ardupilot/libraries/AP_HAL_SITL/sitl_airspeed.cpp

/*
  SITL handling

  This simulates an analog airspeed sensor

  Andrew Tridgell November 2011
 */

#include <AP_HAL/AP_HAL.h>
#if CONFIG_HAL_BOARD == HAL_BOARD_SITL

#include "AP_HAL_SITL.h"
#include "AP_HAL_SITL_Namespace.h"
#include "HAL_SITL_Class.h"
#include "SITL_State.h"
#include <SITL/SITL.h>
#include <AP_Math/AP_Math.h>

extern const AP_HAL::HAL& hal;

using namespace HALSITL;

/*
  convert airspeed in m/s to an airspeed sensor value
 */
void SITL_State::_update_airspeed(float airspeed)
{
    const float airspeed_ratio = 1.9936f;
    const float airspeed_offset = 2013.0f;

    // Check sensor failure
    airspeed = is_zero(_sitl->arspd_fail) ? airspeed : _sitl->arspd_fail;
    // Add noise
    airspeed = airspeed + (_sitl->arspd_noise * rand_float());

    if (!is_zero(_sitl->arspd_fail_pressure)) {
        // compute a realistic pressure report given some level of trapper air pressure in the tube abd our current altitude
        // algorithim taken from https://en.wikipedia.org/wiki/Calibrated_airspeed#Calculation_from_impact_pressure
        float tube_pressure = abs(_sitl->arspd_fail_pressure - _barometer->get_pressure() + _sitl->arspd_fail_pitot_pressure);
        airspeed = 340.29409348 * sqrt(5 * (pow((tube_pressure / 101325.01576 + 1), 2.0/7.0) - 1.0));
    }

    const float airspeed_pressure = (airspeed * airspeed) / airspeed_ratio;
    float airspeed_raw = airspeed_pressure + airspeed_offset;
    if (airspeed_raw / 4 > 0xFFFF) {
        airspeed_pin_value = 0xFFFF;
        return;
    }
    // add delay
    const uint32_t now = AP_HAL::millis();
    uint32_t best_time_delta_wind = 200;  // initialise large time representing buffer entry closest to current time - delay.
    uint8_t best_index_wind = 0;  // initialise number representing the index of the entry in buffer closest to delay.

    // storing data from sensor to buffer
    if (now - last_store_time_wind >= 10) {  // store data every 10 ms.
        last_store_time_wind = now;
        if (store_index_wind > wind_buffer_length - 1) {  // reset buffer index if index greater than size of buffer
            store_index_wind = 0;
        }
        buffer_wind[store_index_wind].data = airspeed_raw;  // add data to current index
        buffer_wind[store_index_wind].time = last_store_time_wind;  // add time to current index
        store_index_wind = store_index_wind + 1;  // increment index
    }

    // return delayed measurement
    delayed_time_wind = now - _sitl->wind_delay;  // get time corresponding to delay
    // find data corresponding to delayed time in buffer
    for (uint8_t i = 0; i <= wind_buffer_length - 1; i++) {
        // find difference between delayed time and time stamp in buffer
        time_delta_wind = abs(
                (int32_t)(delayed_time_wind - buffer_wind[i].time));
        // if this difference is smaller than last delta, store this time
        if (time_delta_wind < best_time_delta_wind) {
            best_index_wind = i;
            best_time_delta_wind = time_delta_wind;
        }
    }
    if (best_time_delta_wind < 200) {  // only output stored state if < 200 msec retrieval error
        airspeed_raw = buffer_wind[best_index_wind].data;
    }

    airspeed_pin_value = airspeed_raw / 4;
}

#endif