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333 lines
10 KiB
333 lines
10 KiB
#!/usr/bin/env python
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#
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# Copyright 2010, 2012 Nick Foster
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#
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# This file is part of gr-air-modes
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#
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# gr-air-modes is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3, or (at your option)
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# any later version.
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#
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# gr-air-modes is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with gr-air-modes; see the file COPYING. If not, write to
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# the Free Software Foundation, Inc., 51 Franklin Street,
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# Boston, MA 02110-1301, USA.
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#
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import math, time
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from air_modes.exceptions import *
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#this implements CPR position decoding and encoding.
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#the decoder is implemented as a class, cpr_decoder, which keeps state for local decoding.
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#the encoder is cpr_encode([lat, lon], type (even=0, odd=1), and surface (0 for surface, 1 for airborne))
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#TODO: remove range/bearing calc from CPR decoder class. you can do this outside of the decoder.
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latz = 15
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def nz(ctype):
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return 4 * latz - ctype
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def dlat(ctype, surface):
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if surface == 1:
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tmp = 90.0
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else:
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tmp = 360.0
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nzcalc = nz(ctype)
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if nzcalc == 0:
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return tmp
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else:
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return tmp / nzcalc
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def nl(declat_in):
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if abs(declat_in) >= 87.0:
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return 1.0
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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)
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def dlon(declat_in, ctype, surface):
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if surface:
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tmp = 90.0
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else:
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tmp = 360.0
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nlcalc = max(nl(declat_in)-ctype, 1)
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return tmp / nlcalc
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def decode_lat(enclat, ctype, my_lat, surface):
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tmp1 = dlat(ctype, surface)
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tmp2 = float(enclat) / (2**17)
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j = math.floor(my_lat/tmp1) + math.floor(0.5 + ((my_lat % tmp1) / tmp1) - tmp2)
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return tmp1 * (j + tmp2)
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def decode_lon(declat, enclon, ctype, my_lon, surface):
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tmp1 = dlon(declat, ctype, surface)
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tmp2 = float(enclon) / (2**17)
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m = math.floor(my_lon / tmp1) + math.floor(0.5 + ((my_lon % tmp1) / tmp1) - tmp2)
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return tmp1 * (m + tmp2)
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def cpr_resolve_local(my_location, encoded_location, ctype, surface):
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[my_lat, my_lon] = my_location
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[enclat, enclon] = encoded_location
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decoded_lat = decode_lat(enclat, ctype, my_lat, surface)
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decoded_lon = decode_lon(decoded_lat, enclon, ctype, my_lon, surface)
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return [decoded_lat, decoded_lon]
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def cpr_resolve_global(evenpos, oddpos, mypos, mostrecent, surface):
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#cannot resolve surface positions unambiguously without knowing receiver position
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if surface and mypos is None:
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raise CPRNoPositionError
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dlateven = dlat(0, surface)
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dlatodd = dlat(1, surface)
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evenpos = [float(evenpos[0]), float(evenpos[1])]
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oddpos = [float(oddpos[0]), float(oddpos[1])]
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j = math.floor(((nz(1)*evenpos[0] - nz(0)*oddpos[0])/2**17) + 0.5) #latitude index
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rlateven = dlateven * ((j % nz(0))+evenpos[0]/2**17)
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rlatodd = dlatodd * ((j % nz(1))+ oddpos[0]/2**17)
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#limit to -90, 90
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if rlateven > 270.0:
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rlateven -= 360.0
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if rlatodd > 270.0:
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rlatodd -= 360.0
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#This checks to see if the latitudes of the reports straddle a transition boundary
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#If so, you can't get a globally-resolvable location.
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if nl(rlateven) != nl(rlatodd):
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raise CPRBoundaryStraddleError
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if mostrecent == 0:
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rlat = rlateven
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else:
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rlat = rlatodd
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#disambiguate latitude
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if surface:
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if mypos[0] < 0:
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rlat -= 90
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dl = dlon(rlat, mostrecent, surface)
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nl_rlat = nl(rlat)
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m = math.floor(((evenpos[1]*(nl_rlat-1)-oddpos[1]*nl_rlat)/2**17)+0.5) #longitude index
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#when surface positions straddle a disambiguation boundary (90 degrees),
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#surface decoding will fail. this might never be a problem in real life, but it'll fail in the
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#test case. the documentation doesn't mention it.
