A Modern VLF Radio Receiver Designed for the Array for VLF Imaging of the D-Region (AVID)

Very-low-frequency (VLF) radio waves are commonly used to remotely sense the D-region ionosphere. In the past few decades, a few prominent designs for VLF receivers have come to the forefront of the community, such as the AWESOME receiver developed at Stanford in the early 2000s. In the last decade, advancements in off-the-shelf electrical components and the obsoletion of others motivated a redesign of this receiver system. The result of this receiver design work is presented in this article with the measured characteristics of over ten complete systems. The newly operational array for VLF imaging of the D-region (AVID), incorporating the updated VLF receiver, is also presented. One early result from AVID is the characterization of the drift rates of the phase of two VLF transmitters. The NLK transmitter at 24.8 kHz was found to drift by −0.07886°/s $\pm ~0.00083^{\circ }$ /s, while the NML transmitter at 25.2 kHz was found to drift by −0.07201°/s $\pm ~0.00019^{\circ }$ /s. We also show that the transmitted power of these transmitters can change over time, by up to ~7% in the case of NLK. Finally, two early case studies are presented, showing AVID’s ability to observe and track: 1) the effects of an M7.2 solar flare on the D-region ionosphere and 2) the onset of a −333 nT Dst geomagnetic storm and the D-region’s response to it. These case studies demonstrate the tools necessary for future work incorporating spatial estimation algorithms to better characterize disturbances to the D-region ionosphere.