Is 10 Sats locked bad enough to cause RTH to miss by 100'?

[gjmphoto]

I took my MPP out today - hit RTH and it came back. I was lucky that as it came back, I was able to find it in the sky and watch as it lined itself up on the wrong side of my house (I was in the back, it started to land in my front yard). It said it was landing and I saw it start to descend towards the trees in my front yard. I quickly took control back and flew it back over the top of my house and landed it back on its landing pad.

WTH? I checked UAV forecast and it was saying that there should have been EST 10 Sats locked.

Could that have caused this problem? What else could be wrong? It missed not by a couple of feet (as

FYI: I used precision landing and let the MPP sit above the target on take off for about 30 secs before moving away.

 

[Starz]

It is better if you wait for a minimum of 12 satellites before takeoff. At that point, it seems to get within about a 6 foot radius of takeoff point. One hundred feet off may be something more than a lack of satellite numbers.

 

[BossBob]

You need to check that the home point that is showing in the app is the same place that your drone is taking off from, even checking that the heading shown is the same as the drones. Only then can you be sure that RTH will work as you want it to.

 

[Simmo]

Upload your flight log using these instructions.
DJI Flight Log Viewer - Phantom Help
The log can be looked at and determine if you have a problem or not?

 

[Kilrah]

I checked UAV forecast and it was saying that there should have been EST 10 Sats locked.

What an app forecasts means nothing, what matters is how many sats were ACTUALLY locked by your aircraft which you see on your info screen and should be looking at.

Most likely you took off without a GPS lock, GPS lock came while in flight on the other side and that's where home point was set.

 

[gjmphoto]

What an app forecasts means nothing, what matters is how many sats were ACTUALLY locked by your aircraft which you see on your info screen and should be looking at.

Most likely you took off without a GPS lock, GPS lock came while in flight on the other side and that's where home point was set.

The app said that the home point had been updated and it was green with GPS lock. The map looked right, the arrow was pointing the right way. I didn't pay attention to the exact number of sats locked when I took off. Is there a way for me to tell now, after the fact, how many sats were locked when I took off?

 

[Kilrah]

Yes, using the logs that someone above suggested you post.
Sounds OK, but maybe GPS reception was bad at the time of landing then.

 

[tcope]

My recommendation is always, don't use RTH unless it is needed. Some will always argue that it is very reliable and nothing ever goes wrong. Posts like this always proves them wrong.

Can't tell what happen unless logs are reviewed.

 

[gjmphoto]

Yes, using the logs that someone above suggested you post.
Sounds OK, but maybe GPS reception was bad at the time of landing then.

Thanks. I'll pull the logs tonight.

I normally don't rely on RTH, but if it happens, I want to be sure it's going to at least get over my head before it starts to descend. Had I not had VLOS when the drone started to drop, and had it at a substantial RTH altitude, it would have ended up in the top of a few trees, possibly before I knew what was going on.

I'll send the logs tonight.

 

[gjmphoto]

OK...I pulled the log...and attached the file to this post.

It seems that the drone had 9 sats when it locked the home position and set the RTH. It then went up to 11 sats, but registered several RED "GPS Position NoMatch" warnings before I began to move. The # of sats increased to 17 as I flew and had 17 when I requested RTH.

If someone could tell me what they see, I'd appreciate it.

Also..I took a screen grab from the log reader and it's clear that where it thought "Home" was was where it tried to land...however, if you look at the map, I put a solid red box where home ACTUALLY was.

 

[Meta4]

It seems that the drone had 9 sats when it locked the home position and set the RTH. It then went up to 11 sats, but registered several RED "GPS Position NoMatch" warnings before I began to move. The # of sats increased to 17 as I flew and had 17 when I requested RTH.

If someone could tell me what they see, I'd appreciate it.

