APM EKF2 alt source

主要看NavEKF2_core下面的selectHeightForFusion()函数。

首先从应用层读取高度，再进行角度补偿，分别有三种高度源：baro rangefinder GPS （可以在地面站中配置选择怎么开启和关闭）

(frontend->_useRngSwHgt > 0) 用户设置的rangefinder最大高度的百分比大于0。

(frontend->_altSource == 1) 其中_altSource 这个值是在地面站可以修改的。

_altSource 值代表用什么做主高度来源。0为气压计，1位测距仪，2为gps。

rangefinder_state.alt_healthy =((rangefinder.status() ==RangeFinder::RangeFinder_Good) && (rangefinder.range_valid_count()>= RANGEFINDER_HEALTH_MAX))；
//这就是判断状态标志的地方，根据在rangefinder驱动里的状态和采集到的有效值数量。我们可以直接把这个值强制为0，就是不管rangefinder怎么采集，都认为数据不健康，不去参与运算。下面的代码就不说了，和我没关系了。

然后就是一些逻辑的切换，如注释：

// select the height measurement to be fused from the available baro, range finder and GPS sources
//EKF2选择高度进行融合
void NavEKF2_core::selectHeightForFusion()
{
    // Read range finder data and check for new data in the buffer
    // This data is used by both height and optical flow fusion processing
	//NRA24数据
    readRangeFinder();
    rangeDataToFuse = storedRange.recall(rangeDataDelayed,imuDataDelayed.time_ms); //若是检测到了有数据更新则给出一个信号

    // correct range data for the body frame position offset relative to the IMU
    // the corrected reading is the reading that would have been taken if the sensor was
    // co-located with the IMU
    if (rangeDataToFuse) {                     //校正 range 的数据
        AP_RangeFinder_Backend *sensor = frontend->_rng.get_backend(rangeDataDelayed.sensor_idx);
        if (sensor != nullptr) {
            Vector3f posOffsetBody = sensor->get_pos_offset() - accelPosOffset;
            if (!posOffsetBody.is_zero()) {
                Vector3f posOffsetEarth = prevTnb.mul_transpose(posOffsetBody);
                rangeDataDelayed.rng += posOffsetEarth.z / prevTnb.c.z;
            }
        }
    }

    // read baro height data from the sensor and check for new data in the buffer
	
    //获得MS5611气压计数据
    readBaroData();
    baroDataToFuse = storedBaro.recall(baroDataDelayed, imuDataDelayed.time_ms); //检查气压buff，得到一个标志

    // select height source
	//选择高度源
    if (extNavUsedForPos) {
        // always use external vision as the hight source if using for position.
	    //外部视觉来得到高度
        activeHgtSource = HGT_SOURCE_EV;
    } 
	//由于_useRngSwHgt>0 和 _altSource=1 只要满足一个就行，故之前设置为-1也没什么影响，只不过没加入这个减小最大range使用值。
	else if (((frontend->_useRngSwHgt > 0) || (frontend->_altSource == 1)) && (imuSampleTime_ms - rngValidMeaTime_ms < 500)) {
        
		if (frontend->_altSource == 1) { //此时在地面站上设置EK2_ALT_SOURCE =1 标记活跃的高度源为：rangefinder
            // always use range finder
            activeHgtSource = HGT_SOURCE_RNG;
        } else {   //若是选择了0，为气压
            // determine if we are above or below the height switch region
			// 确定我们是否在高度开关区域的上方或下方
            
			// 得到按比例运算后的rangefinder高度 0.0001*5000*4%=200cm			
            float rangeMaxUse = 1e-4f * (float)frontend->_rng.max_distance_cm_orient(ROTATION_PITCH_270) * (float)frontend->_useRngSwHgt;
            // What???
			//terrainState 为 terrain position state (m)
            bool aboveUpperSwHgt = (terrainState - stateStruct.position.z) > rangeMaxUse;        //若是rangefinder的高度大于200cm则令此标记为1
            bool belowLowerSwHgt = (terrainState - stateStruct.position.z) < 0.7f * rangeMaxUse; //若是rangefinder的高度小于140cm则令此标记为1
            //若是地形高度是连续的并且缓慢移动，接着地形高度能和rangefinder结合
			
