  Coordinating the operations of separate wireless systems at the wavelength
level can lead to significant improvements in wireless capabilities. We address
a fundamental challenge in distributed radio frequency system cooperation -
inter-node phase alignment - which must be accomplished wirelessly, and is
particularly challenging when the nodes are in relative motion. We present a
solution to this problem that is based on a novel combined high-accuracy
ranging and frequency transfer technique. Using this approach, we present the
design of the first fully wireless distributed system operating at the
wavelength level. We demonstrate the system in the first open-loop coherent
distributed beamforming experiment. Internode range estimation to support phase
alignment was performed using a two-tone stepped frequency waveform with a
single pulse, while a two-tone waveform was used for frequency synchronization,
where the oscillator of a secondary node was disciplined to the primary node.
In this concept, secondary nodes are equipped with an adjunct self-mixing
circuit that is able to extract the reference frequency from the captured
synchronization waveform. The approach was implemented on a two-node dynamic
system using Ettus X310 software-defined radios, with coherent beamforming at
1.5 GHz. We demonstrate distributed beamforming with greater than 90% of the
maximum possible coherent gain throughout the displacement of the secondary
node over one full cycle of the beamforming frequency.
