Why do you need more than 3 values to determine the rate of change? If the time between echo one and echo two is n, and the time between echo two and echo three is m, then the relationship between n and m gives you all the information that you need. If m is less than n, something is approaching. If m is greater than n, something is retreating. If m is slightly smaller than n, whatever it is that is approaching is doing so slowly. If m if significantly smaller than n, then whatever is approaching is doing so quickly.