Add support and tests for BDS 6,1 emergency messages

ACAS RA messages (subtype 2) are not yet implemented. These messages
are the same format as BDS3,0 starting at byte 9

pyModeS/decoder/adsb.py

@@ -34,6 +34,7 @@ from pyModeS.decoder.bds.bds06 import (
 )
 from pyModeS.decoder.bds.bds08 import category, callsign
 from pyModeS.decoder.bds.bds09 import airborne_velocity, altitude_diff
+from pyModeS.decoder.bds.bds61 import is_emergency, emergency, emergency_squawk
 
 
 def df(msg):

pyModeS/decoder/bds/bds61.py

@@ -0,0 +1,98 @@
+# ------------------------------------------
+#   BDS 6,1
+#   ADS-B TC=28
+#   Aircraft Airborne status
+# ------------------------------------------
+
+from __future__ import absolute_import, print_function, division
+
+from pyModeS import common
+
+
+def is_emergency(msg):
+    """
+    Check if the aircraft is reporting an emergency
+    Non-emergencies are either a subtype of zero (no information) or
+    subtype of one and a value of zero (no emergency).
+    Subtype = 2 indicates an ACAS RA broadcast, look in BDS 3,0
+    :param msg:
+    :return: if the aircraft has declared an emergency
+    """
+    if common.typecode(msg) != 28:
+        raise RuntimeError("%s: Not an airborne status message, expecting TC=28" % msg)
+
+    mb = common.hex2bin(msg)[32:]
+
+    subtype = common.bin2int(mb[5:8])
+    emergency_state = common.bin2int(mb[8:11])
+
+    if subtype == 2:
+        raise RuntimeError("%s: Emergency message is ACAS-RA, not implemented")
+
+    if subtype == 0:
+        return False
+    elif subtype == 1 and emergency_state == 0:
+        return False
+    else:
+        return True
+
+
+def emergency(msg):
+    """
+    Return the aircraft emergency
+
+    ===== =======
+    Value Meaning
+    ===== =======
+    1     General emergency
+    2     Lifeguard/Medical
+    3     Minimum fuel
+    4     No communications
+    5     Unlawful communications
+    6     Reserved
+    7     Reserved
+    :param msg:
+    :return: If the aircraft has declared an emergency, the type
+    """
+    if not is_emergency(msg):
+        raise RuntimeError("%s: Aircraft not declared an emergency" % msg)
+    else:
+        mb = common.hex2bin(msg)[32:]
+
+        subtype = common.bin2int(mb[5:8])
+        emergency_state = common.bin2int(mb[8:11])
+
+        return emergency_state
+
+
+def emergency_squawk(msg):
+    """
+    Squawk code of the transmitting aircraft
+    Emergency value 1 will be squawk 7700
+    Emergency value 4 will be squawk 7600
+    Emergency value 5 will be squawk 7500
+    :param msg:
+    :return: aircraft squawk code
+    """
+    if common.typecode(msg) != 28:
+        raise RuntimeError("%s: Not an airborne status message, expecting TC=28" % msg)
+
+    mb = common.hex2bin(msg)[32:]
+    C1 = mb[11]
+    A1 = mb[12]
+    C2 = mb[13]
+    A2 = mb[14]
+    C4 = mb[15]
+    A4 = mb[16]
+    B1 = mb[17]
+    D1 = mb[18]
+    B2 = mb[19]
+    D2 = mb[20]
+    B4 = mb[21]
+    D4 = mb[22]
+
+    byte1 = int(A4 + A2 + A1, 2)
+    byte2 = int(B4 + B2 + B1, 2)
+    byte3 = int(C4 + C2 + C1, 2)
+    byte4 = int(D4 + D2 + D1, 2)
+    return str(byte1) + str(byte2) + str(byte3) + str(byte4)

tests/test_adsb.py

@@ -80,6 +80,11 @@ def test_adsb_velocity():
     assert adsb.altitude_diff("8D485020994409940838175B284F") == 550
 
 
+def test_adsb_emergency():
+    assert not adsb.is_emergency('8DA2C1B6E112B600000000760759')
+    assert adsb.emergency_squawk('8DA2C1B6E112B600000000760759') == '6615'
+
+
 # def test_nic():
 #     assert adsb.nic('8D3C70A390AB11F55B8C57F65FE6') == 0
 #     assert adsb.nic('8DE1C9738A4A430B427D219C8225') == 1