.

 

[Phill2]

From what I've read you need to do a compass calibration before every flight to be safe. You also want to make sure the rth attitude is set to clear any obstacles on the way back. RTH does not bring it in to where you took off from if it's within 30 meters it just lands where it is. Hopefully someone more knowledgeable can chime in.

Mine lands exactly where it took off from

 

[JonEQuest]

Mine has also landed within 5-10' of where it took off from every time for me so far. I have been using RTH just to test it during about half the flights I have taken. I also always come home and land when my battery gives the low warning at 30%. The lowest I have ever been at when landing was 24% and it was 50' from me for the final minutes after the 30% alarm went off. I am new but everyone should decide what RTH parameters work best for you. Hover, Land, or Come Home are the 3 main options. I have my RTH altitude at 200' to make sure I can clear anything within 2 miles of my home. P.S. I am new to DJI batteries but not batteries in general. These batteries will simply shut of when they get low. Always land before that can happen. My lithium powered drill simply stops instantly when the battery is low. That is probably what these are doing when they fall from the sky.

 

[Phill2]

Mine has also landed within 5-10' of where it took off from every time for me so far. I have been using RTH just to test it during about half the flights I have taken. I also always come home and land when my battery gives the low warning at 30%. The lowest I have ever been at when landing was 24% and it was 50' from me for the final minutes after the 30% alarm went off. I am new but everyone should decide what RTH parameters work best for you. Hover, Land, or Come Home are the 3 main options. I have my RTH altitude at 200' to make sure I can clear anything within 2 miles of my home. P.S. I am new to DJI batteries but not batteries in general. These batteries will simply shut of when they get low. Always land before that can happen. My lithium powered drill simply stops instantly when the battery is low. That is probably what these are doing when they fall from the sky.

I too come home at 30% but when I was hovering in my yard to run the battery down it landed itself at 20%
Should I set it lower to get home if a long way off at 30% or is 20% the minimum for flight?

 

[gklugie]

DO NOT RELY ON RTH (This is a choice of last resort if you have lost orientation or control.)

If you cannot fly yet, ........ buy a cheap drone and get some stick time. Buy a PC based simulator program. You need to be in control at all times. To me, this means visual orientation and control. Always.

Lots of fellows buy these things (Phantoms) thinking all systems are perfect and failsafe. They are not. These things are not the military drones you see on TV with pilots "flying" on a big screen monitor half a globe away.

Just because it has a "take off" button, and a "return home" button, does not mean these features always work. Especially the way you think they should work.

Learn to fly. Fly the device.

 

[JonEQuest]

I believe that most of us understand that these are not perfect. I have never relied on RTH but I am definitely more confident that it is there. I started out with a Estes Proto-X FPV and wrecked it dozens of times taking chances and learning so I could avoid making dumb mistakes with my Phantom. I am glad I started with the cheap one. It makes me appreciate how much the P3 does for you so you can enjoy flying. The fact it counters wind alone is a godsend.

 

[User6895]

I am new to flying drones and have yet to fly my p3p out of my line of sight. So have not had a need to use the RTH feature. I like to fly mine myself and land it. Maybe some day I might try flying it out of sight. Do a lot of people fly their drone by camera only?


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[RoyVa]

Hope this helps:
Why Calibrate?
Compass calibration is important to safe, controlled flight. It compensates for changing background magnetic "noise", a.k.a. magnetic inclination and deviation (not to be confused with declination). Inclination and deviation that isn't corrected through compass calibration will cause inconsistencies between GPS and compass that can result in "toilet bowl effect", a swirling motion that can cause the Phantom to fly out of control.

What is Magnetic Inclination and Deviation?
Magnetic deviation is a horizontal variation that comes from the Phantom itself and the equipment you have installed on it as well as the magnetic makeup of the area you are flying in (again not to be confused with declination). Sometimes the deviation will be insignificant, but other times it can be big enough to cause you to lose control. Inclination is a vertical magnetic variation that shifts depending on where you are.

Warning Signs
The Phantom can detect when the compass is providing extremely poor (implausible) data. This typically occurs if you place it near a strong magnetic field or do not calibrate it properly. It will flash red and yellow lights and will not start the motors when this happens. Unfortunately, it can only detect this in extreme conditions and you can still fly with really bad compass data if you're not careful.

Another important safeguard is the compass mod value. This is the total magnetic field as measured by the sensor. You can check this with the Phantom Assistant software. According to DJI, it should be above 750 and below 2,250 but ideally it should be between 1,000 and 1,700. Between 1,200 to 1,500 is very good. Check it away from magnetic influences. If it reads very high or very low, check it again in a different location. If it is still off, it could be magnetized and need degaussing or it could be damaged.

