GPS PreflightCheck: improve failure reporting

- previously it was possible to get a Position Control rejected message
  without further advice what was actually wrong. So now we report warnings
  even if gps is not required for arming (which could be annoying too...).
- the GPS failure message was very generic, making it hard to debug the
  cause. Now we check every bit and send an appropriate warning

All strings were checked not to exceed the maximum length of 50 characters.

src/modules/commander/PreflightCheck.cpp

@@ -600,47 +600,51 @@ static bool ekf2Check(orb_advert_t *mavlink_log_pub, vehicle_status_s &vehicle_s
 	}
 
 	// If GPS aiding is required, declare fault condition if the required GPS quality checks are failing
-	if (enforce_gps_required) {
-		bool ekf_gps_fusion = status.control_mode_flags & (1 << 2);
-		bool ekf_gps_check_fail = status.gps_check_fail_flags > 0;
-
-		if (!ekf_gps_fusion) {
-			// The EKF is not using GPS
-			if (ekf_gps_check_fail) {
-				// Poor GPS quality is the likely cause
-				if (report_fail) {
-					mavlink_log_critical(mavlink_log_pub, "PREFLIGHT FAIL: GPS QUALITY POOR");
-				}
-
-				gps_success = false;
-
-			} else {
-				// Likely cause unknown
-				if (report_fail) {
-					mavlink_log_critical(mavlink_log_pub, "PREFLIGHT FAIL: EKF NOT USING GPS");
-				}
+	if (enforce_gps_required || report_fail) {
+		const bool ekf_gps_fusion = status.control_mode_flags & (1 << estimator_status_s::CS_GPS);
+		const bool ekf_gps_check_fail = status.gps_check_fail_flags > 0;
 
-				gps_success = false;
-				gps_present = false;
-			}
+		gps_success = ekf_gps_fusion; // default to success if gps data is fused
 
-			success = false;
-			goto out;
+		const char *fail_str = enforce_gps_required ? "Fail" : "Warn";
 
-		} else {
-			// The EKF is using GPS so check for bad quality on key performance indicators
-			bool gps_quality_fail = ((status.gps_check_fail_flags & ((1 << estimator_status_s::GPS_CHECK_FAIL_MIN_SAT_COUNT)
-						  + (1 << estimator_status_s::GPS_CHECK_FAIL_MIN_GDOP)
-						  + (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_HORZ_ERR)
-						  + (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_VERT_ERR)
-						  + (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_SPD_ERR))) > 0);
-
-			if (gps_quality_fail) {
-				if (report_fail) {
-					mavlink_log_critical(mavlink_log_pub, "PREFLIGHT FAIL: GPS QUALITY POOR");
+		if (ekf_gps_check_fail) {
+			if (report_fail) {
+				// Only report the first failure to avoid spamming
+				if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_GPS_FIX)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS fix too low", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MIN_SAT_COUNT)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: not enough GPS Satellites", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MIN_GDOP)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS GDoP too low", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_HORZ_ERR)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Horizontal Pos Error too high", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_VERT_ERR)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Vertical Pos Error too high", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_SPD_ERR)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Speed Accuracy too low", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_HORZ_DRIFT)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Horizontal Pos Drift too high", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_VERT_DRIFT)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Vertical Pos Drift too high", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_HORZ_SPD_ERR)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Hor Speed Drift too high", fail_str);
+				} else if (status.gps_check_fail_flags & (1 << estimator_status_s::GPS_CHECK_FAIL_MAX_VERT_SPD_ERR)) {
+					mavlink_log_critical(mavlink_log_pub, "Preflight %s: GPS Vert Speed Drift too high", fail_str);
+				} else {
+					if (!ekf_gps_fusion) {
+						// Likely cause unknown
+						mavlink_log_critical(mavlink_log_pub, "Preflight %s: EKF not using GPS", fail_str);
+						gps_present = false;
+					} else {
+						// if we land here there was a new flag added and the code not updated. Show a generic message.
+						mavlink_log_critical(mavlink_log_pub, "Preflight %s: Poor GPS Quality", fail_str);
+					}
 				}
+			}
+			gps_success = false;
 
-				gps_success = false;
+			if (enforce_gps_required) {
 				success = false;
 				goto out;
 			}