Suggestions anyone?

I have been monitoring mail traffic for IIR, Laser diode or Ultrasonic suggestions that my be able to solve my problem.

I need to detect the speed of an object 2 - 5 feet from the sensor mount.   

Sensor is mounted upward looking at the sky.  

My thought was to use two ultrasonic sensors to detect a pulse width to determine the time the object was within the cone.  

Range data is secondary if  at all.

Need to be inexpensive - doesn't it all....

Suggestions / contacts are greatly appreciated...

Thanks

Mike
Control System Research