Attitude determination using multiple Global Positioning System (GPS) antennas has been investigated extensively in the past decades and has been demonstrated its capability in several researches. However, it is still challenging to incorporate multiple antennas GPS attitude determination system into real navigation systems. The problem is that bad attitude estimates might result from, for example, signal blockage or ionosphere scintillation for multiple antennas GPS attitude determination. If these bad attitude estimates are used, degradation of navigation system performance may result. Lack of reliability makes GPS attitude determination impractical and dangerous for safety of life applications. One possible solution to this problem is to prevent navigation system from use of the multiple antennas GPS attitude estimates when they are not of sufficient quality or to take proper actions. It is thus crucial for navigation systems to identify the bad attitude estimates or to know a reliable error upper bound. Detection of the bad attitude estimate is possible for least squares estimation because there are generally more measurements than required. Consistency of these redundant measurements can be checked and the goodness of estimate is judged. An important application of this consistency check is Receiver Autonomous Integrity Monitoring (RAIM). In addition to consistency check, RAIM further calculates vertical and horizontal protection levels as confidence bounds on the horizontal and vertical errors. In this paper, a method is proposed to calculate protection levels for the least squares GPS attitude determination. The protection levels will serve the upper bound on the attitude errors. The analytical background of multiple antennas GPS attitude determination and the RAIM are also introduced. Data is collected and the proposed method is applied to validate the effectiveness of the proposed protection level calculation method.