diff --git a/src/rtkcmn.c b/src/rtkcmn.c
index da9b34e1a..6c423705d 100644
--- a/src/rtkcmn.c
+++ b/src/rtkcmn.c
@@ -1755,7 +1755,9 @@ static void ast_args(double t, double *f)
 {
     static const double fc[][5]={ /* coefficients for iau 1980 nutation */
         { 134.96340251, 1717915923.2178,  31.8792,  0.051635, -0.00024470},
-        { 357.52910918,  129596581.0481,  -0.5532,  0.000136, -0.00001149},
+        { 357.52910918,  129596581.0481,  -0.5532, /*-*/0.000136, -0.00001149},
+        // some documents say -0.000136, however, the latest says 0.000136. see Eq.(5.36) of IERS 2010.
+        // https://www.iers.org/SharedDocs/Publikationen/EN/IERS/Publications/tn/TechnNote36/tn36_043.pdf#page=25
         {  93.27209062, 1739527262.8478, -12.7512, -0.001037,  0.00000417},
         { 297.85019547, 1602961601.2090,  -6.3706,  0.006593, -0.00003169},
         { 125.04455501,   -6962890.2665,   7.4722,  0.007702, -0.00005939}