IonTV: Using WWV Timing Reference Signals to Observe Ionospheric Variation

For decades, an AM modulated time signal has been broadcast at multiple HF frequencies by the National Institute of Standards and Technology (NIST).  Shortwave radio stations WWV in Colorado and WWVH in Hawaii use these frequencies for the broad dissemination of accurate coordinated universal time information.  As the HF signal traverses the ionosphere, propagation effects ensue, and the high temporal precision of the original transmitted signal provides an attractive potential for wide-sense monitoring of ionospheric variations.  We present the results of an ongoing set of data collections and statistical analysis of the received variation in WWV timing signals aimed at extracting ionospheric propagation effects.  The work includes design of a software defined receiver (SDR) for processing the amplitude modulated dual sideband (AM-DSB) timing signal. By observing the time shift between consecutive seconds of the 10MHz WWV timing signal, reflected from the ionosphere, the change in the effective height of the ionosphere can be estimated.  Simultaneous measurements taken from different observation angles allow a more accurate sensing of ionospheric electron density variability as projected into refractive effects.  The project also has a goal of creating a straightforward and reliable way for hobbyists and citizen scientists to demodulate and process their own NIST timing data. We describe a sample analysis of several blocks of WWV received data, both on remote paths and locally through groundwave propagation near the Colorado transmit array, including simultaneous collects. To process the timing data, several approaches will be described, including a heterodyne SDR with a digital phase-locked-loop (PLL).  Carrier offset tracking using PLL techniques produce Doppler shifts that are associated with traveling ionospheric disturbances and inherent electron density variability.  Demodulation and amplitude/phase analysis of the 100 Hz subcarrier of WWV can also provide precise delta-time information on ionospheric propagation through examination of variability in arrival of the leading edge of 1 pulse-per-second ticks.  Results to date suggest that variation between consecutive second markers is a uniformly distributed Gaussian random variable with at least some of this variation due to ionospheric factors, although systematics must be addressed.