FGFS-4.1/pyModeS
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separate position decoding to airborne and surface types

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This commit is contained in:
junzis 2016-07-08 14:59:45 +02:00
parent 4666de403d
commit faf43134e5
2 changed files with 32 additions and 12 deletions
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36
pyModeS/adsb.py
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@ -153,7 +153,18 @@ def cprlon(msg):
def position(msg0, msg1, t0, t1):
"""Decode position from the combination of even and odd position message
if (5 <= typecode(msg0) <= 8 and 5 <= typecode(msg1) <= 8):
return surface_position(msg0, msg1, t0, t1)
elif (9 <= typecode(msg0) <= 18 and 9 <= typecode(msg1) <= 18):
return airborne_position(msg0, msg1, t0, t1)
else:
raise RuntimeError("incorrect or inconsistant message types")
def airborne_position(msg0, msg1, t0, t1):
"""Decode airborn position from a pair of even and odd position message
131072 is 2^17, since CPR lat and lon are 17 bits each.
Args:
msg0 (string): even message (28 bytes hexadecimal string)
@ -163,11 +174,6 @@ def position(msg0, msg1, t0, t1):
Returns:
(float, float): (latitude, longitude) of the aircraft
"""
if typecode(msg0) < 5 or typecode(msg0) > 18:
raise RuntimeError("%s: Not a position message" % msg0)
if typecode(msg1) < 5 or typecode(msg1) > 18:
raise RuntimeError("%s: Not a position message" % msg1)
msgbin0 = util.hex2bin(msg0)
msgbin1 = util.hex2bin(msg1)
@ -181,7 +187,7 @@ def position(msg0, msg1, t0, t1):
air_d_lat_odd = 360.0 / 59
# compute latitude index 'j'
j = int(math.floor(59 * cprlat_even - 60 * cprlat_odd + 0.5))
j = util.floor(59 * cprlat_even - 60 * cprlat_odd + 0.5)
lat_even = float(air_d_lat_even * (j % 60 + cprlat_even))
lat_odd = float(air_d_lat_odd * (j % 59 + cprlat_odd))
@ -199,13 +205,13 @@ def position(msg0, msg1, t0, t1):
# compute ni, longitude index m, and longitude
if (t0 > t1):
ni = _cprN(lat_even, 0)
m = math.floor(cprlon_even * (_cprNL(lat_even)-1) -
m = util.floor(cprlon_even * (_cprNL(lat_even)-1) -
cprlon_odd * _cprNL(lat_even) + 0.5)
lon = (360.0 / ni) * (m % ni + cprlon_even)
lat = lat_even
else:
ni = _cprN(lat_odd, 1)
m = math.floor(cprlon_even * (_cprNL(lat_odd)-1) -
m = util.floor(cprlon_even * (_cprNL(lat_odd)-1) -
cprlon_odd * _cprNL(lat_odd) + 0.5)
lon = (360.0 / ni) * (m % ni + cprlon_odd)
lat = lat_odd
@ -216,6 +222,11 @@ def position(msg0, msg1, t0, t1):
return round(lat, 5), round(lon, 5)
def surface_position(msg0, msg1, t0, t1):
# TODO: implement surface positon decoding
raise RuntimeError('suface position decoding to be implemented soon...')
def _cprN(lat, is_odd):
nl = _cprNL(lat) - is_odd
return nl if nl > 1 else 1
@ -223,11 +234,12 @@ def _cprN(lat, is_odd):
def _cprNL(lat):
try:
nz = 60
a = 1 - math.cos(math.pi * 2 / nz)
nz = 15
a = 1 - math.cos(math.pi / (2 * nz))
b = math.cos(math.pi / 180.0 * abs(lat)) ** 2
nl = 2 * math.pi / (math.acos(1 - a/b))
return int(nl)
NL = util.floor(nl)
return NL
except:
# happens when latitude is +/-90 degree
return 1

8
pyModeS/util.py
View File

@ -71,3 +71,11 @@ def crc(msg, encode=False):
# last 24 bits
reminder = ''.join(msgbin[-24:])
return reminder
def floor(x):
""" Mode-S floor function
Defined as the greatest integer value k, such that k <= x
eg.: floor(3.6) = 3, while floor(-3.6) = -4
"""
return int(math.floor(x))