Firstly, a constraint is a limitation, within which the system must operate.
Software design, code writing and code testing are sequence of activities in the network which means that you can not logically perform the other activity until the first is completed. This gives a technical constraint on the part of the software developer.
You need a radio transmitter that transmits some kind of radio wave signal, and an array of satelites to compare the postion to other parts of the world. Hope this helps!
