Warzone 2100

In the late 21st century, the world's civilizations are wiped out by a series of nuclear strikes, caused by a network of satellites created by North America that have the task of protecting the US from any nuclear threat, known as NASDA. While most of the survivors form scavenger bands to survive, one group of people, who refer to themselves as "The Project", seeks to rebuild civilization using pre-war technology.
Warzone 2100 places an emphasis on sensors and radar to detect units and to coordinate ground attacks. Counter-battery sensors detect enemy artillery by sensing their projectiles and firing arcs and pinpointing their location to coordinate artillery strikes against enemy artillery.
There is an emphasis on artillery: although many direct- and close-combat weapons and anti-air weapons can be researched and deployed, artillery is a staple of assault on enemy bases and outposts. While the technology tree is clearly defined and consistent, it never appears in-game and, therefore, the player can be left guessing as to what technology is next in the tree. Technology can be acquired by gathering artifacts left behind by certain destroyed enemy structures or units. Researching is composed of largely small and incremental advancements over existing weapons, armor, and chassis types.

Source: Wikipedia, "Warzone 2100", available under the CC-BY-SA License.