-
Notifications
You must be signed in to change notification settings - Fork 70
/
cpr.c
374 lines (340 loc) · 13.2 KB
/
cpr.c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
// Part of readsb, a Mode-S/ADSB/TIS message decoder.
//
// cpr.c - Compact Position Reporting decoder and tests
//
// Copyright (c) 2019 Michael Wolf <[email protected]>
//
// This code is based on a detached fork of dump1090-fa.
//
// Copyright (c) 2014,2015 Oliver Jowett <[email protected]>
//
// This file 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
// any later version.
//
// This file 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 this program. If not, see <http:https://www.gnu.org/licenses/>.
//
// This file incorporates work covered by the following copyright and
// license:
//
// Copyright (C) 2012 by Salvatore Sanfilippo <[email protected]>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include <math.h>
#include <stdio.h>
//
//=========================================================================
//
// Always positive MOD operation, used for CPR decoding.
//
static int cprModInt(int a, int b) {
int res = a % b;
if (res < 0) res += b;
return res;
}
static double cprModDouble(double a, double b) {
double res = fmod(a, b);
if (res < 0) res += b;
return res;
}
//
//=========================================================================
//
// The NL function uses the precomputed table from 1090-WP-9-14
//
static int cprNLFunction(double lat) {
if (lat < 0) lat = -lat; // Table is simmetric about the equator
if (lat > 60) goto L60;
if (lat > 44.2) goto L442;
if (lat > 30) goto L30;
if (lat < 10.47047130) return 59;
if (lat < 14.82817437) return 58;
if (lat < 18.18626357) return 57;
if (lat < 21.02939493) return 56;
if (lat < 23.54504487) return 55;
if (lat < 25.82924707) return 54;
if (lat < 27.93898710) return 53;
if (lat < 29.91135686) return 52;
L30:
if (lat < 31.77209708) return 51;
if (lat < 33.53993436) return 50;
if (lat < 35.22899598) return 49;
if (lat < 36.85025108) return 48;
if (lat < 38.41241892) return 47;
if (lat < 39.92256684) return 46;
if (lat < 41.38651832) return 45;
if (lat < 42.80914012) return 44;
if (lat < 44.19454951) return 43;
L442:
if (lat < 45.54626723) return 42;
if (lat < 46.86733252) return 41;
if (lat < 48.16039128) return 40;
if (lat < 49.42776439) return 39;
if (lat < 50.67150166) return 38;
if (lat < 51.89342469) return 37;
if (lat < 53.09516153) return 36;
if (lat < 54.27817472) return 35;
if (lat < 55.44378444) return 34;
if (lat < 56.59318756) return 33;
if (lat < 57.72747354) return 32;
if (lat < 58.84763776) return 31;
if (lat < 59.95459277) return 30;
L60:
if (lat < 61.04917774) return 29;
if (lat < 62.13216659) return 28;
if (lat < 63.20427479) return 27;
if (lat < 64.26616523) return 26;
if (lat < 65.31845310) return 25;
if (lat < 66.36171008) return 24;
if (lat < 67.39646774) return 23;
if (lat < 68.42322022) return 22;
if (lat < 69.44242631) return 21;
if (lat < 70.45451075) return 20;
if (lat < 71.45986473) return 19;
if (lat < 72.45884545) return 18;
if (lat < 73.45177442) return 17;
if (lat < 74.43893416) return 16;
if (lat < 75.42056257) return 15;
if (lat < 76.39684391) return 14;
if (lat < 77.36789461) return 13;
if (lat < 78.33374083) return 12;
if (lat < 79.29428225) return 11;
if (lat < 80.24923213) return 10;
if (lat < 81.19801349) return 9;
if (lat < 82.13956981) return 8;
if (lat < 83.07199445) return 7;
if (lat < 83.99173563) return 6;
if (lat < 84.89166191) return 5;
if (lat < 85.75541621) return 4;
if (lat < 86.53536998) return 3;
if (lat < 87.00000000) return 2;
else return 1;
}
//
//=========================================================================
//
static int cprNFunction(double lat, int fflag) {
int nl = cprNLFunction(lat) - (fflag ? 1 : 0);
if (nl < 1) nl = 1;
return nl;
}
//
//=========================================================================
//
static double cprDlonFunction(double lat, int fflag, int surface) {
return (surface ? 90.0 : 360.0) / cprNFunction(lat, fflag);
}
//
//=========================================================================
//
// This algorithm comes from:
// http:https://www.lll.lu/~edward/edward/adsb/DecodingADSBposition.html.
