Generating the WAAS Long-Term Corrections

Stephen Peck, Juan Ceva, Darrell Hensel, Paul Young, Robert Fries and Rajendra Malla

Abstract:	Hughes Aircraft is currently designing the algo- rithms of the FAA’s Wide-Area Augmentation System (WAAS). This paper presents the particulars of the generation and validation of the long-term correction message. The Message Type 25 shall provide the WAAS’ users the slow varying satellite ephemeris and clock errors with respect to the broadcast values. Two types of Message Type 25 exist: one in which only the ephemeris position and clock offset bias are sent (veloc- ity code set to zero) and a second type in which the po- sition and velocity ephemeris errors together with the clock bias offset and frequency offsets are sent (veloc- ity code set to one). The first kind of Message Type 25 can accommodate four satellites per message whereas the second type can only accommodate two satellites, resulting in a bandwidth penalty. The current algorithm obtains a precise estimate (~1 meter 3-D RMS) of the GPS orbits and satellite clock biases (~1 nanosecond RMS) and compares these or- bits and clocks with the broadcast orbits and clock biases. The position and velocity errors are computed for the orbits and clocks in such a way that the use of velocity and frequency (clock first derivative) is mini- mized. The paper describes the design goals of the algorithm and describes how slow corrections are computed. In particular the clock correction algorithm is designed to meet the following four criteria: 1. Maintain the long-term clock correction such that the difference between the long-term clock correc- tion and the actual clock solution remains within the limit of the fast-clock correction bandwidth. Additionally, the long-term correction must re- main within its own bandwidth. 2. Keep the long-term clock solution continuous. 3. Avoid large frequency jumps in long-term clock solutions. 4. Maximize use of the velocity-code-set-to-zero message. The results show that velocity-set-to-one messages will be sent 0.8% of the time and a “don’t use” message will have to be sent 0.2%. These low percentages indicate that the WAAS message structure has been designed properly in order to minimize the impact on user per- formance.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	661 - 671
Cite this article:	Peck, Stephen, Ceva, Juan, Hensel, Darrell, Young, Paul, Fries, Robert, Malla, Rajendra, "Generating the WAAS Long-Term Corrections," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 661-671.
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