Required to prevent acquisition of GPS mid flight causing unwanted change in position and velocity
A distinction has been mad between the arm and disarm transition and the decision to use position hold mode (formerly static mode)
When regaining GPS after a timeout, an offset is applied when fusing GPS velocity so that GPS velocity and position data as fused by the EKF is kinematically consistent.
This velocity offset is also accounted for when fusing air data so that wind estimates are not corrupted when the GPS position offset is being pulle back to zero.
The intended behaviour is that the EKF position will be pulled back to the GPS position at a rate of 5m/s for planes and 1 m/s for copters. This avoids large deviations in trajectory when GPS is regained.
1) Un-used public methods to report height and position drifting have been removed
2) A time-out has been added to the airspeed innovation consistency check so that if we are relying on airspeed to constrain velocity drift, a filter divergence or other fault that causes the airspeed to be continually rejected will trigger a change in health status.
3) A timeout of velocity, position or height measurements does not cause a filter fault to be reported. Timeouts can be due to sensor errors and do not necessarily indicate that the filter has failed.
4) Time-outs of various measurements are used to present a consolidated bitmask which inidicates which parts of the solution can be used, eg attitude, height, velocity, relative position, absolute position, etc.
This provides the calling vehicle software the abiity to request the EKF to not use GPS.
An integer is returned that indicates the type of operation available:
0 = request rejected (request will only be accepted if the EKF is in static mode, eg pre-armed)
1 = request accepted, attitude, vertical velocity and position estimates available
2 = request accepted, attitude, height rate, height, horizontal velocity and relative position estimates available
Re-initialisation of the magnetic field states and yaw angle is now only performed a maximum of two times after start-up.
Once when coming out of static modefor the first time (first arm event)
Again (for copter only) when the altitude gain above the arming altitude exceeds 1.5m
this prevents magnetic interference present at arming (eg arming on a metal roof)from corrupting the magnetic field states enough to cause bad heading errors and toilet bowling on copter
(Plane Only) If the yaw and GPS heading disagree by more than 45 degrees on takeoff, then the magnetometer is declared as failed. The heading is then reset based on the difference between GPS ground track and stgate velocity vector.
Magnetometer fusion uses corrected data and bias states are initialised to zero. This allows the compass to be switched in flight.
For persistent compass errors that trigger a timeout, the compass is not permanently failed, however for non-forward fly vehicles the compass weighting is reduced.
GPS measurement variance is doubled if only 5 satellites, and quadrupled if 4 or less.
The GPS glitch rejection thresholds remain the same
This will reduce the impact of GPS glitches on attitude.
This patch reduces the level of 5Hz and 10Hz 'pulsing' heard in motors due to GPS and altimeter fusion which cause a small 5Hz and 10Hz ripple on the output under some conditions. Attitude, velocity and position state corrections from GPS, altimeter and magnetometer measurements are applied incrementally in the interval from receiving the measurement to the predicted time of receipt of the next measurement. Averaging of attitude state corrections is not performed during periods of rapid rotation.
Time stamps are now explicitly initialised to the current IMU time to avoid unwanted activation of timeout logic on filter start and various calls to the hal.scheduler->millis() object have been consolidated.
If the vehicle can fly without a compass (a fly forward vehicle)
then if the compass times out (large errors for more than 10 seconds,
then it will be declared permanently failed and will not be
used until the filter is reset
Divergence is now detected by looking for very large changes in the gyro
bias. This will cause the filter to be reset and declared unhealthy for
10 seconds.
Don't reset filterDiverged status immediately during reset
Set filterDiverged true if covariance blows up
Add fault status reporting
This will enable in-flight magnetometer calibration to be inhibited unconditionally,
This is required for long balloon carriage flights where ground speed can be high
enough to put it into in-air state, but with very poor observability of magnetic field
states causing bad state estimates and heading offsets to develop over time.
The covariance matrix no longer has rows and columns artificially zeroed when in static
mode. Instead booleans indicating whether wind or magentic field state estimation is
active are used to:
a) Set the process noise on these states to zero to stop their variances from increasing
unchecked when not being updated, and
b) Turn off updates for these states when measurement fusion is being performed.
This reduces the likelihood of a badly conditioned covariance matrix forming
during static mode operation.
A filter divergence check has also been added that will declare the filter unhealthy if
position, velocity and magnetic field observations are all failing their innovation
consistency checks. This unhealthy status will persist for 10 seconds after the
condition clears.
AP_NavEKF: Remove unnecessary zeroing of wind covariances
A magnetometer axis that fails the innovation consistency check will cause
all axes not to be used. If this condition continues for 10 seconds, a
magnetometer timeout condition will be declared. When the timeout has
occurred, if it is not a fly forward vehicle, then individual channels
will be used again, but with a reduced weighting.
This patch also cleans up the logic associated with use of the synthetic
sideslip measurement so that it can never be used for a non fly-forward
vehicle type
This patch adds functionality that initialises the wind-speed vector to the
reciprocal of the ground speed vector, and scaled to 6 m/s. On average this gives
a better initial wind velocity estimate on launch by assuming:
a) launch will be into wind
b) wind speed is equal to global average
It also helps prevent a headwind causing initial underestimation of airspeed
causing high autopilot gains and limit cycles on climb-out, until first
turn when the EKF is able to estimate the wind.
This adds new functionality to the detection and compensation of GPS
glitches:
1) A maximum allowable innovation is calculated using the GPS noise
parameter multiplied by the gate, with an additional component allowing
for growth in position uncertainty due to acceleration error since
the last valid measurement
2) Includes per vehicle type values for the acceleration error limit
3) If the innovation length exceeds the maximum allowable, no fusion occurs
4) If no fusion has occurred for long enough such that the position uncertainty
exceeds the maximum set by a per vehicle parameter or a maximum time, an offset
is applied to the GPS data to so that it matches the value predicted by the filter
5) The offset is never allowed to be bigger than 100m
6) The offset is decayed to zero at a rate of 1.0 m/s to allow GPS jumps to
be accommodated gradually
7) The default velocity innovation gate has been tightened up for copter and rover
8) The variance data logging output has been updated to make it more useful
This path reduces duplicated code, eliminates unused variables and
causes the earth magnetic field states to be reset when exiting static mode
which will occur every time copter is armed. This enables copters to be
powered on and initialised inside vehicles or houses, without bad earth
field values affecting flight.
priseborough
Jonathan Challinger
Andrew Tridgell
Emile Castelnuovo
Randy Mackay