ACRO bug (fixed in 2.9.1b): while doing flips in ACRO mode, if you switch to Stabilize while inverted your throttle will go to minimum.  To regain throttle control you need to switch back to ACRO then back to Stabilize again (i.e. switch to stabilize twice).  You never lose control of roll/pitch/yaw.

Loiter/AltHold/Auto/RTL bug: if you switch into these modes with throttle at zero motors will go to minimum until you raise the throttle.

Auto mode altitude bug (fixed in 2.9.1b): setting a waypoint altitude greater than 320m over home altitude may wrap around and instead be interpreted as a low altitude.

ArduCopter 2.9 is now in the mission planner and the downloads area!

The major improvement is we use inertial navigation to improve altitude hold.  This increased reliance on the accelerometers means you must do some additional set-up before flying:

1. Perform the new accelerometer calibration in the mission planner (video).  The auto-trim metho has also changed (video).

2. Add vibration dampening foam between your frame and the APM.  Some suggested materials: DuBrogelhk foam.

 3. If upgrading from 2.8.1, modify the throttle and altitude PID values:

  • Increase your Throttle Rate P, reduce I to zero, increase D
  • Increase Altitude Hold P, reduce I to zero
  • Tune Throttle Accel P and I terms but try to keep P about 1/2 the size of I


Here is the list of major changes (a more detailed list can be found in the release notes):  

  • Alt hold using inertial navigation (Leonard, Randy, Jonathan)
    • AUTO_VELZ_MIN, AUTO_VELZ_MAX parameters control the max climb/descent rate for the autopilot (cm/s)
    • PILOT_VELZ_MAX controls max climb/descent rate for the pilot (in cm/s)
  • Landing improvements (Leonard/Randy).  Copter will descend to 10m or until an object is sensed with the sonar.  Then slows to 50cm/s descent (speed can be adjusted with LAND_SPEED parameter). (video).
  • Surface tracking with sonar (Randy/Leonard).  Copter will attempt to maintain current distance from objects in front of sonar regardless of altitude.  Only used in alt-hold and loiter, not used for missions.  Sonar can be enabled/disabled with CH7 switch. (video)
  • Failsafe improvements (Randy/Craig/John Arne Birkeland) including bug fixes, additional check for PPM encoder failure and implementation of battery failsafe.  Set-up instructions are here.
  • Mediatek gps driver accuracy improvements and use of SBAS [Craig].  Instructions on upgrading your mediatek to firmware 1.9 are here.
  • Traditional Heli improvements (Rob) including (a) bringing heli code back into the fold, (b) enabled rate controller (previously only used angle controllers). (c) fix to rotor speed controllers - now operates by switching off channel 8.  (d) allow wider collective pitch range in acro and alt hold modes vs stabilize mode  (e) bug fix to allow collective pitch to use the entire range of servos
  • Acro trainer (Leonard). Copter will return to be generally upright if you release the sticks in acro mode.
    • ACRO_TRAINER : set to 1 to enable the auto-bring-upright feature
    • ACRO_BAL_ROLL, ACRO_BAL_PITCH : controls rate at which roll returns to level
  • Camera control improvements (Randy/Sandro Benigno):  (a) AP_Relay enabled for APM2  (b) Trigger camera with CH7 or DO_DIGICAM_CONTROL command  (c) Allow pilot override of yaw during missions and fixed CONDITIONAL_YAW command.
  • PPM sum support for transmitters with as few as 5 channels (Randy/Tridge/John Arne Birkeland).
  • Performance and memory useage improvements (Tridge).


As per usual PIDs are optimised for the 3DR/jDrones quad with 850 motors and 10" props. If you're using more powerful motors/props and are seeing bad flight behaviour in stabilize, start by turning down Rate Roll P in 25% steps.

Special thanks to our testing team lead Marco and the dedicated bunch on the 2.8.1 release thread who put their copters at risk while testing the pre-release version.  Some of their videos are here: 1 2 3 4 5 6 7 8

Please feel free to report issues you find in the discussion below and/or add them to the issues list.


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I now have eight flights on my Quad with the GPS re-mounted high above the APM/props.  I also put the GPS on the opposite side of 900 MHz telemetry.  The GPS is mounted on top of a 3/8” diameter carbon fiber tube.  The GPS now allows a solid Loiter as soon as I lift off after short waiting period for the GPS LED to show a 3D lock.   It doesn’t need extra time to acquire the satellite constellation or almanac.  I looked up GPS signal multipath and I wonder if the spinning carbon props don’t scatter the satellite signals.   The pics show the GPS mount and a schematic of GPS multipath signal propagation.


Just wondering if anyone has successfully ran this version, with QGCS (v.1.0.4 beta)? 

I tried to.  While the communication console displays continuous stream of packets, I am getting a "timeout! missing 195 read, 0 write" error when I do a "get". 

What am I missing?

Also is there a wiki on this?

All the best.


The 3DR GPS units have a larger ground plane than the patch antenna to reduce multipath signals from below. Carbon fibre easily absorbs radio energy, so unlikly to reflect the GPS satillite signals.

