diff --git a/src/sgpsdp/sgp4sdp4.h b/src/sgpsdp/sgp4sdp4.h
index 58c36131..389d40d7 100644
--- a/src/sgpsdp/sgp4sdp4.h
+++ b/src/sgpsdp/sgp4sdp4.h
@@ -295,6 +295,7 @@ void            ClearFlag(int flag);
 /* sgp_in.c */
 int             Checksum_Good(char *tle_set);
 int             Good_Elements(char *tle_set);
+int             Convert_Sat_Catnr(const char *catstr);
 void            Convert_Satellite_Data(char *tle_set, tle_t * tle);
 int             Get_Next_Tle_Set(char lines[3][80], tle_t * tle);
 void            select_ephemeris(sat_t * sat);
diff --git a/src/sgpsdp/sgp_in.c b/src/sgpsdp/sgp_in.c
index a794312a..716621aa 100644
--- a/src/sgpsdp/sgp_in.c
+++ b/src/sgpsdp/sgp_in.c
@@ -146,6 +146,41 @@ int Good_Elements(char *tle_set)
     return (1);
 }
 
+/* Decode a 5-character NORAD catalog field.
+   All-digit fields (leading spaces allowed) keep the historic
+   atoi() behavior. IDs >= 100000 use Alpha-5; I and O are
+   skipped as first characters so they are not confused with 1 and 0. */
+int Convert_Sat_Catnr(const char *catstr)
+{
+    char            first;
+    int             base, i;
+
+    if (catstr == NULL)
+        return 0;
+
+    first = catstr[0];
+    if (g_ascii_isdigit(first) || (first == ' '))
+        return atoi(catstr);
+
+    first = g_ascii_toupper(first);
+    if ((first >= 'A') && (first <= 'H'))
+        base = 10 + (first - 'A');
+    else if ((first >= 'J') && (first <= 'N'))
+        base = 18 + (first - 'J');
+    else if ((first >= 'P') && (first <= 'Z'))
+        base = 23 + (first - 'P');
+    else
+        return 0;
+
+    for (i = 1; i < 5; i++)
+    {
+        if (!g_ascii_isdigit(catstr[i]))
+            return 0;
+    }
+
+    return (base * 10000 + atoi(&catstr[1]));
+}
+
 /* Converts the strings in a raw two-line element set  */
 /* to their intended numerical values. No processing   */
 /* of these values is done, e.g. from deg to rads etc. */
@@ -157,7 +192,7 @@ void Convert_Satellite_Data(char *tle_set, tle_t * tle)
     /* Satellite's catalogue number */
     strncpy(buff, &tle_set[2], 5);
     buff[5] = '\0';
-    tle->catnr = atoi(buff);
+    tle->catnr = Convert_Sat_Catnr(buff);
 
     /* International Designator for satellite */
     strncpy(tle->idesg, &tle_set[9], 8);
diff --git a/src/sgpsdp/test-001.c b/src/sgpsdp/test-001.c
index 30440859..12f2cabb 100644
--- a/src/sgpsdp/test-001.c
+++ b/src/sgpsdp/test-001.c
@@ -176,6 +176,42 @@ int main(void)
         }
     }
 
+    /* Alpha-5 catalog numbers (IDs >= 100000). */
+    {
+        char            a5_str[3][80];
+        tle_t           a5_tle;
+
+        if (Convert_Sat_Catnr("25544") != 25544)
+        {
+            printf("Convert_Sat_Catnr(25544) returned %d\n",
+                   Convert_Sat_Catnr("25544"));
+            return 1;
+        }
+        if (Convert_Sat_Catnr("a1286") != 101286)
+        {
+            printf("Convert_Sat_Catnr(a1286) returned %d (expected 101286)\n",
+                   Convert_Sat_Catnr("a1286"));
+            return 1;
+        }
+
+        strcpy(a5_str[0], "OBJECT A1286");
+        strcpy(a5_str[1],
+               "1 A1286U 26249D   26310.97532605  .00001678  00000-0  14252-3 0  9998");
+        strcpy(a5_str[2],
+               "2 A1286  45.2801 232.2047 0090652 210.3603 149.2004 14.99515474   207");
+        if (Get_Next_Tle_Set(a5_str, &a5_tle) != 1)
+        {
+            printf("Get_Next_Tle_Set() rejected an Alpha-5 catalog TLE\n");
+            return 1;
+        }
+        if (a5_tle.catnr != 101286)
+        {
+            printf("Alpha-5 catalog A1286 read as %d (expected 101286)\n",
+                   a5_tle.catnr);
+            return 1;
+        }
+    }
+
     select_ephemeris(&sat);
 
     printf("\nDEEP_SPACE_EPHEM: %d (expected 0)\n\n",
diff --git a/src/tle-update.c b/src/tle-update.c
index b725a815..69c0e16a 100644
--- a/src/tle-update.c
+++ b/src/tle-update.c
@@ -998,7 +998,7 @@ static gint read_fresh_tle(const gchar * dir, const gchar * fnam,
                 catstr[i - 2] = tle_str[1][i];
             }
             catstr[5] = '\0';
-            catnr = (guint) g_ascii_strtod(catstr, NULL);
+            catnr = (guint) Convert_Sat_Catnr(catstr);
 
 
             if (Get_Next_Tle_Set(tle_str, &tle) != 1)