//
// A few remarks:
// 1) 131072 is 2^17 since CPR latitude and longitude are encoded in 17 bits.
//
int decodeCPRairborne(int even_cprlat, int even_cprlon,
int odd_cprlat, int odd_cprlon,
int fflag,
double *out_lat, double *out_lon) {
double AirDlat0 = 360.0 / 60.0;
double AirDlat1 = 360.0 / 59.0;
double lat0 = even_cprlat;
double lat1 = odd_cprlat;
double lon0 = even_cprlon;
double lon1 = odd_cprlon;
double rlat, rlon;
// Compute the Latitude Index "j"
int j = (int) floor(((59 * lat0 - 60 * lat1) / 131072) + 0.5);
double rlat0 = AirDlat0 * (cprModInt(j, 60) + lat0 / 131072);
double rlat1 = AirDlat1 * (cprModInt(j, 59) + lat1 / 131072);
if (rlat0 >= 270) rlat0 -= 360;
if (rlat1 >= 270) rlat1 -= 360;
// Check to see that the latitude is in range: -90 .. +90
if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90)
return (-2); // bad data
// Check that both are in the same latitude zone, or abort.
if (cprNLFunction(rlat0) != cprNLFunction(rlat1))
return (-1); // positions crossed a latitude zone, try again later
// Compute ni and the Longitude Index "m"
if (fflag) { // Use odd packet.
int ni = cprNFunction(rlat1, 1);
int m = (int) floor((((lon0 * (cprNLFunction(rlat1) - 1)) -
(lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5);
rlon = cprDlonFunction(rlat1, 1, 0) * (cprModInt(m, ni) + lon1 / 131072);
rlat = rlat1;
} else { // Use even packet.
int ni = cprNFunction(rlat0, 0);
int m = (int) floor((((lon0 * (cprNLFunction(rlat0) - 1)) -
(lon1 * cprNLFunction(rlat0))) / 131072) + 0.5);
rlon = cprDlonFunction(rlat0, 0, 0) * (cprModInt(m, ni) + lon0 / 131072);
rlat = rlat0;
}
// Renormalize to -180 .. +180
rlon -= floor((rlon + 180) / 360) * 360;
*out_lat = rlat;
*out_lon = rlon;
return 0;
}
int decodeCPRsurface(double reflat, double reflon,
int even_cprlat, int even_cprlon,
int odd_cprlat, int odd_cprlon,
int fflag,
double *out_lat, double *out_lon) {
double AirDlat0 = 90.0 / 60.0;
double AirDlat1 = 90.0 / 59.0;
double lat0 = even_cprlat;
double lat1 = odd_cprlat;
double lon0 = even_cprlon;
double lon1 = odd_cprlon;
double rlon, rlat;
// Compute the Latitude Index "j"
int j = (int) floor(((59 * lat0 - 60 * lat1) / 131072) + 0.5);
double rlat0 = AirDlat0 * (cprModInt(j, 60) + lat0 / 131072);
double rlat1 = AirDlat1 * (cprModInt(j, 59) + lat1 / 131072);
// Pick the quadrant that's closest to the reference location -
// this is not necessarily the same quadrant that contains the
// reference location.
//
// There are also only two valid quadrants: -90..0 and 0..90;
// no correct message would try to encoding a latitude in the
// ranges -180..-90 and 90..180.
//
// If the computed latitude is more than 45 degrees north of
// the reference latitude (using the northern hemisphere
// solution), then the southern hemisphere solution will be
// closer to the refernce latitude.
