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This application serves as a user-interactive animated visualization of a thought experiment of Hermann Bondi's, on the basis of the k-calculus and using radar to measure distances and times, that goes as follows: consider a rod moving with respect to some observer such that the relative motion is characterized by the above described ratio k. In order to measure the length of this rod, the observer must send out two radar pulses spaced such that they reflect from the two ends of the rod, according to the observer, at the same time. The user of the application can change the speed of the rod relative to the observer, the length of the rod (measured in light-seconds), the time at which the radar pulses reflect from the two ends of the rod, the size of the space-time coordinate chart for the observer, and whether the animation's time represents the proper time of the observer or the rod.
For more explanation and some mathematics, see this webpage.
To download this application (Java required for running), click here, unzip the file, and run the .jar file in the folder created. Note that the .jar file is unsigned (but is safe to run)---look up how to open unsigned .jar files on your OS to run it.