ardupilot/ArduPlane/release-notes.txt

Release 3.0.0, April 8th 2014
-----------------------------

The ardupilot development team is proud to announce the release of
version 3.0.0 of APM:Plane. This is a major release with a lot of new
features.

For each release I try to highlight the two or 3 key new features that
have gone in since the last release. That is a more difficult task
this time around because there are just so many new things. Still, I
think the most important ones are the new Extended Kalman Filter (EKF)
for attitude/position estimation, the extensive dual sensors support
and the new AP_Mission library.

We have also managed to still keep support for the APM1 and APM2,
although quite a few of the new features are not available on those
boards. We don't yet know for how long we'll be able to keep going on
supporting these old boards, so if you are thinking of getting a new
board then you should get a Pixhawk, and if you want the best
performance from the APM:Plane code then you should swap to a
Pixhawk now. It really is a big improvement.

New Extended Kalman Filter
--------------------------

The biggest change for the 3.0.0 release (and in fact the major reason
why we are calling it 3.0.0) is the new Extended Kalman Filter from
Paul Riseborough. Using an EKF for attitude and position estimation
was never an option on the APM2 as it didn't have the CPU power or
memory to handle it. The Pixhawk does have plenty of floating point
performance, and Paul has done a fantastic job of taking full
advantage of the faster board.

As this is the first stable release with the EKF code we have decided
to not enable it by default. It does however run all the time in
parallel with the existing DCM code, and both attitude/position
solutions are logged both to the on-board SD card and over
MAVLink. You can enable the EKF code using the parameter
AHRS_EKF_USE=1, which can be set and unset while flying, allowing you
to experiment with using the EKF either by examining your logs with
the EKF disabled to see how it would have done or by enabling it while
flying.

The main thing you will notice with the EKF enabled is more accurate
attitude estimation and better handling of sensor glitches. A Kalman
filter has an internal estimate of the reliability of each of its
sensor inputs, and is able to weight them accordingly. This means that
if your accelerometers start giving data that is inconsistent with
your other sensors then it can cope in a much more graceful way than
our old DCM code.

The result is more accurate flying, particularly in turns. It also
makes it possible to use higher tuning gains, as the increased
accuracy of the attitude estimation means that you can push the
airframe harder without it becoming unstable. You may find you can use
a smaller value for NAVL1_PERIOD, giving tighter turns, and higher
gains on your roll and pitch attitude controllers.

Paul has written up a more technical description of the new EKF code
here:

  http://plane.ardupilot.com/wiki/common-apm-navigation-extended-kalman-filter-overview/


Dual Sensors
------------

The second really big change for this release is support for
dual-sensors. We now take full advantage of the dual accelerometers
and dual gyros in the Pixhawk, and can use dual-GPS for GPS
failover. We already had dual compass support, so the only main
sensors we don't support two of now are the barometer and the airspeed
sensor. I fully expect we will support dual baro and dual airspeed in
a future release.

You might wonder why dual sensors is useful, so let me give you an
example. I fly a lot of nitro and petrol planes, and one of my planes
(a BigStik 60) had a strange problem where it would be flying
perfectly in AUTO mode, then when the throttle reached a very specific
level the pitch solution would go crazy (sometimes off by 90
degrees). I managed to recover in MANUAL each time, but it certainly
was exciting!

A careful analysis of the logs showed that the culprit was
accelerometer aliasing. At a very specific throttle level the Z
accelerometer got a DC offset of 11 m/s/s. So when the plane was
flying along nice and level the Z accelerometer would change from -10
m/s/s to +1 m/s/s. That resulted in massive errors in the attitude
solution.

This sort of error happens because of the way the accelerometer is
sampled. In the APM code the MPU6000 (used on both the APM2 and
Pixhawk) samples the acceleration at 1kHz. So if you have a strong
vibrational mode that is right on 1kHz then you are sampling the "top
of the sine wave", and get a DC offset.

The normal way to fix this issue is to improve the physical
anti-vibration mounting in the aircraft, but I don't like to fix
problems like this by making changes to my aircraft, as if I fix my
aircraft it does nothing for the thousands of other people running the
same code. As the lead APM developer I instead like to fix things in
software, so that everyone benefits.

