Generating and Receiving GPS-Timed Acoustic and Radiofrequency Costas Array Sounding Signals

Understanding the static and dynamic properties of the ionosphere is of great fundamental and technological importance. This work presents a low-cost, GPS-disciplined transmitter/receiver system that uses Costas arrays similar to those in FT8 to achieve simultaneous time-of-flight and Doppler shift measurements. The sharp correlation peaks in both frequency and time domains of the Costas arrays may allow for the study of difficult-to-measure properties of the HF propagation, such as the number of hops, ionospheric propagation type, etc. The narrow bandwidth and optimal autocorrelation properties also make the Costas array a good signal for the moderate noise of the HF bands and allows for QRP (low-power, under 5W) operation. Our hardware design consists of a microcontroller coupled with an AD9850 direct digital synthesis chip and an external 50 MHz crystal oscillator, as well as an Adafruit GPS module with a 1PPS signal. Our device is designed to interface with standard amateur radio transceivers for both transmission and reception. Its open-source architecture and reliance on readily available off-the-shelf parts make it easily reproducible, and suitable for rapid deployment. Some hams and institutions have already expressed interest in deploying units for coordinated data collection. By offering inexpensive, scalable, and accurate ionospheric sounding capabilities, this project aims to support ongoing research of the ionosphere and HF propagation.