diff --git a/Makefile b/Makefile
index 4f8488e..f20f609 100644
--- a/Makefile
+++ b/Makefile
@@ -8,6 +8,12 @@ ext:
 	python setup.py build_ext --inplace
 
 test:
+	make clean
+	@echo ""
+	@echo "[Test with py_common]"
+	python -m pytest tests
+	@echo ""
+	@echo "[Test with c_common]"
 	python setup.py build_ext --inplace
 	python -m pytest tests
 
diff --git a/pyModeS/c_common.pxd b/pyModeS/c_common.pxd
index de13840..2f39078 100644
--- a/pyModeS/c_common.pxd
+++ b/pyModeS/c_common.pxd
@@ -17,8 +17,12 @@ cpdef bint is_icao_assigned(str icao)
 
 cpdef int typecode(str msg)
 cpdef int cprNL(double lat)
+
 cpdef str idcode(str msg)
+cpdef str squawk(str binstr)
+
 cpdef int altcode(str msg)
+cpdef int altitude(str binstr)
 
 cpdef str data(str msg)
 cpdef bint allzeros(str msg)
diff --git a/pyModeS/c_common.pyx b/pyModeS/c_common.pyx
index 77dad81..e774586 100644
--- a/pyModeS/c_common.pyx
+++ b/pyModeS/c_common.pyx
@@ -160,17 +160,7 @@ cpdef long floor(double x):
     return <long> c_floor(x)
 
 cpdef str icao(str msg):
-    """Calculate the ICAO address from an Mode-S message.
-
-    Applicable only with DF4, DF5, DF20, DF21 messages.
-
-    Args:
-        msg (String): 28 bytes hexadecimal message string
-
-    Returns:
-        String: ICAO address in 6 bytes hexadecimal string
-
-    """
+    """Calculate the ICAO address from an Mode-S message."""
     cdef unsigned char DF = df(msg)
     cdef long c0, c1
 
@@ -217,14 +207,7 @@ cpdef bint is_icao_assigned(str icao):
 @cython.boundscheck(False)
 @cython.wraparound(False)
 cpdef int typecode(str msg):
-    """Type code of ADS-B message
-
-    Args:
-        msg (string): 28 bytes hexadecimal message string
-
-    Returns:
-        int: type code number
-    """
+    """Type code of ADS-B message"""
     if df(msg) not in (17, 18):
         return -1
         # return None
@@ -253,45 +236,41 @@ cpdef int cprNL(double lat):
 @cython.boundscheck(False)
 @cython.wraparound(False)
 cpdef str idcode(str msg):
-    """Compute identity (squawk code).
-
-    Applicable only for DF5 or DF21 messages, bit 20-32.
-    credit: @fbyrkjeland
-
-    Args:
-        msg (String): 28 bytes hexadecimal message string
-
-    Returns:
-        string: squawk code
-
-    """
+    """Compute identity (squawk code)."""
     if df(msg) not in [5, 21]:
         raise RuntimeError("Message must be Downlink Format 5 or 21.")
 
-    cdef bytearray _mbin = bytearray(hex2bin(msg).encode())
+    squawk_code = squawk(hex2bin(msg)[19:32])
+    return squawk_code
+
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+cpdef str squawk(str binstr):
+    """Compute identity (squawk code)."""
+
+    if len(binstr) != 13 or set(binstr) != set('01'):
+        raise RuntimeError("Input must be 13 bits binary string")
+
+    cdef bytearray _mbin = bytearray(binstr.encode())
     cdef unsigned char[:] mbin = _mbin
 
     cdef bytearray _idcode = bytearray(4)
     cdef unsigned char[:] idcode = _idcode
 
