Knowledge Bank

Four possible estimators are investigated for the monitoring of crustal deformations from a combination of repeated baseline length measurements and adopted geophysical models, particularly an absolute motion plate model. The first estimator is an extension of the familiar free adjustment. The next two are Bayesian type estimators, one weak and one strong. Finally, a weighted constraint estimator is presented. The properties of these four estimators are outlined and their physical interpretations discussed. A series of simulations are performed to test the four estimators and to determine whether or not to incorporate a plate model for the monitoring of deformations. It is concluded that it is preferable to adopt even a weak but realistic model than none at all. In this case, the weak Bayesian estimator (Best Linear Estimator--BLE) is preferred. It filters the signal (deformations) from the measurement noise in an optimal manner and, furthermore, is not overly sensitive to the errors in the adopted geophysical deformation model. The application of these estimations to the maintenance of a new conventional terrestrial reference system is discussed. The functions of the system are twofold. The first is to monitor the global rotations and translations of the earth with respect to an inertial frame. The second is to monitor the nonglobal motions or deformations of the earth. The relationship between these two functions is outlined.