heliwatch: don't track aircrafts without altitude

heliwatch/src/adsb.rs

@@ -133,6 +133,10 @@ pub fn run(host: &'static str, port: u16, locations: Locations) -> Receiver<Even
                 let hex = format!("{}", icao_address);
                 let entry = entry.read().unwrap();
 
+                if entry.altitude.is_none() {
+                    continue;
+                }
+
                 if entry.altitude.map(|altitude| altitude > MAX_ALTITUDE).unwrap_or(false) ||
                     entry.emitter_category.as_ref().map(|category| IGNORED_CATEGORIES.contains(category)).unwrap_or(false)
                 {