-    cdef unsigned char C1 = mbin[19]
-    cdef unsigned char A1 = mbin[20]
-    cdef unsigned char C2 = mbin[21]
-    cdef unsigned char A2 = mbin[22]
-    cdef unsigned char C4 = mbin[23]
-    cdef unsigned char A4 = mbin[24]
-    # _ = mbin[25]
-    cdef unsigned char B1 = mbin[26]
-    cdef unsigned char D1 = mbin[27]
-    cdef unsigned char B2 = mbin[28]
-    cdef unsigned char D2 = mbin[29]
-    cdef unsigned char B4 = mbin[30]
-    cdef unsigned char D4 = mbin[31]
-
-    # byte1 = int(A4 + A2 + A1, 2)
-    # byte2 = int(B4 + B2 + B1, 2)
-    # byte3 = int(C4 + C2 + C1, 2)
-    # byte4 = int(D4 + D2 + D1, 2)
+    cdef unsigned char C1 = mbin[0]
+    cdef unsigned char A1 = mbin[1]
+    cdef unsigned char C2 = mbin[2]
+    cdef unsigned char A2 = mbin[3]
+    cdef unsigned char C4 = mbin[4]
+    cdef unsigned char A4 = mbin[5]
+    # X = mbin[6]
+    cdef unsigned char B1 = mbin[7]
+    cdef unsigned char D1 = mbin[8]
+    cdef unsigned char B2 = mbin[9]
+    cdef unsigned char D2 = mbin[10]
+    cdef unsigned char B4 = mbin[11]
+    cdef unsigned char D4 = mbin[12]
 
     idcode[0] = int_to_char((char_to_int(A4)*2 + char_to_int(A2))*2 + char_to_int(A1))
     idcode[1] = int_to_char((char_to_int(B4)*2 + char_to_int(B2))*2 + char_to_int(B1))
@@ -300,68 +279,68 @@ cpdef str idcode(str msg):
 
     return _idcode.decode()
 
-    #return str(byte1) + str(byte2) + str(byte3) + str(byte4)
-
 
 @cython.boundscheck(False)
 @cython.wraparound(False)
 cpdef int altcode(str msg):
-    """Compute the altitude.
-
-    Applicable only for DF4 or DF20 message, bit 20-32.
-    credit: @fbyrkjeland
-
-    Args:
-        msg (String): 28 bytes hexadecimal message string
-
-    Returns:
-        int: altitude in ft
-
-    """
+    """Compute the altitude."""
     if df(msg) not in [0, 4, 16, 20]:
         raise RuntimeError("Message must be Downlink Format 0, 4, 16, or 20.")
 
-    # Altitude code, bit 20-32
-    cdef bytearray _mbin = bytearray(hex2bin(msg).encode())
+    alt = altitude(hex2bin(msg)[19:32])
+    return alt
+
+
+@cython.boundscheck(False)
+@cython.wraparound(False)
+cpdef int altitude(str binstr):
+
+    if len(binstr) != 13 or set(binstr) != set('01'):
+        raise RuntimeError("Input must be 13 bits binary string")
+
+    cdef bytearray _mbin = bytearray(binstr.encode())
     cdef unsigned char[:] mbin = _mbin
 
-    cdef char mbit = mbin[25]  # M bit: 26
-    cdef char qbit = mbin[27]  # Q bit: 28
+    cdef char Mbit = binstr[6]
+    cdef char Qbit = binstr[8]
+
     cdef int alt = 0
     cdef bytearray vbin
     cdef bytearray _graybytes = bytearray(11)
     cdef unsigned char[:] graybytes = _graybytes
 
-    if mbit == 48:  # unit in ft, "0" -> 48
-        if qbit == 49:  # 25ft interval, "1" -> 49
-            vbin = _mbin[19:25] + _mbin[26:27] + _mbin[28:32]
+    if bin2int(binstr) == 0:
+        # altitude unknown or invalid
+        alt = -9999
+
+    elif Mbit == 48:  # unit in ft, "0" -> 48
+        if Qbit == 49:  # 25ft interval, "1" -> 49
+            vbin = _mbin[:6] + _mbin[7:8] + _mbin[9:]
             alt = bin2int(vbin.decode()) * 25 - 1000
-        if qbit == 48:  # 100ft interval, above 50175ft, "0" -> 48
-            graybytes[8] = mbin[19]
-            graybytes[2] = mbin[20]
-            graybytes[9] = mbin[21]
-            graybytes[3] = mbin[22]
-            graybytes[10] = mbin[23]
-            graybytes[4] = mbin[24]
-            # _ = mbin[25]
-            graybytes[5] = mbin[26]
-            # cdef char D1 = mbin[27]  # always zero
-            graybytes[6] = mbin[28]
-            graybytes[0] = mbin[29]
-            graybytes[7] = mbin[30]
-            graybytes[1] = mbin[31]
-            # graybytes = D2 + D4 + A1 + A2 + A4 + B1 + B2 + B4 + C1 + C2 + C4
+        if Qbit == 48:  # 100ft interval, above 50175ft, "0" -> 48
+            graybytes[8] = mbin[0]
+            graybytes[2] = mbin[1]
+            graybytes[9] = mbin[2]
+            graybytes[3] = mbin[3]
+            graybytes[10] = mbin[4]
+            graybytes[4] = mbin[5]
+            # M = mbin[6]
+            graybytes[5] = mbin[7]
+            # Q = mbin[8]
+            graybytes[6] = mbin[9]
+            graybytes[0] = mbin[10]
+            graybytes[7] = mbin[11]
+            graybytes[1] = mbin[12]
 
