A New Carrier-Phase Multipath Observable for GPS Real-Time Kinematics, Based on Between Receiver Dynamics

In GPS differential carrier-phase measure ments, multipath is still present and remains as the major contribution to the error budget, especially the low-frequency, shortdelay multipath (specular). There are various approaches to mitigate this effect. The most notable is to build a multipath profile (also called an effective reflector) and to parameterize the multipath measurements that resemble the geometry between the GPS receiver and the profile. However, problems still remain in this approach since the basic measurements used for the estimators do not entirely correspond to all the physical effects of multipath. SNR measurements are used extensively due to their high correlation with the multipath environment and the receiver-satellite geometry. Due to their low precision, however, estimators converge over a long time span. Thus, they turn out to be an impractical measurement for real-time applications. On the other hand, methods to derive and separate the multipath that filter or use the carrier phase, not only may mask out the platform dynamics (especially when higher dynamics are involved), but also may introduce other unexpected biases. These limitations lead us to derive multipath observables for multipath mitigation, merging the synergies from previous approaches. The multipath observables will carry all the multipath spectra with no limitation on the number of multipath signals (including specular reflections and diffraction), and be used as a technique that does not require a priori knowledge of the environment. Since the multipath observables will be derived from the platform dynamics (i.e., GPS carrierphase derived velocity and acceleration estimates that can be designed to be immune to multipath), they can give a good signature of the low-frequency multipath. Our approach for deriving the multipath observables is based on the use of two GPS receivers (a master and a slave) connected to the same clock to remove the common satellite and receiver clock biases after single differencing. It is possible to specify the relative dynamics between the antennas of the two receivers through a rotation motion, like that provided by a momentum wheel with a lever arm with a pre-chosen rotation speed, combined with the platform specific dynamics. This has two main advantages:

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