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ardupilot/libraries/AP_NavEKF2/AP_NavEKF2_RngBcnFusion.cpp

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#include "AP_NavEKF2.h"
#include "AP_NavEKF2_core.h"
/********************************************************
* FUSE MEASURED_DATA *
********************************************************/
// select fusion of range beacon measurements
void NavEKF2_core::SelectRngBcnFusion()
{
// read range data from the sensor and check for new data in the buffer
readRngBcnData();
// Determine if we need to fuse range beacon data on this time step
if (rngBcnDataToFuse) {
if (PV_AidingMode == AID_ABSOLUTE) {
// Normal operating mode is to fuse the range data into the main filter
FuseRngBcn();
} else {
// If we aren't able to use the data in the main filter, use a simple 3-state filter to estimte position only
FuseRngBcnStatic();
}
}
}
void NavEKF2_core::FuseRngBcn()
{
// declarations
ftype pn;
ftype pe;
ftype pd;
ftype bcn_pn;
ftype bcn_pe;
ftype bcn_pd;
const ftype R_BCN = sq(MAX(rngBcnDataDelayed.rngErr , 0.1f));
ftype rngPred;
// health is set bad until test passed
rngBcnHealth = false;
if (activeHgtSource != HGT_SOURCE_BCN) {
// calculate the vertical offset from EKF datum to beacon datum
CalcRangeBeaconPosDownOffset(R_BCN, stateStruct.position, false);
} else {
bcnPosOffset = 0.0f;
}
// copy required states to local variable names
pn = stateStruct.position.x;
pe = stateStruct.position.y;
pd = stateStruct.position.z;
bcn_pn = rngBcnDataDelayed.beacon_posNED.x;
bcn_pe = rngBcnDataDelayed.beacon_posNED.y;
bcn_pd = rngBcnDataDelayed.beacon_posNED.z + bcnPosOffset;
// predicted range
Vector3F deltaPosNED = stateStruct.position - rngBcnDataDelayed.beacon_posNED;
rngPred = deltaPosNED.length();
// calculate measurement innovation
innovRngBcn = rngPred - rngBcnDataDelayed.rng;
// perform fusion of range measurement
if (rngPred > 0.1f)
{
// calculate observation jacobians
ftype H_BCN[24] = {};
ftype t2 = bcn_pd-pd;
ftype t3 = bcn_pe-pe;
ftype t4 = bcn_pn-pn;
ftype t5 = t2*t2;
ftype t6 = t3*t3;
ftype t7 = t4*t4;
ftype t8 = t5+t6+t7;
ftype t9 = 1.0f/sqrtF(t8);
H_BCN[6] = -t4*t9;
H_BCN[7] = -t3*t9;
H_BCN[8] = -t2*t9;
// calculate Kalman gains
ftype t10 = P[8][8]*t2*t9;
ftype t11 = P[7][8]*t3*t9;
ftype t12 = P[6][8]*t4*t9;
ftype t13 = t10+t11+t12;
ftype t14 = t2*t9*t13;
ftype t15 = P[8][7]*t2*t9;
ftype t16 = P[7][7]*t3*t9;
ftype t17 = P[6][7]*t4*t9;
ftype t18 = t15+t16+t17;
ftype t19 = t3*t9*t18;
ftype t20 = P[8][6]*t2*t9;
ftype t21 = P[7][6]*t3*t9;
ftype t22 = P[6][6]*t4*t9;
ftype t23 = t20+t21+t22;
ftype t24 = t4*t9*t23;
varInnovRngBcn = R_BCN+t14+t19+t24;
ftype t26;
if (varInnovRngBcn >= R_BCN) {
t26 = 1.0/varInnovRngBcn;
faultStatus.bad_rngbcn = false;
} else {
// the calculation is badly conditioned, so we cannot perform fusion on this step
// we reset the covariance matrix and try again next measurement
CovarianceInit();
faultStatus.bad_rngbcn = true;
return;
}
Kfusion[0] = -t26*(P[0][6]*t4*t9+P[0][7]*t3*t9+P[0][8]*t2*t9);
Kfusion[1] = -t26*(P[1][6]*t4*t9+P[1][7]*t3*t9+P[1][8]*t2*t9);
Kfusion[2] = -t26*(P[2][6]*t4*t9+P[2][7]*t3*t9+P[2][8]*t2*t9);
Kfusion[3] = -t26*(P[3][6]*t4*t9+P[3][7]*t3*t9+P[3][8]*t2*t9);
Kfusion[4] = -t26*(P[4][6]*t4*t9+P[4][7]*t3*t9+P[4][8]*t2*t9);