             alt = gray2alt(_graybytes.decode())
 
-    if mbit == 49:  # unit in meter, "1" -> 49
-        vbin = _mbin[19:25] + _mbin[26:31]
+    elif Mbit == 49:  # unit in meter, "1" -> 49
+        vbin = _mbin[:6] + _mbin[7:]
         alt = int(bin2int(vbin.decode()) * 3.28084)  # convert to ft
 
     return alt
 
 
-
 cpdef int gray2alt(str codestr):
     cdef str gc500 = codestr[:8]
     cdef int n500 = gray2int(gc500)
@@ -400,15 +379,7 @@ cpdef str data(str msg):
 
 
 cpdef bint allzeros(str msg):
-    """Check if the data bits are all zeros.
-
-    Args:
-        msg (String): 28 bytes hexadecimal message string
-
-    Returns:
-        bool: True or False
-
-    """
+    """Check if the data bits are all zeros."""
     d = hex2bin(data(msg))
 
     if bin2int(d) > 0:
diff --git a/pyModeS/decoder/__init__.py b/pyModeS/decoder/__init__.py
index 2987ed1..c961ffc 100644
--- a/pyModeS/decoder/__init__.py
+++ b/pyModeS/decoder/__init__.py
@@ -1,4 +1,4 @@
-def tell(msg):
+def tell(msg: str) -> None:
     from pyModeS import common, adsb, commb, bds
 
     def _print(label, value, unit=None):
diff --git a/pyModeS/decoder/adsb.py b/pyModeS/decoder/adsb.py
index e74f6c2..1d65849 100644
--- a/pyModeS/decoder/adsb.py
+++ b/pyModeS/decoder/adsb.py
@@ -28,6 +28,7 @@ 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_state, emergency_squawk
 
 
 def df(msg):
diff --git a/pyModeS/decoder/bds/bds05.py b/pyModeS/decoder/bds/bds05.py
index 63f4081..120c31c 100644
--- a/pyModeS/decoder/bds/bds05.py
+++ b/pyModeS/decoder/bds/bds05.py
@@ -139,17 +139,13 @@ def altitude(msg):
         raise RuntimeError("%s: Not a airborn position message" % msg)
 
     mb = common.hex2bin(msg)[32:]
+    altbin = mb[8:20]
 
     if tc < 19:
-        # barometric altitude
-        q = mb[15]
-        if q:
-            n = common.bin2int(mb[8:15] + mb[16:20])
-            alt = n * 25 - 1000
-        else:
-            alt = None
+        altcode = altbin[0:6] + "0" + altbin[6:]
     else:
-        # GNSS altitude, meters -> feet
-        alt = common.bin2int(mb[8:20]) * 3.28084
+        altcode = altbin[0:6] + "0" + altbin[6:]
+
+    alt = common.altitude(altcode)
 
