If you're aligned with The Railroad, this repeatable quest can be pursued from time to time by talking to Tinker Tom. He simply wants an atmospheric sensor placed on a high point at a randomly designated spot in The Commonwealth.
One such destination could be the Raider-infested Back Street Apparel shop, so don't be surprised to find a hostile welcoming committee.
Once you're at the highest point of your destination (usually a roof), place the sensor where the waypoint indicates. Some of the locales are more difficult than others, like highest point of the elevated highway 800 feet up. You can always load your most recent save if you prefer a less challenging spot.
Report back to Tinker Tom to complete the quest.