WAAS MOPS: Practical Examples

The WAAS MOPS describes the method by which a Wide Area Augmentation System (WAAS) transmits its differential GPS corrections and integrity information to users in 250 bit messages. These messages must be decoded and interpreted every second. The corrections even for a single satellite are distributed across several individual messages. The corrections for individual satellites must be combined with receiver measurements and other local information to form the navigation solution and protection bounds. The user must reconstruct and apply all of this information correctly to achieve the required integrity level of 10-7. While the rules for coordinating these messages are defined in the Minimum Operational Performance Standards (MOPS), it is still helpful to provide some background, describe the intent, and show some examples. This paper provides examples of what the message order might look like and how the information should be interpreted. Both nominal and alarm situations are presented. Particular attention is paid to situations in which the user has missed some of the broadcast messages, but still has enough information to continue the procedure. Better understanding the intent of the authors will further assist the user implementation of the MOPS. The MOPS is a large and fairly involved document. To new users in the process of implementing this standard, its rules may seem arbitrary. By showing how the different messages interact and work to protect integrity, the new user may more readily understand how to implement the MOPS compliant messages.