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Sunnyvale, CA | The ionosphere corrections are separate from the other corrections in the WAAS, SF1, SF2,CDGPS, Omnistar VBS, HP, and XP) messages. The WAAS correction method (and those of Omnistar VBS, SF1) systems which have been designed for user receivers that rely on only the GPS L1 signals under, can under some situations, benefit from having a L1-L2 receiver. In that case all of the other corrections form the correction source except the ionosphere corrections are used, and the receiver computes its own ionosphere corrections by measuring the difference in time of arrival between the L1 and L2 signals. There is one problem with this difference measurement at the L1-L2 receiver and that is the L2 signal from most of the GPS satellites is a very weak signal, and it is modulated with the encrypted military data, which makes the difference measurement a bit noisy. Newer GPS satellites are being launched with the L2C signal and these satellites with a stronger, unencrypted signal will provide a better L2 measurement. During times of very high ionosphere activity, the WAAS ionosphere correction can saturate (at an ionosphere correction of greater than 45 meters), and when that happens, the GPS receiver will use the GPS iono model, and users will be very dis-satisfied with the WAAS signal. This is when the L1-L2 WAAS receivers will perform much better than the L1 only WAAS receiver. The high performance satellite correction systems (Omnistar HP, SF2) will use L1-L2 measurements (both code and phase) at the receiver, which overcomes some of the code phase noise of the L1-L2 WAAS measurements. | |
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correction on an earlier post about WAAS
