diff --git a/src/sgpsdp/sgp_in.c b/src/sgpsdp/sgp_in.c
index 973462eb..a794312a 100644
--- a/src/sgpsdp/sgp_in.c
+++ b/src/sgpsdp/sgp_in.c
@@ -76,6 +76,46 @@ int Checksum_Good(char *tle_set)
     return (checksum == check_digit);
 }
 
+/* Checks that the epoch field in columns 19-32 of line 1 (YYDDD.FFFFFFFF)
+   holds a number that Convert_Satellite_Data() can read in full: digits,
+   with spaces allowed only where they are padding, i.e. in place of the
+   leading zeros of the day of year and after the last fraction digit.
+   Checksum_Good() counts a letter as 0, so a letter that replaced a 0
+   would otherwise pass and the epoch would be read up to the letter. */
+static int Epoch_Good(char *tle_set)
+{
+    int             i;
+
+    /* year */
+    if (!g_ascii_isdigit(tle_set[18]) || !g_ascii_isdigit(tle_set[19]))
+        return (0);
+
+    /* day of year; the two leading digits may be padded with spaces */
+    for (i = 20; i < 22; i++)
+        if (!g_ascii_isdigit(tle_set[i]) && (tle_set[i] != ' '))
+            return (0);
+    if ((tle_set[20] != ' ') && (tle_set[21] == ' '))
+        return (0);
+    if (!g_ascii_isdigit(tle_set[22]))
+        return (0);
+
+    /* fraction of day; at least one digit, trailing spaces allowed */
+    if (!g_ascii_isdigit(tle_set[24]))
+        return (0);
+    for (i = 25; i < 32; i++)
+    {
+        if (tle_set[i] == ' ')
+            break;
+        if (!g_ascii_isdigit(tle_set[i]))
+            return (0);
+    }
+    for (; i < 32; i++)
+        if (tle_set[i] != ' ')
+            return (0);
+
+    return (1);
+}
+
 /* Carries out various checks on a TLE set to verify its validity */
 /* tle_set is a character string holding the two lines read    */
 /* from a text file containing NASA format Keplerian elements. */
@@ -99,6 +139,9 @@ int Good_Elements(char *tle_set)
         (tle_set[115] != '.') ||
         (tle_set[123] != '.') || (strncmp(&tle_set[61], " 0 ", 3) != 0))
         return (0);
+    /* Check that the epoch is a well-formed number */
+    if (!Epoch_Good(tle_set))
+        return (0);
 
     return (1);
 }
diff --git a/src/sgpsdp/test-001.c b/src/sgpsdp/test-001.c
index 1a3287ff..30440859 100644
--- a/src/sgpsdp/test-001.c
+++ b/src/sgpsdp/test-001.c
@@ -67,6 +67,22 @@ const dataset_t expected[TEST_STEPS] = {
 };
 
 
+/* check digit of a 69 character TLE line, as computed by Checksum_Good() */
+static char line_checksum(const char *line)
+{
+    int             i, sum = 0;
+
+    for (i = 0; i < 68; i++)
+    {
+        if ((line[i] >= '0') && (line[i] <= '9'))
+            sum += line[i] - '0';
+        else if (line[i] == '-')
+            sum += 1;
+    }
+
+    return (char)('0' + sum % 10);
+}
+
 char            tle_str[3][80];
 sat_t           sat;
 
@@ -127,6 +143,39 @@ int main(void)
         return 1;
     }
 
+    /* a letter in the epoch field must be rejected, even when it
+       replaced a 0 and the checksum still holds */
+    {
+        char            tle_set[139];
+
+        memcpy(tle_set, tle_str[1], 69);
+        memcpy(&tle_set[69], tle_str[2], 69);
+        tle_set[138] = '\0';
+
+        if (!Good_Elements(tle_set))
+        {
+            printf("Good_Elements() rejected the unmodified TLE\n");
+            return 1;
+        }
+
+        tle_set[27] = 'O';      /* epoch 80275.98708465 -> 80275.987O8465 */
+        if (Good_Elements(tle_set))
+        {
+            printf("Good_Elements() accepted a letter in the epoch\n");
+            return 1;
+        }
+
+        tle_set[27] = '0';
+        tle_set[20] = ' ';      /* day of year 5, padded with spaces */
+        tle_set[21] = ' ';
+        tle_set[68] = line_checksum(tle_set);
+        if (!Good_Elements(tle_set))
+        {
+            printf("Good_Elements() rejected a space-padded day of year\n");
+            return 1;
+        }
+    }
+
     select_ephemeris(&sat);
 
     printf("\nDEEP_SPACE_EPHEM: %d (expected 0)\n\n",