From 0e29a4d18a0b477c9761fddb5030200430ede7d2 Mon Sep 17 00:00:00 2001
From: junzis <junzis@users.noreply.github.com>
Date: Fri, 21 Jul 2017 15:57:32 +0200
Subject: [PATCH] minor update

---
 pyModeS/adsb.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/pyModeS/adsb.py b/pyModeS/adsb.py
index b2ddb19..d98a04a 100644
--- a/pyModeS/adsb.py
+++ b/pyModeS/adsb.py
@@ -679,4 +679,8 @@ def altitude_diff(msg):
     msgbin = util.hex2bin(msg)
     sign = -1 if int(msgbin[80]) else 1
     value = util.bin2int(msgbin[81:88]) * 25
-    return sign * value
+
+    if value == 0 or value == 127:
+        return None
+    else:
+        return sign * value