What parameter dictates failsafe behavior when contact with TX is lost?

Hi there,

I’m a brand-new V2450GPS owner (and a brand-new drone owner to boot), and I’m already very much in love. And let me just say, as someone who loves tweaking and adjusting things to max out performance, it’s very heartening to see Skyrocket starting to establish itself as an Android-esque competitor to the Apple-esque Parrots and DJIs of the drone world!

I had a couple of quick questions that I didn’t find an obvious answer to when searching the help materials:

First, what parameter determines when the V2450 will RTL after losing contact with the TX? I found the FS_GCS_ENABLE toggle, but that specifies it isn’t used unless RC_Override is engaged, and I can’t find any info on that parameter. Does it just go by the FENCE parameter? At 100m, I’d assume the FENCE parameter issues an RTL command well before the drone loses contact with the TX.

Secondly, what is considered NORMAL behavior to see during both LOITER mode and RTL mode? The drone appears to wander and correct over an area of 4-5 meters while loitering, and when performing an RTL it usually ends up about 2-3 meters away from the point where I launched it. I’ve only flown this copter about 6 times thus far, and I’ve always had 16-18 satellites connected before launching, so I’m sure the GPS signal is about as good as it gets.

Thanks for any insights, and if there’s somewhere in the help materials that would have answered these questions, please feel free to point me there instead.

I think what you are looking for with regard to TX FS is FS_THR_ENABLE and FS_THR_VALUE.

With regard to GPS accuracy, the number of satellites isn’t the only factor. 2-3 meters is fairly normal. You will often see greater error and wander for the first few minutes after it thinks the GPS is adequate. Other factors such as high vibration and bad compass calibration can, of course also cause trouble. I’ve noticed my SV seems to have more GPS trouble than my larger home-build which has the GPS mounted nice and high above the other electronics.


Kyle, sounds like you are having good fun!

With respect to your questions:

Parameter that controls what happens when TX is lost is FS_THR_ENABLE (set by default to RTL). This is because when TX connection is lost throttle goes below FS_THR_VALUE (set at 975 by default) and that triggers FS_THR_ENABLE.

RC_OVERRIDE is not a copter parameter but a mavlink command/parameter involved with GCS so you can ignore if not using GCS. If you are then FS_GCS_ENABLE controls what to do if GCS connection is lost.

As far as “normal” behavior of loiter and RTL, of course it depends but I’d say 4-5 meter loiter precision is sub-par. Does your copter have a tendency to circle when it wanders within that range? If so could be compass issue and re-calibration might fix that. Also remember that it’s not only satellite count, but really (although that’s related) speed, position and height computed accuracy that matters. So you could technically have some situation where accuracy is worse with say 10 satellites that with 8, That said 16-18 sats should give you good accuracy.

On a typical “day” I get more like < 2m, if not within 1m forf loiter precision. 2-3 meters for RTL is a bit high, but within 2m is what I typically see, sometimes less, sometimes more.

Looks like we answered almost at the same time :slight_smile:

Awesome, thanks to both of you for the quick and helpful replies. The compass seemed okay, I did do the simple accelerometer calibration and the magnetometer calibration, I’m less than confident about the latter however (I always struggle to correctly do the crazy figure 8’s to calibrate my phone’s compass), I might as well re-try that one if I still see it wandering so much. Before that though, it sounds like maybe I just need to try it a few more times (maybe in a few different locations) due to some additional variables I hadn’t considered.

Very helpful re: the FS_THR_ENABLE flag, thanks. I started researching it a bit, but that lead to other questions like “What the hell is PWM and how the $%@* can you measure it in microseconds?” I am assuming whatever PWM is, it would drop towards 975 as the drone loses signal from the TX, and a PWM value < 975 = “the drone is too far away, stupid” and RTL

This would also, of course, mean there is absolutely no reason for me to mess with this setting at ALL, right? I can’t imagine “Sweet I’ve flown this thing like 7 times, time to start disabling safety features and seeing how far away I can fly this thing!” is an ideal scenario. I freaked out a little bit the first time I hit the RTL button and the drone unexpectedly rose from 6’ to 15’ before it came back to me, to give an indication of my experience level here.

I guess on that note, as a beginner drone pilot/enthusiastic tinkerer, is there anything I really need to be doing in the web server options? I’ve set up throw mode as described in the Skyrocket main article here, and that worked great. I tried setting up “Flip” as an action, as well as a toggle for “Simple” mode (I’d like to just learn to correctly fly a drone from the start, but Simple mode sounded great for letting my wife try it out) using the same mode+button setup as you use for Throw mode, but neither seems to do anything so I’m sure I’ve set something up incorrectly or this drone simply can’t execute those commands. Is my time at this stage better sent just getting outside and flying the damn thing?

Yes, by all means, get out and fly and have fun! But also use google or your favorite search engine and continue your research using the wealth of information already available on the Internet! You will quickly find answers to your questions and on the way, acquire knowledge which includes some traditional RC history, technology, the meaning of terms like PWM, and the capabilities of the wonderful general purpose open-source Ardupilot firmware the SV is based on.

In the mean time, I will try to clarify one last thing. The FS_THR_VALUE setting and PWM is not a measure of distance in any way. Rather, it is a threshold value for the received representation of the position of the throttle, below which, the flight controller will assume the receiver isn’t receiving anything from the transmitter at all. In particular, the receiver communicates loss of signal to the flight controller as a specific throttle position which is lower than any throttle value which the transmitter will actually send. FS_THR_VALUE is configurable to allow for different transmitters and receivers which may be configured for different ranges of values. The reason RC receivers usually work this way is because traditional RC hobbyists didn’t want their model airplanes flying on until they ran out of fuel in case they flew out of range of the transmitter. Better to cut the throttle and crash-land as close to the flying range as possible. Drones are smarter than traditional RC airplanes so they use this traditional receiver behavior in different ways.

You can and probably should go read all about how and why the positions of the sticks are often represented by PWM, but for now, just know that, as you guessed, you shouldn’t change FS_THR_VALUE. This advice also applies to most of the other parameters you see on the web page, especially those which you don’t yet fully understand. :slight_smile:


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Interesting, thanks! Again, your response is super appreciated, as I didn’t even understand the relationship between throttle position and PWM well enough to understand what I needed to Google. In my head, I was thinking “not touching the throttle stick” = PWM of “0”, whereas not touching the stick is actually putting the PWM at its middle value. Makes sense! The V2450 uses the “Low throttle” failsafe method then, not the newer “No signal” method?

Also, I have tried Googling this again but I’m still coming up short - is it possible to set up the FLIP command, and enable SIMPLE mode on the V2450? I followed the instructions for setting up THROW mode and it worked perfectly, but I can’t get these other two to work. I bound SIMPLE to “ACTION6” and I bound FLIP to “ACTION4”.

I had figured this would mean I could just trigger ACTION4 while in a hover and the drone would do a flip, and I could use ACTION6 to toggle SIMPLE mode on and off, but neither function seems to work. (These aren’t vital things for me to accomplish, if I’m asking for trouble I can just let them go. I just thought it’d be fun to have those features available for my wife, she wants to try the drone out with me but she’s a lot less interested in really learning how to fly one. I picked SIMPLE mode over SUPER_SIMPLE because she’d be flying it maybe 5 feet away from her at most)