/usr/lib/python2.7/dist-packages/air_modes/altitude.py is in gr-air-modes 0.0.0.e47992d-5ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 | #!/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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