gr-air-modes/python/cpr.py

#!/usr/bin/env python
#
# 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)