Ok, about time I have a say on GPS signals.
The problem doesn't lie with multipath or spinning props or whatever.
The problem is with the very nature off the GPS signals themselves.
They are extremely low power once they reach earth, in fact, they are below
the ambient RF noise.
So that is why a new gps, or a gps that has been off for a few months takes
quite a while to lock on..
The satelites all transmit an "almanac" or "emphemeris" to your receiver,
that tells the receiver which one, or more, of the 24 satelites it COULD
expect to see, if its date and time is correct.
Clever little gubbins in your rx now applies a crude digital "mask" over the
noise, to pick out a simple initial code, that tells it, yes, I receive a
Now it can slowly download crude data from whichever sat it found , to work
out what satelites should be visible, and applies a more sophisticated
"mask" to decode times signals, etc, and from that eventually derive
position, time, etc. And it needs at least 3 sats to work out 3D position.

Ok, here is the problem.
Because of the low signal, manufacturers of gps engines and antennas are
striving for the highest gain and lowest system noise in their designs.
And of course to get the flippen thing as small as possible.
So they don't have very good selectivity, but excellent sensitivity for
random RF noise :-(
Now, you have a rc RX onboard, that creates noise well into the GPS band.
Some goes for the micro driving your APM.
And your video link.
And your telemetry back to base station.
And the cellphone in your pocket.

Initially for space constraints I had my gps mounted on top of my FrSky RX,
some days couldn't get lock, extended the cable and mounted the gps halfway
down the body of the 'plane, never a problem again.
Similar problem with a cheap keychain camera, checked it on a Spectrum
Analiser, yep, signals well into the gps band.
And remember, you won't even SEE the gps signals on a spectrum analiser,
yes, I tried.

About the only solution is to use high quality equipment that doesn't
radiate outside of the band they are designed for,
DON'T use 1.2 GHz links (gps is only another 200 MHz away), and preferably,
get the gps antenna/module as far away from your main electronics as
And yes, the Spektrum telem module does cause gps interference, but not
entirely fatally so (I fly with one, well, my quad does)

Yours truly, Sparky
Although, unkindly, and not entirely deservedly so, also known as

Gustav very nice write up and good advice on GPS operation.

Totally agree, you want the GPS unit as far away from Transmitting Antennas on your quad as far as possible. I was thinking that even by having the GPS unit above the other antenna will help as the ground plane and metal shielding 'protect' the planner antenna. Even adding a earthed shield under the GPS unit to protect the other exposed components from interference will help (as other have done with cases they have made for GPS units)

Jim your quad looks great and easy to see why you have less GPS interference now. My only suggestion if I may, is to increase the twist rate in those cables connecting peripheral devices e.g GPS, telemetry etc. In general this helps reduce signal interferance. Another option if you do not like exposing the GPS so high is to mount it out further on one of the arms. Happy to hear your getting good GPS results now.

I have the ublox 6leh GPS only about 6cm from the 433 telemetry antenna. So far so good. Disabling the 433 does not get me one more satellite. I guess there are no harmonics close to GPS freq.

Why does a 1.32ghz blow every satellite out of the sky?

I agree with everything except: "So that is why a new gps, or a gps that has been off for a few months takes quite a while to lock on..".

This is not an artifact of weak signal levels.  When turned on the GPS receiver first tries to receive the same satellites it last heard.  If not found - this takes a few seconds - it then starts looking for others based on it's internal database.  Sometimes it can take a couple of minutes to find enough satellites to get a fix.  Most small GPS receivers have no internal clock, so they have to listen for any of the 24 satellites until it finds one and gets the time.  Then it can lookup other satellites that should be in range.

But the interference potential from other off-channel sources is correct.  GPS receivers in aircraft cost $1000 or more and have to be able to tolerate lot's of RF fields, including a 1KW 900 MHz pulse from the transponder.

Twisting wires only reduces emitted or induced noise from balanced AC signals.  It does nothing for DC except to make the wire runs more organized.  Data lines can benefit from twisting if there is a return path for each signal path.  (twisting neutralizes common mode emissions or interference).  In an 8-wire Ethernet cable, for example, there are four twisted-pairs of wires - each signal line has its' own return line.  If the signal line is referenced to a common ground then twisting it won't help much.

GPS is at 1.5Ghz to 1.2Ghz see 

And also the 1.2Ghz  Video transmitters are high power signal channel anlog (FM I believe) GPS is low signal strength CDMA (spread spectrum) the spread spectrum helps against the noise. 1.2GHz TX is just like a radio jammer and putting it right next to the antenna, need i say more.

As Gustav said, it's not the frequency that can cause interference, it's the desense effect from a poorly filtered receiver.  You are in an auditorium listening to a speaker on the stage.  When the room is quiet you can hear the speaker just fine.  However, in real life, there's a lot of conversations going on at the same time, which makes it more difficult for you to hear the speaker.  Now imagine someone adjacent to you starts up a conversation - now it's almost impossible to hear the speaker.  Your GPS receiver can't hear the satellites when there is a louder signal nearby which is why a 1.32GHz signal, which is near the GPS L2 frequency of 1227.6GHz, can block satellite reception.

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