Lookout is a much more famous mountain. During the Civil War they fought the "Battle Above the Clouds" there. When you see all those "See Rock City" signs, that's the place they are talking about. It's the mountain you see in all the pictures of Chattanooga and it has a really distinctive shape. But it's on the south side of town and extends down into Georgia, while Signal is north of the TN River.
I'm not sure because of the angle, but I think in this picture that Signal is the mountain to the far right (the brown patch is the rock wall where the Hwy goes up).Lookout Mountain with Signal Over to the Right
The way the river bends I'm thinking thats either Elder Mtn or Suck Creek Mtn. over to the right.. IMHO Signal is not shown in that picture..JMHO