Software Defined Radio Injection-Locking using a GPS signal for multichannel coherent receivers

This paper explores the potential benefits of combining the use of injection-locking techniques with GPS signals as a common clock source when applied to a low-cost Software Defined Radio (SDR) to improve the accuracy of coherent multiple receivers. Coherent systems impose severe requirements on the frequency stability of the signal source at the receiver. In this work, injection-locked oscillators are used as local clock receivers, which inherently synchronizes the SDR analog digital converter (ADCs) sampling times and keeps the local oscillator locked on to the GPS stimulus periodic signal. This paper illustrates the hardware modifications needed for to the injection locking oscillators of eight RTL-SDR radios and the theory behind it, and experimentally measures the degree of coherency in the frequency, phase and time synchronization to verify the proposed method. The coherency demonstrated in the results prove the feasibility of using beamforming, multiple input multiple output (MIMO) and RF transmitter geo-localization.