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Remove altitude-based extra station. I don't now believe there's a way to construct a "fake" station as you don't have the originating time of the transmission as a known quantity.

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Nick Foster 2012-12-11 09:44:21 -08:00
parent c4c63b5b69
commit 3be6e9fd6e
1 changed files with 6 additions and 54 deletions
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60
python/mlat.py
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@ -107,30 +107,15 @@ def mlat_iter(stations, prange_obs, guess = [0,0,0], limit = 20, maxrounds = 100
#create a matrix of partial differentials to find the slope of the error in X,Y,Z,t directions
H = numpy.array([(numpy.array(-stations[row,:])+guess) / prange_est[row] for row in range(len(stations))])
H = numpy.append(H, numpy.ones(len(prange_obs)).reshape(len(prange_obs),1)*c, axis=1)
#print "H: ", H
#now we have H, the Jacobian, and can solve for residual error
solved = numpy.linalg.lstsq(H, dphat)
xerr = solved[0].flatten()
#print "s: ", solved[3]
#print "xerr: ", xerr
guess += xerr[:3] #we ignore the time error for xguess
#print "Estimated position and change: ", guess, numpy.linalg.norm(xerr[:3])
rounds += 1
if rounds > maxrounds:
raise Exception("Failed to converge!")
return (guess, xerr[3])
#gets the emulated Arne Saknussemm Memorial Radio Station report
#here we calc the estimated pseudorange to the center of the earth, using station[0] as a reference point
#in other words, we say "if the aircraft were directly overhead of me, this is the pseudorange to the center of the earth"
#if the dang earth were actually round this wouldn't be an issue
#this lets us use the altitude of the mode S reply as info to construct an additional reporting station
#i haven't really thought about it but I think the geometry (re: *DOP) of this "station" is pretty lousy
#but it lets us solve with 3 stations
def get_fake_prange(surface_position, altitude):
fake_xyz = numpy.array(llh2ecef((surface_position[0], surface_position[1], altitude)))
return [numpy.linalg.norm(fake_xyz)-numpy.linalg.norm(llh2geoid(surface_position))] #use ECEF not geoid since alt is MSL not GPS
#func mlat:
#uses a modified GPS pseudorange solver to locate aircraft by multilateration.
#replies is a list of reports, in ([lat, lon, alt], timestamp) format
@ -142,58 +127,25 @@ def mlat(replies, altitude):
stations = [sorted_reply[0] for sorted_reply in sorted_replies]
timestamps = [sorted_reply[1] for sorted_reply in sorted_replies]
print "Timestamps: ", timestamps
nearest_llh = stations[0]
#nearest_xyz = numpy.array(llh2geoid(stations[0]))
stations_xyz = numpy.array([llh2geoid(station) for station in stations])
stations_xyz = [numpy.array(llh2geoid(station)) for station in stations]
#add in a center-of-the-earth station if we have altitude
# if altitude is not None:
# stations_xyz.append([0,0,0])
stations_xyz = numpy.array(stations_xyz) #convert list of arrays to 2d array
#get TDOA relative to station 0, multiply by c to get pseudorange
#the absolute time shouldn't matter at all except perhaps in
#maintaining floating-point precision; in other words you can
#add a constant here and it should fall out in the solver as time
#error
prange_obs = [[c*(stamp)] for stamp in timestamps]
#add a constant here and it should fall out in the solver as time error
prange_obs = [[c*stamp] for stamp in timestamps]
# if altitude is not None:
# prange_obs.append(get_fake_prange(stations[0], altitude))
#if no alt, use a very large number
#this guarantees monotonicity in the error function
#if no alt, use a reasonably large number (in meters)
#since if all your stations lie in a plane (they basically will),
#there's a reasonable solution at negative altitude as well
if altitude is None:
altitude = 20000
print "Initial pranges: ", prange_obs
print "Stations: ", stations_xyz
firstguess = numpy.array(llh2ecef((nearest_llh[0], nearest_llh[1], altitude)))
prange_obs = numpy.array(prange_obs)
#use the nearest station (we sorted by timestamp earlier) as the initial guess
firstguess = numpy.array(llh2ecef((nearest_llh[0], nearest_llh[1], altitude)))
xyzpos, time_offset = mlat_iter(stations_xyz, prange_obs, firstguess, maxrounds=100)
print "xyzpos: ", xyzpos
llhpos = ecef2llh(xyzpos)
#now, we could return llhpos right now and be done with it.
#but the assumption we made above, namely that the aircraft is directly above the
#nearest station, results in significant error due to the oblateness of the Earth's geometry.
#so now we solve AGAIN, but this time with the corrected pseudorange of the aircraft altitude
#this might not be really useful in practice but the sim shows >50m errors without it
#and <4cm errors with it, not that we'll get that close in reality but hey let's do it right
# if altitude is not None:
# prange_obs[-1] = [numpy.linalg.norm(llh2ecef((llhpos[0], llhpos[1], altitude)))]
# xyzpos_corr, time_offset = mlat_iter(stations, prange_obs, xyzpos) #start off with a really close guess
# llhpos = ecef2llh(xyzpos_corr+nearest_xyz)
return (llhpos, time_offset)
#tests the mlat_iter algorithm using sample data from Farrell & Barth (p.147)
@ -235,7 +187,7 @@ if __name__ == '__main__':
replies = [(station, stamp) for station,stamp in zip(teststations, teststamps)]
ans, offset = mlat(replies, None)
ans, offset = mlat(replies, testalt)
error = numpy.linalg.norm(numpy.array(llh2ecef(ans))-numpy.array(testplane))
rng = numpy.linalg.norm(llh2geoid(ans)-numpy.array(llh2geoid(teststations[0])))
print "Resolved lat/lon/alt: ", ans