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package de.serosystems.lib1090.cpr;
/*
* This file is part of de.serosystems.lib1090.
*
* de.serosystems.lib1090 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 of the License, or
* (at your option) any later version.
*
* de.serosystems.lib1090 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 de.serosystems.lib1090. If not, see .
*/
import de.serosystems.lib1090.Position;
/**
* Decoder for CPR encoded positions
* @author Matthias Schaefer ([email protected])
*/
public class CompactPositionReporting {
/**
* This method can only be used if another position report with a different format (even/odd) is available
* and set with msg.setOtherFormatMsg(other).
* @param pos CPR encoded position
* @param old airborne position message of the other format (even/odd). Note that the time between
* both messages should be not longer than 10 seconds!
* @param reference position to determine right surface position; use null for airborne (will be ignored)
* @return globally unambiguously decoded position for cpr1 or null if two encoded positions cannot be combined or
* if position is otherwise unavailable. Altitude of resulting position is null.
*/
public static Position decodeGlobalPosition(CPREncodedPosition pos, CPREncodedPosition old, Position reference) {
if (pos.isOddFormat() == old.isOddFormat()) return null;
if (pos.isSurface() != old.isSurface()) return null;
if (pos.isSurface() && reference == null) return null;
CPREncodedPosition even = pos.isOddFormat() ? old : pos;
CPREncodedPosition odd = pos.isOddFormat() ? pos : old;
double angle = even.isSurface() ? 90 : 360.0;
// Helper for latitude (Number of zones NZ is set to 15)
double Dlat0 = angle / 60.0;
double Dlat1 = angle / 59.0;
// latitude index
double j = Math.floor((
(59.0 * ((double) even.getEncodedLat())) / ((double) (1 << even.getNumBits())) -
(60.0 * ((double) odd.getEncodedLat())) / ((double) (1 << odd.getNumBits()))) + 0.5);
// global latitudes
double Rlat0 = Dlat0 * (mod(j, 60.0) + ((double) even.getEncodedLat()) / ((double) (1 << even.getNumBits())));
double Rlat1 = Dlat1 * (mod(j, 59.0) + ((double) odd.getEncodedLat()) / ((double) (1 << odd.getNumBits())));
// Southern hemisphere?
if (Rlat0 >= 270.0 && Rlat0 <= 360.0) Rlat0 -= 360.0;
if (Rlat1 >= 270.0 && Rlat1 <= 360.0) Rlat1 -= 360.0;
// ensure that the number of even longitude zones are equal
if (NL(Rlat0) != NL(Rlat1)) return null; // position straddle
double Rlat = pos.isOddFormat() ? Rlat1 : Rlat0;
// Helper for longitude
double NL_helper = NL(Rlat0);// NL(Rlat0) and NL(Rlat1) are equal
// longitude index
double m = Math.floor(
((double) even.getEncodedLon() * (NL_helper - 1.0)) / ((double) (1 << even.getNumBits())) -
((double) odd.getEncodedLon() * NL_helper) / ((double) (1 << odd.getNumBits())) + 0.5);
// global longitude
double n_helper = Math.max(1.0, NL_helper - (pos.isOddFormat() ? 1.0 : 0.0));
double Rlon = (angle / n_helper) * (mod(m, n_helper) + ((double) pos.getEncodedLon()) / ((double) (1 << pos.getNumBits())));
if (pos.isSurface()) {
// check the 4 possible solutions of the surface decoding
Position candidate = new Position(Rlon, Rlat, 0.0);
for (int o : new int[] {90, 180, 270}) {
Position alternative = new Position(Rlon + o, Rlat, 0.0);
if (reference.haversine(alternative) < reference.haversine(candidate))
candidate = alternative;
}
Rlon = candidate.getLongitude();
}
// correct longitude
if (Rlon < -180.0 && Rlon > -360.0) Rlon += 360.0;
if (Rlon > 180.0 && Rlon < 360.0) Rlon -= 360.0;
return new Position(Rlon, Rlat, null);
}
/**
* This method uses a locally unambiguous decoding for airborne position messages. It
* uses a reference position known to be within 180NM (= 333.36km) of the true target
* airborne position and within 45NM for surface positions. The reference point may be
* a previously decoded position that has been confirmed by global decoding (see
* {@link #decodeGlobalPosition(CPREncodedPosition, CPREncodedPosition, Position)}) or
* the receiver position.
* @param pos CPR encoded position
* @param ref reference position
* @return decoded position (without altitude)
*/
public static Position decodeLocalPosition(CPREncodedPosition pos, Position ref) {
if (ref == null) return null;
// latitude zone size
double angle = pos.isSurface() ? 90.0 : 360.0;
double Dlat = pos.isOddFormat() ? angle / 59.0 : angle / 60.0;
// latitude zone index
double j = Math.floor(ref.getLatitude() / Dlat) + Math.floor(
0.5 + mod(ref.getLatitude(), Dlat) / Dlat - ((double) pos.getEncodedLat()) / ((double) (1 << pos.getNumBits())));
// decoded position latitude
double Rlat = Dlat * (j + ((double) pos.getEncodedLat()) / ((double) (1 << pos.getNumBits())));
// longitude zone size
double Dlon = angle / Math.max(1.0, NL(Rlat) - (pos.isOddFormat() ? 1.0 : 0.0));
// longitude zone coordinate
double m = Math.floor(ref.getLongitude() / Dlon) + Math.floor(0.5 + mod(ref.getLongitude(), Dlon) / Dlon
- ((double) pos.getEncodedLon()) / ((double) (1 << pos.getNumBits())));
// and finally the longitude
double Rlon = Dlon * (m + ((double) pos.getEncodedLon()) / ((double) (1 << pos.getNumBits())));
return new Position(Rlon, Rlat, null);
}
/**
* @param Rlat Even or odd Rlat value (CPR internal)
* @return the number of even longitude zones at a latitude
*/
private static double NL(double Rlat) {
if (Rlat == 0) return 59;
else if (Math.abs(Rlat) == 87) return 2;
else if (Math.abs(Rlat) > 87) return 1;
double tmp = 1-(1-Math.cos(Math.PI/(2.0*15.0)))/Math.pow(Math.cos(Math.PI/180.0*Math.abs(Rlat)), 2);
return Math.floor(2*Math.PI/Math.acos(tmp));
}
/**
* Modulo operator in java has stupid behavior
*/
private static double mod(double a, double b) {
return ((a%b)+b)%b;
}
}
