# ------------------------------------------
# BDS 0,5
# ADS-B TC=9-18
# Airborn position
# ------------------------------------------
from pyModeS import common
[docs]def airborne_position(msg0, msg1, t0, t1):
"""Decode airborn position from a pair of even and odd position message
Args:
msg0 (string): even message (28 hexdigits)
msg1 (string): odd message (28 hexdigits)
t0 (int): timestamps for the even message
t1 (int): timestamps for the odd message
Returns:
(float, float): (latitude, longitude) of the aircraft
"""
mb0 = common.hex2bin(msg0)[32:]
mb1 = common.hex2bin(msg1)[32:]
oe0 = int(mb0[21])
oe1 = int(mb1[21])
if oe0 == 0 and oe1 == 1:
pass
elif oe0 == 1 and oe1 == 0:
mb0, mb1 = mb1, mb0
t0, t1 = t1, t0
else:
raise RuntimeError("Both even and odd CPR frames are required.")
# 131072 is 2^17, since CPR lat and lon are 17 bits each.
cprlat_even = common.bin2int(mb0[22:39]) / 131072.0
cprlon_even = common.bin2int(mb0[39:56]) / 131072.0
cprlat_odd = common.bin2int(mb1[22:39]) / 131072.0
cprlon_odd = common.bin2int(mb1[39:56]) / 131072.0
air_d_lat_even = 360.0 / 60
air_d_lat_odd = 360.0 / 59
# compute latitude index 'j'
j = common.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))
if lat_even >= 270:
lat_even = lat_even - 360
if lat_odd >= 270:
lat_odd = lat_odd - 360
# check if both are in the same latidude zone, exit if not
if common.cprNL(lat_even) != common.cprNL(lat_odd):
return None
# compute ni, longitude index m, and longitude
if t0 > t1:
lat = lat_even
nl = common.cprNL(lat)
ni = max(common.cprNL(lat) - 0, 1)
m = common.floor(cprlon_even * (nl - 1) - cprlon_odd * nl + 0.5)
lon = (360.0 / ni) * (m % ni + cprlon_even)
else:
lat = lat_odd
nl = common.cprNL(lat)
ni = max(common.cprNL(lat) - 1, 1)
m = common.floor(cprlon_even * (nl - 1) - cprlon_odd * nl + 0.5)
lon = (360.0 / ni) * (m % ni + cprlon_odd)
if lon > 180:
lon = lon - 360
return round(lat, 5), round(lon, 5)
[docs]def airborne_position_with_ref(msg, lat_ref, lon_ref):
"""Decode airborne position with only one message,
knowing reference nearby location, such as previously calculated location,
ground station, or airport location, etc. The reference position shall
be with in 180NM of the true position.
Args:
msg (str): even message (28 hexdigits)
lat_ref: previous known latitude
lon_ref: previous known longitude
Returns:
(float, float): (latitude, longitude) of the aircraft
"""
mb = common.hex2bin(msg)[32:]
cprlat = common.bin2int(mb[22:39]) / 131072.0
cprlon = common.bin2int(mb[39:56]) / 131072.0
i = int(mb[21])
d_lat = 360.0 / 59 if i else 360.0 / 60
j = common.floor(lat_ref / d_lat) + common.floor(
0.5 + ((lat_ref % d_lat) / d_lat) - cprlat
)
lat = d_lat * (j + cprlat)
ni = common.cprNL(lat) - i
if ni > 0:
d_lon = 360.0 / ni
else:
d_lon = 360.0
m = common.floor(lon_ref / d_lon) + common.floor(
0.5 + ((lon_ref % d_lon) / d_lon) - cprlon
)
lon = d_lon * (m + cprlon)
return round(lat, 5), round(lon, 5)
[docs]def altitude(msg):
"""Decode aircraft altitude
Args:
msg (str): 28 hexdigits string
Returns:
int: altitude in feet
"""
tc = common.typecode(msg)
if tc < 9 or tc == 19 or tc > 22:
raise RuntimeError("%s: Not a airborn position message" % msg)
mb = common.hex2bin(msg)[32:]
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
else:
# GNSS altitude, meters -> feet
alt = common.bin2int(mb[8:20]) * 3.28084
return alt