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Use floor() not trunc() in relative CPR decoding.

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The comment saying to use trunc() is wrong; the original algorithm
is correct.
This commit is contained in:
Oliver Jowett 2015-01-12 16:28:10 +00:00
parent bebc00c257
commit 583984af8a
1 changed files with 5 additions and 9 deletions
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14
mode_s.c
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@ -2585,13 +2585,9 @@ int decodeCPR(struct aircraft *a, int fflag, int surface) {
// This algorithm comes from:
// 1090-WP29-07-Draft_CPR101 (which also defines decodeCPR() )
//
// There is an error in this document related to CPR relative decode.
// Should use trunc() rather than the floor() function in Eq 38 and related for deltaZI.
// floor() returns integer less than argument
// trunc() returns integer closer to zero than argument.
// Note: text of document describes trunc() functionality for deltaZI calculation
// but the formulae use floor().
//
// 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. and we should use floor().
int decodeCPRrelative(struct aircraft *a, int fflag, int surface) {
double AirDlat;
double AirDlon;
@ -2623,7 +2619,7 @@ int decodeCPRrelative(struct aircraft *a, int fflag, int surface) {
// Compute the Latitude Index "j"
j = (int) (floor(latr/AirDlat) +
trunc(0.5 + cprModDouble(latr, AirDlat)/AirDlat - lat/131072));
floor(0.5 + cprModDouble(latr, AirDlat)/AirDlat - lat/131072));
rlat = AirDlat * (j + lat/131072);
if (rlat >= 270) rlat -= 360;
@ -2642,7 +2638,7 @@ int decodeCPRrelative(struct aircraft *a, int fflag, int surface) {
// Compute the Longitude Index "m"
AirDlon = cprDlonFunction(rlat, fflag, surface);
m = (int) (floor(lonr/AirDlon) +
trunc(0.5 + cprModDouble(lonr, AirDlon)/AirDlon - lon/131072));
floor(0.5 + cprModDouble(lonr, AirDlon)/AirDlon - lon/131072));
rlon = AirDlon * (m + lon/131072);
if (rlon > 180) rlon -= 360;