typedef struct {
  char *type_str;  /* One of:'Reflectivity', 'Velocity' or 'Spectrum width' */
  int nsweeps;
        float calibr_const;        /* Calibration constant. */
  float (*f)(Range x);       /* Data conversion function. f(x). */
  Range (*invf)(float x);    /* Data conversion function. invf(x). */
} Volume_header;