//
// e.g. reflat=0, rlat=44, use rlat=44
// reflat=0, rlat=46, use rlat=46-90 = -44
// reflat=40, rlat=84, use rlat=84
// reflat=40, rlat=86, use rlat=86-90 = -4
// reflat=-40, rlat=4, use rlat=4
// reflat=-40, rlat=6, use rlat=6-90 = -84
// As a special case, -90, 0 and +90 all encode to zero, so
// there's a little extra work to do there.
if (rlat0 == 0) {
if (reflat < -45)
rlat0 = -90;
else if (reflat > 45)
rlat0 = 90;
} else if ((rlat0 - reflat) > 45) {
rlat0 -= 90;
}
if (rlat1 == 0) {
if (reflat < -45)
rlat1 = -90;
else if (reflat > 45)
rlat1 = 90;
} else if ((rlat1 - reflat) > 45) {
rlat1 -= 90;
}
// Check to see that the latitude is in range: -90 .. +90
if (rlat0 < -90 || rlat0 > 90 || rlat1 < -90 || rlat1 > 90)
return (-2); // bad data
// Check that both are in the same latitude zone, or abort.
if (cprNLFunction(rlat0) != cprNLFunction(rlat1))
return (-1); // positions crossed a latitude zone, try again later
// Compute ni and the Longitude Index "m"
if (fflag) { // Use odd packet.
int ni = cprNFunction(rlat1, 1);
int m = (int) floor((((lon0 * (cprNLFunction(rlat1) - 1)) -
(lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5);
rlon = cprDlonFunction(rlat1, 1, 1) * (cprModInt(m, ni) + lon1 / 131072);
rlat = rlat1;
} else { // Use even packet.
int ni = cprNFunction(rlat0, 0);
int m = (int) floor((((lon0 * (cprNLFunction(rlat0) - 1)) -
(lon1 * cprNLFunction(rlat0))) / 131072) + 0.5);
rlon = cprDlonFunction(rlat0, 0, 1) * (cprModInt(m, ni) + lon0 / 131072);
rlat = rlat0;
}
// Pick the quadrant that's closest to the reference location -
// this is not necessarily the same quadrant that contains the
// reference location. Unlike the latitude case, all four
// quadrants are valid.
// if reflon is more than 45 degrees away, move some multiple of 90 degrees towards it
rlon += floor((reflon - rlon + 45) / 90) * 90; // this might move us outside (-180..+180), we fix this below
// Renormalize to -180 .. +180
rlon -= floor((rlon + 180) / 360) * 360;
*out_lat = rlat;
*out_lon = rlon;
return 0;
}
//
//=========================================================================
//
// This algorithm comes from:
// 1090-WP29-07-Draft_CPR101 (which also defines decodeCPR() )
//
// Despite what the earlier comment here said, we should *not* be using trunc().
// See Figure 5-5 / 5-6 and note that floor is applied to (0.5 + fRP - fEP), not
// directly to (fRP - fEP). Eq 38 is correct.
//
int decodeCPRrelative(double reflat, double reflon,
int cprlat, int cprlon,
int fflag, int surface,
double *out_lat, double *out_lon) {
double AirDlat;
double AirDlon;
double fractional_lat = cprlat / 131072.0;
double fractional_lon = cprlon / 131072.0;
double rlon, rlat;
int j, m;
AirDlat = (surface ? 90.0 : 360.0) / (fflag ? 59.0 : 60.0);
// Compute the Latitude Index "j"
j = (int) (floor(reflat / AirDlat) +
floor(0.5 + cprModDouble(reflat, AirDlat) / AirDlat - fractional_lat));
rlat = AirDlat * (j + fractional_lat);
if (rlat >= 270) rlat -= 360;
// Check to see that the latitude is in range: -90 .. +90
if (rlat < -90 || rlat > 90) {
return (-1); // Time to give up - Latitude error
}
// Check to see that answer is reasonable - ie no more than 1/2 cell away
if (fabs(rlat - reflat) > (AirDlat / 2)) {
return (-1); // Time to give up - Latitude error
}
// Compute the Longitude Index "m"
AirDlon = cprDlonFunction(rlat, fflag, surface);
m = (int) (floor(reflon / AirDlon) +
floor(0.5 + cprModDouble(reflon, AirDlon) / AirDlon - fractional_lon));
rlon = AirDlon * (m + fractional_lon);
if (rlon > 180) rlon -= 360;
// Check to see that answer is reasonable - ie no more than 1/2 cell away
if (fabs(rlon - reflon) > (AirDlon / 2))
return (-1); // Time to give up - Longitude error
*out_lat = rlat;
*out_lon = rlon;
return (0);
}