Retriever Sentinel

The Strato-Sentinel was a Forerunner Sentinel variant. They were designed to strip mine the resource moon located in the middle of Installation 00, to help in the construction of a Halo ring, such as the newly constructed Installation 04 as seen in the Halo 3 levels The Covenant and Halo, and were also used with the construction of the Portal on Earth, in the present-day country of Kenya.

One of the concepts for the multiplayer level Blackout had the map set in the partially destroyed insides of a Strato-Sentinel orbiting the Ark's strip mining moon.

Trivia

 * Strato-Sentinels were originally planned to appear in Halo 3, but wound up being cut from the game.
 * The word "Strato" in the Sentinel's name is most likely derived from Latin, as "Strato" is the ablative form of "Stratus", meaning "stretched out" or "extended".