  We present a distributed antenna array supporting open-loop distributed
beamforming at 1.5 GHz. Based on a scalable, high-accuracy internode ranging
technique, we demonstrate open-loop beamforming experiments using three
transmitting nodes. To support distributed beamforming without feedback from
the destination, the relative positions of the nodes in the distributed array
must be known with accuracies below $\lambda/15$ of the beamforming carrier
frequency to ensure that the array maintains at least 90\% coherent beamforming
gain at the receive location. For operations in the microwave range, this leads
to range estimation accuracies of centimeters or lower. We present a scalable,
high-accuracy waveform and new approaches to refine range measurements to
significantly improve the estimation accuracy. Using this waveform with a
three-node array, we demonstrate high-accuracy ranging simultaneously between
multiple nodes, from which phase corrections on two secondary nodes are
implemented to maintain beamforming with the primary node, thereby supporting
open-loop distributed beamforming. Upon movement of the nodes, the range
estimation is used to dynamically update the phase correction, maintaining
beamforming as the nodes move. We show the first open-loop distributed
beamforming at 1.5 GHz with two-node and three-node arrays, demonstrating the
ability to implement and maintain phase-based beamforming without feedback from
the destination.