Kfusion[6] = -t26*(t22+P[6][7]*t3*t9+P[6][8]*t2*t9);
Kfusion[7] = -t26*(t16+P[7][6]*t4*t9+P[7][8]*t2*t9);
if (activeHgtSource == HGT_SOURCE_BCN) {
// We are using the range beacon as the primary height reference, so allow it to modify the EKF's vertical states
Kfusion[5] = -t26*(P[5][6]*t4*t9+P[5][7]*t3*t9+P[5][8]*t2*t9);
Kfusion[8] = -t26*(t10+P[8][6]*t4*t9+P[8][7]*t3*t9);
Kfusion[15] = -t26*(P[15][6]*t4*t9+P[15][7]*t3*t9+P[15][8]*t2*t9);
bcnPosOffset = 0.0f;
} else {
// don't allow the range measurement to affect the vertical states in the main filter
Kfusion[5] = 0.0f;
Kfusion[8] = 0.0f;
Kfusion[15] = 0.0f;
}
Kfusion[9] = -t26*(P[9][6]*t4*t9+P[9][7]*t3*t9+P[9][8]*t2*t9);
Kfusion[10] = -t26*(P[10][6]*t4*t9+P[10][7]*t3*t9+P[10][8]*t2*t9);
Kfusion[11] = -t26*(P[11][6]*t4*t9+P[11][7]*t3*t9+P[11][8]*t2*t9);
Kfusion[12] = -t26*(P[12][6]*t4*t9+P[12][7]*t3*t9+P[12][8]*t2*t9);
Kfusion[13] = -t26*(P[13][6]*t4*t9+P[13][7]*t3*t9+P[13][8]*t2*t9);
Kfusion[14] = -t26*(P[14][6]*t4*t9+P[14][7]*t3*t9+P[14][8]*t2*t9);
if (!inhibitMagStates) {
Kfusion[16] = -t26*(P[16][6]*t4*t9+P[16][7]*t3*t9+P[16][8]*t2*t9);
Kfusion[17] = -t26*(P[17][6]*t4*t9+P[17][7]*t3*t9+P[17][8]*t2*t9);
Kfusion[18] = -t26*(P[18][6]*t4*t9+P[18][7]*t3*t9+P[18][8]*t2*t9);
Kfusion[19] = -t26*(P[19][6]*t4*t9+P[19][7]*t3*t9+P[19][8]*t2*t9);
Kfusion[20] = -t26*(P[20][6]*t4*t9+P[20][7]*t3*t9+P[20][8]*t2*t9);
Kfusion[21] = -t26*(P[21][6]*t4*t9+P[21][7]*t3*t9+P[21][8]*t2*t9);
} else {
// zero indexes 16 to 21 = 6
zero_range(&Kfusion[0], 16, 21);
}
Kfusion[22] = -t26*(P[22][6]*t4*t9+P[22][7]*t3*t9+P[22][8]*t2*t9);
Kfusion[23] = -t26*(P[23][6]*t4*t9+P[23][7]*t3*t9+P[23][8]*t2*t9);
// Calculate innovation using the selected offset value
Vector3F delta = stateStruct.position - rngBcnDataDelayed.beacon_posNED;
innovRngBcn = delta.length() - rngBcnDataDelayed.rng;
// calculate the innovation consistency test ratio
rngBcnTestRatio = sq(innovRngBcn) / (sq(MAX(0.01f * (ftype)frontend->_rngBcnInnovGate, 1.0f)) * varInnovRngBcn);
// fail if the ratio is > 1, but don't fail if bad IMU data
rngBcnHealth = ((rngBcnTestRatio < 1.0f) || badIMUdata);
// test the ratio before fusing data
if (rngBcnHealth) {
// restart the counter
lastRngBcnPassTime_ms = imuSampleTime_ms;
// correct the covariance P = (I - K*H)*P
// take advantage of the empty columns in KH to reduce the
// number of operations
for (unsigned i = 0; i<=stateIndexLim; i++) {
for (unsigned j = 0; j<=5; j++) {
KH[i][j] = 0.0f;
}
for (unsigned j = 6; j<=8; j++) {
KH[i][j] = Kfusion[i] * H_BCN[j];
}
for (unsigned j = 9; j<=23; j++) {
KH[i][j] = 0.0f;
}
}
for (unsigned j = 0; j<=stateIndexLim; j++) {
for (unsigned i = 0; i<=stateIndexLim; i++) {
ftype res = 0;
res += KH[i][6] * P[6][j];
res += KH[i][7] * P[7][j];
res += KH[i][8] * P[8][j];
KHP[i][j] = res;
}
}
// Check that we are not going to drive any variances negative and skip the update if so
bool healthyFusion = true;
for (uint8_t i= 0; i<=stateIndexLim; i++) {
if (KHP[i][i] > P[i][i]) {
healthyFusion = false;
}
}
if (healthyFusion) {
// update the covariance matrix
for (uint8_t i= 0; i<=stateIndexLim; i++) {
for (uint8_t j= 0; j<=stateIndexLim; j++) {
P[i][j] = P[i][j] - KHP[i][j];
}
}
// force the covariance matrix to be symmetrical and limit the variances to prevent ill-conditioning.
