/usr/lib/python2.7/dist-packages/air_modes/cpr.py is in gr-air-modes 0.0.0.e47992d-5.
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#
# 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.
#
import math, time
from air_modes.exceptions import *
#this implements CPR position decoding and encoding.
#the decoder is implemented as a class, cpr_decoder, which keeps state for local decoding.
#the encoder is cpr_encode([lat, lon], type (even=0, odd=1), and surface (0 for surface, 1 for airborne))
#TODO: remove range/bearing calc from CPR decoder class. you can do this outside of the decoder.
latz = 15
def nz(ctype):
return 4 * latz - ctype
def dlat(ctype, surface):
if surface == 1:
tmp = 90.0
else:
tmp = 360.0
nzcalc = nz(ctype)
if nzcalc == 0:
return tmp
else:
return tmp / nzcalc
def nl(declat_in):
if abs(declat_in) >= 87.0:
return 1.0
return math.floor( (2.0*math.pi) * math.acos(1.0- (1.0-math.cos(math.pi/(2.0*latz))) / math.cos( (math.pi/180.0)*abs(declat_in) )**2 )**-1)
def dlon(declat_in, ctype, surface):
if surface:
tmp = 90.0
else:
tmp = 360.0
nlcalc = max(nl(declat_in)-ctype, 1)
return tmp / nlcalc
def decode_lat(enclat, ctype, my_lat, surface):
tmp1 = dlat(ctype, surface)
tmp2 = float(enclat) / (2**17)
j = math.floor(my_lat/tmp1) + math.floor(0.5 + ((my_lat % tmp1) / tmp1) - tmp2)
return tmp1 * (j + tmp2)
def decode_lon(declat, enclon, ctype, my_lon, surface):
tmp1 = dlon(declat, ctype, surface)
tmp2 = float(enclon) / (2**17)
m = math.floor(my_lon / tmp1) + math.floor(0.5 + ((my_lon % tmp1) / tmp1) - tmp2)
return tmp1 * (m + tmp2)
def cpr_resolve_local(my_location, encoded_location, ctype, surface):
[my_lat, my_lon] = my_location
[enclat, enclon] = encoded_location
decoded_lat = decode_lat(enclat, ctype, my_lat, surface)
decoded_lon = decode_lon(decoded_lat, enclon, ctype, my_lon, surface)
return [decoded_lat, decoded_lon]
def cpr_resolve_global(evenpos, oddpos, mypos, mostrecent, surface):
#cannot resolve surface positions unambiguously without knowing receiver position
if surface and mypos is None:
raise CPRNoPositionError
dlateven = dlat(0, surface)
dlatodd = dlat(1, surface)
evenpos = [float(evenpos[0]), float(evenpos[1])]
oddpos = [float(oddpos[0]), float(oddpos[1])]
j = math.floor(((nz(1)*evenpos[0] - nz(0)*oddpos[0])/2**17) + 0.5) #latitude index
rlateven = dlateven * ((j % nz(0))+evenpos[0]/2**17)
rlatodd = dlatodd * ((j % nz(1))+ oddpos[0]/2**17)
#limit to -90, 90
if rlateven > 270.0:
rlateven -= 360.0
if rlatodd > 270.0:
rlatodd -= 360.0
#This checks to see if the latitudes of the reports straddle a transition boundary
#If so, you can't get a globally-resolvable location.
if nl(rlateven) != nl(rlatodd):
raise CPRBoundaryStraddleError
if mostrecent == 0:
rlat = rlateven
else:
rlat = rlatodd
#disambiguate latitude
if surface:
if mypos[0] < 0:
rlat -= 90
dl = dlon(rlat, mostrecent, surface)
nl_rlat = nl(rlat)
m = math.floor(((evenpos[1]*(nl_rlat-1)-oddpos[1]*nl_rlat)/2**17)+0.5) #longitude index
#when surface positions straddle a disambiguation boundary (90 degrees),
#surface decoding will fail. this might never be a problem in real life, but it'll fail in the
#test case. the documentation doesn't mention it.
