Apogee Filter

A Kalman filter for apogee detection. The filter tracks a 2-element state vector (altitude and vertical velocity) and provides an apogee detection function.

int filter_init(struct filter *filter)

Initialize the filter.

Zeroes state, sets initial covariance, and loads process/measurement noise from Kconfig (scaled by FILTER_SCALE_DIVISOR).

Parameters:

filter – Pointer to filter structure.

Return values:

  • 0 – on success.

  • -EINVAL – if filter is NULL.

int filter_predict(struct filter *filter, int64_t dt)

Perform the filter prediction step.

Propagates state and covariance forward in time using a constant-velocity model.

Parameters:

  • filter – Pointer to filter structure.

  • dt – Elapsed time in nanoseconds since last prediction.

Return values:

  • 0 – on success.

  • -EINVAL – if filter is NULL or dt <= 0.

int filter_update(struct filter *filter, float z)

Perform the filter measurement update step.

Computes Kalman gain and corrects the state estimate using a barometric altitude measurement.

Parameters:

  • filter – Pointer to filter structure.

  • z – Measured altitude in meters.

Return values:

  • 0 – on success.

  • -EINVAL – if filter is NULL.

int filter_detect_apogee(struct filter *filter)

Detect apogee from the filter’s velocity estimate.

Returns 1 when the estimated vertical velocity crosses zero from positive to non-positive, indicating apogee.

Parameters:

filter – Pointer to filter structure.

Return values:

  • 1 – if apogee detected.

  • 0 – if apogee not detected.

  • -EINVAL – if filter is NULL.

FILTER_SCALE_DIVISOR

Divisor applied to Kconfig milliscale noise parameters.

struct filter
#include <filter.h>

2-state Kalman filter structure for rocket apogee detection.

State vector: x[0] = altitude (m) x[1] = vertical velocity (m/s)

The filter assumes a constant-velocity model and uses barometric altitude as the measurement input.