     return alt
diff --git a/pyModeS/decoder/bds/bds61.py b/pyModeS/decoder/bds/bds61.py
new file mode 100644
index 0000000..05dc649
--- /dev/null
+++ b/pyModeS/decoder/bds/bds61.py
@@ -0,0 +1,83 @@
+# ------------------------------------------
+#   BDS 6,1
+#   ADS-B TC=28
+#   Aircraft Airborne status
+# ------------------------------------------
+
+from pyModeS import common
+
+
+def is_emergency(msg: str) -> bool:
+    """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: 28 bytes hexadecimal message string
+    :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])
+
+    if subtype == 2:
+        raise RuntimeError("%s: Emergency message is ACAS-RA, not implemented")
+
+    emergency_state = common.bin2int(mb[8:11])
+
+    if subtype == 1 and emergency_state == 1:
+        return True
+    else:
+        return False
+
+
+def emergency_state(msg: str) -> int:
+    """Decode aircraft emergency state.
+
+    Value   Meaning
+    -----   -----------------------
+    0       No emergency
+    1       General emergency
+    2       Lifeguard/Medical
+    3       Minimum fuel
+    4       No communications
+    5       Unlawful communications
+    6-7     Reserved
+
+    :param msg: 28 bytes hexadecimal message string
+    :return: emergency state
+    """
+
+    mb = common.hex2bin(msg)[32:]
+    subtype = common.bin2int(mb[5:8])
+
+    if subtype == 2:
+        raise RuntimeError("%s: Emergency message is ACAS-RA, not implemented")
+
+    emergency_state = common.bin2int(mb[8:11])
+    return emergency_state
+
+
+def emergency_squawk(msg: str) -> str:
+    """Decode squawk code.
+
+    Emergency value 1: squawk 7700.
+    Emergency value 4: squawk 7600.
+    Emergency value 5: squawk 7500.
+
+    :param msg: 28 bytes hexadecimal message string
+    :return: aircraft squawk code
+    """
+    if common.typecode(msg) != 28:
+        raise RuntimeError("%s: Not an airborne status message, expecting TC=28" % msg)
+
+    msgbin = common.hex2bin(msg)
+
+    # construct the 13 bits Mode A ID code
+    idcode = msgbin[43:49] + "0" + msgbin[49:55]
+
+    squawk = common.squawk(idcode)
+    return squawk
diff --git a/pyModeS/extra/tcpclient.py b/pyModeS/extra/tcpclient.py
index 0f18c54..26be3be 100644
--- a/pyModeS/extra/tcpclient.py
+++ b/pyModeS/extra/tcpclient.py
@@ -282,6 +282,8 @@ class TcpClient(object):
 
                 # raise RuntimeError("test exception")
 
+            except zmq.error.Again:
+                continue
             except Exception as e:
                 tb = traceback.format_exc()
                 exception_queue.put(tb)
diff --git a/pyModeS/py_common.py b/pyModeS/py_common.py
index d1a4c81..719f8a6 100644
--- a/pyModeS/py_common.py
+++ b/pyModeS/py_common.py
@@ -1,44 +1,46 @@
+from typing import Optional
+
 import numpy as np
 from textwrap import wrap
 
 
-def hex2bin(hexstr):
+def hex2bin(hexstr: str) -> str:
     """Convert a hexdecimal string to binary string, with zero fillings."""
     num_of_bits = len(hexstr) * 4
     binstr = bin(int(hexstr, 16))[2:].zfill(int(num_of_bits))
     return binstr
 
 
-def hex2int(hexstr):
+def hex2int(hexstr: str) -> int:
     """Convert a hexdecimal string to integer."""
     return int(hexstr, 16)
 
 
-def bin2int(binstr):
+def bin2int(binstr: str) -> int:
     """Convert a binary string to integer."""
     return int(binstr, 2)
 
 
-def bin2hex(binstr):
+def bin2hex(binstr: str) -> str:
     """Convert a binary string to hexdecimal string."""
     return "{0:X}".format(int(binstr, 2))
 
 
-def df(msg):
+def df(msg: str) -> int:
     """Decode Downlink Format value, bits 1 to 5."""
     dfbin = hex2bin(msg[:2])
     return min(bin2int(dfbin[0:5]), 24)
 
 
-def crc(msg, encode=False):
+def crc(msg: str, encode: bool = False) -> int:
     """Mode-S Cyclic Redundancy Check.
 
     Detect if bit error occurs in the Mode-S message. When encode option is on,
     the checksum is generated.
 
