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#!/usr/bin/env python
#
# Copyright 2010, 2012 Nick Foster
#
# This file is part of gr-air-modes
#
# gr-air-modes is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3, or (at your option)
# any later version.
#
# gr-air-modes is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with gr-air-modes; see the file COPYING. If not, write to
# the Free Software Foundation, Inc., 51 Franklin Street,
# Boston, MA 02110-1301, USA.
#
# For reference into the methodology used to decode altitude,
# see RTCA DO-181D p.42
from air_modes.exceptions import *
def decode_alt(alt, bit13):
mbit = alt & 0x0040
qbit = alt & 0x0010
if mbit and bit13:
#nobody uses metric altitude: AFAIK, it's an orphaned part of
#the spec. haven't seen it in three years. as a result, replies
#with mbit set can be considered spurious, and so we discard them here.
#bits 20-25, 27-31 encode alt in meters
#remember that bits are LSB (bit 20 is MSB)
#meters_alt = 0
#for (shift, bit) in enumerate(range(31,26,-1)+range(25,19,-1)):
# meters_alt += ((alt & (1<<bit)) != 0) << shift
#decoded_alt = meters_alt / 0.3048
raise MetricAltError
if qbit: #a mode S-style reply
#bit13 is false for BDS0,5 ADS-B squitters, and is true otherwise
if 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
tmp2 = (alt & 0x0020) >> 1
else:
tmp1 = (alt & 0x1FE0) >> 1
tmp2 = 0
decoded_alt = ((alt & 0x0F) | tmp1 | tmp2) * 25 - 1000
else: #a mode C-style reply
#okay, the order they come in is:
#C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4
#the order we want them in is:
#D2 D4 A1 A2 A4 B1 B2 B4
#so we'll reassemble into a Gray-coded representation
if bit13 is False:
alt = (alt & 0x003F) | (alt & 0x0FC0 << 1)
C1 = 0x1000
A1 = 0x0800
C2 = 0x0400
A2 = 0x0200 #this represents the order in which the bits come
C4 = 0x0100
A4 = 0x0080
B1 = 0x0020
D1 = 0x0010
B2 = 0x0008
D2 = 0x0004
B4 = 0x0002
D4 = 0x0001
bigpart = ((alt & B4) >> 1) \
+ ((alt & B2) >> 2) \
+ ((alt & B1) >> 3) \
+ ((alt & A4) >> 4) \
+ ((alt & A2) >> 5) \
+ ((alt & A1) >> 6) \
+ ((alt & D4) << 6) \
+ ((alt & D2) << 5)
#bigpart is now the 500-foot-resolution Gray-coded binary part
decoded_alt = gray2bin(bigpart)
#real_alt is now the 500-foot-per-tick altitude
cbits = ((alt & C4) >> 8) + ((alt & C2) >> 9) + ((alt & C1) >> 10)
cval = gray2bin(cbits) #turn them into a real number
if cval == 7:
cval = 5 #not a real gray code after all
if decoded_alt % 2:
cval = 6 - cval #since the code is symmetric this unwraps it to see whether to subtract the C bits or add them
decoded_alt *= 500 #take care of the A,B,D data
decoded_alt += cval * 100 #factor in the C data
decoded_alt -= 1300 #subtract the offset
return decoded_alt
def gray2bin(gray):
i = gray >> 1
while i != 0:
gray ^= i
i >>= 1
return gray
def encode_alt_modes(alt, bit13):
mbit = False
qbit = True
encalt = (int(alt) + 1000) / 25
if bit13 is True:
tmp1 = (encalt & 0xfe0) << 2
tmp2 = (encalt & 0x010) << 1
else:
tmp1 = (encalt & 0xff8) << 1
tmp2 = 0
return (encalt & 0x0F) | tmp1 | tmp2 | (mbit << 6) | (qbit << 4)
if __name__ == "__main__":
try:
for alt in range(-1000, 101400, 25):
dec = decode_alt(encode_alt_modes(alt, False), False)
if dec != alt:
print("Failure at %i with bit13 clear (got %s)" % (alt, dec))
for alt in range(-1000, 101400, 25):
dec = decode_alt(encode_alt_modes(alt, True), True)
if dec != alt:
print("Failure at %i with bit13 set (got %s)" % (alt, dec))
except MetricAltError:
print("Failure at %i due to metric alt bit" % alt)
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