The solution was to take advantage of the fact that the Pixhawk has
two accelerometers, one is a MPU6000, and the 2nd is a LSM303D. The
LSM303D is sampled at 800Hz, whereas the MPU6000 is sampled at
1kHz. It would be extremely unusual to have a vibration mode with
aliasing at both frequencies at once, which means that all we needed
to do was work out which accelerometer is accurate at any point in
time. For the DCM code that involved matching each accelerometer at
each time step to the combination of the GPS velocity vector and
current attitude, and for the EKF it was a matter of producing a
weighting for the two accelerometers based on the covariance matrix.

The result is that the plane flew perfectly with the new dual
accelerometer code, automatically switching between accelerometers as
aliasing occurred.

Since adding that code I have been on the lookout for signs of
aliasing in other logs that people send me, and it looks like it is
more common than we expected. It is rarely so dramatic as seen on my
BigStik, but often results in some pitch error in turns. I am hopeful
that with a Pixhawk and the 3.0 release of APM:Plane that these types
of problems will now be greatly reduced.

For the dual gyro support we went with a much simpler solution and
just average the two gyros when both are healthy. That reduces noise,
and works well, but doesn't produce the dramatic improvements that the
dual accelerometer code resulted in.

Dual GPS was also quite a large development effort. We now support
connecting a 2nd GPS to the serial4/5 port on the Pixhawk. This allows
you to protect against GPS glitches, and has also allowed us to get a
lot of logs showing that even with two identical GPS modules it is
quite common for one of the GPS modules to get a significant error
during a flight. The new code currently switches between the two GPS
modules based on the lock status and number of satellites, but we are
working on a more sophisticated switching mechanism.

Supporting dual GPS has also made it easier to test new GPS
modules. This has enabled us to do more direct comparisons between the
Lea6 and the Neo7 for example, and found the Neo7 performs very
well. It also helps with developing completely new GPS drivers, such
as the Piksi driver (see notes below).

New AP_Mission library
----------------------

Many months ago Brandon Jones re-worked our mission handling code to
be a library, making it much cleaner and fixing a number of long term
annoyances with the behaviour. For this release Randy built upon the
work that Brandon did and created the new AP_Mission library.

The main feature of this library from the point of view of the
developers is that it has a much cleaner interface, but it also has
some new user-visible features. The one that many users will be glad
to hear is that it no longer needs a "dummy waypoint" after a
jump. That was always an annoyance when creating complex missions.

The real advantage of AP_Mission will come in future releases though,
as it has the ability to look ahead in the mission to see what is
coming, allowing for more sophisticated navigation. The copter code
already takes advantage of this with the new spline waypoint feature,
and we expect to take similar advantage of this in APM:Plane in future
releases.

New Piksi GPS driver
--------------------

One of the most exciting things to happen in the world of GPS modules
in the last couple of years is the announcement by SwiftNav that they
would be producing a RTK capable GPS module called the Piksi at a
price that (while certainly expensive!) is within reach of more
dedicated hobbyists. It offers the possibility of decimeter and
possibly even centimetre level relative positioning, which has a lot
of potential for small aircraft, particularly for landing control and
more precise aerial mapping.

This release of APM:Plane has the first driver for the Piksi. The new
driver is written by Niels Joubert, and he has done a great job. It is
only a start though, as this is a single point positioning driver. It
will allow you to use your new Piksi if you were part of the
kickstarter, but it doesn't yet let you use it in RTK mode. Niels and
the SwiftNav team are working on a full RTK driver which we hope will
be in the next release.

Support for more RC channels
----------------------------

This release is the first to allow use of more than 8 RC input
channels. We now support up to 18 input channels on SBus on Pixhawk,
with up to 14 of them able to be assigned to functions using the
RCn_FUNCTION settings. For my own flying I now use a FrSky Taranis
with X8R and X6R receivers and they work very nicely. Many thanks to
the PX4 team, and especially to Holger and Lorenz for their great work
on improving the SBus code.

Flaperon Support
----------------

This release is the first to have integrated flaperon support, and
also includes much improved flaps support in general. You can now set
a FLAP_IN_CHANNEL parameter to give an RC channel for manual flap
control, and setup a FLAPERON_OUTPUT to allow you to setup your
ailerons for both manual and automatic flaperon control.

