GPS and the difference between lock and fix

[glider]

I've always made it a rule to allow my copters to acclimate for 3-5 minutes (minimum in very cold temps). I've never had an issue with GPS modes sticking to this procedure. The copter could have 15 satellites showing but not have a good position estimate.

Just as with Arducopter, I think it is the same with Yuneec's FC that when it shows 'Ready' (green light blinking on Pixhawk) when GPS lock is attained, does not mean it has a good position estimate fix. Allowing it to sit for a few more minutes while it sorts out the position estimate before taking off should result in more stable flight, particularly when hovering.

I brought this up because was reading at Ardupilot.org a post made by dev Randy McKay:
Copter became hardly controllable and crashed; Log indicates a fail to enter the flight mode (AltHold)

Daniel,

Thanks for the testing and report.

The issue is probably related to the fact that the vehicle was flying before the EKF had a good position estimate. This caused it to fail to enter Loiter mode but more importantly you may have uncovered an issue with AltHold before we get the position estimate. I'll check into this.

For now, I think it would be good to wait for a good position estimate before taking off which you can do by switching into Loiter mode (before takeoff) and waiting for the LEDs to become green. While you're in stabilize mode (on the ground) the light will become flashing green but that actually doesn't mean that it has a good position estimate yet, it only means that it has a good GPS lock. I think maybe we should change this so that it only goes green once it has the good position estimate. That would make it consistent with the autonomous flight modes. I'll discuss with the other devs.​

 

[BobW55]

My understanding,
LOCK = I have solid signal on 3 or more satellites.
FIX = I know where I am.

You could have a Lock on 20 satellites, but the GPS receiver board has not yet FIXED it's location.

 

[glider]

Lock as I recall is >6 satellites on Arducopter before going "green" and would assume is the same for Yuneec, or is it 9?. I would never rely on 6 or even 9 sats for GPS mode, but regardless, when it says 'Ready', it doesn't mean it's really ready IMO. I don't know the inner workings of the Yuneec FC, but a fast lock does not mean a fast fix. After the GPS module sends a lock signal to the FC, it may say 'Ready to Fly', but in my experience you should wait a while for the GPS to get a good fix.

I think many take off as soon as the controller says 'Ready' then can't figure out why it wanders around in hover. It's because it did not get a good fix before taking off. Then when RTL is initiated it lands away from where it was expected (yes I understand the H has dynamic RTL which I'm not too keen on).

Then there is the K-Index many don't consider. It should be in every user manual.

 

[Hoss]

Hmmm...when mine says ready, I'm flying. It's a fun toy and I enjoy it. I guess I've just been lucky.

 

[Rayray]

I agree with the basic premise here. Do not be too quick to take off when the video comes up. I typically wait for 10 or so Sats on the bird, and 7 or so on the ST-16. If you have RealSense, there occasionally may be a delayed warning that RS is not synced, and to restart the bird. No big deal. Try to have the H as level as possible when you start it.

 

[banned user]

mmm interesting, on the ST16 I quickly look at the distance between me and the H and see that it's a correct looking number and stable.

 

[glider]

Just remember it isn't the FC spitting out the numbers, it's the GPS, and all the while the GPS count is increasing, the GPS is altering the position estimate. It may pick up sats quickly, but I doubt it estimates an accurate position any better than a typical low grade M8N. Even the expensive GPS's I use I still allow time to get the best position estimate possible.

Since I fly a lot in high wind and want as stable hover as possible for filming say at the beach and want to keep the wandering to a minimum, I let the GPS count go as high as possible and settle. That's not to say 17 necessarily means better position estimates than say 12, just that it's doing the calculations during that period.

It would be good if HDOP or 2DRMS were included on the ST16 screen as that number indicates how good the position estimate is. Quite honestly I'd swap out the OEM GPS for a Drotek or CSGShop, but promised myself not to hack the H up!

BTW, has anyone placed your hand over the GPS after initializing? Mine throws a flag "no GPS, fly manual". It makes me wonder since I have an early version of the H if the GPS is weak.

 

[PatR]

Glider,

What you mention works the same for larger stuff. Time is your friend when a system is acquiring sats and establishing a location fix. Longer satellite lock times carry with them an increase in positional accuracy. As for putting your hand over the GPS, that also works the same with bigger stuff. GPS signal is weak and it requires very little to disrupt it. The water contained in your hand and body is a very effective signal block. Even back scatter body scanners can't see though it, even if it's only a film of water.

 

[NRRTRAINS]

hi
i also wait and take my time to get the most sats that i can , i go a step farther and i point my Q4K to true north and for some reason it gets more sats . most times 17 on the ST-10 and 15 on the Q .in the photo is a compass and an arrow , just point it north can't hurt .
fly safe