Designed by Greg Koo and Albert Lu, this microwave modulator demonstrated retrodirective backscatter with 4-ary Phase Shift Keying (PSK).
This would be the first of many low-powered, retrodirective designs that the group would develop. Powered by a TI MSP 430 micro-controller, the device consisted of a dynamically switched Van Atta array in which 4 different phase lengths could be inserted into the electrical pathways. The result was a QPSK backscatter signal with large retrodirective gains.