We don't yet have a full wiki page on setting up flaperons, but you
can read about the parameters here:

  http://plane.ardupilot.com/wiki/arduplane-parameters/#Flap_input_channel_ArduPlaneFLAP_IN_CHANNEL

Geofence improvements
---------------------

Michael Day has made an number of significant improvements to the
geo-fencing support for this release. It is now possible to
enable/disable the geofence via MAVLink, allowing ground stations to
control the fence. 

There are also three new fence control parameters. One is
FENCE_RET_RALLY which when enabled tells APM to fly back to the
closest rally point on a fence breach, instead of flying to the center
of the fence area. That can be very useful for more precise control of
fence breach handling.

The second new parameter is FENCE_AUTOENABLE, which allows you to
automatically enable a geofence on takeoff, and disable when doing an
automatic landing. That is very useful for fully automated missions.

The third new geofence parameter is FENCE_RETALT, which allows you to
specify a return altitude on fence breach. This can be used to
override the default (half way between min and max fence altitude).

Automatic Landing improvements
------------------------------

Michael has also been busy on the automatic landing code, with
improvements to the TECS speed/height control when landing and new
TECS_LAND_ARSPD and TECS_LAND_THR parameters to control airspeed and
throttle when landing. This is much simpler to setup than
DO_CHANGE_SPEED commands in a mission.

Michael is also working on automatic path planning for landing, based
on the rally points code. We hope that will get into a release soon.

Detailed Pixhawk Power Logging
------------------------------

One of the most common causes of issues with autopilots is power
handling, with poor power supplies leading to brownouts or sensor
malfunction. For this release we have enabled detailed logging of the
information available from the on-board power management system of the
Pixhawk, allowing us to log the status of 3 different power sources
(brick input, servo rail and USB) and log the voltage level of the
servo rail separately from the 5v peripheral rail on the FMU.

This new logging should make it much easier for us to diagnose power
issues that users may run into.

New SERIAL_CONTROL protocol
---------------------------

This release adds a new SERIAL_CONTROL MAVLink message which makes it
possible to remotely control a serial port on a Pixhawk from a ground
station. This makes it possible to do things like upgrade the firmware
on a 3DR radio without removing it from an aircraft, and will also
make it possible to attach to and control a GPS without removing it
from the plane.

There is still work to be done in the ground station code to take full
advantage of this new feature and we hope to provide documentation
soon on how to use u-Blox uCenter to talk to and configure a GPS in an
aircraft and to offer an easy 3DR radio upgrade button via the Pixhawk
USB port.

Lots of other changes!
----------------------

There have been a lot of other improvements in the code, but to stop
this turning into a book instead of a set of release notes I'll stop
the detailed description there. Instead here is a list of the more
important changes not mentioned above:

  - added LOG_WHEN_DISARMED flag in LOG_BITMASK
  - raised default LIM_PITCH_MAX to 20 degrees
  - support a separate steering channel from the rudder channel
  - faster mission upload on USB
  - new mavlink API for reduced memory usage
  - fixes for the APM_OBC Outback Challenge module
  - fixed accelerometer launch detection with no airspeed sensor
  - greatly improved UART flow control on Pixhawk
  - added BRD_SAFETYENABLE option to auto-enable the safety
    switch on PX4 and Pixhawk on boot
  - fixed pitot tube ordering bug and added ARSPD_TUBE_ORDER parameter
  - fixed log corruption bug on PX4 and Pixhawk
  - fixed repeated log download bug on PX4 and Pixhawk
  - new Replay tool for detailed log replay and analysis
  - flymaple updates from Mike McCauley
  - fixed zero logs display in MAVLink log download
  - fixed norm_input for cruise mode attitude control
  - added RADIO_STATUS logging in aircraft logs
  - added UBX monitor messages for detailed hardware logging of u-Blox status
  - added MS4525 I2C airspeed sensor voltage compensation


I hope that everyone enjoys flying this new APM:Plane release as much
as we enjoyed producing it! It is a major milestone in the development
of the fixed wing code for APM, and I think puts us in a great
position for future development.

Happy flying!