First you can simply post a link to the report that Phantomhelp shows you like this:
DJI Flight Log Viewer - PhantomHelp.com

Next a few basics
The GPS Position nomatch messages are a false alarm and are meaningless - ignore tham.
You need enough sats to be able to get a good fix, but after that, more sats doesn't mean any more accuracy.
GPS always has a small random error that can be up to 10 feet or even more (but usually less).
Forget the app for GPS sat forecasts.
You will always have enough sats as long as you are flying up where the Mavic gets a clear, unobstructed view of the whole sky.
It looks like your launch spot does not have a good skyview and you can see the number of sats increase as you get up clear of obstacles around 80 feet up.

Now to analyse your flight data.
I see something interesting that shows that GPS had nothing to do with what you observed.
RTH brought the Mavic back to 250 ft above home but on descending, it started getting away from home.
The Phantom knew it was off and this is shown in the Home Distance column.

What made it drift?
Why didn't it correct?
When I look at the joystick input, I see that you had the sticks centred while the drone returned and when it started to descend at 4:29.1.
But at 4:38.2 when it had come down about 20 ft, you pushed the right stick forward, then the left stick to the right, then right stick forward and then the left stick to the right again.
You left the sticks centred from 4:49.8 but by then you had pulled/pushed the auto-descent off by 66 feet.
At 4:53.1 you cancelled the descent and took over control and manually landed 44 feet from home.

GPS had no part of it and would have landed you within a few feet of home.
Precision Landing would have put you right on the spot if you had a good target.

 

[Kilrah]

It is often a misconception that when in RTH mode the aircraft just comes back and nothing you do will change it but it's incorrect, your stick actions are still taken into account so that you can "correct" the RTH path, or like in this case disturb it.

 

[gjmphoto]

First you can simply post a link to the report that Phantomhelp shows you like this:
DJI Flight Log Viewer - PhantomHelp.com

Next a few basics
The GPS Position nomatch messages are a false alarm and are meaningless - ignore tham.
You need enough sats to be able to get a good fix, but after that, more sats doesn't mean any more accuracy.
GPS always has a small random error that can be up to 10 feet or even more (but usually less).
Forget the app for GPS sat forecasts.
You will always have enough sats as long as you are flying up where the Mavic gets a clear, unobstructed view of the whole sky.
It looks like your launch spot does not have a good skyview and you can see the number of sats increase as you get up clear of obstacles around 80 feet up.

Now to analyse your flight data.
I see something interesting that shows that GPS had nothing to do with what you observed.
RTH brought the Mavic back to 250 ft above home but on descending, it started getting away from home.
The Phantom knew it was off and this is shown in the Home Distance column.

What made it drift?
Why didn't it correct?
When I look at the joystick input, I see that you had the sticks centred while the drone returned and when it started to descend at 4:29.1.
But at 4:38.2 when it had come down about 20 ft, you pushed the right stick forward, then the left stick to the right, then right stick forward and then the left stick to the right again.
You left the sticks centred from 4:49.8 but by then you had pulled/pushed the auto-descent off by 66 feet.
At 4:53.1 you cancelled the descent and took over control and manually landed 44 feet from home.

GPS had no part of it and would have landed you within a few feet of home.
Precision Landing would have put you right on the spot if you had a good target.

The reason for the change upon descent was that I saw the drone start to land in the wrong place and took over before it went into the trees. I eventually had time to cancel rth but for a while I was forcing it away from the trees as it landed. If you look at the picture I included you'll see the red box was where I really took off from....but the gps thought I was on the other side of my house!

If there was nothing wrong on the logs then the gps was off by about 75 to 100 feet.

 

[Meta4]

If there was nothing wrong on the logs then the gps was off by about 75 to 100 feet.

I can't see any sign of a 75-100 ft error in GPS locations.
You launched from a partially obstructed area with 9-10 sats down below 40 ft and GPS health showing a 4.
When you passed 88 ft, the GPS count rapidly jumped from 11 to18.
When the GPS health gets to 5 at 1:14, the GPS shows it to be 28 ft from the home point it recorded.
At this time, the only joystick input had been pushing the left stick forward to climb.

If there was any issue with the GPS it would that you launched in a location that was not suitable for getting a good GPS signal.
If you do launch in a location where GPS is obstructed, it's easy to check the distance showing and compare that to what you see, or you can climb to get batter GPS signal and reset the home point there where you want the drone to return to.

