INTRODUCTIONBecause GPS signal-in-space integrity monitoring currently is inadequate for safety-of-life applications, real-time fault detection and exclusion (FDE) for navigation systems involving GPS is of considerable importance, especially in achieving primary means of navigation. Since the current assumptions about the GPS constellation and its measurement accuracy do not provide high enough availability of FDE to meet the primary-means objectives for all phases of flight, integration of GPS with inertial sensors for integrity monitoring has received considerable attention [1 -3]. GPS integrity algorithms can be classified into two broad categories based on the method employed for computing the navigation solution -snapshot approaches and filtered approaches. Snapshot approaches are generally based on least-squares (LS) methods, while filtered approaches generally utilize multiple Kalman filters with different fault hypothesis models.In addition to the snapshot vs. filtered classification, GPS integrity algorithms can be classified into two different categories based on the characteristics of their test statistics for the FDE function -range domain methods and position domain methods. For example, the solution separation algorithms in [1,2,4] are position domain methods, while the measurement residual algorithm in [3] is a range domain method.For snapshot algorithms, range domain methods are equivalent to position domain methods since variables in the range domain can be directly related to variables in the position domain and vice versa. For filtered algorithms, however, the relationship between test statistics in the range domain and their corresponding position errors cannot be clearly defined. This is the case because past measurements and statistics of the described system model also play a role in determining the current navigation solution. Therefore, it is more difficult to develop an analytical algorithm that relates the test statistic in the range domain to the horizontal protection level (HPL) or horizontal exclusion level (HEL) in the position domain.In this paper, we propose a hybrid FDE algorithm in which the fault detection function is based on a normalized solution separation scheme, and the fault exclusion function uses a measurement residual technique. Since the test statistic for the fault detection function is in the position domain, HPL can be analytically derived and is independent of failure types. HPL is also predictable without real measurementsan important operational requirement. In addition, with normalization, the proper probability levels for detection threshold, HPL, and horizontal uncertainty level (HUL) can be obtained. For fault exclusion, the test statistic for the fault exclusion function is in the range domain. Hence, there is no analytical equation for HEL that is independent of failure types, and the availability of the fault exclusion function needs to be determined by off-line covariance simulation. However, since HEL is required only for off-line Vol. 50, No. 3, Fall...