ardupilot/libraries/AP_Baro/AP_Baro_Backend.cpp

#include "AP_Baro_Backend.h"
#include <stdio.h>

extern const AP_HAL::HAL& hal;

// constructor
AP_Baro_Backend::AP_Baro_Backend(AP_Baro &baro) : 
    _frontend(baro) 
{
    _sem = hal.util->new_semaphore();    
}

void AP_Baro_Backend::update_healthy_flag(uint8_t instance)
{
    if (instance >= _frontend._num_sensors) {
        return;
    }
    if (!_sem->take_nonblocking()) {
        return;
    }

    // consider a sensor as healthy if it has had an update in the
    // last 0.5 seconds and values are non-zero and have changed within the last 2 seconds
    const uint32_t now = AP_HAL::millis();
    _frontend.sensors[instance].healthy =
        (now - _frontend.sensors[instance].last_update_ms < BARO_TIMEOUT_MS) &&
        (now - _frontend.sensors[instance].last_change_ms < BARO_DATA_CHANGE_TIMEOUT_MS) &&
        !is_zero(_frontend.sensors[instance].pressure);

    _sem->give();
}

void AP_Baro_Backend::backend_update(uint8_t instance)
{
    update();
    update_healthy_flag(instance);
}


/*
  copy latest data to the frontend from a backend
 */
void AP_Baro_Backend::_copy_to_frontend(uint8_t instance, float pressure, float temperature)
{
    if (instance >= _frontend._num_sensors) {
        return;
    }
    uint32_t now = AP_HAL::millis();

    // check for changes in data values
    if (!is_equal(_frontend.sensors[instance].pressure, pressure) || !is_equal(_frontend.sensors[instance].temperature, temperature)) {
        _frontend.sensors[instance].last_change_ms = now;
    }

    // update readings
    _frontend.sensors[instance].pressure = pressure;
    _frontend.sensors[instance].temperature = temperature;
    _frontend.sensors[instance].last_update_ms = now;
}

static constexpr float FILTER_KOEF = 0.1f;

/* Check that the baro value is valid by using a mean filter. If the
 * value is further than filtrer_range from mean value, it is
 * rejected. 
*/
bool AP_Baro_Backend::pressure_ok(float press)
{
    
    if (isinf(press) || isnan(press)) {
        return false;
    }

    const float range = (float)_frontend.get_filter_range();
    if (range <= 0) {
        return true;
    }

    bool ret = true;
    if (is_zero(_mean_pressure)) {
        _mean_pressure = press;
    } else {
        const float d = fabsf(_mean_pressure - press) / (_mean_pressure + press);  // diff divide by mean value in percent ( with the * 200.0f on later line)
        float koeff = FILTER_KOEF;

        if (d * 200.0f > range) {  // check the difference from mean value outside allowed range
            // printf("\nBaro pressure error: mean %f got %f\n", (double)_mean_pressure, (double)press );
            ret = false;
            koeff /= (d * 10.0f);  // 2.5 and more, so one bad sample never change mean more than 4%
            _error_count++;
        }
        _mean_pressure = _mean_pressure * (1 - koeff) + press * koeff; // complimentary filter 1/k
    }
    return ret;
}