The GPS knew exactly where it was once it was up in the clear.
There was no error of 75-100 feet affecting the GPS.

 

[gjmphoto]

I can't see any sign of a 75-100 ft error in GPS locations.
You launched from a partially obstructed area with 9-10 sats down below 40 ft and GPS health showing a 4.
When you passed 88 ft, the GPS count rapidly jumped from 11 to18.
When the GPS health gets to 5 at 1:14, the GPS shows it to be 28 ft from the home point it recorded.
At this time, the only joystick input had been pushing the left stick forward to climb.

If there was any issue with the GPS it would that you launched in a location that was not suitable for getting a good GPS signal.
If you do launch in a location where GPS is obstructed, it's easy to check the distance showing and compare that to what you see, or you can climb to get batter GPS signal and reset the home point there where you want the drone to return to.

The GPS knew exactly where it was once it was up in the clear.
There was no error of 75-100 feet affecting the GPS.

Perhaps I was lucky in the past, but I've launched from that exact location a dozen times before and not had an issue...but the data does show poor GPS health at that time. Still, the screen showed GPS Lock in Green - and told me that it registered my home point - which was clearly NOT where it was sitting (it was about 75 feet to the northeast of where it recorded). Basically, the app lied to me and didn't give me an indication that it was having a problem - instead, it told me that it locked on and recorded its location...incorrectly. That said, I did launch with 9 sats locked - which is definitely my bad.

Your suggestion to watch the horizontal distance and see if it changes as I rise straight up is a good one. I wouldn't have thought of that myself. I'll make that one a habit. I'll also pay more attention to the number of sats and not launch unless I have a dozen or more. GPS Health is not something I know where to find in the app (is there a spot in the app that shows that or is it only in the data?)...or is it the number of bars?

 

[Kilrah]

the app lied to me and didn't give me an indication that it was having a problem

The sound is "the home point has been recorded, please check it on the map"...
It can't know whether it's correct or not, which is why it tells you to check it...

 

[Meta4]

Perhaps I was lucky in the past, but I've launched from that exact location a dozen times before and not had an issue...but the data does show poor GPS health at that time. Still, the screen showed GPS Lock in Green - and told me that it registered my home point - which was clearly NOT where it was sitting (it was about 75 feet to the northeast of where it recorded). Basically, the app lied to me and didn't give me an indication that it was having a problem - instead, it told me that it locked on and recorded its location...incorrectly. That said, I did launch with 9 sats locked - which is definitely my bad.

The data shows GPS health of 4 which is not bad.
The Phantom considered the GPS data good enough to record a home point and let you fly.
Sometimes I see the app update the homepoint shortly after launching if it gets a significantly better fix that makes a difference to the location of the drone.
In your flight this did not happen so we have to assume that the location data was already good before the Mavic went to GPS Health = 5.

I'll also pay more attention to the number of sats and not launch unless I have a dozen or more. GPS Health is not something I know where to find in the app (is there a spot in the app that shows that or is it only in the data?)...or is it the number of bars?

Launching with 9 sats is not necessarily a bad thing.
The actual number of satellites your Mavic is using is a little tricky.
If you had an old DJI drone that used only US GPS sats, DJI would require 6 sats before it would give you the green P-GPS.
But more modern models now use US sats and Russian Glonass sats.
To get the green, you need 6 sats of either system (3+3 won't work).
If your Mavic had 5+4=9 at launch, it would not have gone green and would not have recorded a home point so you would have had either 6+3, 7+2, 8+1 or 9+0.
A handheld GPS requires 4 sats (of the same kind) to give a position fix - as long as their spread is good.
DJI increase that to 6 as a safety margin to ensure a good spread.

I would happily launch from a clear location with 9 sats if the app gives me green.
In an obstructed location like yours, I would still launch on green but be cautious and check that the recorded home point is where I want the drone to come back.