     Args:
-        msg (string): 28 bytes hexadecimal message string
-        encode (bool): True to encode the date only and return the checksum
+        msg: 28 bytes hexadecimal message string
+        encode: True to encode the date only and return the checksum
     Returns:
         int: message checksum, or partity bits (encoder)
 
@@ -75,7 +77,7 @@ def crc(msg, encode=False):
     return result
 
 
-def crc_legacy(msg, encode=False):
+def crc_legacy(msg: str, encode: bool = False) -> int:
     """Mode-S Cyclic Redundancy Check. (Legacy code, 2x slow)."""
     # the polynominal generattor code for CRC [1111111111111010000001001]
     generator = np.array(
@@ -103,7 +105,7 @@ def crc_legacy(msg, encode=False):
     return reminder
 
 
-def floor(x):
+def floor(x: float) -> int:
     """Mode-S floor function.
 
     Defined as the greatest integer value k, such that k <= x
@@ -113,7 +115,7 @@ def floor(x):
     return int(np.floor(x))
 
 
-def icao(msg):
+def icao(msg: str) -> Optional[str]:
     """Calculate the ICAO address from an Mode-S message.
 
     Applicable only with DF4, DF5, DF20, DF21 messages.
@@ -125,6 +127,7 @@ def icao(msg):
         String: ICAO address in 6 bytes hexadecimal string
 
     """
+    addr: Optional[str]
     DF = df(msg)
 
     if DF in (11, 17, 18):
@@ -139,7 +142,7 @@ def icao(msg):
     return addr
 
 
-def is_icao_assigned(icao):
+def is_icao_assigned(icao: str) -> bool:
     """Check whether the ICAO address is assigned (Annex 10, Vol 3)."""
     if (icao is None) or (not isinstance(icao, str)) or (len(icao) != 6):
         return False
@@ -168,7 +171,7 @@ def is_icao_assigned(icao):
     return True
 
 
-def typecode(msg):
+def typecode(msg: str) -> Optional[int]:
     """Type code of ADS-B message
 
     Args:
@@ -184,7 +187,7 @@ def typecode(msg):
     return bin2int(tcbin[0:5])
 
 
-def cprNL(lat):
+def cprNL(lat: float) -> int:
     """NL() function in CPR decoding."""
 
     if np.isclose(lat, 0):
@@ -202,11 +205,8 @@ def cprNL(lat):
     return NL
 
 
-def idcode(msg):
-    """Compute identity (squawk code).
-
-    Applicable only for DF5 or DF21 messages, bit 20-32.
-    credit: @fbyrkjeland
+def idcode(msg: str) -> str:
+    """Compute identity code (squawk) encoded in DF5 or DF21 message.
 
     Args:
         msg (String): 28 bytes hexadecimal message string
@@ -219,20 +219,37 @@ def idcode(msg):
         raise RuntimeError("Message must be Downlink Format 5 or 21.")
 
     mbin = hex2bin(msg)
+    idcodebin = mbin[19:32]
 
-    C1 = mbin[19]
-    A1 = mbin[20]
-    C2 = mbin[21]
-    A2 = mbin[22]
-    C4 = mbin[23]
-    A4 = mbin[24]
-    # _ = mbin[25]
-    B1 = mbin[26]
-    D1 = mbin[27]
-    B2 = mbin[28]
-    D2 = mbin[29]
-    B4 = mbin[30]
-    D4 = mbin[31]
+    return squawk(idcodebin)
+
+
+def squawk(binstr: str) -> str:
+    """Decode 13 bits identity (squawk) code.
+
+    Args:
+        binstr (String): 13 bits binary string
+
+    Returns:
+        int: altitude in ft
+
+    """
+    if len(binstr) != 13 or set(binstr) != set("01"):
+        raise RuntimeError("Input must be 13 bits binary string")
+
+    C1 = binstr[0]
+    A1 = binstr[1]
+    C2 = binstr[2]
+    A2 = binstr[3]
+    C4 = binstr[4]
+    A4 = binstr[5]
+    # X = binstr[6]
+    B1 = binstr[7]
+    D1 = binstr[8]
+    B2 = binstr[9]
+    D2 = binstr[10]
+    B4 = binstr[11]
+    D4 = binstr[12]
 
     byte1 = int(A4 + A2 + A1, 2)
     byte2 = int(B4 + B2 + B1, 2)
@@ -242,11 +259,8 @@ def idcode(msg):
     return str(byte1) + str(byte2) + str(byte3) + str(byte4)
 
 
-def altcode(msg):
-    """Compute the altitude.
-
-    Applicable only for DF4 or DF20 message, bit 20-32.
-    credit: @fbyrkjeland
+def altcode(msg: str) -> Optional[int]:
+    """Compute altitude encoded in DF4 or DF20 message.
 
