From b5454ddaf6bb16278e994594b8c239dbbf0b808d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Est=C3=A9vez?= <daniel@destevez.net>
Date: Fri, 6 Sep 2019 17:15:26 +0200
Subject: [PATCH] Fix eph2clk() and geph2clk() iterative solving

These functions solve the equation
t = t0 - dT(t)
by iteration, where
t0 = time - eph->toc
and then return
dT(t).

The correct iteration to solve this equation is
t_{k+1} = t_0 - dT(t_k).
Formerly, the iteration was written as
t_{k+1} = t_k - dT(t_k),
which is not correct.
---
 src/ephemeris.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/ephemeris.c b/src/ephemeris.c
index a1c57a6f3..6bc972467 100644
--- a/src/ephemeris.c
+++ b/src/ephemeris.c
@@ -178,15 +178,16 @@ extern void alm2pos(gtime_t time, const alm_t *alm, double *rs, double *dts)
 *-----------------------------------------------------------------------------*/
 extern double eph2clk(gtime_t time, const eph_t *eph)
 {
-    double t;
+    double t,t0;
     int i;
     
     trace(4,"eph2clk : time=%s sat=%2d\n",time_str(time,3),eph->sat);
     
-    t=timediff(time,eph->toc);
+    t0=timediff(time,eph->toc);
+    t=t0;
     
     for (i=0;i<2;i++) {
-        t-=eph->f0+eph->f1*t+eph->f2*t*t;
+        t=t0-(eph->f0+eph->f1*t+eph->f2*t*t);
     }
     return eph->f0+eph->f1*t+eph->f2*t*t;
 }
@@ -312,15 +313,16 @@ static void glorbit(double t, double *x, const double *acc)
 *-----------------------------------------------------------------------------*/
 extern double geph2clk(gtime_t time, const geph_t *geph)
 {
-    double t;
+    double t,t0;
     int i;
     
     trace(4,"geph2clk: time=%s sat=%2d\n",time_str(time,3),geph->sat);
     
-    t=timediff(time,geph->toe);
+    t0=timediff(time,geph->toe);
+    t=t0;
     
     for (i=0;i<2;i++) {
-        t-=-geph->taun+geph->gamn*t;
+        t=t0-(-geph->taun+geph->gamn*t);
     }
     return -geph->taun+geph->gamn*t;
 }