Add an altimeter and VSI to the Airball display. We do not yet have a

way to change the altimeter setting.

host/display/aerodynamics.cpp

@@ -49,4 +49,25 @@ double q_to_tas(double q, double p, double t) {
  return sqrt(2.0 * q / dry_air_density(p, t));
}

static constexpr double kAltK = 1.313e-05;
static constexpr double kAltN = 0.1903;
static constexpr double kAltPb = 29.92126;

// https://www.av8n.com/physics/altimetry.htm
// The formulae are given in English units. To maintain a consistent API
// throughout our system, the function inputs and outputs are in SI units. The
// tradeoff is we do a few more unit conversions than needed, in return for a
// hopefully less error-prone set of APIs.
double pressure_to_altitude(double p, double qnh) {
  double p_inHg = p / kPascalsPerInHg;
  double pS_inHg = qnh / kPascalsPerInHg;
  double h_feet =
      (pow(pS_inHg, kAltN) / kAltK) *
      (
          pow(pS_inHg / kAltPb, kAltN) -
          pow(p_inHg / kAltPb, kAltN)
      );
  return kMetersPerFoot * h_feet;
}

} // namespace airball

host/display/aerodynamics.h

@@ -62,6 +62,15 @@ double dry_air_density(double p, double t);
 */
double q_to_tas(double q, double p, double t);

/**
 * Compute the altitude given the barometric pressure.
 *
 * @param p barometric pressure at this point.
 * @param qnh barometer setting at this point (pressure at sea level).
 * @return altitude.
 */
double pressure_to_altitude(double p, double qnh);

} // namespace airball

#endif // AIRBALL_AERODYNAMICS_H

host/display/airdata.cpp

@@ -25,6 +25,7 @@
#include "airdata.h"

#include <iostream>
#include <cstring>

#include "aerodynamics.h"
#include "units.h"
@@ -78,6 +79,7 @@ double find_dpr_to_angle(InterpolationTable& table, double dpr) {

Airdata::Airdata() {
  populate_table(dpr_to_angle);
  memset(climb_rate_buffer_, 0, sizeof(climb_rate_buffer_));
}

void Airdata::update(const airdata_sample* d) {
@@ -88,6 +90,18 @@ void Airdata::update(const airdata_sample* d) {
      single_point_sphere_pressure_coefficient(total_angle);
  ias_ = q_to_ias(free_stream_q_);
  tas_ = q_to_tas(free_stream_q_, d->get_baro(), d->get_temperature());
  double new_altitude = pressure_to_altitude(d->get_baro(), 101300 /* Pascals */);
  for (int i = 1; i < kClimbRatePoints; i++) {
    climb_rate_buffer_[i - 1] = climb_rate_buffer_[i];
  }
  climb_rate_buffer_[kClimbRatePoints - 1] = new_altitude - altitude_;
  altitude_ = new_altitude;
  climb_rate_ = 0;
  for (int i = 0; i < kClimbRatePoints; i++) {
    climb_rate_ += climb_rate_buffer_[i];
  }
  climb_rate_ /= (double) kClimbRatePoints;
  climb_rate_ *= kSamplesPerMinute;
  valid_ = !isnan(alpha_) && !isnan(beta_);
}

host/display/airdata.h

@@ -50,19 +50,32 @@ class Airdata {
  // Free stream dynamic pressure
  double free_stream_q() const {return free_stream_q_;}

  // Current altitude (TODO: No barometer setting yet)
  double altitude() const { return altitude_; }

  // Current climb rate
  double climb_rate() const { return climb_rate_; }

  // Returns true if the data is valid. If not, display a red X indicating system failure.
  bool valid() const {return valid_;}

  // Commands this model to update its contents based on the given sensor data.
  void update(const airdata_sample* d);

private:
  static constexpr int kClimbRatePoints = 5;
  static constexpr int kSamplesPerMinute = 20 * 60;

  InterpolationTable dpr_to_angle;
  double ias_;
  double tas_;
  double alpha_;
  double beta_;
  double free_stream_q_;
  double altitude_;
  double climb_rate_;
  bool valid_;
  double climb_rate_buffer_[kClimbRatePoints];
};

} // namespace airball