Radar (Radio Detection and Ranging) was a surveillance technology that was used to its fullest extent in Second World War, Great World War III, and somewhat in World War IV. As it's de-achronized name implies, radar made use of simple radio and microwave transmissions to reveal key locations to commanders via the EVA network, allowing them to command and guide their forces with precision and efficiency. On top of this, the availability of a radar station granted commanders access to secret communication channels with their respective militaries, opening availability to special technologies and soldier divisions to support their war efforts.
Unfortunately, all variations of radar technology suffered from similar problems. They required an enormous amount of outside data and on-field reporting to provide optimal feedback, as well as needing a considerable amount of energy to function at all. The loss of power to a base always meant the loss of radar feedback, making an army effectively blind to their surroundings. If the power requirements of a base became too taxing for its local power supply, radar would also be lost.
World War II
During the USSR's invasion of Europe, radar technology was used extensively on both sides of the conflict. Both the Allies and the Soviets devoted special divisions of their precursor EVA teams for the sole purpose of recording and reporting radar data, resulting in both sides having ultimately identical equivalents of the same technology. Both versions of radar also had identical energy needs and construction stability, and were referred to as "Radar domes" on both sides of the conflict.
It was in response to the USSR's radar proliferation that the Allies developed the Gap Generators, a device that interfered with the radio and microwave signals of the Russians to the point of making their data illegible. A portable version of the device also existed.
Great World War III
World War III marked a point of divergence when it came to radar technology. While the function of radar remained the same as it did in the previous conflict, the means in which it was advanced differed considerably between Allied and Soviet forces, granting each very different means of surveying their surroundings.
If there was one side during the US invasion that can claim to have perfected the use of radar, it would be the Allies. This is largely due to the United States' devotion to a strong and versatile air-force, an institution so tightly bound to radar Intel that Allied radar facilities were jointly run by EVA operators and the USAF. It was due to this inseparable bond that almost all Allied radar stations also doubled as landing pads for US Harrier jets or Korean Black Eagles, resulting in a hybrid building called the Allied Airforce Command Headquarters. This granted the Allies a considerable advantage when it came to fast and deadly strikes against their foes, as the mere presence of a radar station in conjunction with a War factory would allow Allied commanders to call in decisive and disruptive air-strikes against Soviet forces.
But perhaps the most significant advancement in radar that could be tied to Allied ingenuity was the spy satellite uplink, a device that granted the Allies the ultimate edge in the field of reconnaissance. Through the use of then-experimental orbital satellites, EVA operators would be granted a birds eye view of the entire battlefield, even in places that had not been previously reported on by friendly forces.
While the mere existence of satellite technology may have made the Allies use of Radar seem redundant, the presence of a friendly radar tower was still necessary as a technological middleman for the satellite uplink's data to be transmitted to Allied commanders. This dependency would not be shed until the first tiberium war.
The Allies maintained their use of GAP generators during the US invasion. The device functioned identically to its WWII counterpart, albeit with a slightly wider range of interference and several extra modifications that allowed them to block out the sight of rogue spy satellites.
Conventional Russian radar changed little in the span of time between the second and third world wars. With the notable exception of the mighty Kirov airship, the USSR had little interest in air superiority, believing a powerful ground force to be more practical (and cost-effective) to their battle needs; thus, the advancement of their already reliable recon tech was deemed unnecessary. However, the existence of a radar tower was still considered necessary for a Soviet commander to call in various mid-tier soldiers and vehicles, notably anything that made use of flak. Also, on the off chance that a technician could capture an Allied spy satellite uplink, Soviet forces could quickly modify their radar towers to interpret the satellite data, turning the Allies own advantage against them.
The biggest advancement the Soviet's had in radar was, ironically, not truly "radar" at all, but a form of psychic scanning. Developed by Yuri, the device known as the Psychic Sensor used a complex combination of prototypical computer technology and organic brain chemistry to detect the minds of non-friendly forces, while at the same time relaying their intentions as longitude/latitude data to EVA operators. In this way, any enemy force attacking a Soviet installation would instantly be detected before they even arrived, granting the Soviets a chance to prepare a defense. However, while this strange device did make use of several principles of conventional radar, it could not be considered a true representation of the technology.
Great World War III (Second Iteration)
The restarting of the Third World War via Allied time travel caused some considerable changes in the use of radar for both of the wars original combatants, as well as the introduction of several new forms of radar on the part of Yuri.
The most notable change for Allied radar technology came with the use of Gap generators. Where once these disruptive machines proved effective against Russian surveillance, they proved utterly useless against Yuri's psychic probings, which didn't make use of any electronic wavelength other than those of the human brain. Because of this, several modifications were made to the original generator design, allowing it to "bend" psychic readings around itself, making everything in its protective field as invisible to Yuri as it was to the Russians. Unfortunately the modifications weren't foolproof; potent psychic probing, usually in the form of a "psychic reveal," was enough to temporarily overwhelm the generators systems, making whatever it hid visible for a few precious seconds.
Soviet radar suffered a critical blow thanks to the betrayal and defection of Yuri, who had developed all of the Red Army's psychic technologies. Through various means of sabotage and theft, Yuri was able to effectively strip all of the Soviet Union of its psychic capability, hoarding all of it for himself; among the technologies he stole was the Psychic Sensor.
Facing a severe surveillance disadvantage in the face of two enemies, Russian scientists quickly scrambled for a way to even the odds a little. The result of their collaboration was a simple but effective solution; the spy plane. While hardly as effective as a Spy Satellite or as advanced as a psychic radar, the spy plane was advantageous in that it could be deployed quickly, cheaply, and didn't require any additional consumption of power to function. The plane could also be deployed much earlier then either of its enemy counterparts, granting the Russians a chance to catch their enemies off guard.
While the spy plane didn't require energy to work, it still depended on a radar station to receive it's findings and interpret them. Thus, if a Soviet base had it's radar station destroyed, a new spy plane couldn't be launched for fear of wasting precious fuel.
Yuri's army made use of the psychic sensor as their primary surveillance method. The sensor functioned much the same way as it had with the Russians, but now it possessed several new features that Yuri had "conveniently" forgotten to incorporate in the original blueprints. This new incarnation, called the psychic radar, had the added ability to scan selected locations on the battlefield with a bombardment of psychic energy, a process referred to as a "Psychic Reveal." Outside of this, Yuri's army did not use anything resembling normal radar.
War of the Three Powers
Radar had largely been phased out by the War of the Three Powers. It is unknown what sort of development history the technology had in the new timeline created by Anatoly Cherdenko's erasure of Albert Einstein. By the point of the USSR's collapse in the original timeline, radar had already been largely replaced by satellite recon, with conventional radar demoted to local use in airfields. This did not change with the removal of Einstein, although, puzzlingly, his Gap generator still existed as a component to the new Mirage Tanks. Unlike the original GAP generator though, this version was designed to fend off satellite imaging, not radio waves.
One of the few uses of standard radar that survived to see use in the war came in the form of the Imperial Radar boat, which was essentially a floating radar platform. The destruction of these vessels was crucial to the joint Allied/Soviet victory over many an Imperial Floating Fortress.