ForceSymmetry();
ConstrainVariances();
// update the states
// zero the attitude error state - by definition it is assumed to be zero before each observation fusion
stateStruct.angErr.zero();
// correct the state vector
for (uint8_t j= 0; j<=stateIndexLim; j++) {
statesArray[j] = statesArray[j] - Kfusion[j] * innovRngBcn;
}
// the first 3 states represent the angular misalignment vector.
// This is used to correct the estimated quaternion on the current time step
stateStruct.quat.rotate(stateStruct.angErr);
// record healthy fusion
faultStatus.bad_rngbcn = false;
} else {
// record bad fusion
faultStatus.bad_rngbcn = true;
}
}
// Update the fusion report
rngBcnFusionReport[rngBcnDataDelayed.beacon_ID].beaconPosNED = rngBcnDataDelayed.beacon_posNED;
rngBcnFusionReport[rngBcnDataDelayed.beacon_ID].innov = innovRngBcn;
rngBcnFusionReport[rngBcnDataDelayed.beacon_ID].innovVar = varInnovRngBcn;
rngBcnFusionReport[rngBcnDataDelayed.beacon_ID].rng = rngBcnDataDelayed.rng;
rngBcnFusionReport[rngBcnDataDelayed.beacon_ID].testRatio = rngBcnTestRatio;
}
}
/*
Use range beacon measurements to calculate a static position using a 3-state EKF algorithm.
Algorithm based on the following:
https://github.com/priseborough/InertialNav/blob/master/derivations/range_beacon.m
*/
void NavEKF2_core::FuseRngBcnStatic()
{
// get the estimated range measurement variance
const ftype R_RNG = sq(MAX(rngBcnDataDelayed.rngErr , 0.1f));
/*
The first thing to do is to check if we have started the alignment and if not, initialise the
states and covariance to a first guess. To do this iterate through the available beacons and then
initialise the initial position to the mean beacon position. The initial position uncertainty
is set to the mean range measurement.
