Complex sentence: First, only a tiny radio beacon needed to be carried on the Viking itself, an important feature of the Vanguard Minitrack, "in which the prefix' Mini" applied to the minimum-weight satellite transmitter.
Split into simpler sentences: First, only a tiny radio beacon needs to be carried on the Viking itself. This was to be an important feature of the Vanguard Minitrack, "in which the prefix' Mini" applies to the minimum-weight satellite transmitter.