The "Ruins and Reverberations" quest in Arc Raiders involves a quick visit to the Cielo Sereno Observatory, which is one of the new locations on the Pendola Pass map, introduced to the game with the "Frozen Trail" update. There are just three steps in this quest, and if you know where to go you can do it in a flash, but it's easy to get lost. Here's a full walkthrough on how to complete the "Ruins and Reverberations" quest in Arc Raiders, including how to get a Spotter Relay and where to find the antenna and the crawlspace under the control room.
There are three quest steps for "Ruins and Reverberations," and they all have to be completed in one round: First up, you need to get your hands on a Spotter Relay. Spotters, not to be confused with Snitches, are small, flying Arc enemies that always come in pairs. Two of them always deploy alongside a Bastion or Bombardier, and they do exactly what you'd expect: spot raiders.
Bastions and Bombardiers aren't often seen on Pendola Pass, so your best bet is to go to Buried City or Blue Gate. Take down a Spotter, loot it for the Spotter Relay, then bring it to Pendola Pass with you. The Cielo Sereno Observatory is located in the northeast area of Pendola Pass.
The antenna itself is on the roof, and you should absolutely bring a grappling hook, snaphook, or zipline with you to do this quest, because you're going to need to get onto the roof of the observatory twice. Having one of these mobility tools on you will make this task considerably easier. Firstly, you need to get to the roof to find the antenna.
Use one of the above tools, then head to the southern side of the roof, where there's a section sticking out. The antenna is here, and as long as you have a Spotter Relay in your inventory, you'll be able to interact with it. Next up, however, you need to head inside the observatory and find a crawlspace.
In the middle of the roof, you'll find a skylight you can drop through directly into the control room. Walk out into the corridor and there'll be a panel on the floor that you can drop through. Turn around, so you're facing west, then take the first left turn.
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