From 4a6d3334a7d95ae2bff29ecb53c9b0e3c46101d0 Mon Sep 17 00:00:00 2001
From: Alexander Hirsch <amhirsch825@gmail.com>
Date: Wed, 5 Jun 2019 16:11:50 -0700
Subject: [PATCH] Decoder recognizes supersonic velocity subtype

If the aircraft velocity message indicates a supersonic subtype
(2 or 4), multiply the derived velocity by four. This is in accordance
with Table A-2-9[a, b] in the "Technical Provisions for Mode-S Services
and Extended Squitter"
---
 pyModeS/decoder/bds/bds09.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/pyModeS/decoder/bds/bds09.py b/pyModeS/decoder/bds/bds09.py
index 3ccdae6..44d7a31 100644
--- a/pyModeS/decoder/bds/bds09.py
+++ b/pyModeS/decoder/bds/bds09.py
@@ -59,9 +59,13 @@ def airborne_velocity(msg, rtn_sources=False):
     if subtype in (1, 2):
         v_ew_sign = -1 if mb[13]=='1' else 1
         v_ew = common.bin2int(mb[14:24]) - 1       # east-west velocity
+        if subtype == 2: # Supersonic
+            v_ew *= 4
 
         v_ns_sign = -1 if mb[24]=='1' else 1
         v_ns = common.bin2int(mb[25:35]) - 1       # north-south velocity
+        if subtype == 2: # Supersonic
+            v_ns *= 4
 
         v_we = v_ew_sign * v_ew
         v_sn = v_ns_sign * v_ns
@@ -88,6 +92,8 @@ def airborne_velocity(msg, rtn_sources=False):
 
         spd = common.bin2int(mb[25:35])
         spd = None if spd==0 else spd-1
+        if subtype == 4: # Supersonic
+            spd *= 4
 
         if mb[24]=='0':
             tag = 'IAS'