The Emerald Radar is a game mechanic used by Knuckles the Echidna and Rouge the Bat in Sonic Adventure and its sequel, Sonic Adventure 2. When using this, the user can locate Emerald Shards or other treasures in a level to obtain them in order to complete the said level.
The Emerald Radar appears as three squares each depicted with an Emerald shard (or other treasures depending on the stage) centered in the bottom of the game's HUD. Everytime the player approaches an Emerald Shard, its corresponding Emerald Shard on the Emerald Radar will start glowing at a certain color and a sound will play depending on the range between the user and the Emerald.
The Emerald Radar appeared for the first time in Sonic Adventure where it was only used by Knuckles. The colors of the Emerald Shards in the radar change in accordance to the distance of the shard away from the player's position in the following order of furthest to closest: blue, green, yellow, pink and red. The Radar can also detect multiple shards at the same time and will each have their own.
Sonic Adventure 2
The Emerald Shard reappeared in Sonic Adventure 2 where it was used by Knuckles and Rouge in their levels. In this game though, there are only three colors the Emerald Radar will indicate: (from furthest to closest: green, yellow and red). Unlike its previous appearance, the Emerald Radar only provides an indication for one Emerald Shard at a time.
In the 2P mode of the game, Tikal and Chaos can also use the Emerald Radar although their radars do not share the same extent of detecting Emerald Shards; Tikal's radar is the weakest one while Chaos's one is the strongest among.
In other media
The Emerald Radar also appears in the Sonic the Hedgehog and its spin-offs published by Archie Comics. It is a natural trait of Knuckles, essentially a sixth sense to the echidna which is said that all the best treasure hunters have. It a skill Knuckles gained in the Post-Super Genesis Wave timeline.
Knuckles first used the Emerald Radar to help the Chaotix find a Chaos Emerald needed to undo the Shattered World Crisis. Using his Emerald Radar, Knuckles and the Chaotix traveled from Pumpkin Hill Zone to Aquatic Mine Zone to find the nearby Chaos Emerald before the Hooligans did first. Fortunately, Knuckles and the Chaotix had found a Chaos Emerald, but upon receiving it, they found Chip holding onto it.