 

[gjmphoto]

Alrighty...I got out again today. Had 14-16 sats to lock on to and headed out across a reservoir where I knew there'd be no interference - and where I had flown before several times. Things were definitely better...but still a little fluky...here's what happened, and I'd appreciate any feedback you might have:

1 - On my first run, plenty of GPS health and went out over the reservoir. At the mid point of the lake, I noticed that the map view stopped moving. I could see the playback from the drone, which was perfectly responsive, but the arrow on the map was not moving at all. I then noticed that the arrow was touching an "H", indicating that it was home...but it clearly wasn't. SO I flew it back and landed it. I immediately got a compass error and was asked to recalibrate it. So I did. Then I took it out for a second flight.

2- this flight went just fine...I flew it to the middle, hit RTH to see what it would do and it returned right to the DOT of the spot where I took off. Perfectly.

So I figured it was compass calibration the whole time. UNTIL I got home and looked at the logs. On that first flight, there were no issues recorded, the home point seems to have stayed where it belonged the entire flight! How could the screen show a wrong map when the flight log showed the correct information?

Here's a link to the first flight. - DJI Flight Log Viewer - PhantomHelp.com

Would really appreciate some insight!

- Gary.

 

[Meta4]

1 - On my first run, plenty of GPS health and went out over the reservoir. At the mid point of the lake, I noticed that the map view stopped moving. I could see the playback from the drone, which was perfectly responsive, but the arrow on the map was not moving at all. I then noticed that the arrow was touching an "H", indicating that it was home...but it clearly wasn't. SO I flew it back and landed it. I immediately got a compass error and was asked to recalibrate it. So I did.
2- this flight went just fine...I flew it to the middle, hit RTH to see what it would do and it returned right to the DOT of the spot where I took off. Perfectly.

So I figured it was compass calibration the whole time. UNTIL I got home and looked at the logs. On that first flight, there were no issues recorded, the home point seems to have stayed where it belonged the entire flight! How could the screen show a wrong map when the flight log showed the correct information?

Would really appreciate some insight!

There's not much to say.
You flew out, got confused about something and flew back.
Like almost all flight logs, there's no indication of anything going astray during the flight.
When you thought the home point had moved (it can't unless you reset it), did you check the indicated distance from home?
As you can see from the Phantomhelp report, it continued to give an accurate reading right through the flight (just like it always does).

You seem to be confusing the compass (which only does direction) with GPS (that only does location).
The compass can't have had anything to do with what you imagined.
And your comment about the compass error shows a misunderstanding.
A compass error is not a fault in the compass that needs to be fixed (it's the compass giving you a warning about a magnetic field that could cause problems).
And calibrating the compass doesn't "fix" anything.
DJI's warning message is poorly written and confuses many users.
It says Magnetic interference - Move aircraft or calibrate compass.
The correct action is always to move away from the magnetic field it's warning you about.
No amount of recalibrating will "fix" the problem it's warning you of.

Calibrating the compass only helps the drone identify which magnetic fields are part of the drone so it can ignore them.

 

[gjmphoto]

There's not much to say.
You flew out, got confused about something and flew back.
Like almost all flight logs, there's no indication of anything going astray during the flight.
When you thought the home point had moved (it can't unless you reset it), did you check the indicated distance from home?
As you can see from the Phantomhelp report, it continued to give an accurate reading right through the flight (just like it always does).

You seem to be confusing the compass (which only does direction) with GPS (that only does location).
The compass can't have had anything to do with what you imagined.
And your comment about the compass error shows a misunderstanding.
A compass error is not a fault in the compass that needs to be fixed (it's the compass giving you a warning about a magnetic field that could cause problems).
And calibrating the compass doesn't "fix" anything.
DJI's warning message is poorly written and confuses many users.
It says Magnetic interference - Move aircraft or calibrate compass.
The correct action is always to move away from the magnetic field it's warning you about.
No amount of recalibrating will "fix" the problem it's warning you of.

Calibrating the compass only helps the drone identify which magnetic fields are part of the drone so it can ignore them.

I'm not confused in the least. I know what I saw. The map was unresponsive while the rest of the app was not...but the logs show no issues. Do you have anything productive to add or is telling me I'm confused all you have to offer?