CPR: surface decoding still exhibits occasional errors when m~=0. Suspect it might actually be a bug in the encoder.
This commit is contained in:
Nick Foster
@@ -26,6 +26,8 @@ from air_modes.exceptions import *
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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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@@ -46,18 +48,15 @@ def dlat(ctype, surface):
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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) * pow(math.acos(1.0- (1.0-math.cos(math.pi/(2.0*latz))) / pow( math.cos( (math.pi/180.0)*abs(declat_in) ) ,2.0) ),-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 == 1:
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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 = nl(declat_in)-ctype
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if nlcalc == 0:
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return tmp
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else:
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return tmp / nlcalc
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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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@@ -120,29 +119,29 @@ def cpr_resolve_global(evenpos, oddpos, mypos, mostrecent, surface):
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rlat -= 90
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dl = dlon(rlat, mostrecent, surface)
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nlthing = nl(rlat)
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ni = nlthing - mostrecent
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nl_rlat = nl(rlat)
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#THIS COLLAPSES WHEN M IS NOT A FACTOR OF FOUR
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m = math.floor(((evenpos[1]*(nlthing-1)-oddpos[1]*nlthing)/2**17)+0.5) #longitude index
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if m%4:
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print "IM GONNA DIE"
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print "DL: %f nlthing: %f ni: %f m: %f" % (dl, nlthing, ni, m)
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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 % ni)+enclon/2**17)
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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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print "mylon: %f mylon unwrapped: %f rlon raw: %f" % (mypos[1], wat, rlon)
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zone = lambda lon: 90 * (int(lon) / 90)
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rlon += (zone(wat) - zone(rlon))
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@@ -226,58 +225,64 @@ class cpr_decoder:
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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 = float(2**19)
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scalar = 2.**19
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else:
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scalar = float(2**17)
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scalar = 2.**17
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dlati = float(dlat(ctype, False))
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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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nleo = nl(rlat)-ctype
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if nleo == 0:
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dloni = 360.0
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else:
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dloni = 360.0 / nleo
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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 % scalar)
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xz = int(xz % scalar)
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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 = 10000
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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 = True
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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 = random.uniform(-85, 85)
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even_lon = random.uniform(-180, 180)
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odd_lat = even_lat + 1e-2
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odd_lon = min(even_lon + 1e-2, 180)
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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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#perform a global decode
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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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#print "CPR global decode with only one report: %f %f" % (evenpos[0], evenpos[1])
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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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@@ -290,20 +295,22 @@ if __name__ == '__main__':
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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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# 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-2
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nexteven_lon = min(odd_lon + 1e-2, 180)
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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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