fluttar was a bistatic radar design that used the doppler effect to filter out slow-moving objects, anything below . this feature was developed in response to the performance of the mid-canada line (mcl), which lacked doppler filtering and was rendered useless during bird migrations when the echoes from birds near the antennas swamped the returns from aircraft. fluttar was otherwise similar to the mcl, and like it, lacked the ability to measure the location of a target, only providing an indication that one was somewhere between its two antennas. having multiple doppler filters in the receiver, the approximate velocity and direction (north or south) could be determined.