     Args:
         msg (String): 28 bytes hexadecimal message string
@@ -255,50 +269,78 @@ def altcode(msg):
         int: altitude in ft
 
     """
+    alt: Optional[int]
+
     if df(msg) not in [0, 4, 16, 20]:
         raise RuntimeError("Message must be Downlink Format 0, 4, 16, or 20.")
 
     # Altitude code, bit 20-32
     mbin = hex2bin(msg)
 
-    mbit = mbin[25]  # M bit: 26
-    qbit = mbin[27]  # Q bit: 28
+    altitude_code = mbin[19:32]
 
-    if mbit == "0":  # unit in ft
-        if qbit == "1":  # 25ft interval
-            vbin = mbin[19:25] + mbin[26] + mbin[28:32]
+    alt = altitude(altitude_code)
+
+    return alt
+
+
+def altitude(binstr: str) -> Optional[int]:
+    """Decode 13 bits altitude code.
+
+    Args:
+        binstr (String): 13 bits binary string
+
+    Returns:
+        int: altitude in ft
+
+    """
+    alt: Optional[int]
+
+    if len(binstr) != 13 or set(binstr) != set("01"):
+        raise RuntimeError("Input must be 13 bits binary string")
+
+    Mbit = binstr[6]
+    Qbit = binstr[8]
+
+    if bin2int(binstr) == 0:
+        # altitude unknown or invalid
+        alt = None
+
+    elif Mbit == "0":  # unit in ft
+        if Qbit == "1":  # 25ft interval
+            vbin = binstr[:6] + binstr[7] + binstr[9:]
             alt = bin2int(vbin) * 25 - 1000
-        if qbit == "0":  # 100ft interval, above 50175ft
-            C1 = mbin[19]
-            A1 = mbin[20]
-            C2 = mbin[21]
-            A2 = mbin[22]
-            C4 = mbin[23]
-            A4 = mbin[24]
-            # _ = mbin[25]
-            B1 = mbin[26]
-            # D1 = mbin[27]     # always zero
-            B2 = mbin[28]
-            D2 = mbin[29]
-            B4 = mbin[30]
-            D4 = mbin[31]
+        if Qbit == "0":  # 100ft interval, above 50187.5ft
+            C1 = binstr[0]
+            A1 = binstr[1]
+            C2 = binstr[2]
+            A2 = binstr[3]
+            C4 = binstr[4]
+            A4 = binstr[5]
+            # M = binstr[6]
+            B1 = binstr[7]
+            # Q = binstr[8]
+            B2 = binstr[9]
+            D2 = binstr[10]
+            B4 = binstr[11]
+            D4 = binstr[12]
 
             graystr = D2 + D4 + A1 + A2 + A4 + B1 + B2 + B4 + C1 + C2 + C4
             alt = gray2alt(graystr)
 
-    if mbit == "1":  # unit in meter
-        vbin = mbin[19:25] + mbin[26:31]
+    if Mbit == "1":  # unit in meter
+        vbin = binstr[:6] + binstr[7:]
         alt = int(bin2int(vbin) * 3.28084)  # convert to ft
 
     return alt
 
 
-def gray2alt(codestr):
-    gc500 = codestr[:8]
+def gray2alt(binstr: str) -> Optional[int]:
+    gc500 = binstr[:8]
     n500 = gray2int(gc500)
 
