[hamradio-commits] [dump1090] 30/389: CPR decoding fixed.

[Matthew Ernisse]

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mernisse-guest pushed a commit to branch master
in repository dump1090.

commit b10c8dab35c5e7a20db7f37ded51c13635ecb580
Author: antirez <antirez at gmail.com>
Date:   Fri Jan 11 11:17:46 2013 +0100

    CPR decoding fixed.
---
 dump1090.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/dump1090.c b/dump1090.c
index b5f4a4a..0d523a7 100644
--- a/dump1090.c
+++ b/dump1090.c
@@ -1400,7 +1400,7 @@ double cprDlonFunction(int i, int isodd) {
  *    seconds.
  */
 void decodeCPR(struct aircraft *a) {
-    const double AirDlat0 = 6;
+    const double AirDlat0 = 360.0 / 60;
     const double AirDlat1 = 360.0 / 59;
     double lat0 = a->even_cprlat;
     double lat1 = a->odd_cprlat;
@@ -1408,10 +1408,13 @@ void decodeCPR(struct aircraft *a) {
     double lon1 = a->odd_cprlon;
 
     /* Compute the Latitude Index "j" */
-    int j = ((59*lat0 - 60*lat1) / 131072) + 0.5;
+    int j = floor(((59*lat0 - 60*lat1) / 131072) + 0.5);
     double rlat0 = AirDlat0 * (cprModFunction(j,60) + lat0 / 131072);
     double rlat1 = AirDlat1 * (cprModFunction(j,59) + lat1 / 131072);
 
+    if (rlat0 >= 90 && rlat0 <= 270) rlat0 -= 360;
+    if (rlat1 >= 90 && rlat1 <= 270) rlat1 -= 360;
+
     /* Check that both are in the same latitude zone, or abort. */
     if (cprNLFunction(rlat0) != cprNLFunction(rlat1)) return;
 
@@ -1419,18 +1422,19 @@ void decodeCPR(struct aircraft *a) {
     if (a->even_cprtime > a->odd_cprtime) {
         /* Use even packet. */
         int ni = cprNFunction(rlat0,0);
-        int m = (((lon0 * (cprNLFunction(rlat0)-1)) -
-                  (lon1 * cprNLFunction(rlat0))) / 131072) + 0.5;
+        int m = floor((((lon0 * (cprNLFunction(rlat0)-1)) -
+                        (lon1 * cprNLFunction(rlat0))) / 131072) + 0.5);
         a->lon = cprDlonFunction(rlat0,0) * (cprModFunction(m,ni)+lon0/131072);
         a->lat = rlat0;
     } else {
         /* Use odd packet. */
         int ni = cprNFunction(rlat1,1);
-        int m = (((lon0 * (cprNLFunction(rlat1)-1)) -
-                  (lon1 * cprNLFunction(rlat1))) / 131072) + 0.5;
+        int m = floor((((lon0 * (cprNLFunction(rlat1)-1)) -
+                        (lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5);
         a->lon = cprDlonFunction(rlat1,1) * (cprModFunction(m,ni)+lon1/131072);
         a->lat = rlat1;
     }
+    if (a->lon > 180) a->lon -= 360;
 }
 
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