            // If the terrain height is consistent and we are moving slowly, then it can be
            // used as a height reference in combination with a range finder
            // apply a hysteresis to the speed check to prevent rapid switching
			// 设置滞后
			// 速度
			//计算水平速度的模
            float horizSpeed = norm(stateStruct.velocity.x, stateStruct.velocity.y); 
			//若是水平速度大于设置的最大速度(此时设置的是2m/s)并且速度是可用，或者高度发散则此标记记为1
            bool dontTrustTerrain = ((horizSpeed > frontend->_useRngSwSpd) && filterStatus.flags.horiz_vel) || !terrainHgtStable;
			
            float trust_spd_trigger = MAX((frontend->_useRngSwSpd - 1.0f),(frontend->_useRngSwSpd * 0.5f));
			//判断标志，信任rangefinder
            bool trustTerrain = (horizSpeed < trust_spd_trigger) && terrainHgtStable;

            /*
             * Switch between range finder and primary height source using height above ground and speed thresholds with
             * hysteresis to avoid rapid switching. Using range finder for height requires a consistent terrain height
             * which cannot be assumed if the vehicle is moving horizontally.
            */
			//切换高度源 （暂时只考虑高度），若是已经超过最大高度200cm并且选择的是Rangefinder则，若是已经超过最大水平速度2m/s并且选择的是Rangefinder则
            if ((aboveUpperSwHgt || dontTrustTerrain) && (activeHgtSource == HGT_SOURCE_RNG)) { //当选择了NRA24后
                // cannot trust terrain or range finder so stop using range finder height
                if (frontend->_altSource == 0) {
                    activeHgtSource = HGT_SOURCE_BARO;
                } else if (frontend->_altSource == 2) {
                    activeHgtSource = HGT_SOURCE_GPS;
                }
            } else if (belowLowerSwHgt && trustTerrain && (activeHgtSource != HGT_SOURCE_RNG)) {
                // reliable terrain and range finder so start using range finder height
                activeHgtSource = HGT_SOURCE_RNG;
            }
        }
	//高度源为2 ，GPS高度
    } else if ((frontend->_altSource == 2) && ((imuSampleTime_ms - lastTimeGpsReceived_ms) < 500) && validOrigin && gpsAccuracyGood) {
        activeHgtSource = HGT_SOURCE_GPS;
	// 不然选择3 Range Beacon.
    } else if ((frontend->_altSource == 3) && validOrigin && rngBcnGoodToAlign) {
        activeHgtSource = HGT_SOURCE_BCN;
    } else {
	//否则为0，则选择气压计定高
        activeHgtSource = HGT_SOURCE_BARO;
    }

    // Use Baro alt as a fallback if we lose range finder, GPS or external nav
	//用气压高度作为一个fallback 来告诉飞控是否丢失了range finder的数据
    bool lostRngHgt = ((activeHgtSource == HGT_SOURCE_RNG) && ((imuSampleTime_ms - rngValidMeaTime_ms) > 500)); //若是超过500ms没收到则令此标志为1
    bool lostGpsHgt = ((activeHgtSource == HGT_SOURCE_GPS) && ((imuSampleTime_ms - lastTimeGpsReceived_ms) > 2000)); //若是超过2s没收到gps数据则令此标志为1
    bool lostExtNavHgt = ((activeHgtSource == HGT_SOURCE_EV) && ((imuSampleTime_ms - extNavMeasTime_ms) > 2000)); //外部视觉定高
   //若是当前选择的定高源丢失了则用气压定高   
   if (lostRngHgt || lostGpsHgt || lostExtNavHgt) {
        activeHgtSource = HGT_SOURCE_BARO;
    }

    // if there is new baro data to fuse, calculate filtered baro data required by other processes
    //若是有新的气压计数据，经过滤波
	if (baroDataToFuse) {
        // calculate offset to baro data that enables us to switch to Baro height use during operation
        if  (activeHgtSource != HGT_SOURCE_BARO) {
            calcFiltBaroOffset();
        }
        // filtered baro data used to provide a reference for takeoff
        // it is is reset to last height measurement on disarming in performArmingChecks()
		//留着
        if (!getTakeoffExpected()) {
            const float gndHgtFiltTC = 0.5f;
            const float dtBaro = frontend->hgtAvg_ms*1.0e-3f;
            float alpha = constrain_float(dtBaro / (dtBaro+gndHgtFiltTC),0.0f,1.0f);
            meaHgtAtTakeOff += (baroDataDelayed.hgt-meaHgtAtTakeOff)*alpha;
        }
    }
    //
    // If we are not using GPS as the primary height sensor, correct EKF origin height so that
    // combined local NED position height and origin height remains consistent with the GPS altitude
    // This also enables the GPS height to be used as a backup height source
	//若是不把GPS高度作为主要高度则作为备用高度
    if (gpsDataToFuse &&
            (((frontend->_originHgtMode & (1 << 0)) && (activeHgtSource == HGT_SOURCE_BARO)) ||
            ((frontend->_originHgtMode & (1 << 1)) && (activeHgtSource == HGT_SOURCE_RNG)))
            ) {
        correctEkfOriginHeight();
    }

