From b36540e2a17c49bf36bc824bc1e4488306d1a1a0 Mon Sep 17 00:00:00 2001
From: Xavier Olive <1360812+xoolive@users.noreply.github.com>
Date: Tue, 31 May 2022 09:55:07 +0200
Subject: [PATCH] fix corner case

---
 pyModeS/decoder/bds/bds09.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pyModeS/decoder/bds/bds09.py b/pyModeS/decoder/bds/bds09.py
index 90cf95d..737f31a 100644
--- a/pyModeS/decoder/bds/bds09.py
+++ b/pyModeS/decoder/bds/bds09.py
@@ -81,10 +81,10 @@ def airborne_velocity(msg, source=False):
 
         spd = common.bin2int(mb[25:35])
 
-        spd = None if spd == 0 else spd - 1
-
         if subtype == 4:  # Supersonic
             spd *= 4
+            
+        spd = None if spd == 0 else spd - 1
 
         if mb[24] == "0":
             spd_type = "IAS"