*/
if (!rngBcnAlignmentStarted) {
if (rngBcnDataDelayed.beacon_ID != lastBeaconIndex) {
rngBcnPosSum += rngBcnDataDelayed.beacon_posNED;
lastBeaconIndex = rngBcnDataDelayed.beacon_ID;
rngSum += rngBcnDataDelayed.rng;
numBcnMeas++;
// capture the beacon vertical spread
if (rngBcnDataDelayed.beacon_posNED.z > maxBcnPosD) {
maxBcnPosD = rngBcnDataDelayed.beacon_posNED.z;
} else if(rngBcnDataDelayed.beacon_posNED.z < minBcnPosD) {
minBcnPosD = rngBcnDataDelayed.beacon_posNED.z;
}
}
if (numBcnMeas >= 100) {
rngBcnAlignmentStarted = true;
ftype tempVar = 1.0f / (float)numBcnMeas;
// initialise the receiver position to the centre of the beacons and at zero height
receiverPos.x = rngBcnPosSum.x * tempVar;
receiverPos.y = rngBcnPosSum.y * tempVar;
receiverPos.z = 0.0f;
receiverPosCov[2][2] = receiverPosCov[1][1] = receiverPosCov[0][0] = rngSum * tempVar;
lastBeaconIndex = 0;
numBcnMeas = 0;
rngBcnPosSum.zero();
rngSum = 0.0f;
}
}
if (rngBcnAlignmentStarted) {
numBcnMeas++;
if (numBcnMeas >= 100) {
// 100 observations is enough for a stable estimate under most conditions
// TODO monitor stability of the position estimate
rngBcnAlignmentCompleted = true;
}
if (rngBcnAlignmentCompleted) {
if (activeHgtSource != HGT_SOURCE_BCN) {
// We are using a different height reference for the main EKF so need to estimate a vertical
// position offset to be applied to the beacon system that minimises the range innovations
// The position estimate should be stable after 100 iterations so we use a simple dual
// hypothesis 1-state EKF to estimate the offset
Vector3F refPosNED;
refPosNED.x = receiverPos.x;
refPosNED.y = receiverPos.y;
refPosNED.z = stateStruct.position.z;
CalcRangeBeaconPosDownOffset(R_RNG, refPosNED, true);
} else {
// we are using the beacons as the primary height source, so don't modify their vertical position
bcnPosOffset = 0.0f;
}
} else {
if (activeHgtSource != HGT_SOURCE_BCN) {
// The position estimate is not yet stable so we cannot run the 1-state EKF to estimate
// beacon system vertical position offset. Instead we initialise the dual hypothesis offset states
// using the beacon vertical position, vertical position estimate relative to the beacon origin
// and the main EKF vertical position
// Calculate the mid vertical position of all beacons
ftype bcnMidPosD = 0.5f * (minBcnPosD + maxBcnPosD);
// calculate the delta to the estimated receiver position
ftype delta = receiverPos.z - bcnMidPosD;
// calcuate the two hypothesis for our vertical position
ftype receverPosDownMax;
ftype receverPosDownMin;
if (delta >= 0.0f) {
receverPosDownMax = receiverPos.z;
receverPosDownMin = receiverPos.z - 2.0f * delta;
} else {
receverPosDownMax = receiverPos.z - 2.0f * delta;
receverPosDownMin = receiverPos.z;
}
bcnPosOffsetMax = stateStruct.position.z - receverPosDownMin;
bcnPosOffsetMin = stateStruct.position.z - receverPosDownMax;
} else {
// We are using the beacons as the primary height reference, so don't modify their vertical position
bcnPosOffset = 0.0f;
}
}
// Add some process noise to the states at each time step
for (uint8_t i= 0; i<=2; i++) {
receiverPosCov[i][i] += 0.1f;
}
// calculate the observation jacobian
ftype t2 = rngBcnDataDelayed.beacon_posNED.z - receiverPos.z + bcnPosOffset;
ftype t3 = rngBcnDataDelayed.beacon_posNED.y - receiverPos.y;
ftype t4 = rngBcnDataDelayed.beacon_posNED.x - receiverPos.x;
ftype t5 = t2*t2;
ftype t6 = t3*t3;
ftype t7 = t4*t4;
ftype t8 = t5+t6+t7;
if (t8 < 0.1f) {
// calculation will be badly conditioned
return;
}
ftype t9 = 1.0f/sqrtF(t8);
