Complex sentence: The first radar images taken from the Earth showed very bright (radar-reflective) highlands christened Alpha Regio, Beta Regio, and Maxwell Montes; improvements in radar techniques later achieved an image resolution of 1--2 kilometres.
Split into simpler sentences: The first radar images taken from the Earth showed very bright (radar-reflective) highlands, which were christened Alpha Regio, Beta Regio, and Maxwell Montes. Improvements in radar techniques later resulted in an image resolution of 1--2 kilometres.