    // Select the height measurement source
	//选择高度测量源
    if (extNavDataToFuse && (activeHgtSource == HGT_SOURCE_EV)) { //视频定高
        hgtMea = -extNavDataDelayed.pos.z;
        posDownObsNoise = sq(constrain_float(extNavDataDelayed.posErr, 0.1f, 10.0f));
    } else if (rangeDataToFuse && (activeHgtSource == HGT_SOURCE_RNG)) {  
        // using range finder data
        // correct for tilt using a flat earth model
	   //rangefinder作为高度源
        if (prevTnb.c.z >= 0.7) {
            // calculate height above ground
			// 计算地表高度
            hgtMea  = MAX(rangeDataDelayed.rng * prevTnb.c.z, rngOnGnd);
            // correct for terrain position relative to datum
            hgtMea -= terrainState;
            // enable fusion
			// 使能融合
            fuseHgtData = true;
            velPosObs[5] = -hgtMea;
            // set the observation noise
			// 设置观测噪声
            posDownObsNoise = sq(constrain_float(frontend->_rngNoise, 0.1f, 10.0f));
            // add uncertainty created by terrain gradient and vehicle tilt
			// 添加不确定因素
            posDownObsNoise += sq(rangeDataDelayed.rng * frontend->_terrGradMax) * MAX(0.0f , (1.0f - sq(prevTnb.c.z)));
        } else {
            // disable fusion if tilted too far
			// 若是倾斜的厉害则不进行融合
            fuseHgtData = false;
        }
    } else if  (gpsDataToFuse && (activeHgtSource == HGT_SOURCE_GPS)) { //gps源
        // using GPS data
        hgtMea = gpsDataDelayed.hgt;
        // enable fusion
        velPosObs[5] = -hgtMea;
        fuseHgtData = true;
        // set the observation noise using receiver reported accuracy or the horizontal noise scaled for typical VDOP/HDOP ratio
        if (gpsHgtAccuracy > 0.0f) {
            posDownObsNoise = sq(constrain_float(gpsHgtAccuracy, 1.5f * frontend->_gpsHorizPosNoise, 100.0f));
        } else {
            posDownObsNoise = sq(constrain_float(1.5f * frontend->_gpsHorizPosNoise, 0.1f, 10.0f));
        }
    } else if (baroDataToFuse && (activeHgtSource == HGT_SOURCE_BARO)) { //若是设置为气压模式
        // using Baro data
		//用气压数据
        hgtMea = baroDataDelayed.hgt - baroHgtOffset;
        // enable fusion
		//使能融合
        velPosObs[5] = -hgtMea;
        fuseHgtData = true;
        // set the observation noise
		//观测噪声，观测噪声从哪里输入？通过地面站输入的  AP_GROUPINFO("RNG_M_NSE", 18, NavEKF2, _rngNoise, 0.5f),
        posDownObsNoise = sq(constrain_float(frontend->_baroAltNoise, 0.1f, 10.0f));
        // reduce weighting (increase observation noise) on baro if we are likely to be in ground effect
        if (getTakeoffExpected() || getTouchdownExpected()) {
            posDownObsNoise *= frontend->gndEffectBaroScaler;
        }
        // If we are in takeoff mode, the height measurement is limited to be no less than the measurement at start of takeoff
        // This prevents negative baro disturbances due to copter downwash corrupting the EKF altitude during initial ascent
        if (motorsArmed && getTakeoffExpected()) {
            hgtMea = MAX(hgtMea, meaHgtAtTakeOff);
        }
    } else {
        fuseHgtData = false;
    }

    // If we haven't fused height data for a while, then declare the height data as being timed out
    // set timeout period based on whether we have vertical GPS velocity available to constrain drift
    hgtRetryTime_ms = (useGpsVertVel && !velTimeout) ? frontend->hgtRetryTimeMode0_ms : frontend->hgtRetryTimeMode12_ms;
    if (imuSampleTime_ms - lastHgtPassTime_ms > hgtRetryTime_ms) {
        hgtTimeout = true;
    } else {
        hgtTimeout = false;
    }
}