What Does Calibration Actually Do?
Calibration measures the magnetic fingerprint of the surrounding area. By turning the compass 360 degrees, the Phantom can see where the compass reading doesn't smoothly increase or decrease. It uses this information to build an adaption table so that when the Phantom turns during flight, the reading is smooth and linear.

When Should I Calibrate?
You do not need to calibrate before every flight and in some cases you definitely should not calibrate. That doesn't mean you shouldn't ever bother doing it. It only takes one time for it to go very wrong. The most important aspect of compass calibration is making sure the magnetic "noise" around your Phantom is consistent between calibration and during flight.

• DO Calibrate
  • If you go to a new location that is a good distance (i.e. >100 miles) from the last place you calibrated the compass.
  • If the terrain has changed significantly i.e. going from prairie to mountainous.
  • If you change any equipment on your Phantom.
  • If you just installed new firmware.
  • If you just degaussed your compass (BTW, don't degauss unless you are absolutely positively sure you need to).
  • If you have taken all the precautions to make sure there are no localized magnetic fields near you.

• DO NOT Calibrate
  • If you're in an urban area surrounded by concrete, buildings, and hidden or overhead power lines / pipes / etc.
  • If you're on the beach or on a boat.
  • If you're in immediate proximity to metallic objects or anything magnetic.

• Pre-Calibration Checklist
  • Everything used in flight should be powered up during calibration, e.g. GoPro, tracker, etc.
  • Remove all metal from within 5 - 10ft radius, e.g. watch, phone, belt buckles, coins, controller, etc.
  • Calibrate on grass or dirt and not on concrete, asphalt, in or on a building or structure.
  • Calibrate on a level surface if possible.

• How to Calibrate (P1 and P2. P3 check the manual.)
  • Power up your Phantom and accessories as normal.
  • What for the Phantom to complete home position and heading recording (2 sets of green flashes)
  • Flip S1 five times between the top two positions. Check to see the Phantom lights are solid yellow.
  • Pick up the Phantom and turn it smoothly and steadily a full 360 degrees until the lights turn solid green.
  • Point the front of the Phantom straight down and repeat until the lights turn off and resume normal flashing.
    Note: Don't be concerned if your gimbal reacts poorly to being face down, keep turning as normal.
  • Optional: power off and restart Phantom.
  • Enjoy your flight!

If for any reason, you do not complete any of the above steps smoothly and evenly, restart the process.

 

[Meta4]

I am new to flying drones and have yet to fly my p3p out of my line of sight. So have not had a need to use the RTH feature.

The time to learn how RTH works and how to initiate it, is now ... before you need it.
The day you need RTH, you have to know how to use it.
Practice in safe controlled conditions to be come familiar with RTH and how to cancel it to resume control yourself.

 

[Erroll]

Calibrate on a level surface if possible.

Very comprehensive write up!

Out of curiosity, how does not being on a level surface affect calibration?

 

[Dirty Bird]

Good thinking. What I recommend is climbing straight up with the camera level. Pan around slowly ensuring ALL obstacles within your flight area are BELOW the horizon. Gain sufficient altitude to meet this requirement, note the altitude, & set this as your RTH altitude. Then you can be confident you won't hit something during a RTH event.

I am a total newbie and this is my first post. I just got my P3A 2 weeks ago and have only flown it about 10 times. What I quickly realized is that my RTH altitude needs to be a good bit higher than ANYTHING in my area. For example I live near a church with a ~100' tall steeple. I thought, "what if I use RTH or lose signal and my bird come straight for that steeple?" I need to be higher, 110' is not enough because it is not perfect at judging altitude all the time so set my RTH altitude at about 40m or 130' vs. the stock 30m setting. I am asking the more experienced fliers, is there anything else I should take into consideration for safer RTH?

 

[WetDog]

Good thinking. What I recommend is climbing straight up with the camera level. Pan around slowly ensuring ALL obstacles within your flight area are BELOW the horizon. Gain sufficient altitude to meet this requirement, note the altitude, & set this as your RTH altitude. Then you can be confident you won't hit something during a RTH event.

That sometimes works. I've found that due to the difficulty in determining height and indeed angles with a small camera with a wide angle lens, you have to really overestimate the RTH height that way. Safe from the standpoint of running into trees, but at least where I fly - in a forested area with 130 foot trees as the norm - I end up being in the flight path of planes. Especially if the ground isn't flat. Which is, amazingly, often the case. I will carefully map out areas and make sure I'm not missing a tall tree or a gentle upslope and calculate the RTH value accordingly. You can set it on the fly and if you are above the set RTH value if the bird loses signal, IT WILL NOT DESCEND to that value, but stay at the current altitude. So you can poke around and get accurate values in relative safety. Just make sure that you turn the craft around and look towards the landing zone every once in a while.