     # in 100-ft step must be converted first
-    gc100 = codestr[8:]
+    gc100 = binstr[8:]
     n100 = gray2int(gc100)
 
     if n100 in [0, 5, 6]:
@@ -314,9 +356,9 @@ def gray2alt(codestr):
     return alt
 
 
-def gray2int(graystr):
+def gray2int(binstr: str) -> int:
     """Convert greycode to binary."""
-    num = bin2int(graystr)
+    num = bin2int(binstr)
     num ^= num >> 8
     num ^= num >> 4
     num ^= num >> 2
@@ -324,12 +366,12 @@ def gray2int(graystr):
     return num
 
 
-def data(msg):
+def data(msg: str) -> str:
     """Return the data frame in the message, bytes 9 to 22."""
     return msg[8:-6]
 
 
-def allzeros(msg):
+def allzeros(msg: str) -> bool:
     """Check if the data bits are all zeros.
 
     Args:
@@ -347,7 +389,7 @@ def allzeros(msg):
         return True
 
 
-def wrongstatus(data, sb, msb, lsb):
+def wrongstatus(data: str, sb: int, msb: int, lsb: int) -> bool:
     """Check if the status bit and field bits are consistency.
 
     This Function is used for checking BDS code versions.
diff --git a/tests/test_adsb.py b/tests/test_adsb.py
index 64599ab..c6e6de0 100644
--- a/tests/test_adsb.py
+++ b/tests/test_adsb.py
@@ -80,6 +80,12 @@ def test_adsb_velocity():
     assert adsb.altitude_diff("8D485020994409940838175B284F") == 550
 
 
+def test_adsb_emergency():
+    assert not adsb.is_emergency("8DA2C1B6E112B600000000760759")
+    assert adsb.emergency_state("8DA2C1B6E112B600000000760759") == 0
+    assert adsb.emergency_squawk("8DA2C1B6E112B600000000760759") == "6615"
+
+
 # def test_nic():
 #     assert adsb.nic('8D3C70A390AB11F55B8C57F65FE6') == 0
 #     assert adsb.nic('8DE1C9738A4A430B427D219C8225') == 1
diff --git a/tests/test_c_common.py b/tests/test_c_common.py
index 4d14034..7b703d6 100644
--- a/tests/test_c_common.py
+++ b/tests/test_c_common.py
@@ -1,62 +1,60 @@
-from pyModeS import c_common
+try:
+    from pyModeS import c_common
+
+    def test_conversions():
+        assert c_common.hex2bin("6E") == "01101110"
+        assert c_common.bin2hex("01101110") == "6E"
+        assert c_common.bin2hex("1101110") == "6E"
+
+    def test_crc_decode():
+
+        assert c_common.crc("8D406B902015A678D4D220AA4BDA") == 0
+        assert c_common.crc("8d8960ed58bf053cf11bc5932b7d") == 0
+        assert c_common.crc("8d45cab390c39509496ca9a32912") == 0
+        assert c_common.crc("8d74802958c904e6ef4ba0184d5c") == 0
+        assert c_common.crc("8d4400cd9b0000b4f87000e71a10") == 0
+        assert c_common.crc("8d4065de58a1054a7ef0218e226a") == 0
+
+        assert c_common.crc("c80b2dca34aa21dd821a04cb64d4") == 10719924
+        assert c_common.crc("a800089d8094e33a6004e4b8a522") == 4805588
+        assert c_common.crc("a8000614a50b6d32bed000bbe0ed") == 5659991
+        assert c_common.crc("a0000410bc900010a40000f5f477") == 11727682
+        assert c_common.crc("8d4ca251204994b1c36e60a5343d") == 16
+        assert c_common.crc("b0001718c65632b0a82040715b65") == 353333
+
+    def test_crc_encode():
+        parity = c_common.crc("8D406B902015A678D4D220AA4BDA", encode=True)
+        assert parity == 11160538
+
+    def test_icao():
+        assert c_common.icao("8D406B902015A678D4D220AA4BDA") == "406B90"
+        assert c_common.icao("A0001839CA3800315800007448D9") == "400940"
+        assert c_common.icao("A000139381951536E024D4CCF6B5") == "3C4DD2"
+        assert c_common.icao("A000029CFFBAA11E2004727281F1") == "4243D0"
+
+    def test_modes_altcode():
+        assert c_common.altcode("A02014B400000000000000F9D514") == 32300
+
+    def test_modes_idcode():
+        assert c_common.idcode("A800292DFFBBA9383FFCEB903D01") == "1346"
+
+    def test_graycode_to_altitude():
+        assert c_common.gray2alt("00000000010") == -1000
+        assert c_common.gray2alt("00000001010") == -500
+        assert c_common.gray2alt("00000011011") == -100
+        assert c_common.gray2alt("00000011010") == 0
+        assert c_common.gray2alt("00000011110") == 100
+        assert c_common.gray2alt("00000010011") == 600
+        assert c_common.gray2alt("00000110010") == 1000
+        assert c_common.gray2alt("00001001001") == 5800
+        assert c_common.gray2alt("00011100100") == 10300
+        assert c_common.gray2alt("01100011010") == 32000
+        assert c_common.gray2alt("01110000100") == 46300
+        assert c_common.gray2alt("01010101100") == 50200
+        assert c_common.gray2alt("11011110100") == 73200
+        assert c_common.gray2alt("10000000011") == 126600
+        assert c_common.gray2alt("10000000001") == 126700
 