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if mostrecent == 0:
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enclon = evenpos[1]
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else:
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enclon = oddpos[1]
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rlon = dl * ((m % max(nl_rlat-mostrecent,1)) + enclon/2.**17)
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#print "DL: %f nl: %f m: %f rlon: %f" % (dl, nl_rlat, m, rlon)
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#print "evenpos: %x, oddpos: %x, mostrecent: %i" % (evenpos[1], oddpos[1], mostrecent)
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if surface:
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#longitudes need to be resolved to the nearest 90 degree segment to the receiver.
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wat = mypos[1]
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if wat < 0:
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wat += 360
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zone = lambda lon: 90 * (int(lon) / 90)
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rlon += (zone(wat) - zone(rlon))
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#limit to (-180, 180)
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if rlon > 180:
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rlon -= 360.0
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return [rlat, rlon]
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#calculate range and bearing between two lat/lon points
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#should probably throw this in the mlat py somewhere or make another lib
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def range_bearing(loc_a, loc_b):
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[a_lat, a_lon] = loc_a
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[b_lat, b_lon] = loc_b
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esquared = (1/298.257223563)*(2-(1/298.257223563))
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earth_radius_mi = 3963.19059 * (math.pi / 180)
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delta_lat = b_lat - a_lat
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delta_lon = b_lon - a_lon
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avg_lat = ((a_lat + b_lat) / 2.0) * math.pi / 180
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R1 = earth_radius_mi*(1.0-esquared)/pow((1.0-esquared*pow(math.sin(avg_lat),2)),1.5)
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R2 = earth_radius_mi/math.sqrt(1.0-esquared*pow(math.sin(avg_lat),2))
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distance_North = R1*delta_lat
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distance_East = R2*math.cos(avg_lat)*delta_lon
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bearing = math.atan2(distance_East,distance_North) * (180.0 / math.pi)
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if bearing < 0.0:
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bearing += 360.0
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rnge = math.hypot(distance_East,distance_North)
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return [rnge, bearing]
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class cpr_decoder:
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def __init__(self, my_location):
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self.my_location = my_location
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self.evenlist = {}
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self.oddlist = {}
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self.evenlist_sfc = {}
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self.oddlist_sfc = {}
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def set_location(self, new_location):
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self.my_location = new_location
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def weed_poslists(self):
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for poslist in [self.evenlist, self.oddlist]:
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for key, item in poslist.items():
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if time.time() - item[2] > 10:
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del poslist[key]
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for poslist in [self.evenlist_sfc, self.oddlist_sfc]:
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for key, item in poslist.items():
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if time.time() - item[2] > 25:
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del poslist[key]
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def decode(self, icao24, encoded_lat, encoded_lon, cpr_format, surface):
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if surface:
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oddlist = self.oddlist_sfc
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evenlist = self.evenlist_sfc
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else:
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oddlist = self.oddlist
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evenlist = self.evenlist
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#add the info to the position reports list for global decoding
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if cpr_format==1:
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oddlist[icao24] = [encoded_lat, encoded_lon, time.time()]
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else:
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evenlist[icao24] = [encoded_lat, encoded_lon, time.time()]
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[decoded_lat, decoded_lon] = [None, None]
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#okay, let's traverse the lists and weed out those entries that are older than 10 seconds
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self.weed_poslists()
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if (icao24 in evenlist) \
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and (icao24 in oddlist):
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newer = (oddlist[icao24][2] - evenlist[icao24][2]) > 0 #figure out which report is newer
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[decoded_lat, decoded_lon] = cpr_resolve_global(evenlist[icao24][0:2], oddlist[icao24][0:2], self.my_location, newer, surface) #do a global decode
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else:
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raise CPRNoPositionError
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if self.my_location is not None:
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[rnge, bearing] = range_bearing(self.my_location, [decoded_lat, decoded_lon])
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else:
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rnge = None
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bearing = None
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return [decoded_lat, decoded_lon, rnge, bearing]
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#encode CPR position
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def cpr_encode(lat, lon, ctype, surface):
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if surface is True:
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scalar = 2.**19
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else:
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scalar = 2.**17
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#encode using 360 constant for segment size.
