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Pixhawk gps lock

#1
I have this pixhawk quad and I finally got it flying properly and tuned up and all. I have not set up my GPS system yet tho. I have heard bad things about people losing their drones because of gps. Before I dive into it and go and fly, how do I know if I have a high-quality GPS lock before I take off?
 

clolsonus

Active member
#2
I should start out by saying I don't run pixhawk, but your post has gone a whole 30 minutes without being replied to which is unacceptable! :) So if you are going to add a gps and do anything autonomous, then you also need to install a radio modem and have a live connection to qgroundcontrol (or mission planner depending on which firmware you run.) With your ground control software you can see a live report of the number of satellites your gps is tracking. If it's a fairly current ublox8, I'd like to see 7-8 satellites before I'm comfortable flying. Often in flight (with the ublox8) I'll see 12-18 satellites and then I feel really comfortable the system is locked in.)
One issue with any of these systems is self-jamming. You might have an FPV transmitter and your data link transmitter on your quad. Even though these aren't necessarily transmitting on gps frequencies, having a transmitter antenna close to your gps antenna can wipe out the gps with too much noise. Good separation between your gps antenna and all your transmitting antennas is important. My personal rule of thumb is at least 6" between gps antenna and a 100mw transmitting antenna (more if I'm transmitting at higher power than 100mw.) It can be challenging to create this much space on a small quad, so you have to start making compromises, and that's where problems can happen.
So those are the two big issues I'm aware of with flyaways: (1) taking off before you get a good gps fix, and (2) some degree of partial self jamming your own gps receiver.
Historically flyaways have also happened due to firmware bugs or poor handling of gps errors, but I would think most of those issues would have been worked through in most popular firmwares by now.
I do mostly fixed wing UAV work at the U of MN UAV lab so I know a couple things, but only within my narrow little slice of life. We have our own in-house developed flight controller and firmware/software stack which we like, but it is mostly focused on fixed wing aircraft right now. Hopefully a pixhawk/quad expert can jump in and offer more specific advice!
 

makattack

Winter is coming
Moderator
Mentor
#3
Good information from @clolsonus even without direct px4 experience. I would only add that if you're methodical about slowly incorporating the sensors, you should be ok. Sound like you're already doing that, making sure it works well in acro, and stabilized flight modes. Next I would test and make sure altitude hold works well, check the logs to make sure the compass isn't acting funny on all your flights. Make sure you have a good compass calibration at that, preferably at your flying site, or the next best case, an open area away from large metal or magnetic sources in approximate closeness to where you will fly.
 
#4
Ok. thanks, I have a mavlink transmitter and receiver. Altho I think they both transmit and receive. One goes in my computer and the other plugs into my pixhawk. What you are saying is I need to get the mavlink linked between my drone and a laptop in order to view the satalights and stuff in mission planner?
 

makattack

Winter is coming
Moderator
Mentor
#5
Yes, getting the telemetry radios connected is probably going to be critical with GPS flight. That's because you'll have telemetry logs to look at, even if something goes wrong. That of course means you'll need a ground control setup that is more involved than just your TX, so it's more hardware. I tend to use mobile device (my android phone) for this purpose.

The other nice thing about the 3dr telemetry radios is that they operate at UHF frequencies, so you can get good range especially if you upgrade the antennas, but even the rubber duckies they normally come with will give more range than microwave RC RF systems. While they are lower power, you will need to have a ham radio license to operate them.
 
#7
Do I need a constant mavlink lock to fly in Loiter mode? Or can I get away without it? I don't need GPS stabilization to fly but it is nice to have.
 

makattack

Winter is coming
Moderator
Mentor
#8
I have no idea why "fpv" would be used to describe the 3DR / mavlink telemetry radios. But, if you have two of them, and they have a USB interface, in addition to a serial/uart port, then most likely it is a telemetry radio.

You can configure what happens if you have telemetry link vs not. It's really just a redundant control link in addition to giving telemetry. So, if you have a telemetry link, you can command a loiter from your ground station in addition to any mapped tx/rx channels you have configured to do the same.