diff --git a/mlat/client/coordinator.py b/mlat/client/coordinator.py
index c5c3a13..b6ccc9a 100644
--- a/mlat/client/coordinator.py
+++ b/mlat/client/coordinator.py
@@ -413,34 +413,33 @@ def received_df17(self, message, now):
             # not a position message
             return
 
-        ac.last_position_time = now
+        if message.even_cpr:
+            ac.even_message = message
+        else:
+            ac.odd_message = message
+
+        if not ac.even_message or not ac.odd_message:
+            return
+        if abs(ac.even_message.timestamp - ac.odd_message.timestamp) > 5 * self.freq:
+            return
 
         if message.altitude is None:
             return    # need an altitude
+
+        ac.last_position_time = now
+
         if message.nuc < 6:
             return    # need NUCp >= 6
 
         ac.recent_adsb_positions += 1
 
-        if self.server.send_split_sync:
-            if not ac.requested:
-                return
+        if not ac.requested:
+            return
 
+        if self.server.send_split_sync:
             # this is a useful reference message
             self.server.send_split_sync(message)
         else:
-            if message.even_cpr:
-                ac.even_message = message
-            else:
-                ac.odd_message = message
-
-            if not ac.requested:
-                return
-            if not ac.even_message or not ac.odd_message:
-                return
-            if abs(ac.even_message.timestamp - ac.odd_message.timestamp) > 5 * self.freq:
-                return
-
             # this is a useful reference message pair
             self.server.send_sync(ac.even_message, ac.odd_message)
 
diff --git a/mlat/client/version.py b/mlat/client/version.py
index ce4758b..022aade 100644
--- a/mlat/client/version.py
+++ b/mlat/client/version.py
@@ -18,4 +18,4 @@
 
 """Just a version constant!"""
 
-CLIENT_VERSION = "0.2.14"
+CLIENT_VERSION = "0.2.15"
diff --git a/modes_message.c b/modes_message.c
index 724fab8..29af1c8 100644
--- a/modes_message.c
+++ b/modes_message.c
@@ -456,6 +456,12 @@ static int decode(modesmessage *self)
 
                 if (! (self->altitude = decode_ac12((self->data[5] << 4) | ((self->data[6] & 0xF0) >> 4))))
                     return -1;
+
+                // crude check if there is any CPR data, if either cpr_lat or cpr_lon is mostly zeros, set invalid
+                if ((self->data[7] == 0 && (self->data[8] & 0x7F) == 0) || (self->data[9] == 0 && self->data[10] == 0)) {
+                    self->valid = 0;
+                    //fprintf(stderr, "%02x %02x %02x %02x\n", self->data[7], self->data[8], self->data[9], self->data[10]);
+                }
             }
         }
         break;