 
-def test_conversions():
-    assert c_common.hex2bin("6E") == "01101110"
-    assert c_common.bin2hex("01101110") == "6E"
-    assert c_common.bin2hex("1101110") == "6E"
-
-
-def test_crc_decode():
-
-    assert c_common.crc("8D406B902015A678D4D220AA4BDA") == 0
-    assert c_common.crc("8d8960ed58bf053cf11bc5932b7d") == 0
-    assert c_common.crc("8d45cab390c39509496ca9a32912") == 0
-    assert c_common.crc("8d74802958c904e6ef4ba0184d5c") == 0
-    assert c_common.crc("8d4400cd9b0000b4f87000e71a10") == 0
-    assert c_common.crc("8d4065de58a1054a7ef0218e226a") == 0
-
-    assert c_common.crc("c80b2dca34aa21dd821a04cb64d4") == 10719924
-    assert c_common.crc("a800089d8094e33a6004e4b8a522") == 4805588
-    assert c_common.crc("a8000614a50b6d32bed000bbe0ed") == 5659991
-    assert c_common.crc("a0000410bc900010a40000f5f477") == 11727682
-    assert c_common.crc("8d4ca251204994b1c36e60a5343d") == 16
-    assert c_common.crc("b0001718c65632b0a82040715b65") == 353333
-
-
-def test_crc_encode():
-    parity = c_common.crc("8D406B902015A678D4D220AA4BDA", encode=True)
-    assert parity == 11160538
-
-
-def test_icao():
-    assert c_common.icao("8D406B902015A678D4D220AA4BDA") == "406B90"
-    assert c_common.icao("A0001839CA3800315800007448D9") == "400940"
-    assert c_common.icao("A000139381951536E024D4CCF6B5") == "3C4DD2"
-    assert c_common.icao("A000029CFFBAA11E2004727281F1") == "4243D0"
-
-
-def test_modes_altcode():
-    assert c_common.altcode("A02014B400000000000000F9D514") == 32300
-
-
-def test_modes_idcode():
-    assert c_common.idcode("A800292DFFBBA9383FFCEB903D01") == "1346"
-
-
-def test_graycode_to_altitude():
-    assert c_common.gray2alt("00000000010") == -1000
-    assert c_common.gray2alt("00000001010") == -500
-    assert c_common.gray2alt("00000011011") == -100
-    assert c_common.gray2alt("00000011010") == 0
-    assert c_common.gray2alt("00000011110") == 100
-    assert c_common.gray2alt("00000010011") == 600
-    assert c_common.gray2alt("00000110010") == 1000
-    assert c_common.gray2alt("00001001001") == 5800
-    assert c_common.gray2alt("00011100100") == 10300
-    assert c_common.gray2alt("01100011010") == 32000
-    assert c_common.gray2alt("01110000100") == 46300
-    assert c_common.gray2alt("01010101100") == 50200
-    assert c_common.gray2alt("11011110100") == 73200
-    assert c_common.gray2alt("10000000011") == 126600
-    assert c_common.gray2alt("10000000001") == 126700
+except:
+    pass