There are several problems with measuring tennis serve speed with a hand speed radar gun: direction of the serve is off with a direction of the radar gun introduces cosine error factor (lowers the actual speed); initial distance of ball speed capture point is closer than the server position (lowers actual speed as well). This is why Association of Tennis Professionals (ATP) does not recognize tennis serve measurements recorded in any event other than ATP level 250 and higher because these top level tournaments are equipped with systems costing thousands of dollars. But if you know how to lower the error margin with radar gun you can do it (but it’s not as inexpensive and reliable as “Tennis Serve Tracker”) and below are the videos of real time side by side comparison between speed radar gun and the app - see for yourself!