diff --git a/demod/mod/meisei100mod.c b/demod/mod/meisei100mod.c
index 14deaff..7409576 100644
--- a/demod/mod/meisei100mod.c
+++ b/demod/mod/meisei100mod.c
@@ -395,6 +395,7 @@ int main(int argc, char **argv) {
         rs_init_BCH64(&gpx.RS);
     }
 
+    gpx.sn = -1;
 
     bitofs += shift;
 
@@ -661,8 +662,12 @@ int main(int argc, char **argv) {
                                 printf("\n");
 
                                 if (option_jsn && err_frm==0) {
-                                    printf("{ \"frame\": %d, \"id\": \"IMS100-%.0f\", \"datetime\": \"%04d-%02d-%02dT%02d:%02d:%06.3fZ\", \"lat\": %.5f, \"lon\": %.5f, \"alt\": %.5f, \"vel_h\": %.5f, \"heading\": %.5f }\n",
-                                           gpx.frnr, gpx.sn, gpx.jahr, gpx.monat, gpx.tag, gpx.std, gpx.min, gpx.sek, gpx.lat, gpx.lon, gpx.alt, gpx.vH, gpx.vD );
+                                    char id_str[] = "xxxxxx\0\0\0\0\0\0";
+                                    if (gpx.sn > 0 && gpx.sn < 1e9) {
+                                        sprintf(id_str, "%.0f", gpx.sn);
+                                    }
+                                    printf("{ \"frame\": %d, \"id\": \"IMS100-%s\", \"datetime\": \"%04d-%02d-%02dT%02d:%02d:%06.3fZ\", \"lat\": %.5f, \"lon\": %.5f, \"alt\": %.5f, \"vel_h\": %.5f, \"heading\": %.5f }\n",
+                                           gpx.frnr, id_str, gpx.jahr, gpx.monat, gpx.tag, gpx.std, gpx.min, gpx.sek, gpx.lat, gpx.lon, gpx.alt, gpx.vH, gpx.vD );
                                     printf("\n");
                                 }
 
diff --git a/meisei/meisei_ims.c b/meisei/meisei_ims.c
index 7c764c0..259a2d6 100644
--- a/meisei/meisei_ims.c
+++ b/meisei/meisei_ims.c
@@ -543,6 +543,7 @@ int main(int argc, char **argv) {
         rs_init_BCH64();
     }
 
+    gpx.sn = -1;
 
     bufpos = 0;
     bit_count = 0;
@@ -832,8 +833,12 @@ int main(int argc, char **argv) {
                                     printf("\n");
 
                                     if (option_jsn && err_frm==0) {
-                                        printf("{ \"frame\": %d, \"id\": \"IMS100-%.0f\", \"datetime\": \"%04d-%02d-%02dT%02d:%02d:%06.3fZ\", \"lat\": %.5f, \"lon\": %.5f, \"alt\": %.5f, \"vel_h\": %.5f, \"heading\": %.5f }\n",
-                                               gpx.frnr, gpx.sn, gpx.jahr, gpx.monat, gpx.tag, gpx.std, gpx.min, gpx.sek, gpx.lat, gpx.lon, gpx.alt, gpx.vH, gpx.vD );
+                                        char id_str[] = "xxxxxx\0\0\0\0\0\0";
+                                        if (gpx.sn > 0 && gpx.sn < 1e9) {
+                                            sprintf(id_str, "%.0f", gpx.sn);
+                                        }
+                                        printf("{ \"frame\": %d, \"id\": \"IMS100-%s\", \"datetime\": \"%04d-%02d-%02dT%02d:%02d:%06.3fZ\", \"lat\": %.5f, \"lon\": %.5f, \"alt\": %.5f, \"vel_h\": %.5f, \"heading\": %.5f }\n",
+                                               gpx.frnr, id_str, gpx.jahr, gpx.monat, gpx.tag, gpx.std, gpx.min, gpx.sek, gpx.lat, gpx.lon, gpx.alt, gpx.vH, gpx.vD );
                                         printf("\n");
                                     }