From 0b30db353daab873eb8add9588a237495b97263a Mon Sep 17 00:00:00 2001 From: Nick Foster Date: Wed, 17 Oct 2012 09:13:10 -0700 Subject: [PATCH] CPR: surface decoding still exhibits occasional errors when m~=0. Suspect it might actually be a bug in the encoder. --- python/cpr.py | 93 +++++++++++++++++++++++++++------------------------ 1 file changed, 50 insertions(+), 43 deletions(-) diff --git a/python/cpr.py b/python/cpr.py index 7c29013..decefc0 100755 --- a/python/cpr.py +++ b/python/cpr.py @@ -26,6 +26,8 @@ from air_modes.exceptions import * #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): @@ -46,18 +48,15 @@ def dlat(ctype, surface): def nl(declat_in): if abs(declat_in) >= 87.0: return 1.0 - 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)) + 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 == 1: + if surface: tmp = 90.0 else: tmp = 360.0 - nlcalc = nl(declat_in)-ctype - if nlcalc == 0: - return tmp - else: - return tmp / nlcalc + nlcalc = max(nl(declat_in)-ctype, 1) + return tmp / nlcalc def decode_lat(enclat, ctype, my_lat, surface): tmp1 = dlat(ctype, surface) @@ -120,29 +119,29 @@ def cpr_resolve_global(evenpos, oddpos, mypos, mostrecent, surface): rlat -= 90 dl = dlon(rlat, mostrecent, surface) - nlthing = nl(rlat) - ni = nlthing - mostrecent + nl_rlat = nl(rlat) - #THIS COLLAPSES WHEN M IS NOT A FACTOR OF FOUR - m = math.floor(((evenpos[1]*(nlthing-1)-oddpos[1]*nlthing)/2**17)+0.5) #longitude index - if m%4: - print "IM GONNA DIE" - - print "DL: %f nlthing: %f ni: %f m: %f" % (dl, nlthing, ni, m) + 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 % ni)+enclon/2**17) + 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 - print "mylon: %f mylon unwrapped: %f rlon raw: %f" % (mypos[1], wat, rlon) zone = lambda lon: 90 * (int(lon) / 90) rlon += (zone(wat) - zone(rlon)) @@ -226,58 +225,64 @@ class cpr_decoder: #encode CPR position def cpr_encode(lat, lon, ctype, surface): if surface is True: - scalar = float(2**19) + scalar = 2.**19 else: - scalar = float(2**17) + scalar = 2.**17 - dlati = float(dlat(ctype, False)) + #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)) - nleo = nl(rlat)-ctype - if nleo == 0: - dloni = 360.0 - else: - dloni = 360.0 / nleo - + #encode using 360 constant for segment size. + dloni = dlon(lat, ctype, False) xz = math.floor(scalar * ((lon % dloni)/dloni) + 0.5) - yz = int(yz % scalar) - xz = int(xz % scalar) + yz = int(yz) & (2**17-1) + xz = int(xz) & (2**17-1) return (yz, xz) #lat, lon if __name__ == '__main__': import sys, random - - rounds = 10000 + + 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 = True + 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 = random.uniform(-85, 85) - even_lon = random.uniform(-180, 180) - odd_lat = even_lat + 1e-2 - odd_lon = min(even_lon + 1e-2, 180) + 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) - #perform a global decode + #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) - #print "CPR global decode with only one report: %f %f" % (evenpos[0], evenpos[1]) 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: @@ -290,20 +295,22 @@ if __name__ == '__main__': 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) +# 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-2 - nexteven_lon = min(odd_lon + 1e-2, 180) + 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)