ftype t10 = rngBcnDataDelayed.beacon_posNED.x*2.0f;
ftype t15 = receiverPos.x*2.0f;
ftype t11 = t10-t15;
ftype t12 = rngBcnDataDelayed.beacon_posNED.y*2.0f;
ftype t14 = receiverPos.y*2.0f;
ftype t13 = t12-t14;
ftype t16 = rngBcnDataDelayed.beacon_posNED.z*2.0f;
ftype t18 = receiverPos.z*2.0f;
ftype t17 = t16-t18;
ftype H_RNG[3];
H_RNG[0] = -t9*t11*0.5f;
H_RNG[1] = -t9*t13*0.5f;
H_RNG[2] = -t9*t17*0.5f;
// calculate the Kalman gains
ftype t19 = receiverPosCov[0][0]*t9*t11*0.5f;
ftype t20 = receiverPosCov[1][1]*t9*t13*0.5f;
ftype t21 = receiverPosCov[0][1]*t9*t11*0.5f;
ftype t22 = receiverPosCov[2][1]*t9*t17*0.5f;
ftype t23 = t20+t21+t22;
ftype t24 = t9*t13*t23*0.5f;
ftype t25 = receiverPosCov[1][2]*t9*t13*0.5f;
ftype t26 = receiverPosCov[0][2]*t9*t11*0.5f;
ftype t27 = receiverPosCov[2][2]*t9*t17*0.5f;
ftype t28 = t25+t26+t27;
ftype t29 = t9*t17*t28*0.5f;
ftype t30 = receiverPosCov[1][0]*t9*t13*0.5f;
ftype t31 = receiverPosCov[2][0]*t9*t17*0.5f;
ftype t32 = t19+t30+t31;
ftype t33 = t9*t11*t32*0.5f;
varInnovRngBcn = R_RNG+t24+t29+t33;
ftype t35 = 1.0f/varInnovRngBcn;
ftype K_RNG[3];
K_RNG[0] = -t35*(t19+receiverPosCov[0][1]*t9*t13*0.5f+receiverPosCov[0][2]*t9*t17*0.5f);
K_RNG[1] = -t35*(t20+receiverPosCov[1][0]*t9*t11*0.5f+receiverPosCov[1][2]*t9*t17*0.5f);
K_RNG[2] = -t35*(t27+receiverPosCov[2][0]*t9*t11*0.5f+receiverPosCov[2][1]*t9*t13*0.5f);
// calculate range measurement innovation
Vector3F deltaPosNED = receiverPos - rngBcnDataDelayed.beacon_posNED;
deltaPosNED.z -= bcnPosOffset;
innovRngBcn = deltaPosNED.length() - rngBcnDataDelayed.rng;
// update the state
receiverPos.x -= K_RNG[0] * innovRngBcn;
receiverPos.y -= K_RNG[1] * innovRngBcn;
receiverPos.z -= K_RNG[2] * innovRngBcn;
receiverPos.z = MAX(receiverPos.z, minBcnPosD + 1.2f);
// calculate the covariance correction
for (unsigned i = 0; i<=2; i++) {
for (unsigned j = 0; j<=2; j++) {
KH[i][j] = K_RNG[i] * H_RNG[j];
}
}
for (unsigned j = 0; j<=2; j++) {
for (unsigned i = 0; i<=2; i++) {
ftype res = 0;
res += KH[i][0] * receiverPosCov[0][j];
res += KH[i][1] * receiverPosCov[1][j];
res += KH[i][2] * receiverPosCov[2][j];
KHP[i][j] = res;
}
}
// prevent negative variances
for (uint8_t i= 0; i<=2; i++) {
if (receiverPosCov[i][i] < 0.0f) {
receiverPosCov[i][i] = 0.0f;
KHP[i][i] = 0.0f;
} else if (KHP[i][i] > receiverPosCov[i][i]) {
KHP[i][i] = receiverPosCov[i][i];
}
}
// apply the covariance correction
for (uint8_t i= 0; i<=2; i++) {
for (uint8_t j= 0; j<=2; j++) {
receiverPosCov[i][j] -= KHP[i][j];
}
}
// ensure the covariance matrix is symmetric
for (uint8_t i=1; i<=2; i++) {
for (uint8_t j=0; j<=i-1; j++) {
ftype temp = 0.5f*(receiverPosCov[i][j] + receiverPosCov[j][i]);
receiverPosCov[i][j] = temp;
receiverPosCov[j][i] = temp;
}
}
if (numBcnMeas >= 100) {
// 100 observations is enough for a stable estimate under most conditions
// TODO monitor stability of the position estimate
rngBcnAlignmentCompleted = true;
}
}
}
/*
Run a single state Kalman filter to estimate the vertical position offset of the range beacon constellation
Calculate using a high and low hypothesis and select the hypothesis with the lowest innovation sequence
*/
void NavEKF2_core::CalcRangeBeaconPosDownOffset(ftype obsVar, Vector3F &vehiclePosNED, bool aligning)
{
// Handle height offsets between the primary height source and the range beacons by estimating
// the beacon systems global vertical position offset using a single state Kalman filter
// The estimated offset is used to correct the beacon height when calculating innovations
