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 bytes hexadecimal string)
msg1 (string): odd message (28 bytes hexadecimal string)
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:]
# 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)
def surface_position_with_ref(msg, lat_ref, lon_ref):
"""Decode surface 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 45NM of the true position.
Args:
msg (string): even message (28 bytes hexadecimal string)
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 = 90.0 / 59 if i else 90.0 / 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 = 90.0 / ni
else:
d_lon = 90.0
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)
def surface_position(msg0, msg1, t0, t1, lat_ref, lon_ref):
"""Decode surface position from a pair of even and odd position message,
the lat/lon of receiver must be provided to yield the correct solution.
Args:
msg0 (string): even message (28 bytes hexadecimal string)
msg1 (string): odd message (28 bytes hexadecimal string)
t0 (int): timestamps for the even message
t1 (int): timestamps for the odd message
lat_ref (float): latitude of the receiver
lon_ref (float): longitude of the receiver
Returns:
(float, float): (latitude, longitude) of the aircraft
"""
msgbin0 = common.hex2bin(msg0)
msgbin1 = common.hex2bin(msg1)
# 131072 is 2^17, since CPR lat and lon are 17 bits each.
cprlat_even = common.bin2int(msgbin0[54:71]) / 131072.0
cprlon_even = common.bin2int(msgbin0[71:88]) / 131072.0
cprlat_odd = common.bin2int(msgbin1[54:71]) / 131072.0
cprlon_odd = common.bin2int(msgbin1[71:88]) / 131072.0
air_d_lat_even = 90.0 / 60
air_d_lat_odd = 90.0 / 59
# compute latitude index 'j'
j = common.floor(59 * cprlat_even - 60 * cprlat_odd + 0.5)
# solution for north hemisphere
lat_even_n = float(air_d_lat_even * (j % 60 + cprlat_even))
lat_odd_n = float(air_d_lat_odd * (j % 59 + cprlat_odd))
# solution for north hemisphere
lat_even_s = lat_even_n - 90.0
lat_odd_s = lat_odd_n - 90.0
# chose which solution corrispondes to receiver location
lat_even = lat_even_n if lat_ref > 0 else lat_even_s
lat_odd = lat_odd_n if lat_ref > 0 else lat_odd_s
# check if both are in the same latidude zone, rare but possible
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_even)
ni = max(common.cprNL(lat_even) - 0, 1)
m = common.floor(cprlon_even * (nl - 1) - cprlon_odd * nl + 0.5)
lon = (90.0 / ni) * (m % ni + cprlon_even)
else:
lat = lat_odd
nl = common.cprNL(lat_odd)
ni = max(common.cprNL(lat_odd) - 1, 1)
m = common.floor(cprlon_even * (nl - 1) - cprlon_odd * nl + 0.5)
lon = (90.0 / ni) * (m % ni + cprlon_odd)
# four possible longitude solutions
lons = [lon, lon + 90.0, lon + 180.0, lon + 270.0]
# make sure lons are between -180 and 180
lons = [(l + 180) % 360 - 180 for l in lons]
# the closest solution to receiver is the correct one
dls = [abs(lon_ref - l) for l in lons]
imin = min(range(4), key=dls.__getitem__)
lon = lons[imin]
return round(lat, 5), round(lon, 5)