Tracker: correct altitude pressure offset calculation

the value nav_status.altitude_offset is expected to be a correction for differences between the barometers.  The user calibrates this value with a MAV_CMD_PREFLIGHT_CALIBRATION call.

Without this patch we were passing in the raw barometric pressure values for the tracker and the tracked vehicle.  It seems get_altitude_difference is expecting a sea-level pressure as its first argument now, as it subtracts the field elevation from the pressure-difference altitude calculations.

Change our call to provide a sea-level-adjusted value

AntennaTracker/tracking.cpp

@@ -152,7 +152,7 @@ void Tracker::tracking_update_position(const mavlink_global_position_int_t &msg)
  */
 void Tracker::tracking_update_pressure(const mavlink_scaled_pressure_t &msg)
 {
-    float local_pressure = barometer.get_pressure();
+    float local_pressure = barometer.get_sealevel_pressure(barometer.get_pressure());
     float aircraft_pressure = msg.press_abs*100.0f;
 
     // calculate altitude difference based on difference in barometric pressure
@@ -166,6 +166,7 @@ void Tracker::tracking_update_pressure(const mavlink_scaled_pressure_t &msg)
 			nav_status.altitude_offset = -alt_diff;
 			nav_status.alt_difference_baro = 0;
 			nav_status.need_altitude_calibration = false;
+            gcs().send_text(MAV_SEVERITY_INFO, "Pressure alt delta=%f", alt_diff);
 #if HAL_LOGGING_ENABLED
             logger.Write_NamedValueFloat("NAV_ALT_OFS", nav_status.altitude_offset);
 #endif