The Union Aerospace Corporation (UAC) is a significant military and space travel corporation of the future.
During Hitler's third (or fourth attempt) to use the Spear of Destiny he used it to cast a spell to see into the future, to see how to develop technology from the future. Some of this technology originated from UAC.
During the Mars Incident in 2145-46, over a couple of centuries later, a Marine (B.J. Blazkowicz III) would defeat a Cyberdemon ("Harbinger") created by Dr. Kronos. He went missing after the event, however, but later went on to help fight demons on Phobos, Deimos and later Earth as well.
Stan Blazkowicz later traveled to Tycho base with fellow Marines, to the Moon to follow up on the Mars Incident, where he used UAC technology against the demons and Harbinger Cyberdemon from Hell, while teleporting to other planets of the galaxy.
- Some of the later levels of Mission 3: Ultimate Challenge are marked with the UAC logo. This is a nod to the Doom Universe.
- Stan Blazkowicz and B.J. Blazcowicz (Doom RPG) are decendents of B.J. Blazkowicz and had the most direct interactions with UAC.
- One of the secret levels in Doom II has Nazis from the past in the future, in which the Marine must fight, there is also a dead Billy Blaze (one of the Blazkowicz family members ancestors/descendents). Although these references are likely more of an easter egg, rather than a reference to the events of Mission 3: Ultimate Challenge or Commander Keen.
- One of the level packs for Wolfendoom (The Portal and Astrostein 1-3) a fan mod plays with and extends the idea of Hitler's attempts at time travel, and interfering with the future, and interacting into UAC technology, and with the forces of hell.
- The Activision version of the game contains the UAC logos (on large crate boxes in the UAC base level) and images of the Sun (Doom computer screens first seen in the Phobos Hanger level). The Super CD version does not. These two alternate changes seem to indicate alternate versions of the same game. In the Super CD version, the Sun screen can be seen. These minor edits seem to indicate that both are still connected to the DOOM universe. The Sun graphic itself can be seen in the first level of Doom (Hanger) on video screens in the level.