Radar is Apple's internal bug reporting and tracking system. iOS URL schemes are of the form "rdar://problem/123456".
There are two different methods to get Radar; the first one installs the RadarWeb webapp. Start Safari, navigate to "rdar://" (without quotes), then hit the bookmark button and "Add to Home Screen". Wait a few seconds for the favicon to appear in Springboard. Another way is to download the app from Switchboard, which is for iPad only. Internal firmwares also include a version of Radar.
- 2.1.3 (Switchboard)