fix altitude decoding

pyModeS/c_common.pxd

@@ -19,6 +19,7 @@ cpdef int typecode(str msg)
 cpdef int cprNL(double lat)
 cpdef str idcode(str msg)
 cpdef int altcode(str msg)
+cpdef int altitude(str binstr)
 
 cpdef str data(str msg)
 cpdef bint allzeros(str msg)

pyModeS/c_common.pyx

@@ -256,7 +256,6 @@ 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
@@ -280,7 +279,7 @@ cpdef str idcode(str msg):
     cdef unsigned char A2 = mbin[22]
     cdef unsigned char C4 = mbin[23]
     cdef unsigned char A4 = mbin[24]
-    # _ = mbin[25]
+    # X = mbin[25]
     cdef unsigned char B1 = mbin[26]
     cdef unsigned char D1 = mbin[27]
     cdef unsigned char B2 = mbin[28]
@@ -288,11 +287,6 @@ cpdef str idcode(str msg):
     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)
-
     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))
     idcode[2] = int_to_char((char_to_int(C4)*2 + char_to_int(C2))*2 + char_to_int(C1))
@@ -321,47 +315,62 @@ cpdef int altcode(str msg):
     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
+    alt = altitude(hex2bin(msg)[19:32])
-    cdef bytearray _mbin = bytearray(hex2bin(msg).encode())
+
+    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 Mbit = binstr[6]
-    cdef char qbit = mbin[27]  # Q bit: 28
+    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 bin2int(binstr) == 0:
-        if qbit == 49:  # 25ft interval, "1" -> 49
+        # altitude unknown or invalid
-            vbin = _mbin[19:25] + _mbin[26:27] + _mbin[28:32]
+        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
+        if Qbit == 48:  # 100ft interval, above 50175ft, "0" -> 48
-            graybytes[8] = mbin[19]
+            graybytes[8] = mbin[0]
-            graybytes[2] = mbin[20]
+            graybytes[2] = mbin[1]
-            graybytes[9] = mbin[21]
+            graybytes[9] = mbin[2]
-            graybytes[3] = mbin[22]
+            graybytes[3] = mbin[3]
-            graybytes[10] = mbin[23]
+            graybytes[10] = mbin[4]
-            graybytes[4] = mbin[24]
+            graybytes[4] = mbin[5]
-            # _ = mbin[25]
+            # M = mbin[6]
-            graybytes[5] = mbin[26]
+            graybytes[5] = mbin[7]
-            # cdef char D1 = mbin[27]  # always zero
+            # Q = mbin[8]
-            graybytes[6] = mbin[28]
+            graybytes[6] = mbin[9]
-            graybytes[0] = mbin[29]
+            graybytes[0] = mbin[10]
-            graybytes[7] = mbin[30]
+            graybytes[7] = mbin[11]
-            graybytes[1] = mbin[31]
+            graybytes[1] = mbin[12]
             # graybytes = D2 + D4 + A1 + A2 + A4 + B1 + B2 + B4 + C1 + C2 + C4
 
             alt = gray2alt(_graybytes.decode())
 
-    if mbit == 49:  # unit in meter, "1" -> 49
+    elif Mbit == 49:  # unit in meter, "1" -> 49
-        vbin = _mbin[19:25] + _mbin[26:31]
+        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)

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
+        altcode = altbin[0:6] + "0" + altbin[6:]
-        q = mb[15]
-        if q:
-            n = common.bin2int(mb[8:15] + mb[16:20])
-            alt = n * 25 - 1000
-        else:
-            alt = None
     else:
-        # GNSS altitude, meters -> feet
+        altcode = altbin[0:6] + "0" + altbin[6:]
-        alt = common.bin2int(mb[8:20]) * 3.28084
+
+    alt = common.altitude(altcode)
 
     return alt

pyModeS/py_common.py

@@ -206,7 +206,6 @@ def idcode(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
@@ -226,7 +225,7 @@ def idcode(msg):
     A2 = mbin[22]
     C4 = mbin[23]
     A4 = mbin[24]
-    # _ = mbin[25]
+    # X = mbin[25]
     B1 = mbin[26]
     D1 = mbin[27]
     B2 = mbin[28]
@@ -243,10 +242,9 @@ def idcode(msg):
 
 
 def altcode(msg):
-    """Compute the altitude.
+    """Compute the altitude in DF 4 and 20.
 
     Applicable only for DF4 or DF20 message, bit 20-32.
-    credit: @fbyrkjeland
 
     Args:
         msg (String): 28 bytes hexadecimal message string
@@ -261,44 +259,69 @@ def altcode(msg):
     # Altitude code, bit 20-32
     mbin = hex2bin(msg)
 
-    mbit = mbin[25]  # M bit: 26
+    altitude_code = mbin[19:32]
-    qbit = mbin[27]  # Q bit: 28
 
-    if mbit == "0":  # unit in ft
+    alt = altitude(altitude_code)
-        if qbit == "1":  # 25ft interval
+
-            vbin = mbin[19:25] + mbin[26] + mbin[28:32]
+    return alt
+
+
+def altitude(binstr):
+    """Decode 13 bits altitude 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")
+
+    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
+        if Qbit == "0":  # 100ft interval, above 50187.5ft
-            C1 = mbin[19]
+            C1 = binstr[0]
-            A1 = mbin[20]
+            A1 = binstr[1]
-            C2 = mbin[21]
+            C2 = binstr[2]
-            A2 = mbin[22]
+            A2 = binstr[3]
-            C4 = mbin[23]
+            C4 = binstr[4]
-            A4 = mbin[24]
+            A4 = binstr[5]
-            # _ = mbin[25]
+            # M = binstr[6]
-            B1 = mbin[26]
+            B1 = binstr[7]
-            # D1 = mbin[27]     # always zero
+            # Q = binstr[8]
-            B2 = mbin[28]
+            B2 = binstr[9]
-            D2 = mbin[29]
+            D2 = binstr[10]
-            B4 = mbin[30]
+            B4 = binstr[11]
-            D4 = mbin[31]
+            D4 = binstr[12]
 
             graystr = D2 + D4 + A1 + A2 + A4 + B1 + B2 + B4 + C1 + C2 + C4
             alt = gray2alt(graystr)
 
-    if mbit == "1":  # unit in meter
+    if Mbit == "1":  # unit in meter
-        vbin = mbin[19:25] + mbin[26:31]
+        vbin = binstr[:6] + binstr[7:]
         alt = int(bin2int(vbin) * 3.28084)  # convert to ft
 
     return alt
 
 
-def gray2alt(codestr):
+def gray2alt(binstr):
-    gc500 = codestr[:8]
+    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 +337,9 @@ def gray2alt(codestr):
     return alt
 
 
-def gray2int(graystr):
+def gray2int(binstr):
     """Convert greycode to binary."""
-    num = bin2int(graystr)
+    num = bin2int(binstr)
     num ^= num >> 8
     num ^= num >> 4
     num ^= num >> 2