if mostrecent == 0:
enclon = evenpos[1]
else:
enclon = oddpos[1]
rlon = dl * ((m % max(nl_rlat-mostrecent,1)) + enclon/2.**17)
#print "DL: %f nl: %f m: %f rlon: %f" % (dl, nl_rlat, m, rlon)
#print "evenpos: %x, oddpos: %x, mostrecent: %i" % (evenpos[1], oddpos[1], mostrecent)
if surface:
#longitudes need to be resolved to the nearest 90 degree segment to the receiver.
wat = mypos[1]
if wat < 0:
wat += 360
zone = lambda lon: 90 * (int(lon) / 90)
rlon += (zone(wat) - zone(rlon))
#limit to (-180, 180)
if rlon > 180:
rlon -= 360.0
return [rlat, rlon]
#calculate range and bearing between two lat/lon points
#should probably throw this in the mlat py somewhere or make another lib
def range_bearing(loc_a, loc_b):
[a_lat, a_lon] = loc_a
[b_lat, b_lon] = loc_b
esquared = (1/298.257223563)*(2-(1/298.257223563))
earth_radius_mi = 3963.19059 * (math.pi / 180)
delta_lat = b_lat - a_lat
delta_lon = b_lon - a_lon
avg_lat = ((a_lat + b_lat) / 2.0) * math.pi / 180
R1 = earth_radius_mi*(1.0-esquared)/pow((1.0-esquared*pow(math.sin(avg_lat),2)),1.5)
R2 = earth_radius_mi/math.sqrt(1.0-esquared*pow(math.sin(avg_lat),2))
distance_North = R1*delta_lat
distance_East = R2*math.cos(avg_lat)*delta_lon
bearing = math.atan2(distance_East,distance_North) * (180.0 / math.pi)
if bearing < 0.0:
bearing += 360.0
rnge = math.hypot(distance_East,distance_North)
return [rnge, bearing]
class cpr_decoder:
def __init__(self, my_location):
self.my_location = my_location
self.evenlist = {}
self.oddlist = {}
self.evenlist_sfc = {}
self.oddlist_sfc = {}
def set_location(self, new_location):
self.my_location = new_location
def weed_poslists(self):
for poslist in [self.evenlist, self.oddlist]:
for key, item in poslist.items():
if time.time() - item[2] > 10:
del poslist[key]
for poslist in [self.evenlist_sfc, self.oddlist_sfc]:
for key, item in poslist.items():
if time.time() - item[2] > 25:
del poslist[key]
def decode(self, icao24, encoded_lat, encoded_lon, cpr_format, surface):
if surface:
oddlist = self.oddlist_sfc
evenlist = self.evenlist_sfc
else:
oddlist = self.oddlist
evenlist = self.evenlist
#add the info to the position reports list for global decoding
if cpr_format==1:
oddlist[icao24] = [encoded_lat, encoded_lon, time.time()]
else:
evenlist[icao24] = [encoded_lat, encoded_lon, time.time()]
[decoded_lat, decoded_lon] = [None, None]
#okay, let's traverse the lists and weed out those entries that are older than 10 seconds
self.weed_poslists()
if (icao24 in evenlist) \
and (icao24 in oddlist):
newer = (oddlist[icao24][2] - evenlist[icao24][2]) > 0 #figure out which report is newer
[decoded_lat, decoded_lon] = cpr_resolve_global(evenlist[icao24][0:2], oddlist[icao24][0:2], self.my_location, newer, surface) #do a global decode
else:
raise CPRNoPositionError
if self.my_location is not None:
[rnge, bearing] = range_bearing(self.my_location, [decoded_lat, decoded_lon])
else:
rnge = None
bearing = None
return [decoded_lat, decoded_lon, rnge, bearing]
#encode CPR position
def cpr_encode(lat, lon, ctype, surface):
if surface is True:
scalar = 2.**19
else:
scalar = 2.**17
#encode using 360 constant for segment size.
