A bi-static radar configuration measures the direct time-of-flight of a transmitted RF pulse and is capable of measuring this time-of-flight with a jitter on the order of about one pico-second, or about 0.01 inch of free space distance for an electromagnetic pulse over a range of about one to ten feet....http://www.google.com.tw/patents/US5510800?utm_source=gb-gplus-share專利 US5510800 - Time-of-flight radio location system