// A high and low estimate is calculated to handle the ambiguity in height associated with beacon positions that are co-planar
// The main filter then uses the offset with the smaller innovations
ftype innov; // range measurement innovation (m)
ftype innovVar; // range measurement innovation variance (m^2)
ftype gain; // Kalman gain
ftype obsDeriv; // derivative of observation relative to state
const ftype stateNoiseVar = 0.1f; // State process noise variance
const ftype filtAlpha = 0.01f; // LPF constant
const ftype innovGateWidth = 5.0f; // width of innovation consistency check gate in std
// estimate upper value for offset
// calculate observation derivative
ftype t2 = rngBcnDataDelayed.beacon_posNED.z - vehiclePosNED.z + bcnPosOffsetMax;
ftype t3 = rngBcnDataDelayed.beacon_posNED.y - vehiclePosNED.y;
ftype t4 = rngBcnDataDelayed.beacon_posNED.x - vehiclePosNED.x;
ftype t5 = t2*t2;
ftype t6 = t3*t3;
ftype t7 = t4*t4;
ftype t8 = t5+t6+t7;
ftype t9;
if (t8 > 0.1f) {
t9 = 1.0f/sqrtF(t8);
obsDeriv = t2*t9;
// Calculate innovation
innov = sqrtF(t8) - rngBcnDataDelayed.rng;
// calculate a filtered innovation magnitude to be used to select between the high or low offset
OffsetMaxInnovFilt = (1.0f - filtAlpha) * bcnPosOffsetMaxVar + filtAlpha * fabsF(innov);
// covariance prediction
bcnPosOffsetMaxVar += stateNoiseVar;
// calculate the innovation variance
innovVar = obsDeriv * bcnPosOffsetMaxVar * obsDeriv + obsVar;
innovVar = MAX(innovVar, obsVar);
// Reject range innovation spikes using a 5-sigma threshold unless aligning
if ((sq(innov) < sq(innovGateWidth) * innovVar) || aligning) {
// calculate the Kalman gain
gain = (bcnPosOffsetMaxVar * obsDeriv) / innovVar;
// state update
bcnPosOffsetMax -= innov * gain;
// covariance update
bcnPosOffsetMaxVar -= gain * obsDeriv * bcnPosOffsetMaxVar;
bcnPosOffsetMaxVar = MAX(bcnPosOffsetMaxVar, 0.0f);
}
}
// estimate lower value for offset
// calculate observation derivative
t2 = rngBcnDataDelayed.beacon_posNED.z - vehiclePosNED.z + bcnPosOffsetMin;
t5 = t2*t2;
t8 = t5+t6+t7;
if (t8 > 0.1f) {
t9 = 1.0f/sqrtF(t8);
obsDeriv = t2*t9;
// Calculate innovation
innov = sqrtF(t8) - rngBcnDataDelayed.rng;
// calculate a filtered innovation magnitude to be used to select between the high or low offset
OffsetMinInnovFilt = (1.0f - filtAlpha) * OffsetMinInnovFilt + filtAlpha * fabsF(innov);
// covariance prediction
bcnPosOffsetMinVar += stateNoiseVar;
// calculate the innovation variance
innovVar = obsDeriv * bcnPosOffsetMinVar * obsDeriv + obsVar;
innovVar = MAX(innovVar, obsVar);
// Reject range innovation spikes using a 5-sigma threshold unless aligning
if ((sq(innov) < sq(innovGateWidth) * innovVar) || aligning) {
// calculate the Kalman gain
gain = (bcnPosOffsetMinVar * obsDeriv) / innovVar;
// state update
bcnPosOffsetMin -= innov * gain;
// covariance update
bcnPosOffsetMinVar -= gain * obsDeriv * bcnPosOffsetMinVar;
bcnPosOffsetMinVar = MAX(bcnPosOffsetMinVar, 0.0f);
}
}
// calculate the mid vertical position of all beacons
ftype bcnMidPosD = 0.5f * (minBcnPosD + maxBcnPosD);
// ensure the two beacon vertical offset hypothesis place the mid point of the beacons below and above the flight vehicle
bcnPosOffsetMax = MAX(bcnPosOffsetMax, vehiclePosNED.z - bcnMidPosD + 0.5f);
bcnPosOffsetMin = MIN(bcnPosOffsetMin, vehiclePosNED.z - bcnMidPosD - 0.5f);
// calculate the innovation for the main filter using the offset with the smallest innovation history
if (OffsetMaxInnovFilt > OffsetMinInnovFilt) {
bcnPosOffset = bcnPosOffsetMin;
} else {
bcnPosOffset = bcnPosOffsetMax;
}
}
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