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The availability of synchro-phasor data has raised the possibility of a linear state estimator if the inputs are only complex currents and voltages and if there are enough such measurements to meet observability and redundancy requirements. Moreover, the new digital substations can perform some of the computation at the substation itself resulting in a more accurate two-level state estimator. The main contribution in this paper is that this two-level processing removes the bad data and topology errors, which are major problems today, at the substation level. In Part I of the paper, we describe the layered architecture of databases, communications, and the application programs that are required to support this two-level linear state estimator. In Part II, we describe the mathematical algorithms that are different from those in the existing literature. As the availability of phasor measurements at substations will increase gradually, this paper describes how the state estimator can be enhanced to handle both the traditional state estimator and the proposed linear state estimator simultaneously. This provides a way to immediately utilize the benefits in those parts of the system where such phasor measurements become available and provides a pathway to transition to the "smart" grid of the future.