dlati = dlat(ctype, False)
yz = math.floor(scalar * ((lat % dlati)/dlati) + 0.5)
rlat = dlati * ((yz / scalar) + math.floor(lat / dlati))
#encode using 360 constant for segment size.
dloni = dlon(lat, ctype, False)
xz = math.floor(scalar * ((lon % dloni)/dloni) + 0.5)
yz = int(yz) & (2**17-1)
xz = int(xz) & (2**17-1)
return (yz, xz) #lat, lon
if __name__ == '__main__':
import sys, random
rounds = 10001
threshold = 1e-3 #0.001 deg lat/lon
#this accuracy is highly dependent on latitude, since at high
#latitudes the corresponding error in longitude is greater
bs = 0
surface = False
lats = [i/(rounds/170.)-85 for i in range(0,rounds)]
lons = [i/(rounds/360.)-180 for i in range(0,rounds)]
for i in range(0, rounds):
even_lat = lats[i]
#even_lat = random.uniform(-85, 85)
even_lon = lons[i]
#even_lon = random.uniform(-180, 180)
odd_lat = even_lat + 1e-3
odd_lon = min(even_lon + 1e-3, 180)
decoder = cpr_decoder([odd_lat, odd_lon])
#encode that position
(evenenclat, evenenclon) = cpr_encode(even_lat, even_lon, False, surface)
(oddenclat, oddenclon) = cpr_encode(odd_lat, odd_lon, True, surface)
#try to perform a global decode -- this should fail since the decoder
#only has heard one position. need two for global decoding.
icao = random.randint(0, 0xffffff)
try:
evenpos = decoder.decode(icao, evenenclat, evenenclon, False, surface)
raise Exception("CPR test failure: global decode with only one report")
except CPRNoPositionError:
pass
#now try to do a real decode with the last packet's odd complement
#watch for a boundary straddle -- this isn't fatal, it just indicates
#that the even and odd reports lie on either side of a longitudinal boundary
#and so you can't get a position
try:
(odddeclat, odddeclon, rng, brg) = decoder.decode(icao, oddenclat, oddenclon, True, surface)
except CPRBoundaryStraddleError:
bs += 1
continue
except CPRNoPositionError:
raise Exception("CPR test failure: no decode after even/odd inputs")
if abs(odddeclat - odd_lat) > threshold or abs(odddeclon - odd_lon) > threshold:
print "F odddeclat: %f odd_lat: %f" % (odddeclat, odd_lat)
print "F odddeclon: %f odd_lon: %f" % (odddeclon, odd_lon)
raise Exception("CPR test failure: global decode error greater than threshold")
# else:
# print "S odddeclat: %f odd_lat: %f" % (odddeclat, odd_lat)
# print "S odddeclon: %f odd_lon: %f" % (odddeclon, odd_lon)
nexteven_lat = odd_lat + 1e-3
nexteven_lon = min(odd_lon + 1e-3, 180)
(nexteven_enclat, nexteven_enclon) = cpr_encode(nexteven_lat, nexteven_lon, False, surface)
#try a locally-referenced decode
try:
(evendeclat, evendeclon) = cpr_resolve_local([even_lat, even_lon], [nexteven_enclat, nexteven_enclon], False, surface)
except CPRNoPositionError:
raise Exception("CPR test failure: local decode failure to resolve")
#check to see if the positions were valid
if abs(evendeclat - nexteven_lat) > threshold or abs(evendeclon - nexteven_lon) > threshold:
print "F evendeclat: %f nexteven_lat: %f evenlat: %f" % (evendeclat, nexteven_lat, even_lat)
print "F evendeclon: %f nexteven_lon: %f evenlon: %f" % (evendeclon, nexteven_lon, even_lon)
raise Exception("CPR test failure: local decode error greater than threshold")
print "CPR test successful. There were %i boundary straddles over %i rounds." % (bs, rounds)
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