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dlati = dlat(ctype, False)
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yz = math.floor(scalar * ((lat % dlati)/dlati) + 0.5)
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rlat = dlati * ((yz / scalar) + math.floor(lat / dlati))
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#encode using 360 constant for segment size.
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dloni = dlon(lat, ctype, False)
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xz = math.floor(scalar * ((lon % dloni)/dloni) + 0.5)
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yz = int(yz) & (2**17-1)
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xz = int(xz) & (2**17-1)
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return (yz, xz) #lat, lon
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if __name__ == '__main__':
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import sys, random
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rounds = 10001
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threshold = 1e-3 #0.001 deg lat/lon
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#this accuracy is highly dependent on latitude, since at high
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#latitudes the corresponding error in longitude is greater
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bs = 0
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surface = False
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lats = [i/(rounds/170.)-85 for i in range(0,rounds)]
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lons = [i/(rounds/360.)-180 for i in range(0,rounds)]
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for i in range(0, rounds):
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even_lat = lats[i]
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#even_lat = random.uniform(-85, 85)
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even_lon = lons[i]
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#even_lon = random.uniform(-180, 180)
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odd_lat = even_lat + 1e-3
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odd_lon = min(even_lon + 1e-3, 180)
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decoder = cpr_decoder([odd_lat, odd_lon])
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#encode that position
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(evenenclat, evenenclon) = cpr_encode(even_lat, even_lon, False, surface)
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(oddenclat, oddenclon) = cpr_encode(odd_lat, odd_lon, True, surface)
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#try to perform a global decode -- this should fail since the decoder
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#only has heard one position. need two for global decoding.
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icao = random.randint(0, 0xffffff)
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try:
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evenpos = decoder.decode(icao, evenenclat, evenenclon, False, surface)
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raise Exception("CPR test failure: global decode with only one report")
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except CPRNoPositionError:
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pass
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#now try to do a real decode with the last packet's odd complement
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#watch for a boundary straddle -- this isn't fatal, it just indicates
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#that the even and odd reports lie on either side of a longitudinal boundary
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#and so you can't get a position
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try:
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(odddeclat, odddeclon, rng, brg) = decoder.decode(icao, oddenclat, oddenclon, True, surface)
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except CPRBoundaryStraddleError:
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bs += 1
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continue
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except CPRNoPositionError:
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raise Exception("CPR test failure: no decode after even/odd inputs")
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if abs(odddeclat - odd_lat) > threshold or abs(odddeclon - odd_lon) > threshold:
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print "F odddeclat: %f odd_lat: %f" % (odddeclat, odd_lat)
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print "F odddeclon: %f odd_lon: %f" % (odddeclon, odd_lon)
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raise Exception("CPR test failure: global decode error greater than threshold")
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# else:
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# print "S odddeclat: %f odd_lat: %f" % (odddeclat, odd_lat)
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# print "S odddeclon: %f odd_lon: %f" % (odddeclon, odd_lon)
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nexteven_lat = odd_lat + 1e-3
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nexteven_lon = min(odd_lon + 1e-3, 180)
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(nexteven_enclat, nexteven_enclon) = cpr_encode(nexteven_lat, nexteven_lon, False, surface)
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#try a locally-referenced decode
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try:
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(evendeclat, evendeclon) = cpr_resolve_local([even_lat, even_lon], [nexteven_enclat, nexteven_enclon], False, surface)
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except CPRNoPositionError:
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raise Exception("CPR test failure: local decode failure to resolve")
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#check to see if the positions were valid
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if abs(evendeclat - nexteven_lat) > threshold or abs(evendeclon - nexteven_lon) > threshold:
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print "F evendeclat: %f nexteven_lat: %f evenlat: %f" % (evendeclat, nexteven_lat, even_lat)
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print "F evendeclon: %f nexteven_lon: %f evenlon: %f" % (evendeclon, nexteven_lon, even_lon)
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raise Exception("CPR test failure: local decode error greater than threshold")
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print "CPR test successful. There were %i boundary straddles over %i rounds." % (bs, rounds)
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