diff --git a/python/altitude.py b/python/altitude.py
index 1824194..3519642 100755
--- a/python/altitude.py
+++ b/python/altitude.py
@@ -48,7 +48,7 @@ def decode_alt(alt, bit13):
 			#in this representation, the altitude bits are as follows:
 			# 12 11 10 9 8 7 (6) 5 (4) 3 2 1 0
 			# so bits 6 and 4 are the M and Q bits, respectively.
-			tmp1 = (alt & 0x3F80) >> 2
+			tmp1 = (alt & 0x1F80) >> 2
 			tmp2 = (alt & 0x0020) >> 1
 		else:
 			tmp1 = (alt & 0x1FE0) >> 1